HTO/HL Launch with Ramjet Assist

This article documents the results of a back-of-the-envelope sizing study similar to that of reference 1.  The differences are (1) horizontal takeoff,  and (2) the orbital vehicle is a small rocket spaceplane instead of a minimal space capsule.  For this study,  the ultimate payload of the spaceplane is a pilot and two passengers,  all spacesuited with about a day’s life support supplies,  plus about 100 pounds (each) of luggage for the two passengers.

For purposes of quickly locating reference 1,  see "Manned Launch to LEO Using Ramjet Missile Technology",  dated 10-27-13.  

The study in reference 1 emphasized the necessity of strict frontal area proportioning to enable hypersonic flight with ramjet propulsion in a cluster vehicle.  For this study I assumed ramjet takeover to be Mach 1.5,  and the staging speed to be Mach 5,  again at 60,000 feet (18.3 km).  All rocket propulsion was assumed to be LOX-liquid methane (LCH4),  and I used some ramjet performance data calculated for RJ-5 synthetic (kerosene-like) fuel,  although the design intent here is that the ramjet use LCH4 as its fuel. 

Past attempts at a design like this proved too difficult to believe success was possible,  based on a simple two-stage airplane concept.  In that concept,  the second stage was rocket-only,  and the first stage was assumed to be rocket plus ramjet in a convenient parallel-burn configuration.  The wing and rocket takeoff hardware always proved too draggy and too heavy.  Even with a staging velocity of M6 at 60,000 feet,  the velocity increment to orbit was too demanding for anything but LOX-LH2 propulsion in the second stage.  By changing those basic concepts,  to something seemingly a little more complicated,  I was able to use LOX-LCH4 in all items,  with very conservative inert fractions that support reusability. 

Trajectory,  Flight Phases,  and Configuration

I reduced the mass ratio required of the spaceplane by raising the energy of its stage point to 4.08 km/s at about 150 km.  The rocket flight from there to orbit is thus all-exoatmospheric,  but still subject to gravity losses.  That made LOX-LCH4 rocket propulsion feasible in the spaceplane at inert mass fractions that could be credibly reusable.  This is the final ascent “burn” shown in figure 1.  Weights are given in figure 2.  The details of the spaceplane rough-out design are given in figure 3. 

That approach then required a rocket booster stage to take the spaceplane from ramjet staging conditions to the spaceplane engine ignition point.  Because this combination starts off in hypersonic atmospheric flight,  I chose a serial stage integration with the same body diameter as the spaceplane.  That keeps the stack’s hypersonic drag to a minimum,  making it very more likely that the ramjet propulsion strap-ons can push this “payload” to at least Mach 5.  That stage point is shown in figure 1 as Mach 5 at 60,000 feet (1.48 km/s at 18.3 km).  Weights are given in figure 2. 

Details of the booster stage are given in figure 4.  This item includes a guidance and control package and an ablative entry heat shield up forward,  and a recovery chute package aft,  for ocean splashdown recovery,  as indicated in figures 1 and 4. 

Both the booster stage and the spaceplane are presumed to be propelled with 1000 psia chamber LOX-LCH4 engines with exit bells sized for perfect expansion at the backpressure extant at 60,000 feet.  I used a standard chart to determine expansion ratio and optimum thrust coefficient,  assuming the gas specific heat ratio was 1.2,  and which chart includes the nozzle kinetic energy efficiency associated with an average 15-degree half angle in the bell.  Reference 2 suggests a chamber c*=6120 ft/s for 1000 psia with LOX-LCH4.

Both of these phases were analyzed with the simple rocket equation,  with the actual velocity change ratioed-up by factor 1.10 to “account” empirically for gravity and drag losses.  The spaceplane is a hypersonic glider for re-entry,  with a heat shield that is part refractory,  part ablative,  and lands as a dead-stick glider.  I sized its wing for a 90 KCAS (knots calibrated air speed) landing speed without high-lift devices,  factor 1.2 away from it estimated stall speed (as in aircraft design practice).    This is crude,  but “ballpark correct”.

To reach the ramjet staging point from ramjet takeover (see again figure 1),  I decided to use ramjet strap-on pods similar to those in reference 1,  and shown here in figure 5.  The RJ-5 performance figures I used would be conservative representations of performance fueled by LCH4,  by a few-to-several percent.  I averaged ramjet Isp (predicted from RJ-5) and did a rocket-equation estimate,  where the velocity increment was factored up by (I hope) a conservative value of 1.5.  This is a very rough estimate at best,  but it produced similar values to those I got for the rather similar pod design in reference 1.  So,  it should be “in the ballpark”.  Weights are in figure 2.  By themselves,  the pods are 50% fuel and 50% inerts.  There is plenty of inert margin to cover the recovery gear,  and the robustness for reusability. 

I used LCH4 density to size out the required ramjet fuel tank volume,  annular to the inlet duct as shown in figure 5.  This pod has the same translating-spike supersonic inlet that maintains shock-on-lip from Mach 1.5 takeover to staging at Mach 5.  These pods are mounted on each side of the payload core for supersonic body lift,  and sized for a tiny thrust margin over cluster drag at Mach 5.  I used the same circumferential fin stowage idea as in reference 1.   

The cluster configuration with the ramjet pods must fly hypersonically,  the same situation as in the reference 1 study.  Exposed fins have been minimized to reduce drag so that the ramjets can reach the desired speed.  The real problem is hypersonic aeroheating,  most especially the extreme localized heating from shock wave impingement upon adjacent structures,  which is inherent in cluster vehicle designs.  The choice of Mach 5 or 6 makes little difference,  both are extreme conditions.  There will need to be tough ablative replaceable panels located in the impingement zones,  to act as sacrificial protection against the damage the shock waves will do otherwise.

I also used the same biplane swivel-stowed wing idea as in reference 1,  so that these pods can return to launch site and land,  as indicated in figure 1.    Pods like this have very little power-on drag,  that being only skin friction plus some cowl lip pressure drag.  That is how a cluster vehicle can be pushed to Mach 5 or 6,  even in the thinner air at 60,000 feet.  The key is cutting payload core drag to a minimum,  and then sizing ramjet pod diameters large enough to push it to the speed needed.  Pod landing speed is under 100 KCAS,  without any high-lift devices.   

These strap-on ramjet pods have ejectable booster nozzles,  but no solid-propellant integral boosters,  unlike those in reference 1,  otherwise the basic design approach is identical.  The idea here was to use the ramjet chambers as the takeoff liquid-propellant rocket booster engines,  on the theory that the ejectable nozzles at a (guessed) 100 lb each would be lighter than yet another set of rocket engines of rather large thrust for fast takeoff and climb acceleration. 

Thus,  the takeoff "rocket" booster need only be a large wing and propellant tankage,  plus landing gear and a small fanjet engine to help it return to launch site for landing.  This is the very first flight segment shown in figure 1.  The takeoff booster package is shown in figure 6.  It has a delta wing big enough for under-100 KCAS landing without high-lift devices,  yet with a leading edge sweep for min drag at Mach 1.5,  even with a subsonic wing airfoil section. 

This configuration has a practical takeoff speed at full gross weight of about 200 KCAS (which sized the wing area),  similar to a very high-performance fighter.  But,  it would be way too draggy to fly under anything but massive rocket thrust,  at speeds above Mach 1.5.  That’s why I staged-off both the wing and the boost propellant at ramjet takeover,  and then made them into a single component for easier recovery. 

This takeoff booster is positioned underneath the rocket core and ramjet strap-ons,  as indicated in figure 2,  such that centers of gravity and pressure match up longitudinally.  The wing is high-mounted,  with a low-mounted horizontal tail that has twin vertical fins.    The takeoff booster propellant is in the fuselage,  and is pumped up to the ramjet strap-on pods,  where it is burned at high thrust.  Ejectable nozzles are sized for optimal sea-level expansion from 1000 psia.  Note from figure 6 the lower Isp than that for the near-vacuum engines in the spaceplane (figure 3) and booster rocket stage (figure 4). 

