Injector Sizing: not dumb questions at all

[Gary Derian]

A couple opinions and maybe a few facts here, might be worth $.03.  There
needs to be some time for air and fuel to mix.  Injecting fuel near TDC,
like a diesel, limits engine rpm because of the need to mix air and fuel.
The first Bosch mechanical fuel injection injected directly into the
cylinder (Mercedes 300 SL) but was timed so the end of the injection
occurred at 60 deg ATDC during the intake stroke.  The mechanical injection
pump could inject all the necessary fuel in a relatively short time.  At
part throttle, the injection started after TDC to avoid the reversion.  At
full throttle, the injection began before the intake valve opened.  Good
running engines have been built with constant flow fuel injection.  I think
timing the injection pulse to the intake stroke has some benefit at low rpm
and part throttle but near max hp, there is enough cylinder turbulence to
effectively mix and vaporize the fuel no matter how it gets into the
cylinder, as long as it gets there during the intake stroke.

Having the exhaust jump start the intake flow during overlap works but only
at full throttle with a racing engine that has tuned intake and exhaust and
is operating near peak torque.  The exhaust tuning provides low pressure and
the intake tuning provides high pressure during overlap.  The pressure wave
that creates the exhaust tuning pulse comes from the blowdown just as the
exhaust valve opens.  The intake pressure wave comes from the high
depression when the intake valve is wide open and the piston velocity is at
maximum.

Gary Derian <gderian at cybergate.net>