Highest Compression Ratio for E85 2

[PhdDave]

What is the highest compression ratio that E85 is able to run at without undue harm? What are the chances that the automotive manufacturers will make a car that can only run on E85, and gain power by using a lighter engine of smaller displacement?

 

[GregLocock]

Yes, I was just pondering the customer acceptance for a car that would only run properly on E85.

The other alternative would be to limit the throttle opening on a high compression ratio car that was an NA, if the octane level was too low.

Either way, I can just imagine the reaction.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Eric68]

Maybe I am a little "turbo naive", but how would that help efficiency at part throttle? (not that part throttle is important to me LOL) It would solve the WOT problem, especially if you could measure octane prior to combustion and be a little more proactive with the waste gate and spark curve than a knock sensor.

You would be able to pack more air into the cylinder with boost and increase cylinder pressure at WOT, but you would be in the same boat when cruising? Doesn't higher static compression still squeeze the charge harder at part throttle?

 

[dcasto]

I guess we need this

http://www.daledetrich.com/variable.htm

It kinda reminds me of the Model A or T Fords. Because there were no standards for gasoline, the Fords had a lever on the column that would rotate the distributer and change the ignition timing. You'd start the car and listen for it knocking then make adjustment as you were driving.

 

[Eric68]

There you go . . . now all we need is a V8 version that will stand up to 500-600 HP for 200,000 miles lol

 

[patprimmer]

Ummm

Throttling the engine reduces manifold pressure, so the same compression sees lower cylinder pressures at part throttle than at WOT. This effectively eliminates knock at part throttle.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dcasto]

I don't think that the partial throttle stops knock. Don't you remember cars in the late 70's. They would run for 3 minutes after you closed the throttle and turned off the ignition, and they "dieseled". You still compress 9 to 1 and the compression ratio makes heat (temperature) in the cylinder AND the engine is holding in some heat, so the fuel auto-ignited.

 

[patprimmer]

So are you saying that the same heat is generated by compressing air at 5 psi by 9 as is generated by compressing air at 15 psi by 9.

I would have thought the run on situation was purely from residual heat causing a glow plug effect.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dcasto]

The thermal heat of compression is defined as:

Tout = Tin * ratio ^ ((k-1)/k)
T in absolute T, k= Cp/Cv, ratio = Pd/Ps, Ps and Pd are absolute pressues.

 

[Eric68]

Interesting discussion. Part throttle certainly has less cylinder pressure due to being throttled, but there is more that changes than just cylinder pressure at part throttle.

For one, most engines increase ignition advance significantly and lean the mixture at cruise (like 50+ degrees). Also, probably the most significant factor in my opinion is load. There is very little load on an engine at cruise -- even my 500 HP street/strip motor is only developing 10-15 HP when cruising flat ground at 55 mph. It is very difficult to make an engine detonate under these circumstances. Increase the load though (like pulling up a hill with a trailer), and combine that with a lot of advance and lean mixtures and an engine can certainly detonate at part throttle.

By the way, run-on after shutting an engine off is not detonation. Detonation occurs when you have secondary flame front in the chamber and the two flame fronts collide. Run on, is a single (abnormal) ignition source that ignites fuel after the ignition power is turned off. Sounds nasty when it happens but it is not nearly as destructive as detonation. (grew up listening to Mom's 72 Valiant run on for several minutes sometimes -- I can still remember my Dad cursing the old slant 6 hoping it would blow up and finally shut off)

 

[dcasto]

At low load the throttle is shut and vacuum is high and nearly no fuel is available to ignite, as a matter of fact the cylinder conditions could be so lean the fuel won't ignite.

A question just poped in my head, is this condition why some (carburated or even FI)vehicles "backfire" when the throttle is closed? Its a similar or opposite situation with turbo engines that use a blow off valve. When going into curve a there is a sudden closure of the TB, the air is vented after the MAF sensor and the engines thinks it has lots of air so the engine goes rich and some fuel is not burned in the cylinder and hence we get to see flames.