This takeoff booster turns after staging off,  decelerating subsonic,  and travels back to landing at the launch point by means of a small fanjet inside the tail cone,  “with a big flush inlet” not sized here.  That fanjet could burn LCH4 residuals,  but it is probably just as easy to include some jet fuel for this. 

The takeoff cluster is rather draggy,  so I used factor 1.5 as a (hopefully) conservative way to address the velocity increment indicated from the trajectory.  This went into a simple rocket equation estimate for mass ratio.  Weights are in figure 2.  The takeoff booster by itself is 60% propellants and 40% inerts,  not too unlike transport and bomber aircraft. 

Concluding Remarks

These are “ballpark” results only,  and provide only a “realistic” startpoint for more traceable back-of-the-envelope estimating methods,  which would in turn be the startpoint for trajectory-code models. 

Payload fractions for the delivered persons and luggage,  and for the dry-tank spaceplane,  are also shown in figure 2.  These are remarkably good,  considering that every component is intended to be high-lifetime reusable. 

Ramjet assist evidently “bought” the weight allowances necessary for reusability,  at no increase in launch weight,  looking at the spaceplane-as-payload 3.72% of launch weight case. 

I did not adjust the weight statements for the ejecta lost as ramjet takeover obtains (maybe 500-600 lb covering eject nozzles and frangible-glass port covers).  At this level of analysis,  those are “lost in the uncertainty”. 

I did not include any propellant allowance for deorbiting the spaceplane.  That actually could be done with attitude thrusters or a small solid propellant cartridge.  The size of those items is “lost in the uncertainty” at this level of analysis. 

I did not include any means in the spaceplane for go-around at landing,  or cross-range maneuvering during descent.  That is a serious safety and practicality lack,  and is probably a small but significant item at this level of analysis. 

These components are actually too slender,  an artifact of the assumed minimal credible diameter of 5 feet assumed for the spaceplane fuselage.  That diameter is consistent with serial seating of the 3 occupants.  Making that diameter larger would result in more realistic length/diameter ratios for all of the components. 

References

1 G. W. Johnson,  “Manned Launch to LEO Using Ramjet Missile Technology”,  article posted 11-3-2013,  on http://exrocketman.blogspot.com

2 Pratt and Whitney,  “Aeronautical Vest-Pocket Handbook”,  12^th edition 21^st printing,  December 1969.

Figure 1 – Basic Trajectory and Phases of Flight

Figure 2 – Configuration and Weight Statements

Figure 3 – Upper-Stage Spaceplane

Figure 4 – Rocket Booster Stage

Figure 5 – Ramjet Strap-On Pods

Figure 6 – Take-Off “Booster” (Wing and Propellant)

Aviation Alternative Fuel Compatibility Issues

This is a very complex question.  There are three big parts to it:  (1) a huge legacy fleet of aircraft with components known to react badly to alcohols,  (2) pilots not trained to use the different fuels which really do respond quite differently in so many ways,  and (3) an already-disruptive transition from low-aromatic higher-leaded 100/130 grade aviation gasoline to the high aromatic lower-leaded 100LL grade of aviation gasoline.  During that transition to 100LL,  many of the same types of seal failures were seen as are seen converting to ethanol-containing fuels. 

With ethanol already in motor gasoline,  usually at the 8-10% level,  exposures have already occurred with reported impacts that vary widely.  This is for the airplanes that can use the motor gasoline STC.  That STC actually prohibits the use of ethanol blends,  which are now about all that is available for motor gasoline. 

Therefore,  that conversion is not so popular now,  in part because of the prohibition,  and in part because the problems encountered when “using the ethanol-containing motor gasoline anyway”.  These problems trace to materials in common aviation use that are incompatible with ethanol.  I have grouped them in the discussions below.  For non-aviation items,  see "An Update on Ethanol Fuel Use" dated 11-2-13.  

See Update 9-26-16 at end of article.

Metals

Intergranular corrosion of aluminum alloys is worse with the aluminum-copper Duralumins.  These are the primary structural materials in aircraft,  including aluminum tubing for fuel lines.  That is why “wet-wing” tanks and aluminum tube fuel lines need a surface coating to resist alcohol-induced corrosion.  Unfortunately,  the Alodine process usually recommended for these items requires their removal from the aircraft,  a labor-intensive and expensive thing.  This could be remedied by changes in production for new aircraft,  but the legacy fleet is very large,  and it has changed little with the passage of decades. 

You would be better-off using a compatible bladder-lined tank,  and Alodining just the aluminum lines (which raises fuel bladder compatibility as discussed below). 

Some of the steels exhibit some corrosion sensitivities,  too,  but not nearly as bad as the Duralumins.  The zinc casting alloys seem more sensitive to steam than alcohol,  as regards intergranular corrosion. 

Non-Duralumin aluminum casting materials for carburetors and similar devices are less sensitive to these effects.  These generally hold up rather well with ethanol.  I have several from air-cooled engine cars that are just fine after years of calendar time,  and hundreds of hours of operation,  with E-85 ethanol. 

Metering Devices and Related Items

The real issue about fuel metering devices is twofold:  (1) seals and (2) small passages.  Seal compatibility is discussed below.  There are potentially-serious problems with small passages,  whether the fuel is gasoline,  alcohol,  or blends of them.  When gasoline (motor or aviation) evaporates,  it leaves behind a coating of gum and varnish on the solid surfaces (motor gasoline is far worse,  but both do it).  These deposits can plug small passages,  as anybody who has ever cleaned-out a “dirty carburetor” on a car or lawnmower already knows.  It can happen to aircraft metering devices,  too.

The problem is slightly different when ethanol evaporates inside a metering device.  It leaves no gum and varnish,  and the corrosive effects of direct liquid contact are not the bigger source of troubles.  It is the ethanol vapor that attacks the aluminum,  leaving behind a fine gritty aluminum oxide,  and a low-density,  “sticky-gooey” aluminum hydroxide gel-like material.  The grit is a really big wear problem for any moving parts,  and the gooey gel plugs up the passage. 

Oddly enough,  gasoline turns out to be a decent solvent for the ethanol-induced deposits,  and ethanol is a decent solvent for the gum and varnish that gasoline leaves behind!  That raises the hope that blends of the two might behave better than either fuel “neat”,  but no one really knows. 

The real point is that engines should not be parked without use for months at a time,  with fuel in the fuel delivery system.  Bad things will happen with either fuel.  If you know the engine will not be run for a long time,  the better choice is cut off the fuel to it at the tank valve,  and drain the lines,  pumps,  and metering device dry.  That’s true with any fuel,  and therefore very good advice.

Composite Structural Materials

The most common choices are epoxy- and polyester- or vinyl ester-based materials.  The choice of carbon or glass fiber really isn’t the issue,  it is degradation of the matrix polymer that can lead to softening and failure.  Many epoxies are good with gasoline wetting,  so that one may build a “wet wing” fuel tank with it.  Some of these are OK with ethanol,  others are not. 

You have to “dunk test” a sample of your specific composite material to find out.  The softening of the epoxy will show up in a couple of days to a week.  The same is true of polyesters and vinyl esters.  You simply must check before risking ethanol exposure (or gasoline for you homebuilders).  If you must use ethanol,  and you find your matrix is susceptible,  then you must use a fuel bladder,  and it had better not ever leak!

Sealing Materials and Non-Metallic Components

Neoprene,  ethylene propylene,  and Buna-S work well with ethanol,  just as they do with gasoline.  The fluorosilicones and the Vitons are not compatible with ethanol.  Nobody knows for sure about the Buna-N materials.  These are all materials used in O-rings and similar seals,  throughout the aviation fleet.  The polysulfides often used as fuel tank sealants don’t work with ethanol.  (Some of these same elastomer seal material problems cropped up with the transition from 100/130 to 100LL,  due to the high aromatics content,  which is also a corrosive solvent.)

