Rich combustion in SI engines

[SomptingGuy]

Does anyone have a plot of torque (or equivalent) vs equivalence ratio for an SI engine that goes well beyond the usual range? I'm looking to see what happens well rich of stoichiometric (e.g. 1.5 and above).

Thanks.

 

[R1100S]

No good answer, but, just out of curiousity, why?

 

[SomptingGuy]

Why?

For use in transient simulation, where your control system can't quite keep up with the driver's right foot.

From years past (before the days when TWC enforced stoich running), it was clear that slightly rich was better than stoich - so I wondered what happens when you go super-rich. At what point does putting more fuel in mean getting less work out?

 

[patprimmer]

So I guess you mean for very short term.

Traditionally this was done with an accelerator pump.

More power was generally obtained by going about stoich times 1.2 after which power would fall off.

The accelerator pump temporarily increased fuel to overcome transients. I have no way to calculate the actual A:F ratio shortly after the pump activated, but I guess maybe
about 20 to 80 ccs of extra fuel is pumped into a 350 CI V8 as the throttle goes from closed to wide open. I guess most is used to cover lag in main jets and maintain 1 to 1.2 times stoich, but residual fuel in manifold will be carried over after mains have caught up with air. Wild guess say 1.4 stoich for a second or so. Like I say, a wild guess.

To much pump, or for to long makes engine go flat, but no real misfire.

Not enough pump, or to late pump squirt caused lean out backfire.

As I say, no real data, just wild guess from some old time experience with carburettors


Regards

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[SomptingGuy]

Thanks. Thermo-chemical equilibrium calcs show apparent heat release to roll off gently off above about 1.15-1.2 times stoich. But there's no substitute for experience, even if it's not backed up by measured data.

 

[GregLocock]

Have you looked in the LARC archive? they did a lot on carburetted engines.

Also, do you mean mixture as in petrol added divided by air in, or do you mean actual vapourised mixture? there is a huge difference, chokes and accelerator pumps create puddles and sheets of liquid fuel in the intake, some of which gets caught up and blown into the cylinder, and some just sits there.

Cheers

Greg Locock

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[patprimmer]

I would have thought that it accumulated there for a short time, but was being carried over even after the mains were flowing, thus enriching the engine for a short time.

How much and for how long is anybodies guess, and would be highly subject to manifold design, air speeds and temperatures

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Tmoose]

I think the actual ratio involved in combustion must not get much richer than 8-1 or the fire goes out. As other have said, during cold start vaporization is poor, so extra fuel is needed to get the apparent mixture up in the combustible range. Similarly When non-cv carb butterflies rapidly open, the fuel delivery lags behind air flow, so the pump shot just "makes up" for it.