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?

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

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/ball 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.

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.

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