There are some nylons that react badly to ethanol by swelling and weakening.  Teflon is pretty much impervious to everything.  The Nitrophyl and cork float materials are incompatible with ethanol,  although polypropylene and polyethylene seem suitable.  All sorts of fuel metering,  filtering,  and pumping is done with devices incorporating these materials in their working parts. 

Polyurethane (foam or otherwise) is not compatible with ethanol.  No one knows about the Acetal polymers.  These can be found in some fuel tank gaging devices. 

Fuel Bladders

Neoprene works,  polyurethane does not.  Polyurethane is by far the most common fuel tank bladder out there in the aviation fleet.  Very unfortunate for those wishing to do an ethanol conversion!

Polar Solvent Effects in Pumps

Once the incompatible shaft seal material fails in an electric fuel pump,  the fuel can leak into the electric motor itself.  Nonpolar gasoline does not cause electrical arcing problems,  polar ethanol does.  Both are considerable fire hazards when exposed to electric sparks.  You have to avoid this with the right seal material in the first place.  The problem is the legacy fleet and the store of parts that supports it:  many or most of these devices are not compatible with ethanol fuel. 

Capacitative Fuel Gaging Devices

These cannot work when the fuel is a polar solvent.  Ethanol is polar,  gasoline is nonpolar.  Ethanol fuel doesn’t work at all with capacitative gaging,  and neither do ethanol-containing blends. 

Changes to Operating Characteristics and Pilot Training

These depend upon how the conversion was done,  and the nature of the fuel or blend to be flown.  I know about the characteristics of some of the conversions,  but not all of them.

                Cold start

“Neat” denatured ethanol meeting ASTM specification D-4806 “Fuel Grade Ethanol” is 95% ethanol,  5% gasoline or petroleum alkylate,  and under 0.5% water.  It will not cold start below about 50-60 F ambient temperature,  because the volatility is too low (very low vapor pressure,  a high latent heat of evaporation,  and essentially a constant boiling temperature instead of distillation curve behavior).  To use it requires re-plumbing the engine start primer to a separate canister containing some gasoline. 

This separate start canister change can be avoided if the fuel contains 15-or-more% gasoline instead of 5%,  as in automotive E-85 and aviation AGE-85.  The automotive grade isn’t really 85% ethanol in winter.  It can be as low as 70% ethanol,  and is usually about 75% ethanol, during winter,  for even better starting than “real E-85”.  AGE-85 is specified to be 85% ethanol,  14% gasoline,  and 1% biodiesel. 

                Mixture Control

The conversion of just about any aviation carburetor,  and the Bendix RSA-5 series of injection devices,  leads inherently to a flex-fuel system capable of metering all the intermediate blends.  I honestly don’t know about conversions of the other fuel injection systems. 

For a carburetor,  you want sufficient flow capacity for ethanol in both the main jet and idle systems,  at otherwise the same air flow pressure signals.  Basically,  the jets get bigger.  Because it is an aviation carburetor,  the pilot has (and is expected to use effectively) a mixture control.  It is a change in the operation of this mixture control that the pilot will see;  the rest appears unchanged to him. 

The same thing is true for Bendix Airmotive RSA-5-based fuel injection.  You need to drill out the idle valves slightly for greater idle flow capacity,  and you use enlarged fuel injectors at the cylinders,  all at the same regulated fuel line pressures as before. 

When operating on ethanol with a properly-executed conversion,  “full rich” will be all the way forward with the mixture control,  just like it was before.  If you pull it all the way lean,  the engine will die for lack of fuel,  just like it did before. 

On gasoline,  however,  “full rich” is only about halfway forward.  If you go all the way forward,  the engine will die from overrich mixture.  All the way back is still lean-out,  just as before.  Only the sensitivity is different:  a shorter travel of the control gets the full effect.  The risk is the lack of a mechanical stop for “full rich”.  

On intermediate blends,  “full rich” will be between those forward and halfway positions,  more or less linearly proportional to the blend.  Mishandling the control can cause the engine to die from overrich mixtures.  Greater pilot awareness of,  and a gut feel for,  how the engine is running is simply required.  This does take experience with engines;  it is not for the novice.  But,  it is not hard,  and can be learned quickly.

Ethanol burns differently than gasoline (soot-free flame),  and can be leaned far more aggressively than gasoline.  Blends fall somewhere in between the “neat fuel” extremes. 

With gasoline,  you need to enrich the mixture very slightly from the “perfect” mixture point in order to control hardware temperatures and avoid engine damage risks.  This usually takes the form of finding the peak exhaust gas temperature,  and then enriching slightly to drop that exhaust gas temperature by some small margin,  usually around 50 to 100 degrees F.  You can just “peak” the sound of the engine,  and then bump the mixture control a tad richer from there,  and achieve essentially the same result,  just not as repeatably. But it works just fine,  all the same.  

With ethanol,  hardware temperatures will be lower for the just about the same exhaust gas temperatures.  This is because of the reduced heat load on engine parts from the adjacent flame,  since there is little soot in the flame to radiate.  You can simply find the peak exhaust gas temperature,  or even just “peak” the sound of the engine.  Such mixtures are nearer the excess-oxygen point,  but the hardware is running cooler,  which acts to offset the oxygen risk. 

                Fuel Samples and Water Bottoms

With ethanol in the blend,  you will never see water bottoms,  which is completely at variance with all pilot experience operating on gasoline fuel.  With ethanol in the blend,  any such water bottom in the tank goes into solution,  right up to the phase separation point. 

If separation occurs,  the water and all the ethanol go into the bottom layer,  being denser.  The now-dry hydrocarbon floats on top.  Any dye in the fuel goes with the hydrocarbon.  It is easy to see the layers,  even if both components are clear.  But your typical tank bottom sample will be clear with phase separation in the tank,  when the 100LL (or 100LL blend with ethanol) ought to be blue. 

Very simply put,  do not ever fly with a phase-separated tank!  That boundary gets shaken up in flight,  so that you draw globules of one layer or the other,  not the mixture,   into the engine.  Power surges and possibly detonation can result. 

Unfortunately,  predicting that phase separation point is difficult at best.  But there is a simple go/no-go field test.  It uses the clear-tube fuel quantity gaging tube that most aircraft now have. 

                Is My Tank Separated As It Is?

If you gage a tank with the tube,  you essentially take a “core sample” of the fuel in the tank.  Experience shows that any separation boundary in the tank is preserved in the “core sample” taken with the gaging tube.  You can see it.  If you do,  drain it down to one layer (the dry 100LL gasoline),  and refuel on top of that.  (If you are using automotive gasoline,  drain it all and simply replace it with fresh fuel.  It has lost its ethanol content,  which was a large part of its octane rating.) 

                Will My Tank Separate Upon Refueling?

Your tank’s fuel blend may not be separated,  but still might separate if you refuel on top of it with one or the other fuels used neat.  There is a way to determine that risk,  which requires no knowledge of the blend proportions in your tank,  or in the fuel that you will add.  It does require than you can quantify the number of gallons still in your tank and how many gallons you are about to add.  In other words,  you need to know the “calibration” of your fuel quantity indicator. 

Use your bottom sample and a sample of what you propose to add,  in the closest approximation to the volume ratio that will obtain if you refuel.  Shake it up and watch it for separation for a couple of minutes.  If your sample separates,  so will your tank,  so don’t refuel that way.  Instead,  drain down and refuel with one fuel.  If it doesn’t separate,  neither will your refueled tank,  so top off your tank and fly on happy. 

                What If the Fuel Is “Old”? Does It Get Too Wet?

