Injector Sizing: REALLY dumb question

Greg Hermann bearbvd at sni.net
Fri Aug 28 03:33:25 GMT 1998


>OK, 'nuther dumb newbie question.  How can injector timing have any
>effect on torque??  Torque, I thought, is directly related to BMEP,
>which would in turn be almost directly related to the amount of air
>ingested into the cylinder (assuming for the moment that the injector
>duration is adjusted so A/F ratio is constant throughout the RPM
>range).
>	The only thing I can think of related to injector timing which would
>affect the amount of air getting into the cylinder would be, if fuel
>is squirted into the manifold before the valve opens, the
>vaporization of the fuel there would tend to displace air from that
>portion of the manifold and lower the final charge density.

BINGO #1
>	Looking at it this way I could easily tempt myself into believing
>that the best approach would be to delay injection until the last
>possible moment (uhh...), or even better, to inject the fuel into the
>chamber directly only after the intake valve is closed.

BINGO #2-- Care to ask how much money the oem's are spending trying to
figure out how to do direct injection for a not too astronomical cost???
AND for just this reason?? Remember, the one thing that talks in Detroit is
money!!

>	OTOH, if you're squirting methanol in, for example,
>the cooling effect due to the high heat of evaporation easily
>dominates the displacing effect of the vapor, so that you are much
>better off overall if the vaporization is already complete by the
>time the intake valve closes, because the cooling effect of the
>mixture caused by the evap of the methanol draws additional air/fuel
>mixture in behind it.

If you really get into the literature in depth, there is a serious, and
unresolved, debate over whether the high heat of vaporization of methanol
helps more due to charge cooling or by reducing compression work after the
intake valve closes. Most reasonable people admit that it is most likely
some of both.
The one thing that is sure is that evaporating fuel by cooling off intake
tract parts does not do much good at all in either respect. You just waste
heat from the next power stroke (and or do additional compression work to
heat them back up again.

>	Gasoline lies somewhere between the two ends of this continuum, on
>the one end the fuel with no heat of vaporization whatever that
>you'd like to inject only after the intake valve closes, and on the
>other end the fuel with a very high heat of evap. that you'd like to
>inject well before so that the evap & cooling is complete by the time
>the charge reaches the cylinder.

There is more than enough compression work done in each cycle to vaporize
much more than all of whatever kind of fuel you care to run (including any
nitrous mixed with it) in any amount which you could get to burn. Saving on
this work makes gains quicker than increasing charge density (and V E ) the
percentage that is available from charge cooling can. Play with an
indicator card for a while, and you will see what I mean. Direct injection
will, therefore, work better, even with alky. Been there, done that, it
would be pretty hairy to post. And if you quit holding engine conditions
constant, saving compression work on the one hand lets you do more (by
letting you raise the compression ratio, without detonation,  and thus get
a better expansion ratio (which is where the power really lies) With a
better Carnot (theoretical) efficiency for the cycle, then you are really
getting somewhere.

>	I don't know exactly where gasoline falls on that continuum, so it's
>not clear to me how y'all are coming to conclusions about when's the
>best time to inject the fuel.

Back to your comment about BMEP: it is indeed related to torque, and
directly so. Two things that affect  it significantly (in a negative way)
are the amount of work done on the compression stroke and the amount of
heat rejected to the cooling system during the power stroke. Cooling intake
tract parts by evaporating fuel on them increases to both of these negative
influences on BMEP. Therefore, evaporating the fuel in the right place at
the right time (not sizzling it on intake tract parts) CAN increase TORQUE.
QED!

Data proof---take a look at the BSFC numbers for an engine run on a big
Holley, and then run on Webers., and both jetted for highest power. The
Webers will give more peak power (5 to 10%) with about 15% LOWER BSFC!!!!
Wanna bet on why????
>
Regards , Greg





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