[sustran] low compression vs high compression engines

J.H. Crawford joel at xs4all.nl
Tue Apr 14 19:39:47 JST 1998

I'll have a stab at this.

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

Not all motorcycle engines are 2-stroke, although most of the 
smaller ones are. Four-stroke engines are always valved and
always provide excellent "scavenging" of the previous burned
charge. As already said, small 2-stroke engines are almost
never valved, and scavenging, while better than it was a
few decades ago, is still poor. Non-valved engines require
crankcase compression in order to feed the air-fuel mixture
through the intake reeds, and this precludes the splash/
force-feed crankcase lubrication universally used in 4-stroke
and large, valved 2-stroke engines. This means that the
crankcase lubricant has to be supplied by mixing it with
the fuel, which is what gives 2-stroke engines their
characteristic putrid exhaust stink. While the ratio of
fuel-to-oil was greatly reduced in the 50s and 60s, it
seems now to hang at about 50:1 or 100:1.

I'm not sure about compression ratios, although I have the
idea that most 2-stroke engines are fairly low compression.
Four-stroke engines are made with a fairly wide of compression
ratios. Diesel engines are always high compression engines,
with a minimum ratio of 16:1, running up to about 24:1. This
is required in order to ignite the fuel, there being no spark.
This is, I believe, the cause of the problems with NOx,
which is formed during the buring of fuel under high pressure.
The output of hydrocarbons and carbon monoxide from Diesel
engines is extremely low in comparison with the raw exhaust
of gasoline engines (without a catalytic converter, which,
once warmed up to its ignition temperature, burns the excess
hydrocarbons and carbon monoxide to water and carbon dioxide).

>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.

One thing to watch out for here is that horsepower was figured
differently up until some time in the 1970s (I believe). The
consumer movement forced auto manufacturers to provide more
accurate (i.e., lower) horsepower ratings. This was a step-wise
change, and I'm not sure exactly when it occurred. I have the
idea that horsepower ratings declined by about 40% from one
year to the next.

>"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.

It is not intrinsically necesary to raise compression ratios
to increase power output, but it is the easiest means.
It is worth noting that many European cars from the 50s and 60s
(when fuel was extremely expensive in real terms) were mostly
high-compression engines because these provided the highest
output from a small, lightweight engine, which in turn
helped to keep the cars small and light.

>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.

The real issue here, I believe, is that engines achieve
their best fuel efficiency at maximum rated torque.
(The most useful measure is pounds of fuel per horsepower-hour.)
With large Diesel engines, maximum rated torque usually
occurs at maximum rated speed, which is to say at maximum
rated horsepower. That is why Diesel railroad locomotives
engines are usually only run at two speeds: full output and idle.

Smaller, lighter, less efficient automotive engines usually
achieve maximum torque at about half their maximum rated
speed. They also achieve their best fuel economy at this
speed, but usually at much less than wide-open throttle.
The reason for this is that these engines inject extra
fuel under full-load conditions in order to get the most
possible power out of a small block. The result is that
these engines deliver terrible fuel efficiency under
full-load conditions.

If you were to design a car that would not go faster
than, say, 60 MPH, and would take, say, 20 seconds
to achieve this speed, you could give it a much lower
power engine than is customary today, and you could
optimize it to deliver excellent fuel efficiency at
highway speeds. Its rotational speed would probably
be considerably lower than is normal today. The older
engines (from the 1950s) were slow turning, low
compression engines. If you remember the 1955 
straight-6 Chevrolet from that period, it had
relatively low horsepower but a very flat torque
curve, so it worked fine with just a 3-speed
transmission. Today's high-output engines are
very "peaky" and need 5-speed transmissions to
deliver high acceleration.

>In 1946 passenger cars averaged about 15 miles per gallon; 

I think the actual peak would have been closer to around
20 MPG in the mid-50s.

>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."

There is no direct correlation between amount of fuel
burned and pollution created. A modern high-compression
engine delivers a very clean exhaust when the catalytic
converter is running because partitally combusted exhaust
gasses are fully burned in the converter. I'm not sure
about the NOx problem, however. Nor about some of the
newer concerns with polycyclic aromatic hydrocarbons, etc.

>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

There are other ways of raising the octane rating than
adding tetraethy lead, and these are used in premium
unleaded gasoline (probably also in all unleaded gasoline,
but I'm not sure).

>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."

Yes, but as already stated, all other things being equal,
the higher the combustion temperature, the higher the
thermodynamic efficiency of the engine.

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

Might be interesting if you could post this.

Sorry for the long-winded post.


J.H. Crawford    Crawford Systems    joel at xs4all.nl    http://www.mokum.com/

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