It is particularly important to investigate whether your tank is separated,  and whether it will separate upon refueling,  if the aircraft has sat idle for months with an ethanol blend in it.  

Aircraft tanks are vented,  and can accumulate moisture from the atmosphere in the air space on top of the fuel,  as the tank “breathes” every 24-hour cycle.  (With straight gasoline,  this is where water bottoms come from.)

The so-called azeotrope mixture of straight ethanol fuel is 5% water.  That is an unrealistic upper bound for what a real vented tank can absorb,  even over a very long period of time.  More realistic results are 1-2% water after 6 months to a year sitting,  even in the very humid air along the Texas Gulf Coast. 

I’ve never seen more than 2% water myself.  It’s usually no more than 1% around Waco McLennan County,  even after a year sitting.  If the aircraft sits idle that long,  the really significant risks are fuel evaporation deposits in the metering device as described above,  not water absorbed in the fuel. 

Putting straight 100LL on top of E-95 ethanol residuals with 2% water can indeed pose a refueling separation risk,  sometimes.  Sometimes not,  maybe quite often not.  The point is,  you simply do not know.  It depends upon how much fuel gets added compared to how much is still in the tank and how wet that has become.  That’s why the bottom sample prediction test described just above is so important when operating with blends. 

In the extensive experimental blend fuel work that I have done with cars,  blends near E-30 to E-40 typically require more than a 15-20% water addition to force a phase separation.  I typically use a 30-35% water addition when I run the forced separation test checking blend strengths,  just to make the test fully reliable.  It’s pretty much the same behavior at E-10 levels,  and at E-85 levels. 

                Power and Economy

Different investigators report different outcomes from power tests on ethanol versus gasoline.  It depends on whether you aggressively-lean with the ethanol,  as discussed above.  If you do,  the lower-compression engines may show around 5-10% better power and efficiency on ethanol.  The higher-compression engines may show 10-20% better power and efficiency on ethanol.  But you must aggressively lean,  or you won’t see much of this effect.  Maybe none. 

If you don’t aggressively lean and thus you don’t see the power and efficiency improvement,  then your fuel flow rates will essentially match the volumetric heating value ratio of the fuels.  Heating value is proportional to the ideal or stoichiometric air fuel ratio by mass,  so for gasoline (14.5:1) versus ethanol (9:1),  your ethanol flow rate will be about 14.5/9 = 1.61 times larger than your gasoline flow rate,  at otherwise the same power setting and conditions. 

If you do aggressively lean the ethanol (and you safely can),  then for a power improvement of 10% at lower compression,  you get an ethanol/gasoline flow ratio near 14.5/(9*1.10) = 1.46.  If you have a power increase of 20% at higher compression,  then the flow ratio is near 14.5/(9*1.2) = 1.34.  That sensitivity to leaning strategy is why different investigators using different leaning strategies get such widely-varying results. 

Blends are going to fall somewhere in-between,  and quite probably not in a linear fashion with blend ratio.  The leaning strategy also needs to vary with blend,  as there is more and more hardware-heating soot radiating in the flame,  as gasoline content increases.  To the best of my knowledge,  those tests to optimize leaning strategy with blend,  have not been done.  The risk is engine damage (usually burned valves and seats) if you get it wrong.  It takes a while to incur the damage. 

Fuel Density and Gross Weight

Gasoline (motor or aviation) is typically variable in density,  but usually about 6.1 to 6.2 lb/US gallon.  E-85 is pretty close to 6.5 lb/US gallon,  E-95 a little bit denser still.  That’s not much of a density difference,  and therefore not much of a risk for gross weight,  unless you are already (or habitually) very near the maximum limit.  Just use the tank volume and the higher blend density to figure the higher fuel weight.  If that puts you over gross weight limits,  then you offload a little cargo or fuel.  Simple as that. 

Concluding Comments

There’s enough risks and difficulties associated with using ethanol or ethanol blends in the aviation fleet,  that you probably don’t want to do this unless you have some other compelling need.  The most difficult risks are materials compatibility problems.  Corrosion can occur,  and must be mitigated.  Doing the conversion itself is not very hard,  nor is learning how to fly with it. 

Don’t do this at all if you have a capacitative fuel gaging system.  Period. 

One of the biggest issues is fuel flow capacity,  when ethanol requires substantially-higher flow rates.  Most of the time,  you will simply have to convert gravity flow systems to pump-fed systems.  Just up-sizing the fuel lines is nearly always inadequate by itself,  although it still needs to be done,  even with adding fuel pumps.  The convenient time to Alodine the fuel lines is when you up-size them,  though. 

You will have to select pumps and check valves with ethanol-compatible seals,  namely neoprene.  If you don’t pay close attention to this,  you will get into trouble later when the seals leak,  particularly in an electric fuel pump.  Check for neoprene diaphragms and polypropylene plastic parts inside engine-driven fuel pumps,  and inside carburetors or RSA-5 devices. 

Teflon-lined fuel hoses on the engine are the best choice,  although neoprene or Buna-S rubber is OK.  Be sure any fuel bladder is neoprene,  not polyurethane.  You also need to check the compatibility of the materials in your mechanical fuel quantity indicator (see “horror story” below).

Be sure your fuel sample device is polypropylene.  And be sure to acquire a tank-gaging sample tube,  especially if you intend to fly blends. 

The available STC’s are for either gasoline or ethanol,  not blends.  To the best of my knowledge,  blends are still only allowed in experimental category.  If you are experimental,  you have more latitude to use automotive fuel components,  which are generally a lot more ethanol-compatible these days.  Experimental guys therefore have it a little easier. 

                “Horror Story” about Fuel Quantity Indicators

There are two structural transparency plastics:  Lexan and Plexiglas.  Both are acrylic plastics,  just different manufacturers.  Long-term liquid ethanol contact can cause surface crazing in both of them (a web of tiny cracks).  But,  they are both quickly and completely destroyed by warm ethanol vapor contact.

When converting an old Piper “Pawnee” to use E-95 ethanol,  I ran across a mechanical fuel gage under a clear blister,  which was also the vapor collection space for the tank vent.    This works fine with gasoline,  but repeatedly failed in about half an hour with ethanol on a nice warm day.  The blister literally collapses into a wadded-up mess.  (It’s not a cheap item,  either.) 

That blister had an integral flange with drilled holes.  It bolted directly to the airframe on top of a paper gasket.  I had to replace it with glass,  which you simply cannot bolt down that way,  and for which custom blown parts are very,  very expensive. 

The solution was a steel base plate to which the ring lid of a Mason jar was welded.  A little silicone adhesive on the threads of the Mason jar sealed it to the ring lid.  This worked,  and is safe,  as demonstrated by test to the FAA. 

By putting the tailwheel in a chair to level the deck,  we made paint marks on the jar as we filled the tank.  That way,  we actually did end up with a more-accurate fuel quantity indicator than the original,  which was nothing but an inaccurately-positioned decal.

And that’s why there is a grocery store Mason jar in the STC for using ethanol in a Piper Pawnee!  

Update 11-4-13:

Taking into account my automotive,  small engine,  and aircraft experiences,  I would assess things as follows:

The automotive industry has long adopted materials compatible with high aromatic and ethanol content in gasoline.  That plus flex-fuel cars has driven the supporting parts industry in that direction for decades.  As a result,  you can reliably use blends or convert to E-85,  pretty much in anything from modern to very old.  And I do mean many decades old!

It appears to me that the 4-stroke small engine folks have also largely made the transition to ethanol-compatible materials.  I’m not so sure about the 2-stroke small engine folks,  and I have not run any stiff blend experiments in 2-stroke because of my preconceptions regarding lubrication.  E-10 seems to be OK in 2-stroke,  though,  and based on that,  I don’t have much concern about E-15 (it’s just not that different). 

