air-assisted injectors, sort of

[Tom Sharpe]

Sorry for the late reply. Here's another $.02. If I can get a faster flame
front, I can reduce timing and torque loss from compressing burning gasoline
BTDC. I might even be able to reduce detonation.......If I could make the
charge burn really fast (not detonate), I could time the engine ATDC.....
You guys seem way ahead of me, so keep us all posted... Tom.

Chris Conlon wrote:

> Greg wrote:
>
> > The neatest thing about it is that I see no reason not to stab the
> > Orbital air assisted injectors (the actual fuel injectors, which,
> > in turn, plug into them,  are standard stuff) into the existing
> > bungs on an existing TPI engine and go from there.
> > ...
> > Orbital claims maximum droplet size of 8 microns coming out of
> > the injector. Obviously, if these units can direct inject, they
> > are quick enough to match injection with inhale in a TPI system
> > without resorting to staged injectors.
>
> Presumably this means that these injectors have a minimum usable
> "on" time a good bit lower than the usual 1-2 msec or so? (Or does
> the engine have a fairly low ratio of power at WOT to power at idle?)
>
> There are still several relationships and effects that I'm curious
> about:
>
> Does a decreased droplet size lower the "effective" octane of a fuel
> (by increasing flame speed, leading to an increased chance of
> detonation)? Or is it all vapor by TDC anyway, regardless of droplet
> size?
>
> To what extent is the fuel/air really homogeneous by TDC? On one hand
> you have "stratified charge" engines, and OTOH your residual volume
> is fairly small in an average-CR engine.
>
> Ignoring the issue of detonation, how ignitable are lean mixtures? Or
> rather, how lean can you make the mixture and still be able to ignite
> it? What if you allow for a seriously powerful ignition, and/or bump
> the CR up a lot? It seems a bit weird to me that lean mixtures are
> more prone to detonation (which indicates to me a mixture near
> optimum, or near one of perhaps 2 optimums) and also considered to
> be hard to ignite - or is that only at a *much* leaner A/F ratio?
>
> (I realize this is heading towards a diesel or GDI engine, I just
> wonder if any of those efficiencies can be brought to a TPI setup.)
>
> Also I'd really love to find some reference that maps flame speed
> and the detonation line, flame temperature, charge density and A/F
> ratio for gasoline over a wide range. (Not just near stoich.)  In
> the past I was involved in making explosives, and all this kind of
> data was easy to find, but this was mostly for solid and some liquid
> explosives. I did get to make some air-fuel explosives, but not many,
> and never really had nice complete data as for more conventional
> systems.
>
> My experience there left me with a few good rules of thumb; I'm sure
> some of them apply here but doubt that they all do:
>
> Fine grain size (in a mixture) increases flame speed. Grain shape
> (in a composite) will have a profound effect on flame speed, and
> often on an explosive's tendency to detonate. (We probably only
> get one grain shape, spherical, which is regressive-burning, i.e.
> the burn rate slows as it goes.)
>
> Almost anything that burns really well can be made to detonate if
> you have enough of it, under enough pressure, hot enough, and with
> a strong enough initiation. Gas in an engine is obviously not too
> very far from this limit under normal conditions.
>
> There's a grey area (of pressure, density, temperature, flame speed)
> within which an explosive might burn or it might detonate. Some
> explosives easily go from burning to detonation, some do not. Pressure
> helps make this transition. (Gas engines, again, are often in this
> grey area.) Sometimes this grey area may be a very wide area indeed.
> A very strong initiation (shock wave) can decisively ensure that the
> explosive will detonate, not burn.
>
> In a mixture, mixtures close to stoich are usually *in the neighborhood
> of* the most-easily-detonated mixture.
>
> Ok if you've suffered my post this far I'll give up my silly/brilliant
> idea for people to poke holes in or run with:
>
> Take the engine of your choice
> Add the air-assisted injectors, or outright LPG injection. You're
>   gonna need all the octane you can get
> Raise the CR way way up
> Have your chosen head-work Guru redo the head, bearing in mind
>   the high CR.
> Run the hottest ignition you can find or build (probably build)
> Ceramic coat the piston/bore/head
> Run the engine way lean
> Run water/methanol injection to control detonation / EGT / NOx formation
>
> I like the idea of squeezing a lot out of a given displacement, and out
> of a given amount of fuel... what I don't know is if this is at all
> workable. Maybe I'll have to wait for GDI after all.
>
>    Comments appreciated,
>    Chris C.