Thanks, Rob! (Actually, I do consider myself an equipment designer - I just have to pay to do it instead of getting paid. But that's another story - and a sore point with me, even if it is fun - because I'd much prefer that the folks I buy stuff from do it before I get it... /forums/images/graemlins/mad.gif
Anyway, first off, I don't know nearly as much about 2-strokes as I do 4-strokes, although the trucking company my dad owned did have a couple of 2-stroke Detroits for a while.
Basically, the theory with two strokes is that they should be able to produce twice the power, because they're producing power with every other stroke instead of every fourth. But, if you think about it, that would only account for a power-to-weight-ratio advantage, because fuel is still going to be injected every other stroke instead of every four, too. So much for fuel efficiency advantages, right off the bat, and things go down hill quickly from there for the two-stroke. Still, hp-to-weight advantages are nothing to be sneezed at, especially with one of the diesel's biggest disadvantages being lower hp-to-weight ratio than gas engines, so further exploration of the issue is warranted, right? So here's the rest of my take on it:
In practice, the two-stroke can only produce about 1.5 times the power of an identically-sized four-stroke, for reasons I'll get to (up to 1.7, some used to claim, though maybe that's improved in the last twenty years, but I don't think so). The big reason, again, is volumetric efficiency. Part of a two-stroke's stroke is wasted because the air intake ports near the bottom of the cylinders effectively end the stroke right there, from a power production standpoint. So now we have a situation where we're chucking the same amount of fuel into an engine that's only producing 1.5 times the power - definitely a bad idea.
A few other issues: Twice the heat produced in the same size package puts stress on everything, and means it has to be designed heavier if it's going to last - there goes some of the only advantage we started out with, hp-to-weight ratio. (Also add bigger radiators and oil-coolers - same issue, same result.) The scavenging blower that starts forcing fresh air into the cylinders just before they reach bottom must be mechanically powered (a supercharger instead of a turbocharger) if it's going to be truly effective (and efficient at getting the burned gasses out of the cylinders), so there goes some more of your power. There's always going to be more unburned gasses left in the cylinder than with a four-stroke, so there goes a little more of your efficiency. The design of the cylinders inherently makes them harder to cool, increasing the heat stress, increasing the need for cooling, increasing the design weight, etc. etc. The lower combustion efficiency increases the amount of soot produced, which works on the oil, which increases wear. Add to all of this the fact that two-strokes become less and less efficient as the rpm increases. They're really only reasonably efficient at relatively slow rpms - they just sound like they're going fast. Now you have an engine that can't efficiently run as fast as a comparable four-stroke, so you have to design it bigger to compensate, and there goes some more of the only advantage we started out with. We could go on and on, but basically we have a classic case of reality making mince-meat of theory. Which is probably why you don't see many two-stroke diesels...
