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Posted by D. Sherman on Wednesday, April 02, 2008 at 23:52:38 :
In Reply to: Anybody done the math lately? posted by Bob in N. GA on Wednesday, April 02, 2008 at 21:02:53 :
Ultimately, if the free market does what free markets are supposed to do, the prices of diesel and gasoline will adjust themselves until both are the same in terms of average cents per brake-hp-hour in the engines that typically run them.
Diesels, of course, are inherently more efficient because they run close to twice the compression ratio of a gas engine. However, the main reason diesel originally became popular was because they would burn a lower-grade fuel. The cheaper fuel combined with the higher efficiency made for considerably lower fuel costs. Now that the fuel is more expensive, though, the only advantage diesel has is thermodynamic efficiency. Given that a reliable long-lasting diesel engine has higher initial cost and considerably more weight than a comparable-hp gas engine, diesels start out with two hurdles to overcome (up-front cost, and heavier vehicle weight). Diesel is still ahead, but only marginally, and I don't expect that to last much longer.
What nobody is thinking about here, however, is that there's no inherent reason why gasoline or alcohol has to be burned in a spark-ignition (Otto cycle) engine at 8 or 10 to 1 compression ratio. A Diesel cycle engine can be made to work on those fuels as well, and at that point you instantly have all the thermodynamic efficiency advantages of a diesel engine combined with the lower fuel cost per BTU of a gas engine. This is really something engine designers need to put some effort into.
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