 

[GregLocock]

One typical cause of backfire is that unburnt hot rich mixture travels down the exhaust and mixes with fresh air from an exhaust leak. On protos backfire on closed throttle is almost invariably a poor seal at the manifold to downpipe joint.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Eric68]

I think Greg nailed it -- one other thing to remember is that a lean misfire in the cylinder can result in a backfire in the exhaust too. If it won't fire in the cylinder because it is too lean, the charge may still burn in the exhaust and result in a rich smelling exhaust. I think this is more common with performance engines with big nasty cams that are subject to reversion (like at idle).

 

[payntir]

Max Schauk {sp?) used ethanol (neat, e100 i believe) as a replacement for aviation fuel. crossed the atlantic with it. Various "maximum" e85 CRs from 13:1 to 15:1 with different engines. the composition of the 15% petroleum product and the engine tune are the determining factors. petroleum blenders have found that profit can be increased by using the lowest possible octane gasoline with 85% ethanol to bring the octane up to that of unleaded regular. octane rating systems seem to be nebulous concerning ethanol. seems to be too much $ at stake, a lot of research to spec.
as distilled, ethanol is an azeotrope, with 5% water. anhydrous ethanol is needed to blend with gasoline for e85, adding expense, decreasing net energy. i suggest optimizing engine to operate on hydrous azeotropic ethanol. water injection has long been used to reduce cylinder temperatures. compression ratios should approach 18:1. As regards supercharging as a form of variable compression, ford motor 1997 mustang super stallion utilized supercharging. power increase about 10% on e-85, highly tuned engine, 5.4 liter, 590 hp on e85 dynamic compression, boost pressure, i dunno

 

[stanlsimon]

A couple of issues which didn't seem to be addressed previously were EtOH's air/fuel ratio, flame front speed, and also NOX emissions of higher compression engines.
If anyone has any information on these subjects I would be interested.

I am interested in modifications to an existing V8,( high compression heads, pistons, connecting rods, carb revisions, etc. One Web site claims up to 19:1 cr is safe.
This is not all new technology as the gearheads have already been down this track. The emphasis has been on maximizing HP/CID. It would be nice for engineers to have a monopoly on this knowledge, unfortunately, such is not always the case.

 

[patprimmer]

19:1 is just possible with M100 running very rich and not to much advance and a long duration camshaft.

With E85 it is no way possible. Some bench racers only hear the word alcohol and react as though
1) They know what they are talking about.
2) That all alcohol is M100.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[TDIMeister]

"A couple of issues which didn't seem to be addressed previously were EtOH's air/fuel ratio, flame front speed, and also NOX emissions of higher compression engines."

Stoichiometric AFR of neat EtOH: 9.0. For blends, you can find out the StAFR by determining the mass fraction of each fuel component and its corresponding StAFR. NOTE: Mass fraction is not the same as volume fraction, by means of which fuels are usually blended. Laminar burning speed is faster than gasoline (look up if you want exact values). NOx emission is ambiguous because of additional variables like charge cooling.

Several posts back tried to calculate the heat of compression using isentropic relations. The value of the specific heat ratio (kappa) used was wrong. For pure air, it's about 1.4. In a fuel-air mixture, that value is lower. 1.3 can be used as an approximation for stoichiometric mixtures of most hydrocarbon fuels and air for engines that are not direct injected. So in the equation (T2/T1)=(v1/v2)^(k-1) the value in the exponent should be around 0.3, not 5/2. Within any reasonable effective (dynamic) compression ratio used, autoignition is not an issue.

Next, it must be said that it is a very common mistake to conclude compression ratio in a piston engine as a ratio of pressures. It's NOT. Although the relation (T2/T1)=(v1/v2)^(k-1)=(P2/P1)^((k-1)/k) holds true, end temperature is INDEPENDENT of throttling, etc. It does change due to leakage and heat transfer of a real compression process, that is neglected in isentropic (reversible, adiabatic) relations. Going back to the temperature independence of throttling, if P1 drops, P2 also drops accordingly only as a function of the volumetric compression ratio (v1/V2) raised by k. In a closed control volume, the compression ratio is a VOLUMETRIC parameter. Don't EVER use pressure ratios unless you're dealing with a turbine engine or you have specific pressure trace data or you'll look silly.

