Re: Carnot cycle and gasoline octane


David Lemper wrote:
It is said that :
1. a high compression internal combustion engine is more efficit as
it goes out further on the Carnot cycle.
2. High octane gas ( eg 91 ) has less energy density than 87 octane.

Questions :
1. Will a high compression engine get more miles per gallon than a
lower compression engine in the same vehicle ?
2. Considering the increased cost of 91 octane, does a high
compression engine produce a saving or added cost to the
economy ? Is there any data ?

David Lemper
//

Dave,

The compression ratio of a motor has an impact on the thermal
efficiency of the air/fuel burn and is approx equal to:

e=1-1/r^.3 where e=thermal efficiency and r=compression ratio

Therefore, increasing the compression ration (CR) has the potential to
make the engine more efficient for the same amount of fuel--which would
mean more miles per gallon. Of course, its not quite that simple.

Other engine factors that weigh-in include detonation/pre-ignition
points, ignition timing and air fuel ratios. All have to be balanced
for best performance.

Octane aids in diminishing the air/fuel mixture from igniting
spontaniously before the proper time in the compression cycle. Higher
fuel octane numbers allow higher CR's (and higher thermal efficiencies)
before this occures. Alcohol fuels have higher octane ratings generally
than gasoline (99 for ethanol/106 for methanol) and even though they
have less heating value, the higher equivalent octane ratings provide
thermal incentives in high CR racing engines.

So to answer your first question--yes, with conditions.

Second question--

Ethanol is used as an octane booster for cheaper gas. Today, regular 87
octane gas has generally about 10% ethanol in it for this purpose.
Ethanol is cheaper than gasoline--(regardless of what you've heard),
therefore to answer your 91octane question would really depend on the
composition of the 91octane fuel and other factors to numerous to
mention. In general terms, if a 12:1 CR gave 100% relative power, then
8:1 would yield about 88%, or 12% less power for the same fuel usage. I
leave it to you to play with the numbers to answer your second
question.

Hope this helps,

Tut

.

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