Burning Lean = more heat?

I understand why burning rich reduces combustion temperatures. I don't
understand why burning lean increases combustion temps. Web and usenet
google searches haven't turned up much, and there is a lot of contradictory
theories.

What gives?

thanks,
Andy

Probably too simple, but a lean mixture has a higher ratio of o2 and
subseqently the resulting combustion generates more heat up to the point of
a misfire or incomplete combustion. My old old race car would turn in it's
best times on a given day, at the leanest I could run the car and still
maintain acceleration at the 1/4 mile speed traps. Next step lean it would
fall down right at the finish line.
"Andrew Crabtree" > wrote in message
...
> I understand why burning rich reduces combustion temperatures. I don't
> understand why burning lean increases combustion temps. Web and usenet
> google searches haven't turned up much, and there is a lot of
> contradictory theories.
>
> What gives?
>
> thanks,
> Andy
>
>
>
>



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Andrew Crabtree > wrote in article
>...
> I understand why burning rich reduces combustion temperatures. I don't
> understand why burning lean increases combustion temps. Web and usenet
> google searches haven't turned up much, and there is a lot of
contradictory
> theories.
>
> What gives?
>


There are only so many BTUs in gasoline.

Leaner mixtures actually burn cooler since there are fewer BTUs available
in a given fuel/air charge, but.....

......a richer mixture cools the cylinder more as it is introduced, so, even
though you are burning more fuel - thus more BTUs - the cylinder
temperatures run cooler due to the cooling effect of the rich fuel charge.

An engine will produce the most power (and the most heat) running at stoichiometry, when the amount of air is exactly the amount needed to burn all of the fuel. This ratio of air to fuel is about 14.7 by weight for gasoline. Before the days of emission controls most engines ran slightly lean at part throttle to ensure complete fuel burning for good economy, and slightly rich at wide open throttle. Since the introduction of three way catalytic converters the engine must run at stoichiometry for the NOx reduction to work and is so critical that an oxygen sensor must be used to control the mixture. Since relatively high compression ratios are used with low octane gas to get good numbers on the government mileage tests the only way for an engine to run without detonation at wide open throttle is to greatly retard the spark and run very rich, as low as 10 to 1 air to fuel ratio. Under this condition only about 2/3 to the fuel is burned, the other third is used to keep combustion temperature down and is not burned. Since all gasoline engines now use knock sensors the easiest way to increase power and efficiency is to use higher octane gas, allowing for wide open throttle with more ignition advance and a mixture that is still rich, but not nearly as extreme as with lower octane gas.


* wrote:
>
> Andrew Crabtree > wrote in article
> >...
> > I understand why burning rich reduces combustion temperatures. I don't
> > understand why burning lean increases combustion temps. Web and usenet
> > google searches haven't turned up much, and there is a lot of
> contradictory
> > theories.
> >
> > What gives?
> >
>
> There are only so many BTUs in gasoline.
>
> Leaner mixtures actually burn cooler since there are fewer BTUs available
> in a given fuel/air charge, but.....
>
> .....a richer mixture cools the cylinder more as it is introduced, so, even
> though you are burning more fuel - thus more BTUs - the cylinder
> temperatures run cooler due to the cooling effect of the rich fuel charge.

--
Mike Walsh
West Palm Beach, Florida, U.S.A.

On Wed, 1 Jun 2005, Mike Walsh wrote:

> An engine will produce the most power (and the most heat) running at
> stoichiometry

False. An engine will produce emissions most compatible with catalytic
cleanup when running at stoichiometry. Greatest power is produced at a
mixture richer than stoich (between 12 and 13 to 1) and greatest heat at a
mixture leaner than stoich.


> Since relatively high compression ratios are used with low octane gas to
> get good numbers on the government mileage tests the only way for an
> engine to run without detonation at wide open throttle is to greatly
> retard the spark and run very rich, as low as 10 to 1 air to fuel ratio.

Er...no. No 10:1 AFR on any modern passenger car gasoline-powered engine.
And you're forgetting three very important letters (E, G and R).

> Since all gasoline engines now use knock sensors the easiest way to
> increase power and efficiency is to use higher octane gas,

Nope. There is no benefit to the use of fuel of a higher octane than
that for which any particular engine management system is calibrated,
and most systems are calibrated for regular-grade gasoline (87 AKI
at sea level).

Mike Walsh > wrote in message
...
>
> An engine will produce the most power (and the most heat) running at
stoichiometry,

Absolutely not. A gasoline engine will produce the most power at an A/F
ratio of about 12.6 (1.17 x stoich). By definition this is also the point at
which the most heat, i.e. number of BTU, is produced. However this is not
the point at which the maximum flame temperature occurs. Flame temperature
peaks at an A/F ratio very slightly richer than stoichiometric - about 1.05
stoich or an A/F ratio of 14:1

when the amount of air is exactly the amount needed to burn all of the
fuel. This ratio of air to fuel is about 14.7 by weight for gasoline. Before
the days of emission controls most engines ran slightly lean at part
throttle to ensure complete fuel burning for good economy, and slightly rich
at wide open throttle. Since the introduction of three way catalytic
converters the engine must run at stoichiometry for the NOx reduction to
work and is so critical that an oxygen sensor must be used to control the
mixture. Since relatively high compression ratios are used with low octane
gas to get good numbers on the government mileage tests the only way for an
engine to run without detonation at wide open throttle is to greatly retard
the spark and run very rich, as low as 10 to 1 air to fuel ratio.

