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Dave, sorry I've not replied sooner, and I think my rather sweeping statement does deserve some explanation.
The first thing is to understand there are many different factors that affect how efficient a diesel engine is and therefore how clean it will run. The biggest single factor however is how well the fuel and air mix when injected. Poor mixing results in poor combustion and that means a dirty exhaust and frequently poor fuel consumption. The real problem is how to get the fuel and air mixed. Back in the early days of diesels this was done in many different ways. Alongside the direct injection system which dominates today where the combustion chamber is formed as a bowl or toroid in the piston crown, there were numerous other direct and indirect injection systems ranging from the swirl chamber (the longest lasting indirect system found on most smaller engines until relatively recently), pre-combustion chamber, cylinder head mounted air cells (such as the Lanova system used on some Fairbanks Morse engines), piston mounted air cells (extensively used by Cummins) and a host of others. These all aimed to achieve one result. To intimately and evenly mix the injected fuel with the air in the cylinder in a few thousandths of a second. The indirect injection systems were extremely successful at this, and as a rule of thumb in the 1930s, 40s & 50s the cleanest and most powerful engines used some form of indirect injection system. More recently (from the 1940s onwards), the direct injection system was progressively developed to become almost as clean as the indirect systems, but it still has not quite reached the same levels. So, if the indirect is so much cleaner and has better combustion, why has it been replaced by the direct injection system? In a nutshell, indirect injection systems tend to be less efficient as their combustion chambers tend to have a very large surface area and they rely in forcing air through small passages to create high degrees of turbulence, which means they lose a lot of heat from compression and combustion to the cooling system. As a result they can be difficult to start and have higher fuel consumption. So, direct injection has taken over. Today, the best direct injection engines have highly developed combustion chambers that create a lot of turbulence towards the end of the compression stroke along with fuel injectors operating at very high pressures ensuring good atomisation and penetration (i.e. they can fire smaller droplets right across the combustion chamber whereas older low pressure systems had to fire larger droplets to ensure they would get across). The result is that many modern direct injection engines run cleanly and efficiently.
How does this relate to 2 stroke engines? First, it has to be understood that the GM 2 stroke was designed in the 1930s and continued with relatively few changes until it ceased production some 50 years plus later when they found it impossible to make it meet the latest emmissions standards. GM are not the only ones to have done this. Gardner in the UK built probably the best 4 stroke diesel truck & bus engines in the UK based on a 1930s design. They were so well regarded that truck buyers would pay quite a lot extra to have a Gardner engine in their trucks or busses in place of cheaper alternatives from Cummins, Perkins or the truck maker themselves. However, in the early 90's Euro emmissions regulations on diesels started to tighten up, and Gardner found they could not get their engine through the new tests and would need to start again from scratch. But, instead of designing a new engine, they decided it was better to simply stop making new engines and concentrate on supporting all the existing engines instead. Bearing in mind both of these engines were very old designs towards the end of their lives it is little wonder that they could not be brought into line with the new regulations. It is also worth bearing in mind this happened before diesel engines gained the benefit of electronically controlled unit injectors and common rail injection allowing more precise and more flexible fuel metering and timing.
Now for a little guesswork. I suspect that one of the problems with making the Detroit run clean is that it smokes under load at low revs. This would be due to poor scavenging and air charging at low speeds meaning that with the original mechanical injection system too much fuel was being injected for the air in the cylinder. As the revs rose, the scavenging and air charging would improve and the engine would get cleaner until it reached the point where it was going too fast for the air to get through the cylinder and properly scavenge it. If this is the case, a lot could be cured by using an electronically controlled injection system that could better match the fuel quantity to the oxygen in the cylinder. In addition to that, with a uniflow engine like the Detroit, modern variable valve timing technology (a very simple one that merely advances and retards the cam timing based on speed is all that would be required) would be able to match the exhaust valve operation to the inlet ports to give optimum scavenging and cylinder charging at all speeds. The final result would be an engine that would be capable of running a lot cleaner than a traditional 2 stroke. I believe that combining modern combustion chamber designs (based on those from 4 stroke engines), modern electronically controlled high pressure injection and variable valve timing would result in an engine that could be made to run as cleanly as a 4 stroke. If the blower is made a little bigger so that it delivers too much air and some of that is allowed to pass right through the cylinder and into the exhaust that excess air could be used to clean things up further by providing additional oxygen to the catalysts to complete combustion. The overall result would then be a 2 stroke engine that, in theory at least, could run cleaner than a 4 stroke. Of course, much of this is theory and there's a large chunk of guesswork in there, but I believe the prinicples are sound enough and cannot see why a clean 2 stroke could not be built.
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