I don’t think the boat motor folks have made the transition to ethanol-compatible materials yet,  not even on the 4-stroke side of the house.  It’s boat motors and 2-stroke equipment that I hear the most horror stories about.  (I haven’t really investigated this,  so my perception is only just that: a perception.)  If there was going to be a problem with water bottoms,  I would expect to see it in boats,  though. 

The light aircraft industry is still using the same materials they were using half a century (or more) ago.  Many of these are not even good for high-aromatic gasoline,  not to mention ethanol.  Commercial aircraft are today nearly all turbine,  for which biodiesel content in the jet fuel is proving to be a good fit (that’s a another whole topic area not discussed here).

Bottom lines:

Automotive:  feel free to experiment with stiff blends and neat ethanol.  The materials in common use for the last few decades seem to support ethanol compatibility,  by and large.  No mods necessary up to about E-42,  although I don’t recommend over E-35 because of minor cold start problems.  Conversions to straight E-85 are easy,  but will be successful only if you do all 3 required items (mixture,  timing,  extra intake heat).

4-stroke small engine:  feel free to experiment with stiff blends up to E-35 without mods.  I’d not recommend neat ethanol or E-85 conversions,  because the carburetors do not have removable jets and are therefore so very hard to modify.  It’s very hard to change the timing in a magneto ignition,  too.  The supporting parts seem to me to be largely ethanol-compatible in recent years.

2-stroke small engine:  I personally would not experiment with stiff blends in these,  because of lubrication fears (oil-in-fuel,  with an oil solvent also in the fuel?),  and because the supporting parts do not yet seem to be very ethanol-compatible.  Wait till the supporting parts industry has made the transition. 

4-stroke boat:  you can try stiff blends in unmodified engines,  but be aware that supporting parts may well be incompatible with ethanol.  You will have to determine the neoprene/polypropylene issues for yourself.  I’d rather wait until the supporting parts industry has made the transition,  before I went above about E-15.  If your parts are compatible,  up to E-35 should work just fine. 

2-stroke boat:  same as 2-stroke small engine,  and for the same reasons. 

Gasoline airplane:  you need an otherwise-compelling reason to do ethanol blends and conversions,  and you will run into incompatible materials and metal corrosion problems everywhere you look.  But it can be done.  And it does work. 

Turbine aircraft:  out of scope here.  Biodiesel blends do work just fine,  though.  

Update 9-26-16:

              From Biofuels Digest 9-25-16:

              DOE Study

In Colorado, a study conducted by DOE’s National Renewable Energy Laboratory (NREL), found that the petroleum components of ethanol-blended gasoline become degraded and unfit for use in an engine long before the ethanol portion takes up enough water to cause phase separation in the fuel tank. “Phase separation” occurs when an excessive amount of water is introduced into the fuel tank leading the ethanol and water to mix and sink to the bottom of the tank. In other words, gasoline becomes “stale” and unusable before water uptake by the ethanol component becomes a concern.

As part of the study, NREL scientists stored gasoline-ethanol blends ranging from E0 (0% ethanol) to E85 (83% ethanol) in actual lawn mower fuel tanks over several months in a climate-controlled chamber meant to replicate hot, humid environments like Houston and Orlando. The samples were tested at regular intervals for evidence of gasoline weathering and water uptake. In every case, the hydrocarbon components of the fuel became unfit for use in an engine before water uptake became a concern.

For gasoline-ethanol blends, it often took more than three months for phase separation to occur, meaning the fuel had already weathered to a point it was unusable. “In a small engine fuel tank in a constantly high-temperature, high-humidity environment, it takes three months or longer for E10 and other ethanol blends to take up enough water for phase separation,” the study found. “This confirms the statement by Mercury Marine that water uptake in E10 blends ‘…does not happen at a level or rate that is relevant.’”

President and CEO Bob Dinneen offered the following comments on the new study:

“Simply put, critics who continue to suggest E10 is a problem for small engines and boat motors are all wet. This research from NREL clearly demonstrates that gasoline goes bad long before the ethanol in the tank could cause any problems due to moisture uptake.

“Every manufacturer of small and off-road engines has approved the use of E10 in their equipment for many years. If owners of this equipment simply follow the manufacturers’ recommendations for fuel, maintenance, and winterization, they won’t have any issues at all. But, as this study shows, letting gasoline sit in your tank for extended periods of time is likely to cause some issues—irrespective of whether the gasoline contains ethanol or not.”

My take:  the water problems that small engine and boat motor owners report are due to the item being left out in the rain,  not the humidity.  The fuel tank caps have air vents.  The rain gets in that way.  Plus,  many folks are using fuel more than 3 months old.  Both a recipes for bad performance and maintenance problems,  even with zero ethanol in the fuel.  

An Update on Ethanol Fuel Use

I keep running into claims that ethanol destroys engines or fuel systems,  primarily from the small engine,  boat motor,  and airplane people.  Airplanes are a separate subject,  since the federal aircraft regulations resist the change to less-susceptible materials,  even those known to be needed with the newer low-lead aviation gasolines (100LL).  As for the rest,  there are 4-stroke and 2-stroke types,  and there are automotive and lawn equipment types. See also "Aviation Alternative Fuel Compatibility Issues" dated 11-3-13.  

See Update 9-26-16 at end of article

2-Stroke Equipment

I cannot speak to the effects of stiff ethanol blends in 2-stroke engines,  other than routine use of what is now sold as “unleaded regular”.  That fuel is a nominal maximum-E-10 material,  that usually tests near E-8 when I check it.  I use it in my weed-eaters and my chain saw,  which are all 2-stroke. 

I have not ever added any extra ethanol to my 2-stroke fuels,  since these also carry the engine lubricating oil in solution.  I often clean greasy parts with ethanol,  so I naturally worry about stiff blend effects on critical engine lubrication.  10% ethanol seems to me to be no problem,  however. 

I have two weed-eaters,  one (a Ryobi) very old,  the other (a Sears Craftsman) only about a year old as of this writing.  My chainsaw (also Sears Craftsman) is about 4-5 years old.  All of these run on “unleaded regular” with 32:1 oil.  “Unleaded regular” has been a nominal E-10 material throughout the life of all but the oldest (Ryobi) weed-eater. 

There have been lots of complaints about dirt clogging things,  and fuel components “dissolving”,  and the “need for engine overhauls”,  ever since the advent of E-10 as “unleaded regular”.  Honestly,  I have seen none of these troubles,  but then I keep my machines clean,  and stored out of the weather. 

I believe that most of these “troubles” can be traced to the solvent action of the ethanol acting upon pre-existing serious dirt and water contamination,  which really should not have been there in the first place.  I also believe there has been a lot of predatory “repair” activity because of these effects:  expensive engine overhauls that did not need to be done.

If there are pre-existing “water bottoms” in the fuel tank,  then the ethanol in a blend fuel will pull it into solution,  right up to the fairly-unpredictable phase-separation point.  An engine set-up for gasoline (even an E-10),  will not run on the wet ethanol in the bottom layer of a phase-separated tank.  You do not need an overhaul,  all you need is to drain the tank and fuel lines,  and replace with fresh fuel.

If there is pre-existing “dirt” in the fuel tank and lines (usually gum and varnish deposits),  the solvent action of the ethanol in a stiff blend will “mobilize” this debris all at once,  which lets it travel downstream into fuel pumps,  fuel filters,  and carburetors. 

It usually won’t hurt the pump too bad (although check valves can leak because of grit blocking full valve closure).  But,  filters can clog up completely,  and the small passages and metering jets in carburetors can get plugged up.  You do not need an engine overhaul for such problems.  You need a clean fuel tank and flushed lines,  a carburetor kit,  and maybe a rebuild kit for any fuel pump that you might have.  (Most equipment is gravity-fed without a pump,  however.)

