[outofsalt]

“Many people think high-octane gasoline is more powerful than low octane gasoline. This is not true. The energy produced from a gallon of high and low octane gasoline is almost the same. Any minor variation depends on what additives are used by refiners and blenders. The key features of high-octane gasoline are a higher ignition temperature and a slower burning rate.

The higher ignition temperature of high octane gasoline reduces the chance of detonation from “hot spots” within the engine’s cylinders and minimizes pre-ignition. A slower burn rate allows for more efficient use of the ignited fuel’s pressure buildup to be converted to mechanical energy instead of heat. That is why a high performance engine will run smoother and will feel more powerful when high-octane gasoline is used.

Using a low-octane gasoline whose ignition temperature is too low causes pre-ignition. Low-octane automotive gasoline (87-octane) has a typical ignition temperature of 300 degrees Celsius; high-octane (93-octane) automotive gasoline has a typical ignition temperature of 400 degrees Celsius. Aviation gasoline is blended to ignite at 500 degrees Celsius. High compression and high cylinder temperature will cause the fuel to ignite before the sparkplug fires.” http://www.aviationpros.com/article/10387611/octane-101-autogas-vs-avgas

[EMI_Guy]

Most excellent response.

I can’t precisely quote the equation at this moment, but it states where “r” is the compression ratio, efficiency is proportional. Raise “r” and the economy increases provided...you’re feeding the correct fuel into a higher r engine.

By their very nature diesels kick the tar out of gassers with respect to economy: vastly higher r, more specific heat value than gasoline and slow burning making for great BMEPs on the piston crown.

[conservatism_IS_compassion]

“Many people think high-octane gasoline is more powerful than low octane gasoline. This is not true. The energy produced from a gallon of high and low octane gasoline is almost the same. Any minor variation depends on what additives are used by refiners and blenders. The key features of high-octane gasoline are a higher ignition temperature and a slower burning rate. . .”

I can’t precisely quote the equation at this moment, but it states where “r” is the compression ratio, efficiency is proportional. Raise “r” and the economy increases provided...you’re feeding the correct fuel into a higher r engine.

By their very nature diesels kick the tar out of gassers with respect to economy: vastly higher r, more specific heat value than gasoline and slow burning making for great BMEPs on the piston crown.

To try to explain the fundamentals, think of it this way:

1. Gasoline engines, Diesel engines, gas turbine engines, Steam engines - any engine which uses heat to expand/pressurize a gas and to produce power - has to
   1. start with ("take in,” in the case of internal combustion engines) cool air or water

   2. use a pump to raise the pressure of the air (or water)

   3. heat the air (or water, in the steam engine case)

   4. allow the air (or steam) to expand against a resistance, producing work, and

   5. throw off as much remaining heat as possible (and there will be heat which must be discarded) to return to step 1

2. Engines produce work only in “proportion” to the increases in pressure and temperature at which they operate.

3. This argues for compressing the air in a gasoline or diesel to very high pressure, which produces high initial temperature before combustion should begin. And in a diesel design, the fuel is not mixed with the air (and thus does not start burning) until it is time for the expansion stroke of the piston to begin (unfortunately there is a nontrivial lag in the combustion process, so that by the time the first bit of fuel injected burns, there is more fuel injected, which combusts explosively at the high pressure/temperature of the combustion, resulting in “diesel knock.”) But in general the diesel concept is consistent with the design of very high compression, thus very high temperature/pressure ratios, and thus excellent efficiency in producing work from fuel. It can therefore fairly be said that if you are not using a diesel, you are not doing your very best to get high efficiency.

4. Primarily because of “diesel knock,” and the weight penalties that imposes on the design of diesel engines for a given power capability, spark-ignition gasoline engines are preferred by automobile drivers. But the compression ratio which is serviceable in a gasoline engine is much lower than for a diesel because the advantages of the gasoline engine over the diesel are pretty much predicated on avoiding the “knock” problem. And the limit of the serviceable compression ratio of a gasoline engine depend on the minimum octane rating of gasoline for which it is designed. If you design for 87 octane, you use a lower compression ratio than you do if you design for 92 octane.

5. An engine designed to be able to operate smoothy on 87 octane gasoline does not run any better on 92 octane gasoline. There is no significant difference in the heat value of the octane grades, and no difference in the hardware you are using it in. Thus, no predicted improvement in performance.

6. OTOH using 87 octane gas in an engine designed for 92 octane gas is destructive of the engine. At the onset of the Energy Crisis, General Motors committed engineering malpractice in a very similar way, by the simple expedient of modifying a gasoline engine to very high compression ratio, and substituting diesel fuel injectors for spark plugs. The result was wonderful performance on paper - but, entirely predictably, very poor service in real life.