[sustran] two-stroke versus four-stroke

[Eric Bruun]

About the relative efficiency of two-stroke engines:

High- versus low-compression:

Higher compression does lead to different exhaust products and
tetra-ethyl lead was used to prevent "knocking" at high compression
ratios. However, high compression is thermodynamically more 
efficient, meaning that more useful work is gotten from a unit
of fuel. So the secret is to make high compression engines that
optimize the fuel combustion so as to avoid both low compression
related pollution and high compression related pollution. This is
done with electronic fuel injection and with valve/combustion
chamber design and valve timing.  

Carburetors versus fuel injection:

The problems with the two strokes that are in common use are several.
They are built for low price with obsolete technology and tooling. 
One problem is that they have old-fashioned carburetors instead
of fuel injection, even a better carburetion system would 
improve pollution and power output, fuel injection even more.

Valves versus no valves
 
But the big problem with small two strokes (ship and loco engines 
are different) is that they have no valves. Valves are central
to controlling the intake, combustion, and exhaust processes
carefully over a wide range of RPMs and load conditions.  Valves
also add weight, expense and maintenance costs. Again, cheapness
wins out.

What to do

Since cheapness is driving the process, probably the only way to
get cleaner engines is to put tough regulations in place, including
periodic spot checks of vehicles.  Otherwise people will just
buy the cheapest vehicles -- why buy one that costs more, is
heavier, has less power, and needs to have its valves adjusted 
regularly just because it pollutes less?