The only trouble I ever experienced was with the older Ryobi weed-eater,  and I cannot trace it conclusively to ethanol in the E-10 that passes for “unleaded regular”.  This machine is several years old,  with a very poor sealing design for the fuel lines coming out of the fuel tank.  I think age and heat have caused the polyethylene tank to shrink,  and the too-cheap plastic fuel tubing to harden.  I honestly think this would have happened even if there had been no ethanol in the fuel,  precisely because I have seen it before there ever was an E-10,  meaning I have seen it for many decades. 

4-Stroke Lawn and Garden Equipment

The very same “housekeeping” considerations apply here as for 2-stroke equipment just above.  You do not want any dirt or water bottoms anywhere in your fuel system.  However,  you can run up to 35% ethanol in your fuel,  and pretty much expect the same power and fuel consumption as on straight gasoline. 

The stiffer blends will be even more prone to pick up dirt and water bottoms,  causing the same troubles discussed above.  You simply avoid those problems,  it is easy to do.  Keep your equipment stored out of the weather,  and periodically clean the contamination out of your tanks and carburetor float bowls.  You can do this job yourself,  and you don’t even need a new float bowl gasket every time,  contrary to what “they” tell you. 

Here is my experience with a John Deere SX-75 riding lawnmower dating to 1987,  a Troy-Bilt wood chipper about 4-5 years old now,  a Sears Craftsman small push-type power mower dating to sometime in the late 1970’s,  and a Yard Machines riding mower of recent vintage,  that I acquired “used” about a year ago.  All of these run on nominal E-35 blend,  ranging from about E-28 to about E-38 in actual blend strength.  This is a lot stiffer than the nominal E-10 that is “unleaded regular” today. 

The John Deere has run on E-35 since the spring of 2008.  It has a polyethylene fuel tank,  neoprene fuel lines,  a polypropylene fuel cutoff valve,  and neoprene seals and polypropylene parts in its aluminum carburetor.  That’s 5 years’ exposure to date,  without one single failure of any fuel system part.  I have never overhauled its engine,  and judging by the way it runs,  I won’t need to for a long time yet.  I do blend 20% Lucas Oil Stabilizer into its engine oil. 

The Troy-Bilt wood chipper has run on E-35 blends since I bought it 4-5 years ago.  It has similar fuel system materials to the John Deere.  I have experienced no troubles at all with it. 

My very-old Craftsman push mower has run since about spring of 2008 on E-35 blends.  It has the same selection of fuel system materials as the John Deere.  I have had no troubles with fuel system parts other than a primer bulb on the carburetor that failed this year,  after having been installed 2-3 years ago. 

It failed by cracking,  but the cracks were on the outside surface propagating inward,  not the inside surface exposed to the fuel.  I have to conclude that this replacement part failed prematurely from inferior material selection,  with UV light and ozone in the atmosphere being the real cause of part failure.  The previous part was “real” neoprene,  and had lasted for some 3 decades in the same circumstances. 

The Yard Machines riding mower has run for about a year on E-35 blend fuel without a parts failure.  It does not have a simple carburetor.  Idle is controlled by devices that I do not yet understand,  and seems to behave as if the mixture is too lean on cold start,  when using blends.  I compensate by using the choke until the machine is fully warmed to full working temperature.  That takes care of it,  quite well enough. 

Automotive-Type Equipment (5 Vehicles)

I have so far exposed 5 vehicles to high-concentration ethanol in their fuels,  3 of them blends in unmodified vehicles,  and 2 of them straight E-85 in modified vehicles.  The unmodified blend fuel vehicles are a 1995 Ford F-150 XLT (the “ethanol Ford”),  a 1998 Nissan Sentra GXE (the “ethanol Nissan”),  and a 1960 VW beetle.  The modified straight E-85 vehicles are a 1973 VW Beetle (the “ethanol VW”),  and a 1944 Farmall-H tractor (the “ethanol Farmall”). 

The age of all of these vehicles belies the EPA’s concern about exposing older vehicles to ethanol.  I have experienced no troubles in any of them that are traceable to ethanol exposure. 

                Ethanol VW on Straight E-85

The 1973 ethanol VW had a high-time (worn-out) engine and transmission when I started this experiment.  What I found was that there are 3 things required for a successful conversion in a modern over-square,  high-speed engine:   (1) modified mixture ratio at idle,  off-idle,  and full speed,  (2) a significant advance in ignition timing,  and (3) extra intake manifold heating to compensate for a harder-to-vaporize fuel.  This particular engine was 1585 cc displacement. 

My first conversion to E-85 fuel was accomplished 10-29-2006 at odometer mile 231,626.  I went through several iterations and a couple of carburetors before I hit upon the items that worked for modified carburetion and timing,  plus the use of 20% Lucas Oil Stabilizer,  by about 12-17-2006 at odometer 232,269.  In that winter weather and the following seasons,  I found and finalized my added intake heat rig by about 8-12-2007 at odometer 235,105. 

It wasn’t until all 3 items were finalized that relative performance indices (ethanol vs gasoline) were finally established.  The energy conversion efficiency was verified by two independent measures to be nearly factor 1.2 larger on E-85 relative to the E-10 that is unleaded regular.  This partially offsets the drop in volumetric heating value,  so that fuel mileage on E-85 is about 80-85% that on gasoline,  not the 70% one would expect from straight heating value ratios. 

I drove that car on straight E-85 until it was no longer needed,  and then I returned it to mothballs in its ethanol configuration on 6-9-2011 at odometer 250,321.  That’s an operational exposure to very high-concentration ethanol for about 4.5 years,  and about 19,000 miles.  If there was a problem with ethanol damaging anything in the fuel system or engine,  I would have found it in an interval that long!  There were no problems at all. 

This vehicle has a steel fuel tank-without-any-terneplate lining,  steel and neoprene fuel lines,  a polypropylene cutoff valve that I added long ago,  and an aluminum carburetor and aluminum fuel pump,  both with neoprene seals and polypropylene parts (including the bowl float).  There was also a nylon float-retainer part inside the float bowl.  I do not know all of the materials in the fuel tank quantity indicator device,  but it still seems to function without troubles.  The indicator readout on the dashboard seems to be worn out,  because I can influence its reading by hitting it with my knuckles.   Otherwise,  none of this ever showed any hint of any kind of failure. 

                Ethanol Farmall on Straight E-85

This vintage-1944 machine has a steel fuel tank that was severely rusted inside when I bought it used about 25 years ago,  a cast iron carburetor with a tin float,  and neoprene O-rings around the brass adjusting screws.  The fuel system has steel and neoprene lines,  plus a polypropylene cutoff valve that I added long ago.  The fuel strainer is glass and aluminum,  with neoprene seals. 

This old engine is a low-speed,  under-square design very insensitive to ignition timing.  So,  the E-85 conversion only required (1) mixture adjustment,  and (2) extra intake heat.  In this case,  I only had to drill out the seat for the already-adjustable main jet.  There was plenty of idle adjustment available to handle ethanol.  The extra intake heat took the form of a simple sheet-metal air-guide baffle around the carburetor/intake manifold,  to trap extra heated air coming off the radiator. 

I did this conversion 10-17-2006.  It has run on nothing but E-85 ever since then (to 11-2-2013 as of this writing).  That’s an exposure time of 7 years.  In all that time I have never had one single fuel system or engine part failure.  The tractor runs with less smoke and better power than it ever did in all the prior years running on gasoline. 

Here’s the really striking result:  since I began running this machine on E-85,  the pre-existing corrosion inside that ancient steel tank has nearly disappeared!  The supposedly-corrosive ethanol has evidently actually mitigated the pre-existing corrosion inside that tank!

