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Posted by D. Sherman on Thursday, April 03, 2008 at 14:10:16 :
In Reply to: Re: Anybody done the math lately? posted by Todd Wilson on Thursday, April 03, 2008 at 01:03:07 :
I don't see any reason why emissions should automatically be different with a gas-fired diesel versus a "diesel"-fired diesel. Soot tends to mostly come from larger fuel molecules that don't burn as completely, so a lighter fuel will tend to be less sooty. NOx comes from high combustion temperatures, which doesn't depend on the fuel type. CO and HC come from incomplete combustion. PAH and aldehydes are supposedly the worst pollutants from a conventional diesel engine, but I don't know just what fuel properties contribute to them.
I would look at the military multifuel as a starting point or a "proof of concept" engine, not as an ultimate design. It's a 50 year old design. The main design requirement was that it run on any liquid fuel. Pollution was not an issue in 1960. The originals smoked a lot, so they added the turbo to meet California smog rules, making what was called the "clean air engine". It's still too smoky for today's smog regs, but so is every diesel engine from that era. I suspect a modern computer system and maybe some other design tweaks could improve that engine's emissions quite a bit.
The basic principle is interesting. Instead of injecting all the fuel as a finely-atomized spray, it atomizes part of it, but injects most of it as a solid jet into a spherical pocket in the top of the piston. The hot piston then vaporizes the volatile hydrocarbons and "cracks" the heavier ones, producing a flammable vapor that burns relatively slowly during the combustion cycle. This way, you don't have a blow-torch effect of a huge flame coming out of the injector and melting a hole in the piston like would happen if you tried to burn gasoline in a conventional diesel engine. The amount of fuel that's atomized is just enough to get the fire started, but not enough to do any damage. Obviously, "slow" is relative, when we're talking about chemistry that happens during 10 or 20 degrees of crankshaft rotation, but it's enough to even out the combustion of a wide variety of fuels. The nice thing about this design is that neither the injectors nor the pistons are any more expensive or have any extra parts than ordinary ones.
The density compensator helps by changing the pump timing depending on the viscosity of the fuel. This is where a more modern electronic control system could probably help quite a bit. Maybe a computer could also make use of a knock sensor and an exhaust O2 sensor to further tweak the timing and fuel quantity for better economy and emissions.
I'd just like to see somebody with the resources to do it right take a crack at this approach. Obviously making specialized injectors and pistons is beyond what even a well-equipped garage can do, but it would be easy for the research department at a major engine builder.
They need to stop being locked into the idea that a diesel engine requires "diesel" fuel. A spark-ignition engine is inherently much more picky about fuel. You need a fuel that you can mix with air and heat to a high temperature (via compression) without it igniting, and then ignite it reliably with a spark. A diesel engine simply needs a fuel that will burn when atomized into hot air. That's a much easier requirement to meet. The fact that gasoline will destroy a normal diesel engine is really a defect in the specific design of the engine, not any inherent fault with the diesel principles.