Re: Diesel vs. Gas

Posted by Joe Evans on November 16, 2000 at 19:56:37 from (209.41.235.118):

In Reply to: Diesel vs. Gas posted by brian on November 16, 2000 at 12:57:53:

Not all diesels exhibit the "noise" you are referring to but indeed most do. We have a Perkins 4cyl diesel Miller welder that does not knock at all. Diesel engine components are built much heavier than gasoline components generally. Years ago when tractor makers had gas and diesel versions of the same tractor using essentially the same engine components, the design of the base engine was so robust that they could get by quite successfully with it. What you are hearing is the fuel "cooking off". If you take a compression reading of a gas engine, you'll find figures of around 125 to 150 PSI. Do the same with a diesel and you'll find 300 to 350 PSI readings because of the high compression ratio of a diesel design. Diesels need to be built heavier to contain this force. The Olds 351 diesels of years ago were converted gas engines, and they turned out to be a unmitigated disaster for GM. Our company had three trucks with these engines--you name it, it broke! For every pound of compression there is roughly a 2 degree Fahrenheit rise in temperature. So 350 psi yields about 700 degrees in a diesel combustion chamber. Since diesel fuel cooks off at about 700 degrees this works out real nicely. The high compression coupled with the inherently higher BTU value of diesel fuel per volume than gasoline yields a hgher BMEP (brake mean effective pressure) within a diesel cylinder. I don't know what the generated PSI in a diesel cylinder is when it fires, but I would guess it is in the thousands of pounds. This all means the diesel engine does more work per unit of fuel than gasoline. My 27.5 HP Kubota sips fuel at a rate of .75 gallon per hour when I'm mowing. No way an equivalent HP gas engine will do this. One other note. Most diesels technically do not use injectors but nozzles. Injectors which are typically found on GM two strokers are essentially "syringes" that inject fuel into the combustion chamber when their plunger is activated by a cam. Nozzles accomplish the same thing but get their fuel and timing from a timed pump.

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