                Ethanol Ford on Various Stiff Blends

This is a 1995 vintage Ford F-150 XLT pickup truck,  with the 302 (5 L) V-8 engine and electronic fuel injection.  The fuel injection takes the form of a fuel rail maintained by a fixed regulator at constant pressure difference above absolute intake manifold pressure.  Mixture control is by injector duty-cycle “open”,  controlled closed-loop to a feedback signal from an exhaust oxygen sensor,  with an open-loop stored-map cold start feature.  I have used 20% Lucas Oil Stabilizer in the engine oil for this vehicle since long before ever trying ethanol blend fuels. 

It was a fairly high-time engine when I started using blend fuels in the rear tank only 12-16-2007 at odometer 177,332.  I investigated blends from near E-10 to about E-50 like this.  Starting 4-3-2008 at odometer 181,140,  I used blends in both tanks.  I had pretty much settled on nominal E-35 blends in both tanks by 8-4-2008 at 184,215.  I still use these nominal E-35 blends routinely as of this writing (11-2-2013 at about 201,800 odometer). 

To date,  that’s about 6 years’ exposure to stiff ethanol blends (some 25,000 miles) on otherwise factory-stock materials.  In all of that time,  I have never had a fuel system or engine part failure.  That would be steel tanks and lines,  neoprene lines and neoprene seals everywhere,  aluminum fuel injector body,  and polypropylene and neoprene parts in the fuel injection body and the fuel injectors.  I do not know what the fuel tank quantity indicating devices are made of,  but they have never even changed their calibration in all these years of exposure to ethanol. 

                Ethanol Nissan on E-30-ish Blends

This 1998 vintage vehicle has a fuel-injected in-line 4-cylinder 1.6 liter engine.  It was very high-time when I started using stiff blend (nominal E-30) fuels in it,  essentially unmodified (although I did add a fuel rail tap for testing blend strength).  I was already using large amounts of Lucas Oil Stabilizer in this vehicle long before the blend experiments,  because of a serious oil leak in a front crankshaft seal. 

I drove it far enough,  in a condition like that,  for the old engine to wear completely-out at an odometer reading far beyond any reasonable expectation for life,  and then I overhauled that engine.  I still drive it on stiff blend fuels today.  I did not replace the catalytic converter at overhaul,  which is now far beyond any reasonable expectation for useful service life.  Yet it still functions!  Total exposure time is about 4 years,  and about 48,000 odometer miles. 

I began using blend fuel 8-2-2009 at odometer 186, 514.  This was an estimated E-43 blend for the initial tank.  After a bit,  I pretty much standardized on an E-30 blend,  although I once accidentally took it to an E-50 blend and saw the same kind of fuel mileage degradation that I saw in the ethanol Ford at that same kind of blend strength. 

I have used blends near E-30 in this vehicle through overhaul to the present day,  which is now almost 234,000 miles odometer (about 20,000 since overhaul).  The fuel system materials and fuel injection control are pretty much identical to the ethanol Ford,  just a different manufacturer.  I have replaced the instrument cluster (due to old age wear-out of the dashboard indicators),  but the tank calibrations have remained unchanged with the passage of time,  for each of the two different instrument clusters.  That tells me that the in-tank fuel quantity measuring equipment is unaffected by exposure to stiff fuel blends. 

I have not seen any problems induced by the use of stiff ethanol fuel blends.  In point of fact,  at overhaul,  the inside of the engine was comparatively very clean.  And as for the effects of using Lucas Oil Stabilizer,  the rod and main bearings,  which were “worn to the red metal” at overhaul at 214,447 odometer miles,  were not the cause of the overhaul.  In point of fact,  it was wearout of the timing chain tensioner.  It had worn through the phenolic,  so that the chain was “cutting” the steel backplate of the chain tensioner.  The Lucas additive really does arrest bearing insert wear!

Based on the timing of major accessory wear-out,  I estimated the intended design lifetime of the Nissan to be only about 150,000 miles.  Based on federal motor vehicle regulations,  the catalytic converter should last longer than the vehicle,  say,  about 200,000 miles.  Mine has seen significant ethanol fuel content since 186,000 miles,  which changes that expected outcome.  

Ethanol tends to strip carbon off of exhaust components (as demonstrated by tailpipe cleanliness in the ethanol Ford),  which would include the catalyst bed in the catalytic converter in the Nissan.  Accordingly,  my “evident catalytic converter life” of 214,000 miles-to-date is far beyond any ordinary reasonable expectation on gasoline.  Since the “mode of death” for cat converters is carbon soot buildup,  I have to conclude the ethanol content has extended my cat converter life by acting to strip out some of the soot buildup. 

This is in direct opposition to the “conventional wisdom” of the EPA:  that ethanol “might hurt” emissions control equipment,  something oil company lobbyists convinced them of.  In at least this one case,  ethanol content has acted to extend the operational life of emissions equipment. 

So,  “they” (EPA and the oil lobbyists) are wrong;  there is no reason to fear E-15 blends no matter the vintage of the vehicle,  and really no reason to fear blends up to about E-35,  based on my data.  There are some other researchers who have shown at least-E-20 to be OK.  So,  I am not alone,  although I may have gone farther than anyone else in terms of blend strength. 

                1960 VW on E-35 Blend

I pulled this vehicle out of about 15 or 16 continuous years in mothballs,  in the belief that I needed it.  I “woke it up” unmodified factory stock on E-35 blend quite successfully.  Then I found I did not need this vehicle on the road,  and returned it to mothballs.  There were no problems.  The materials were about the same as in the 1973 ethanol VW,  except that the older vehicle has no fuel quantity indicator at all. 

Mileage Trends Established with the Ethanol Ford (and Nissan) Data

Most of this is Ford data,  simply confirmed by experiences in the Nissan.  Mileage has been better than what one would expect based upon a simple volumetric heating value ratio.  Up to about 40-45% ethanol,  mileage seemed indistinguishable from that on plain gasoline.  Above that blend strength,  fuel mileage dropped very sharply.  And,  behavior was like late timing:  very smooth,  just fuel-consumptive.

It takes the averaging of multiple tanks over carefully-controlled driving cycles to establish these trends.  The “scatter” from tank-to-tank,  even with a controlled driving cycle,  is far larger than the trends reported here.  Judging it from one tank to the next is a very serious technical error,  contrary to the popular wisdom.  And,  not controlling the driving cycle very carefully,  greatly expands the natural scatter from tank-to-tank at otherwise “the same” conditions.  None of this is intuitively obvious. 

I have published mileage data plots for the Ford and the Nissan previously.  See reference 1 and reference 2.  Both show miles per gallon indistinguishable from plain gasoline up to around 40-42% ethanol in the fuel.  Above that blend strength,  mileage falls rather sharply.  Yet,  even at lower mileage,  the ethanol VW definitely showed experimental energy conversion efficiencies roughly factor 1.1 to factor 1.2 higher on ethanol,  than on gasoline.  Somehow,  these seemingly-conflicting results combine and unify into one picture. 

The simplest model is a ratio of volumetric heating values,  as a function of ethanol content.  That is not a realistic model,  as Figure 1 (below) clearly shows (the blue curve).    The second simplest model is a linear extrapolation between gasoline efficiency at factor 1 to ethanol efficiency at factor 1.1,  (red curve in Figure 1).  This one is better,  but still does not explain the observed constant mileage from 0% to 42% ethanol. 

The next simplest model is to suppose that the extra released heat from the higher-efficiency ethanol feeds back into the efficiency factor for burning the gasoline component.  If you assume this effect is linearly-proportional to ethanol content in the fuel (and it may not be),  then you get a curve shape as in the green curve of Figure 1.  When I assume a proportionality constant of 0.4 for that feedback effect,  I get the plotted curve,  which pretty-well matches the Ford and Nissan experiences. 

