Injector Sizing: REALLY dumb question

[Tony Bryant]

From:           	"Zack" <zubenubi at inetport.com>
To:             	diy_efi at efi332.eng.ohio-state.edu
Date sent:      	Thu, 27 Aug 1998 20:01:10 -0600
Subject:        	Re: Injector Sizing: REALLY dumb question
Priority:       	normal
Send reply to:  	diy_efi at efi332.eng.ohio-state.edu

> 
> 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).

Here's what I've inferred from my reading (mainly Heywood's ICEF)

Its all related to fuel vapourization. Only small droplets and vapour 
will actually burn. Big drops will largely survive the combustion 
process and come out as HC emissions. Therefore if less than half 
your fuel is in small enough droplets or vapour to burn, you've got a 
very lean burn, with tons of HC emissions to boot. Lean of course 
is wasting some of the air, and therefore some of the potential 
power.

For these reasons I've heard (unconfirmed rumours) that F1 cars 
heat the fuel to 300deg before injection. No droplets at all - all 
vapour. You can't argue with 900HP from 3L normally aspirated.

IMHO (again) The optimal injection timing varies with operating 
conditions. Squiting & puddling a ton of gas on top of a relatively 
cold valve wastes the atomisation effect of the injector wheres 
squiting past a "red hot" intake valve wastes that potential 
vapourisation effect.

i.e. Low RPM, High VE or cold engine -> directly into cylinder
     Any RPM, Low VE on hot engine -> Sit on valve.
    High RPM, High VE -> don't have any choice, must inject during 
both conditions

I suspect displacing air on a gasoline effect is a minor effect.