Hmmmm, while it is tempting to look at the excruciatingly careful tuning of
racing engines and equate that with low emission levels, I'm afraid that the
truth is not quite that simple.
In drag racing, the object is to produce as much power as possible within
the various class limitations, without compromising the ability of an engine
to last long enough to complete its task. This is done by designing the
engine to burn as much fuel as possible with each fuel charge in the
combustion chamber (NOT necessarily to burn ALL of the fuel in the charge),
and to burn that fuel as quickly as possible without breaking something. In
drag racing, engines are typically tuned quite rich, since power output
drops off as the mixture approaches stoichiometric (the ratio for "perfect"
combustion). A richer mixture tends to increase CO levels.
Economy is seldom a consideration in drag racing, though it certainly can be
in many endurance-type races. In endurance racing, where excessive pit stops
cost valuable time, economy is an important consideration, balanced, of
course, with power output in the desired RPM ranges, etc. However, a fuel
efficient engine is NOT necessarily a clean-burning engine. For example,
engines with high compression ratios or running leaner mixtures can be very
fuel efficient, but the high combustion pressures/temperatures and/or extra
available oxygen typically produce higher levels of various oxides of
nitrogen...a major component in low-level ozone production and photochemical
smog.
To develop maximum power, a typical air-fuel ratio would be about 12:1, but
for maximum thermal output (efficiency) from the fuel, a typical ratio would
be more like 17:1. We're talking mass here, rather than volume.
When an engine is running in a leaner, more efficient configuration, there
is relatively more oxygen present, so N2 (nitrogen) molecules are able to be
oxidized into various oxides of nitrogen. The extra heat from the leaner
mixture also contributes to this process.
As mentioned, high compression engines develop more combustion heat and
pressure, which also helps form oxides of nitrogen, even if the mixture is a
bit on the rich side.
When an engine is running in a richer configuration to develop more power,
there tends to be a shortage of oxygen, and a higher percentage of the
available hydrocarbons are only partially oxidized into CO rather than being
fully oxidized into CO2.
Modern computer-controlled engines, in combination with a catalytic
convertor, are designed to come up with the best compromise between high
efficiency, low emissions, and maximum power. The relative importance of
these mutually incompatible aims are determined by the government, our
health, our pocketbooks, and our egos.
If I've overlooked anything obvious, or made any mistakes, please let me
know
Regards,.
Alan
===
To unsubscribe from the mil-veh mailing list, send the single word
UNSUBSCRIBE in the body of a message to <mil-veh-request@skylee.com>.