A bit OT: car fuel consumption.

[The Natural Philosopher]

Mostly wrong. All cars have catalytic converter and lambda sensors and anti-knock sensors these days. Those together will lean the mixture out to the point of almost detonation.

[The Natural Philosopher]

yes, but its also about too lean and too advanced ignition. High octane fuels will run leaner and with more advance, and that's what the ECU should deliver .

Todays (since 2000) car have the fuel level controlled all the time by means of the knock sensors,= - literally second by second changes in spark timing and injection duration are the order of the day.

Better fuel should net you better mpg and a slightly smoother power delivery.

[Steve Walker]

Higher octane rating means that the ignition can be advanced further, for a more complete, powerful and efficient burn, without starting to knock. Cars having a knock sensor can make this adjustment themselves, those without can only be adjusted externally.

[Vir Campestris]

I thought they aimed at a stoichiometric mixture to minimise the emissions of CO, HC and NOx?

But ICBW.

Anecdotally I've switched to 97 - my car is old enough that I'm not sure it would like E10 - and it's giving about 10% better MPG.

Andy

[The Natural Philosopher]

Yes, that is the point of 'almost at knocking'

Essentially the leaner it is and the more advance it has the less CO and usually the more power but you run into detonation risk, so the mixture is richened enough and spark retarded to stop that and then let the cat deal with the residual unburnts

That is probably about normal. 7.5% is the 'theoretical' figure

[Dave Plowman (News]

Yes. A lean mixture results in excessive hydrocarbons. Why 'lean burn' engines, that gave better MPG at cruise, died out when cats. arrived.

[NY]

I'd have expected that you'd get excessive hydrocarbons (which are presumably the alkanes and alkenes in unburned fuel) if the mixture was too rich, rather than too lean. Evidently not.

How is it that diesel engines, which run *very* lean (far more lean than stoichiometric), don't suffer from excessive hydrocarbons? Is it due to the different method of ignition, or is it because of differences in the composition of the fuel?

[Tim+]

Place not your faith in the Plowman. In this case he?s got things wrong.

Tim

[Dave Plowman (News]

Given it was all some 30 years ago, you'll excuse me for confusing hyrdocarbons with NOX. ;-)

However, the only way to reduce all emissions (other than CO) is a series of catalytic converters. And lean burn simply doesn't work with those, unless you dramatically reduce the performance that engine is capable of.

You can design a cat. to deal with the excess NOX produced by lean burn over stoic. But not (as far as I know) one which can do that and also handle the emissions produced with an AFR for maximum torque.