air-assisted injectors, sort of

[Greg Hermann]

>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?)

More like the injector has a rather longer time to fill the pre-chamber
with a metered amount of fuel, then the compressed air takes the
(pre-metered amount of) fuel past the air valve quite quickly. The actual
amount of air used for this is not that critical, so higher flow rates and
shorter duty cycles are ok for the air valves.
>
>There are still several relationships and effects that I'm curious
>about:
>
>Does a decreased droplet size lower the "effective" octane of a fuel
No.
>(by increasing flame speed,

no effect

Or is it all vapor by TDC anyway, regardless of droplet
>size?

With progressively larger droplets, not all are necessarily vaporized or
burned.  Leading to HC emissions and less fuel economy. It is a proven fact
that larger fuel droplets can survive the combustion process intact.
>
>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.

Combustion chamber design and charge stratification is a whole nother
(closely related) topic.
>
>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?

17 or 18 to 1 is the limit of flammability for a homogeneous mixture, no
matter how hot the spark.


I just wonder if any of those efficiencies can be brought to a TPI setup.

I wasn't thinking of bringing the lean mixture efficiencies to a TPI
setup--you need the stratified charge effect for that. All I would hope for
is the improved fuel economy and power due to better fuel atomization and
reduction of compression work by evaporating a higher percentage of the
fuel during the compression stroke. I think that both gains would be
significant--and, although this is unexplored territory, no-one has come
forward with any wagers against it yet.
>

>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:
>
As to your thoughts on detonation:

My SUBJECTIVE feelings on it are that there is a critical P-T-mixture ratio
for any given fuel above which detonation will occur, but that there is
also a (material and conditions specific) time delay involved. Once you get
the mixture above the critical point, the countdown starts, then after a
certain time , it detonates. This feel is based on a lot of engine tuning.
Also a bit of (field) experience working with explosives (is the FBI still
in the wings wondering about our 747's??).

My experience with engines seems to indicate that if you can get the burn
over with quickly (as a result of short flame travel, mixture motion, and
mixture turbulence, as opposed to fast flame travel) the chances of getting
detonation are VASTLY reduced,  the need for ignition advance is also
reduced, and BSFC is improved.

I have actually worked with an 11.5 : 1  C.R. engine whose chambers were
conducive to the above mentioned things happening, and I observed the
following: It would run on 101 octane (unleaded) with 45 degrees of total
advance with no detonation whatsoever, but it made its best power with only
32 degrees of total advance. Otherwise the same, but with different chamber
shapes, the same engine did not make as much power, and could knock its
brains out on 104 octane. Power was still increasing with increasing
advance with more than 40 degrees, but detonation prevented going any
further whilst looking for more power.

Does your experience working with explosives back up any of what I have
(kind of lamely) tried to say here?

Regards, Greg