Injector Sizing: not dumb questions at all

[Greg Hermann]

Comments on and review of Gary's latest sage response:

>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.

True.

>The first Bosch mechanical fuel injection injected directly into the
>cylinder (Mercedes 300 SL)

True.

 but was timed so the end of the injection
>occurred at 60 deg ATDC during the intake stroke.

Credible, but sounds awfully early. For start of injection, this would
sound right.

 The mechanical injection
>pump could inject all the necessary fuel in a relatively short time.

Sure could. No doubt.

At part throttle, the injection started after TDC to avoid the reversion.

See above. I would believe 60 degrees after in a heartbeat.

 At full throttle, the injection began before the intake valve opened.

So that the fuel diluted the exhaust and made the pipes shoot flames for
extra thrust??? This is beyond INCREDIBLE! Perhaps, if your statement is
accurate, this effect is what launched their car into the crowd at Le Mans,
And Merc, having figured out how they had erred, might really have quit
racing over this screw up rather than for the reasons stated at the
time!!!!!

Good running engines have been built with constant flow fuel injection.

Hilborn, Enderle, et al come to mind.  Big Daddy and Snake, Parnelli and
A.J. all ran pretty well indeed. But we want better, not just good. We have
access to better technology than they did, if we will just learn how to use
it to the fullest extent possible!!

I think timing the injection pulse to the intake stroke has some benefit at
low rpm
>and part throttle

It does.

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.

There is, and it will cover most of the sins you seem bent on committing.
You will never know any better until the guy who understands what to do
that is better whups you! However, the same benefits that are available to
improve BMEP and torque at low speed are available at full snort if we can
figure out how to get enough properly atomized fuel into the air stream at
the most advantageous time. Why in hell do you think that TIMED LUCAS PORT
INJECTION (mechanical, with short, properly timed squirts at the right
time) supplanted constant flow setups?? (Maybe cuz it made more power????)

>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.

Make that read any angine operating with decent gas velocity in the ports.
Remember (or learn) in an intake manifold at high vacuum, velocities are
still high---density is what goes lower.  Initial eshaust velocity is Mach
1 at any load that gives a pressure ratio just below 2:1 (cylinder p to
port p) when the e valve opens. Full throttle doesn't make it go any faster
(called choked flow in technospeak.) Also, remember (or learn) that Mach 1
is a function only of temperature, not of density or pressure, for any
given gas! Funny thing here: pr at e valve opening is higher (as is temp)
on a low comp ratio engine that on a high cr (race) motor.  Check out the
vlave problems that the NASCAR truck guys went through trying to run Ti
exhaust valves, as on Winston Cup motors, on their 9:1 (by rule) truck
motors. Problems solved when they went back to Inconel valves. Something
about how a bigger EXPANSION ratio takes more enthalpy out of the gasses!
So the exhaust starts to work better sooner on a mild engine with decent
manifolds than it does on a race motor.

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.

This is the so-called "Kinetic" effect. various sonic pulses and
reflections also are extremely (if not a lot more) important here.

The intake pressure wave comes from the high
>depression when the intake valve is wide open and the piston velocity is at
>maximum.

Fuel standoff more likely comes from the reflected compression pulse
created when the fast moving gas in the i port runs into the closing intake
valve. This pulse is much sharper. With one throttle per hole, a really
viscious standing wave can be set up between i valve and  (somewhat open)
throttle butterfly under some speed conditions. I strongly suspect that
such waves are the source of fuel standoff.

Regards, Greg