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?

 

[reidh]

Dave,

The maximum CR for E85 depends on so many factors, its probably not worth listing them all.

Regardless, an engine that is designed to run solely on E85 could have a higher CR than a flex-fuel engine. We have to remember though the increased octane rating of E85 will never make up for the reduced energy density.

Reidh

 

[NickE]

reidh- you do mean in mileage? Cause with a bit of FI and good fueling you can get more power for sure.

 

[lugnuts67]

there is less energy in E85 than an equal amount of gasoline. period. However, because you can run higher compression ratios, you can achieve higher cylinder pressures and produce high HP. You need a good bit more juice though. A pontiac guy here in the south converted his supercharged drag car and posted his findings at

http://hardcorepontiacs.com/forum/169-e85-report.html

He said that he used almost twice the fuel as he would with race gas but it cost 1/3 as much.

Also, if you are going to give it a try I recommend Quickfuel's metering blocks - they make the job much easier even though the E85 is more forgiving as far as jetting for max power goes.

 

[globi5]

This engine runs with a compression ratio of 28 on E95:

http://www.greencarcongress.com/2007/05/scania_to_unvei.html

(probably not spark ignition though).

But I remember reading of a spark ignition engine with a compression ratio of 19 on pure ethanol.

In order to deal with the higher pressure, an engine will have to be strengthened and therefore gain weight.
A significant power (per weight) increase (to allow for a smaller engine) may or may not be achieved with a naturally aspirated engine. It will work, if the engine was supercharged.

 

[crysta1c1ear]

For use in my next posting

Adiabatic compression:
P₁V₁^k=P₂V₂^k

Gas Law:
P₁V₁[÷]T₁=P₂V₂[÷]T₂

Divide the first equation by the second:
T₁V₁^(k-1)=T₂V₂^(k-1)

Regroup temperatures and volumes:
T₁[÷]T₂=(V₂[÷]V₁)^(k-1)

Move powers to other side:
(T₁[÷]T₂)^{1[÷](k-1)}=V₂[÷]V₁

I don't really like all this gamma or k stuff - the ratio of the specific heats, so let me simplify it.

C_v is the number of degrees of freedom in the gas, and C_p is two more. So k=(dof+2)/dof and k-1=2/dof. So 1[÷](k-1) = dof/2. Before combustion air molecules don't reach the sort of temperatures needed to make them vibrate, so the number of degrees of freedom is say 5 and power is therefore 5/2.

Now if state 1 is autoignition and state 2 is the start of compression with ambient air let's write the equation again.

CR=(T_auto[÷]T_amb)^5[÷]2

 

[crysta1c1ear]

But I remember reading of a spark ignition engine with a compression ratio of 19 on pure ethanol.

When did the intake valves close on the engine with the 19:1 compresstion ratio?

I ask, because I read an autoignition temperature for ethanol of 630 Kelvin and estimate the maximum you could compress air before reaching the autoignition temperature of ethanol as being ...

      PRINT (630/300)^(5/2)

Maximum compression ~ 6.4 ???

I know that sounds really low. Bear in mind it is from start of compression to autoignition and not from V_Max to V_min so my calculated compression ratio would only start at intake valve closing for example.

I am wondering if my autoignition temperature of 630K for ethanol is wrong, or my theory, or what?

And I am wondering how the 19:1 ethanol engine doesn't autoignite? Was the engine spark ignition, but direct fuel injection, thus preventing the fuel from igniting under pressure, as it cannot light until it enters the engine?

 

[patprimmer]

Alcohol being methanol or ethanol will run with quite rich mixture. This rich mixture cools the charge by evaporative cooling so offsetting some of the adiabatic temperature rise.

Also the engine may have less than 100%Ve and as you say, late intake valve closing will reduce effective CR.

Heat added from and lost to the block and head surfaces will also effect charge temperature as in the early part of the cycle the charge will gain heat, but at a point in the compression stroke it will start to lose heat.

I have seen over 20:1 on methanol. I have never pushed ethanol to the verge of detonation.

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.

 

[crysta1c1ear]

Thanks Pat
I read 680K for an autoignition temperature for methanol, compared to 630K for ethanol.

 

[globi5]

I don't know whether the intake valve actually did close late on this engine.

But, if I also remember correctly the latent heat of evaporation of ethanol is at least 3 times higher than that of gasoline.

So, not only does ethanol have a higher octane rating and run richer, it also requires more heat (and takes it from the adiabatically heated air).

If you also calculate the heat required for the ethanol phase change (liquid to vapor), you'll notice that the compression ratio can be significantly higher than 6.4.
(9kg hot air and 1kg liquid, cold ethanol).

 

[dcasto]

I'd bet E85 is 85% ethanol octane=100(?) and 15% 6rvp natural gasoline octane = 40, net about 90 octane, so compression ratio could be 11:1 or so.

 

[crysta1c1ear]

I did an estimate the other day, which I won't put here as the calculations aren't handy, but from memory, at 9:1 air fuel ratio, ethanol evaporating could take about 130 kelvin out of the air. That sounds a lot. Can it be right? And similar calculations (so similar mistakes if wrong) gave iso-octane removing about 30 kelvin.

