3 ms·

How are they preventing engine knock ? At 14:1 compression you'd have to be running premium gas, which would pretty much cancel the efficiency saving, in a $$ p

How are they preventing engine knock ? At 14:1 compression you'd have to be running premium gas, which would pretty much cancel the efficiency saving, in a $$ perspective.

crypto5 10y ago
Mazda has 14:1 for regular gas.
Unklejoe 10y ago
I think direct injection allows engines to run higher compression ratios without knocking. I'm not sure exactly why, but I assume it's because the fuel sprays directly on top of the piston which could allow it to cool any hot spots which would have normally caused detonation.
- jschwartzi 10y ago
  The fuel probably has to fully vaporize before it can ignite. This happens in the manifold for a normal EFI, or in the carburetor for an older engine. There's some short period of time that this is happening, and so if you time the injection to happen with enough time to vaporize the fuel before ignition it probably doesn't have enough time to pre-ignite before spark. This is speculation.
asimuvPR 10y ago
Direct injection makes a significant difference by delivering cooler fuel at the correct timing point. The fuel is also delivered at a higher pressure and with better atomization.
micaksica 10y ago
Don't advance ignition timing so far at 14:1 compression, and DI helps.
Gibbon1 10y ago
Not sure, but would assume a gas engine can run with a higher compression ratio when under partial load than full load. If you can increase the compression ratio on the fly then you can keep the compression ratio at optimum over the whole power band. Goes double if you have a turbo since the boost provide by the turbo goes from nil to lots at the high end of the power band. I think there isn't much improvements to be had for engines at full power. But lots of room for improvement at partial power. Interesting bit, hybrid cars have really low emissions. Which says to me they're avoiding running their engines at power/rpm ranges that create high emissions.

How are they preventing engine knock ? At 14:1 compression you'd have to be running premium gas, which would pretty much cancel the efficiency saving, in a $$ perspective.

crypto5 10y ago

Mazda has 14:1 for regular gas.

Unklejoe 10y ago

I think direct injection allows engines to run higher compression ratios without knocking. I'm not sure exactly why, but I assume it's because the fuel sprays directly on top of the piston which could allow it to cool any hot spots which would have normally caused detonation.

jschwartzi 10y ago
The fuel probably has to fully vaporize before it can ignite. This happens in the manifold for a normal EFI, or in the carburetor for an older engine. There's some short period of time that this is happening, and so if you time the injection to happen with enough time to vaporize the fuel before ignition it probably doesn't have enough time to pre-ignite before spark. This is speculation.

asimuvPR 10y ago

Direct injection makes a significant difference by delivering cooler fuel at the correct timing point. The fuel is also delivered at a higher pressure and with better atomization.

micaksica 10y ago

Don't advance ignition timing so far at 14:1 compression, and DI helps.

Gibbon1 10y ago

Not sure, but would assume a gas engine can run with a higher compression ratio when under partial load than full load. If you can increase the compression ratio on the fly then you can keep the compression ratio at optimum over the whole power band. Goes double if you have a turbo since the boost provide by the turbo goes from nil to lots at the high end of the power band. I think there isn't much improvements to be had for engines at full power. But lots of room for improvement at partial power. Interesting bit, hybrid cars have really low emissions. Which says to me they're avoiding running their engines at power/rpm ranges that create high emissions.