To the original poster: Your question is similar to that asked in the recent thread asking about maximum CR in a propane engine. The answer is that there is no single value nor range of values that can adequately cover every variable in a engine.

A factor that also comes into play are the objectives for the engine design. If the engine is to live almost exclusively at part load, and you want maximum fuel efficiency, there is an upper limit of compression ratio beyond which efficiency drops off again due to a multitude of factors like heat transfer (worsening ratio of combustion chamber surface area to volume); leakage through the rings; friction; increasing role of crevice volumes and quench zones; etc. For an engine developed for part load, this maximum compression ratio should be strived for, but again, there is no hard number, but from a theoretical derivation of a dual-combustion cycle with a defined peak pressure limit, thermal efficiencies cease to increase significantly past a (dynamic) CR of about 14:1. The point is that it's not constructive to raise the compression ratio simply for the sake of striving for the biggest number. However, if the emissions profile permits obviating a three-way catalyst, in addition to an optimally increased compression ratio, a small benefit in BSFC can be achieved by operating the engine slightly lean.

If the engine is to be run mostly at full-load and you are still concerned about fuel efficiency, you must balance increasing the compression ratio and timing advance with necessary fuel enrichment to prevent knocking. You might save far more fuel by running at a lower compression ratio and degree of spark advance, but maintaining roughly stoichiometric AFR, instead of running a higher CR and forced to enrich the mixture to a lambda of 0.8 that I've seen for some gasoline engines.

Running on E85, short of a major breakthough in engine efficiency (i.e. 45% increase), you will NEVER come out with a case of an engine that has the same or better mileage (read fuel consumption) on E85 as on gasoline, due to the approx. 45% shortfall in LHV. Run away from anyone who tries to convince you otherwise!!!

 

[BrianPetersen]

tdimeister has the math right, and Eric68 (and payntir) are on the right track for the practical experience.

Although I have not done it myself (no local source for E85 and the particular race engine that I have does not have parts available to raise the compression ratio enough), I have been told by others who build race engines that you can get away with things by running an engine on E85 that you could never do even on the best and highest-octane racing gasolines, nevermind ordinary pump gasoline. They say that "in the engine", E85 acts like roughly 112 octane fuel. Step away from the calculator ... E85 is a great fuel for a race engine, if the rulebook will allow it. Nominal octane ratings for a given compression ratio obtained in a research engine are not necessarily reflective for a "real" engine with a "real" combustion chamber shape and running at "real" (i.e. high) engine speeds, particularly when the fuel chemistry is way, way different from what the octane test apparatus was designed for.

For a daily driver application in which best efficiency is the main objective rather than maximum possible power, if you were to build a dedicated E85 engine, I think you would want to downsize displacement by around 10% - 15% and up compression by several points, probably into the 13:1 - 14:1 range for an engine with cylinders the size of your average auto engine. Or, keep displacement the same, raise geometric compression even higher, and fiddle with the intake cam closure timing to get some Atkinson cycle happening (less power, i.e. back to the same as baseline gasoline engine, but more efficiency). It should be possible for an optimized E85 engine to approach the nominal "fuel economy" of a gasoline engine by taking advantage of the high octane rating, BUT, such an engine will not be capable of running on gasoline.

The variable turbo pressure scenario (Saab) is good for taking advantage of the octane to make more power, but I don't see that approach improving the part-load efficiency. It's still limited by having to be capable of running on gasoline at an acceptable power level.

 

[stanlsimon]

State of Minnesota tested Mileage of vehicles using e85 and conventional fuel, found no significant difference in fuel economy.

 

[patprimmer]

Then the state of Minnesota must have a very liberal definition of significant as ethanol contains significantly less energy per unit mass or volume than normal pump petrol.



Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[globi5]

What you do get with the heat of vaporization is more dense air and fuel, more pounds in the cylinder, more explosion, more power.


This sentence is contradictory.
At a given pressure one cannot increase density without decreasing temperature.

p*V=n*R*T

Heat of vaporization does lower the temperature at TDC.