Originally Posted by Diesel_Dave
but I don't see how 9:1 with gasoline can be anything but bad.
What am I missing here?
Its common for them to run 10:1 A/F on turbo gas engines under full load. They do this for a few reasons. The very rich fuel mixture greatly reduces the chance of preignition and detonation. All that fuel cools the intake charge and any liquid droplets of fuel would go into the combustion chamber and cool the hot spots that will form on the piston, spark plug creamic and grounding arm. The very rich mix its self also cools combustion temps.
Your gasoline engine's air fuel mixture will auto ignite well under 600'F, you want to keep your total compression temperature at the time of spark plug firing well below this.
Running the calucation for a gas engine with a 10:1 compression ratio with 73'F intake air at 1bar gives me compression temp of about 890'F.
Run the numbers for my diesel, 22:1 gives me over 1400'F of compression heating.
Thats why turbo engines blow apart some times when they go lean, they loose that fuel cooling provided by the 10:1A/F mix, when the leaner A/F mix is compressed by the piston it gets hotter than it did with the 10:1A/F, when the air fuel mix hits >530'F it goes boom, no more engine.
When you change your starting values for Temp and Pressure like for a turbo diesel engine with no intercooler your adiabatic compression numbers go wild.
You may be wondering why the 10:1 C/R engine's adiabatic heating calc yealded a number that was higher than the auto ignition point of gasoline?
Adiabaitaic compression is figured only for the compressing of air, not air and fuel, factoring in the fuel evaporation could be done but the effect the very rich fuel mixture has on the compressability charastics of air and how the liquid droplets act in the hot combustion chamber is not known to me.
Also the spark plug ignites the A/F mix in an N/A engine well before the piston is at top dead center. So when the spark plug fires, at 30'BTDC or more your C/R at that moment might be 7:1 or 8:1.
Also the faster your N/A engine spins the less your cylinders are filled (in most cases, not all) typically your cylinders are 90% full +/-5% at full power. Some times with more engine speed your volumemetric efficiency will go above 100%, this doesn't happen on street driven N/A engines.
So a 10:1 C/R on paper isn't really 10:1 C/R inside the engine.
In a turbo gas engine, as you build boost your ignition retards from its peak advance and moves back closer to TDC, so instead of lighting your A/F at 30-38' BTDC like in an N/A gasser a gasser with a power adder such as turbo, supercharger or nitrous can have its timing as low as 10'BTDC when its running at full power. So a turbo has more heat, more compression to deal with and there for is more trouble, this is where things get tricky.
I say Diesel engines are easier.