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Steve Beck

CI engine and SI engine?

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Look up the definitions, and you will have your answer.

DrD

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Spark ignition engine is like a gasoline car motor. Compression ignition engine is like a diesel truck engine. The fuel properties are not what you might expect. For spark ignition, you want a fuel that won't ignite without a spark. If it did, that would be pre-ignition or detonation (commonly called knock and dieseling, respectively). For a compression ignition engine, you want fuel that will ignite without a spark. High octane fuel has less energy per unit mass than low octane fuel and also diesel. For a high performance engine, you don't pay extra to get high octane gasoline for a more powerful fuel, you pay for the anti-knock or anti-pre-ignition properties of the fuel so that you can have a higher compression ratio (closer to a compression ignition engine) without the fuel igniting before the piston is at the right location and the spark plug actually fires. In spite of what is portrayed in movies where moonshiners pour white lightning into the tank and get twice the horsepower, ethanol is a poor motor fuel, having an octane rating of 89, which was considered garbage when I was in high school and drove a real muscle car. I ran 117 octane aviation fuel in my Olds 442 W-30 with the 4.55:1 rear end and top speed of 108 mph. Even methanol only has an octane rating of 91. People run methanol ("alki-holl") at the tractor pull, not because of it's octane rating, but because it has an OH and doesn't require as much air to burn more fuel. You can feed fuel easier than push air into an engine. You should also know that pure normal (straight chain) C8H18 has an octane rating of zero. Pure iso-octane has an octane rating of 100 so we should really call it "iso-octane rating". The pressure and temperature vs. crank angle for spark and compression engines are also quite different, which means they react differently and also produce different combustion products, some of which are undesirable emissions. The fuel explodes rapidly in a spark ignition engine and is limited by injection rate in a compression engine so that combustion extends over a longer time. Internal combustion engines are a fascinating field of study and why I went into engineering in the first place. Sadly, it was my junior year of college before I encountered a professor who could answer any of my many questions. Don't give up; rather, press on!

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