The autoignition temperatures I had were 630K for ethanol and 690K for isooctane.

octane 690+30=720
ethanol 630+130=760

So air that might reach 760 due to compression (in the absence of fuel) could reach 630 due to ethanol evaporation (a drop of 130) and just start to autoignite.

What I am saying is at stoichiometric AFRs ethanol looks like at could support significantly higher compression ratios than isooctane due to the large amounts of evaporative cooling.

If you look at the enthalpy of vaporization of ethanol (around 40 kJ/mol but varies depending on source in the literature!) and that of octane, the numbers are broadly similar. But the figures per molecule only tell part of the story, as an ethanol molecule has two carbons and octane has eight. With that in mind, it comes as no surprise then that evaporating the ethanol required for combustion drops the temperature about 4 times as much as evaporating octane.
(You have to evaporate about four times as many ethanol molecules.)

So if the temperature drops are as much as I estimated the other night, 130 kelvin (ethanol) and 30 kelvin (octane), it would explain why low autoignition temperature ethanol can have a higher compression ratio than high autoignition temperature isooctane.

 

[dcasto]

crystal1clear, I don't think your analysis is correct. I use pure methane gas as a fuel. When the methane is injected with the air, the combined temperature is the same. Methane has an Octane rating of over 100 and because of this we can run up to 11:1 compression ratio. Maximum compression ratio is more a function of the fuels octane rating which is inturn a function of how the molecules stucture and burning rate. Look at normal octane, its physical properties are nearly the same as isooctane, yet it has a 0 octane value and if used as a fuel, the compression ratio for an engine would have to be about 4:1

The original OP was for E85, unless you know what the other 15% is, you can't committ to a compression ratio. Like my original answer, E85 probabily has a net octane of 90. I haven't see the spec's but I'll bet the new flex fuel cars are less than 10 : 1 compression ratio and the knock sensors retard the timing big time if you put plain ol 85 octane in it!

 

[globi5]

dcasto,
if a higher latent heat of evaporation wasn't just as effective as using a fuel with a higher octane number, then water injection would not be applied.

 

[spdracer22]

As a reference... In an auto engineering trade mag I picked up a SAE World, there's an article about a prototype Lotus Elise running E85...

If I remember correctly, it's 11-11.5:1 CR, supercharged, with 6 injectors (4 standard location, 2 pre-S/C). They were also running it with the knock control disabled. The two pre-S/C injectors aided charge cooling, and I believe they said it could have run just fine without the air-air intercooler altogether. I think they picked up 60 extra horsepower with just the addition of the two injectors and more advanced timing. Those were the only changes (injectors/timing) over the base car.

From what I've read, E85 has an octane rating of about 100, thus the higher timing that can be used (not to mention the better charge cooling effects). BUT, mixes as low as E70 can be labeled for sale as "E85" for better winter/cold starting performance, so you have to be careful if you're building/tuning an engine based solely on E85.

My thoughts on the flex fuel cars is that they probably run their standard gasoline-only engine compression ratio, but increase/decrease timing and fuel flow based on the fuel mix. Since the change is mostly software-based, and most people will run plain old E10 anyway, it seems to be the most logical choice (to me) for an OEM.

 

[patprimmer]

Octane rating is simply a measure of knock resistance. The mechanism of the knock is not considered.

There are a whole host of other factors that effect the onset of detonation.

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.

 

[crysta1c1ear]

Look at normal octane, its physical properties are nearly the same as isooctane, yet it has a 0 octane value and if used as a fuel, the compression ratio for an engine would have to be about 4:1

Autoignition temperatures
n-octane 500
iso-octane 690
methane 850

Compression ratio required to compress 300K air to
500 -> 3.6
690 -> 8.0
850 -> 13.5
calculated using
CR=(T_auto÷T_amb)^5÷2

 

[PhdDave]

To clear up things,if I were to build an engine to run only on E85 rather than on gas and/or E85 and my target was 100HP engine output, would I be able to save weight on the car with the smaler engine and gain miles per gallon?

 

[patprimmer]

No

The potential decrease in engine size is minimal if any.

Ethanol contains less energy per units weight or volume than petrol and the power increase comes from it requiring a richer mixture as well as higher potential compression ratio.

The percentage increase in fuel required is greater than the power increase, therefore the fuel consumption goes up, not down.

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.

 

[Mechaniac]

If your question is would a smaller engine built specifically for the advantages of E85 generate better fuel efficiency than a larger one built as a gasoline engine but burning E85, both generating a peak output of 100 HP, then yes. Most certainly the smaller engine would consume less fuel under normal use. Is it less MPG than a gasoline engine built for the same purpose, no, but closer than the currently produced "FlexFuel" engines that are a compromise on E85. I've been told on another forum that the current Car & Driver Magazine has an article on some OEM manufacturers doing just what you're considering. That is designing small E85 only engines with specific outputs much higher than possible on Gasoline but better fuel efficiency than a larger FlexFuel engine. I'm going to have to pick up a copy and check it out myself as I have converted my '01 Chevy Silverado 3/4 ton pick-up with a 8.1 liter V8 Gasoline engine over to E85. It's only been just over a week but power has gone up an exciting amount and fuel efficiency isn't that bad compared to gas. It's at least 85%. If I were to rebuild with higher compression and optimized ignition timing both could only get better.

Vernon