These effects are very fuel species-dependent:  for the very same alcohol efficiency and feedback factors,  the far lower heating value of methanol gives an entirely different curve shape with blend strength.  This is shown in Figure 2.  I didn’t do it,  but similar different curve shapes would obtain from fuels like isopropanol and butanol,  even something like ethyl tertiary butyl ether (ETBE).  One would have to determine the relative fuel energy conversion efficiency factors from neat fuel tests.  The feedback factor has to come from blend fuel tests (whichever value matches the observed shape). 

Since the feedback factor of 0.4 that I have,  came only from ethanol testing,  only Figure 1 is “reliable”,  and only to the extent that the feedback effect is linear with blend strength.  Figure 2 is nothing but an educated guess for the moderately-similar methanol.  I did not feel educated in the least,  regarding guesses for propanol,  butanol,  or ETBE,  so I did not create any plots for them at all. 

Concluding Remarks

What I have to offer here is really in respect to ethanol-gasoline blend fuels,  not “neat” fuels.  Ethanol is only mildly corrosive,  compared to high-aromatic gasoline’s very mildly corrosive effects.  Methanol is far more corrosive.  Propanol and butanol are considered to be relatively non-corrosive.  The experimentation with which I am familiar found ETBE to be rather non-corrosive. 

You have to balance corrosivity effects against heating value and mileage effects.  The heating value effects are relative to some sort of a realistic energy-recovery model,  not just the ratio of volumetric heating values (as is “customary”,  but is wrong).  It’s a judgment call,  no matter what. 

Corrosivity effects are inherently empirical.  I hope the accounts here add to that empirical database.  The “conventional wisdom” is simply wrong about ethanol and corrosivity,  as regards commercial and automotive applications. 

Aircraft applications I will cover separately. 

See also references 3 and 4.

Update 11-4-13:

Taking into account my automotive,  small engine,  and aircraft experiences,  I would assess things as follows:

The automotive industry has long adopted materials compatible with high aromatic and ethanol content in gasoline.  That plus flex-fuel cars has driven the supporting parts industry in that direction for decades.  As a result,  you can reliably use blends or convert to E-85,  pretty much in anything from modern to very old.  And I do mean many decades old!

It appears to me that the 4-stroke small engine folks have also largely made the transition to ethanol-compatible materials.  I’m not so sure about the 2-stroke small engine folks,  and I have not run any stiff blend experiments in 2-stroke because of my preconceptions regarding lubrication.  E-10 seems to be OK in 2-stroke,  though,  and based on that,  I don’t have much concern about E-15 (it’s just not that different). 

I don’t think the boat motor folks have made the transition to ethanol-compatible materials yet,  not even on the 4-stroke side of the house.  It’s boat motors and 2-stroke equipment that I hear the most horror stories about.  (I haven’t really investigated this,  so my perception is only just that: a perception.)  If there was going to be a problem with water bottoms,  I would expect to see it in boats,  though. 

The light aircraft industry is still using the same materials they were using half a century (or more) ago.  Many of these are not even good for high-aromatic gasoline,  not to mention ethanol.  Commercial aircraft are today nearly all turbine,  for which biodiesel content in the jet fuel is proving to be a good fit (that’s a another whole topic area not discussed here).

Bottom lines:

Automotive:  feel free to experiment with stiff blends and neat ethanol.  The materials in common use for the last few decades seem to support ethanol compatibility,  by and large.  No mods necessary up to about E-42,  although I don’t recommend over E-35 because of minor cold start problems.  Conversions to straight E-85 are easy,  but will be successful only if you do all 3 required items (mixture,  timing,  extra intake heat).

4-stroke small engine:  feel free to experiment with stiff blends up to E-35 without mods.  I’d not recommend neat ethanol or E-85 conversions,  because the carburetors do not have removable jets and are therefore so very hard to modify.  It’s very hard to change the timing in a magneto ignition,  too.  The supporting parts seem to me to be largely ethanol-compatible in recent years.

2-stroke small engine:  I personally would not experiment with stiff blends in these,  because of lubrication fears (oil-in-fuel,  with an oil solvent also in the fuel?),  and because the supporting parts do not yet seem to be very ethanol-compatible.  Wait till the supporting parts industry has made the transition. 

4-stroke boat:  you can try stiff blends in unmodified engines,  but be aware that supporting parts may well be incompatible with ethanol.  You will have to determine the neoprene/polypropylene issues for yourself.  I’d rather wait until the supporting parts industry has made the transition,  before I went above about E-15.  If your parts are compatible,  up to E-35 should work just fine. 

2-stroke boat:  same as 2-stroke small engine,  and for the same reasons. 

Gasoline airplane:  you need an otherwise-compelling reason to do ethanol blends and conversions,  and you will run into incompatible materials and metal corrosion problems everywhere you look.  But it can be done.  And it does work. 

Turbine aircraft:  out of scope here.  Biodiesel blends do work just fine,  though.  

Update 9-26-16:

              From Biofuels Digest 9-25-16:

              DOE Study

In Colorado, a study conducted by DOE’s National Renewable Energy Laboratory (NREL), found that the petroleum components of ethanol-blended gasoline become degraded and unfit for use in an engine long before the ethanol portion takes up enough water to cause phase separation in the fuel tank. “Phase separation” occurs when an excessive amount of water is introduced into the fuel tank leading the ethanol and water to mix and sink to the bottom of the tank. In other words, gasoline becomes “stale” and unusable before water uptake by the ethanol component becomes a concern.

As part of the study, NREL scientists stored gasoline-ethanol blends ranging from E0 (0% ethanol) to E85 (83% ethanol) in actual lawn mower fuel tanks over several months in a climate-controlled chamber meant to replicate hot, humid environments like Houston and Orlando. The samples were tested at regular intervals for evidence of gasoline weathering and water uptake. In every case, the hydrocarbon components of the fuel became unfit for use in an engine before water uptake became a concern.

For gasoline-ethanol blends, it often took more than three months for phase separation to occur, meaning the fuel had already weathered to a point it was unusable. “In a small engine fuel tank in a constantly high-temperature, high-humidity environment, it takes three months or longer for E10 and other ethanol blends to take up enough water for phase separation,” the study found. “This confirms the statement by Mercury Marine that water uptake in E10 blends ‘…does not happen at a level or rate that is relevant.’”

President and CEO Bob Dinneen offered the following comments on the new study:

“Simply put, critics who continue to suggest E10 is a problem for small engines and boat motors are all wet. This research from NREL clearly demonstrates that gasoline goes bad long before the ethanol in the tank could cause any problems due to moisture uptake.

“Every manufacturer of small and off-road engines has approved the use of E10 in their equipment for many years. If owners of this equipment simply follow the manufacturers’ recommendations for fuel, maintenance, and winterization, they won’t have any issues at all. But, as this study shows, letting gasoline sit in your tank for extended periods of time is likely to cause some issues—irrespective of whether the gasoline contains ethanol or not.”

My take:  the water problems that small engine and boat motor owners report are due to the item being left out in the rain,  not the humidity.  The fuel tank caps have air vents.  The rain gets in that way.  Plus,  many folks are using fuel more than 3 months old.  Both a recipes for bad performance and maintenance problems,  even with zero ethanol in the fuel.  

References

1.      1. Gary W. Johnson,  “Stiff Blend Effects in Gasoline Cars”,  posted 11-122-2010 on http://exrocketman.blogspot.com

2.      2. Gary W. Johnson,  “Nissan Mileage Results on Blends”,  posted 11-17-2010 on http://exrocketman.blogspot.com

3.       3. Gary W. Johnson,  “Biofuels in General and Ethanol in Particular”,  posted 8-9-2012 on http://exrocketman.blogspot.com

4.     4. Gary W. Johnson,  “Ethanol and Emissions Control Functionality”,  posted 11-15-2012 on http://exrocketman.blogspot.com

Figure 1 – Ethanol Model

Figure 2 – Methanol Model

Figure 3 – Modeling Equations and Data