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post #8 of Old 01-19-2003
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Gas Vs. Deisel (are we crazy?)

Thank you for your thoughtful answer. I knew about the relative difference in the specific heat output of Diesel fuel vs gasoline and that gasoline produces less heat, but I had also understood that diesel engines do not combust the fuel as completely as gasoline engines and that the relatively large unburned hydrocarbons typically found in a diesel engine''s exhaust, meant that not as much energy was extracted from the fuel bringing the two fuels into parity in terms of energy extracted.

Beyond that, while heat is one measure of a fuel''s output, when you talk about reciprocating engines, the expansion and speed of expansion of the combusted fuel has more significance. It has been my understanding that gasoline actually imparts a greater amount of expansive force much more quickly than diesel during combustion for a given weight allowing it to produce more shaft horsepower.

Now some of this may be quite out of date. Most of my knowledge on this subject was self-taught during a period in the 1970''s when I was racing, and helping design and build racing motorcycles, including their engines. In those days I was scouring primary source research material on engine design. During that era, Volkswagen was developing small automotive diesels and thier research was available in SAE papers. Since they also owned NSU, (who was heavily involved with 2 stroke engine design in those days and racing engines were two stroke) I had gotten and read through almost all of Volkswagen''s available research papers. In actual practice, Volkswagen was not able to get better fuel economy out of thier small diesels compared to equal output gas engines.

Diesels are employed in trucks because they produce enormous amounts of torque in relationship to the horsepower that is being produce. Trucks need huge amounts of torque to move their heavy loads without having even more gears required. It is not a matter of fuel economy.

As to our comparative fuel economy figures, I apologize for starting that line of discussion because my example (or perhaps our combined examples)probably represents too small a ''scientific sample'' to really be meaningful.

In any event, any difference in fuel consumption for someone going over to the Bahamas is probably insignificant in the big picture. Obviously if fuel consumption is a concern then outboards should be elimated because dispite their apparent efficiency due to prop orientation, generally do not achieve the same fuel economy of a similar horsepower output inboard.

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