[sustran] low compression vs high compression engines

[Roberto Verzola]

I would first of all like to thank Gerry Hawkes and Eric Bruun for
their quick replies. However, the issue remains unclear to me:

1. Are motorcycle engines low-compression engines? (I seem to have
come across this information somewhere, but I don't exactly remember
now where...).

If they are:

2. Do low-compression engines (like motorcycles) produce more
pollution than high-compression engines? Here, as I said earlier,
Barry Commoner made what seems to me a very lucid explanation
why the more recent, more efficient high-compression engines are
actually worse polluters than the older low-compression engines that
they replaced. Here's Commoner: (The Closing Circle, p.168)

"In the internal combustion engine, gasoline is mixed with air in the
cylinders, and the mixture is then ignited at a suitable moment, by
means of an electric spark. Just before it is ignited the fuel-air
mixture is compressed by the cylinder piston. The cylinder pressure
has a great deal to do with the amount of power that the engine can
deliver; generally the greater the pressure the higher the power
output. For reasons which are yet to be fully explained, the
automobile industry long ago became committed to increasing the
engine's power. In 1925, when the first records became availalbe, the
average passenger car engine delivered 55 horsepower; by 1946 the
average was 100 horsepower. Between 1946 and 1958 average horsepower
was driven upward by the engineers to reach 240 in 1958. In that year,
in response to foreign competition, United States manufacturers
introduced the "compact" car, which a smaller engine. As a result,
between 1958 and 1961, average horsepower dropped from 240 to 175.
Then a curious nearly biological phenomenon occurred -- the "compact"
cars gradually grew in size and in engine power, so that between 1961
and 1968, average horsepower climbed again, to 250.

"To achieve increase horsepower, it was necessary to increase engine
compression; the relevant measure, "compression ratio", rose from 5.9
in 1946 to 9.3 in 1961. It then dipped briefly, along with horsepower,
but, recovering from that aberration, climbed upward again, reaching
an average of 9.5 in 1968. Thus, the low-powered, low-compression
engine was displaced between 1946 and 1968. This technological
displacement, like many others in that period, has strongly
intensified the impact of automobile travel on the environment.

Commoner then proceeds to explain why the high-compression high-power
engines caused more pollution:

1. "...high-power engines are less efficient in their use of fuel --
especially when run at low speeds, as they are in the car-choked city
streets -- the amount of gasoline burned per mile traveled increased.
In 1946 passenger cars averaged about 15 miles per gallon; by 1968 the
average was about fourteen miles per gallon. This meant more fuel
combustion -- and therefore more air pollution from gasoline
combustion products -- per vehicle mile traveled."

2. "At high cylinder pressures, the explosive combustion is uneven,
causing a jarring "knock" which decreases engine power. To suppress
engine knock, it became necessary to add tetraethyl lead to the
gasoline."

3. "As compression ratio increased, so did the engine's operating
temperature; this has sharply increased the amount of nitrogen oxides
emitted per unit of engine use."

In the text, he cited actual statistical figures showing how
high-compression, high-power engines produced more pollution.


Obet