Hi Gord,

Not so much on us automotive engineers today... A lot of modern
automotive engines are run right along the knock limit for
efficiency, with knock control constantly operating. To do this,
there has to be a knock event every now and then for the knock
control to be able to detect the limit. (Knock control just detects
knock events and retards the ignition. When there is no knock,
ignition is advanced again until the next knock occurs.) This
normally works fine. However on some (not all) engine types, on high
load testbed runs this has recently led to very rare statistical
occurence of "super-knock" events, with disastrous results.


Yes, that's quite interesting to me, and it backs my opinion of
using low test fuel in my cars...I never use high test fuel at
all, mind you, I only use standard domestic vehicles but I
consider high test wasteful in modern engines with 'knock
control'.

Well, it depends...

Normally you won't damage an engine with knock control by using lower
grade fuel than recommended, however within limits. If you use very low
octane fuel on a high compression or a turbocharged engine, ignition
might occur before the spark just from the compression, as we
discussed. There is nothing knock control can do about that. Also,
depending on your driving profile, you might find that fuel consumption
increases with the lower grade fuel, as retarded ignition reduces
engine efficiency. This is especially valid at high engine loads and
low engine speed, where the biggest retard is neccessary. Wether the
increase in consumption will eat up the price advantage of the lower
grade can only be determined by experiment.

Yes, I understand that, another good system that GAMI is looking
at is their accurate fuel injectors to enable automobile engines
to be run lean.

Now that is a new one to me! I know about GAMIs injectors for GA piston
engines and all the LOP-operation stuff, wich are around for a while
now, and which I think are a big improvement. I didn't know that they
are planning on entering the automotive market and I'm quite surprised.
Automotive engines, different from aviation, run stoechiometric
basically anywhere on the map exept full load. (stoechiometric mixture
is more or less peak EGT). Due to their design, mostly liquid cooled,
temperatures are not a problem. Of course you could run such an engine
lean (LOP), but you won't pass any emissions test for sure. I don't
think you really need special injectors to do it, but you would need to
retune the ECU. And the fuel savings compared to running stoechiometric
are only a fraction of what you save if otherwise you have to run rich.

The Japanese car makers had lean running engines on the market for a
while, and some of the direct injection gaoline engines being developed
now also run lean by creating a stratified charge. But that's far
beyond an aftermarket improvement.

We run (perhaps ran might be more accurate) large
recip aircraft engines at '10% lean from best power' (by manually
leaning them during cruise) for many thousands of hours and they
worked fine in that condition, matter of fact they'll continue to
run fine as much as about 30% lean before they get unstable, they
seem to love lean mixtures!...

Do you by chance know whether these had direct injection (injecting the
fuel into the cylinder instead of the manifold)? To my knowledge there
have been direct injection piston engines among the big radials, but I
haven't found any further information about it so far.

regards,
Friedrich

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