Nonsense. No engine, modern or otherwise runs at anything like that rich at
WOT, nor is ignition advance greatly retarded. For a good compromise between
power and economy most production engines are set up to run about 13 to 13.5
A/F ratio at WOT.

Under this condition only about 2/3 to the fuel is burned, the other third
is used to keep combustion temperature down and is not burned. Since all
gasoline engines now use knock sensors the easiest way to increase power and
efficiency is to use higher octane gas, allowing for wide open throttle with
more ignition advance and a mixture that is still rich, but not nearly as
extreme as with lower octane gas.

Higher octane fuel is only of benefit if detonation is occuring at the
optimum power settings for ignition and fueling on a lower octane fuel. This
is not the case for the vast majority of standard production engines.
--
Dave Baker - Puma Race Engines (www.pumaracing.co.uk)

In article >,
"Dave Baker" > wrote:

> Mike Walsh > wrote in message
> ...
> >
> > An engine will produce the most power (and the most heat) running at
> stoichiometry,
>
> Absolutely not. A gasoline engine will produce the most power at an A/F
> ratio of about 12.6 (1.17 x stoich). By definition this is also the point at
> which the most heat, i.e. number of BTU, is produced. However this is not
> the point at which the maximum flame temperature occurs. Flame temperature
> peaks at an A/F ratio very slightly richer than stoichiometric - about 1.05
> stoich or an A/F ratio of 14:1
>
> when the amount of air is exactly the amount needed to burn all of the
> fuel. This ratio of air to fuel is about 14.7 by weight for gasoline. Before
> the days of emission controls most engines ran slightly lean at part
> throttle to ensure complete fuel burning for good economy, and slightly rich
> at wide open throttle. Since the introduction of three way catalytic
> converters the engine must run at stoichiometry for the NOx reduction to
> work and is so critical that an oxygen sensor must be used to control the
> mixture. Since relatively high compression ratios are used with low octane
> gas to get good numbers on the government mileage tests the only way for an
> engine to run without detonation at wide open throttle is to greatly retard
> the spark and run very rich, as low as 10 to 1 air to fuel ratio.
>
> Nonsense. No engine, modern or otherwise runs at anything like that rich at
> WOT, nor is ignition advance greatly retarded. For a good compromise between
> power and economy most production engines are set up to run about 13 to 13.5
> A/F ratio at WOT.
>
> Under this condition only about 2/3 to the fuel is burned, the other third
> is used to keep combustion temperature down and is not burned. Since all
> gasoline engines now use knock sensors the easiest way to increase power and
> efficiency is to use higher octane gas, allowing for wide open throttle with
> more ignition advance and a mixture that is still rich, but not nearly as
> extreme as with lower octane gas.
>
> Higher octane fuel is only of benefit if detonation is occuring at the
> optimum power settings for ignition and fueling on a lower octane fuel. This
> is not the case for the vast majority of standard production engines.
> --
> Dave Baker - Puma Race Engines (www.pumaracing.co.uk)

Good post Dave.

Andrew Crabtree > wrote in message
...
> I understand why burning rich reduces combustion temperatures. I don't
> understand why burning lean increases combustion temps. Web and usenet
> google searches haven't turned up much, and there is a lot of
contradictory
> theories.

It depends what you mean by lean. Leaner than what? Leaner than
stoichiometric, leaner than the optimum A/F ratio for maximum power?

Maximum flame temperature in a gasoline/air engine occurs at about 1.05 x
stoich or 14:1 A/F ratio. Maximum power, and therefore total heat generated,
occurs richer than this at 1.17 stoich (12.6 A/F)

The point at which flame temperature peaks depends very much on the fuel and
the oxidant. For gasoline burning in pure oxygen rather than air it is at
about 1.15 x stoich. For hydrogen burning in oxygen it is very close to
stoich and for acetylene in oxygen it is at 1.75 x stoich.

There is no simple rule that says a fuel has to produce its peak flame
temperature at stoichiometric just because that is theoretically where all
the fuel molecules combine exactly with available oxygen ones.
--
Dave Baker - Puma Race Engines (www.pumaracing.co.uk)

Andrew Crabtree wrote:
> I understand why burning rich reduces combustion temperatures. I don't
> understand why burning lean increases combustion temps. Web and usenet
> google searches haven't turned up much, and there is a lot of contradictory
> theories.
>
> What gives?
>
> thanks,
> Andy
>
>
>
There is an optimum mixture ratio for max heat, close to stoich. Going
further lean again reduces temp. Any good book on IC engine design
should give this. Try a college library.

Running rich reduces temps by vaporizing fuel. Any conversion of
a liquid to a gas will absorb heat and excess fuel, in the form of
atomized droplets, does this handily though it's an awful polluter.
Aircraft engines regularly run full rich on takeoff for this reason,
since they're aircooled and tend to be rather powerful engines for
their weight. Rich can mean mixtures as rich as 8:1, and mixture in
most piston engines is manually controlled. We find best power at
around 12:1, highest exhaust gas and cylinder head temps at 15:1, and
the thing won't run past 18:1.

Dan

Dan