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| 2005/9/26-28 [Transportation/Car] UID:39882 Activity:nil |
9/26 How come diesel engines run more efficiently than regular
gasoline engines? Is it because diesel fuels contain more
energy/volume? If you burn the same quantity of diesel fuel
as regular gasoline, will both release different amounts of
heat?
\_ Yes diesel is more energy dense.
\_ and it's also denser too.
\_ "On average, 1 gallon (3.8 L) of diesel fuel contains approximately
155x10^6 joules (147,000 BTU), while 1 gallon of gasoline contains
132x10^6 joules (125,000 BTU)."
http://auto.howstuffworks.com/diesel3.htm
\_ They have a much more effient thermodynamic cycle,
higher compression, and they don't draw a vacuum on partial
throttle. --jwm
\_ "A gasoline engine compresses at a ratio of 8:1 to 12:1, while a
diesel engine compresses at a ratio of 14:1 to as high as 25:1.
The higher compression ratio of the diesel engine leads to better
efficiency." http://auto.howstuffworks.com/diesel1.htm
But I don't understand how higher compression ratio leads to
better efficiency. Maybe there is more air to burn per unit of
fuel, so closer to 100% of the fuel is burnt.
\_ Compression ratio matters not so much because you're
compressing more air into the gasoline vapor, but because
after ignition the exhaust gas is at a higher pressure and
pushes on the piston for a longer distance. Imagine after
pulling in the fuel-air mixture it is at pressure P. One
engine does some work to compress to 10P while the other does
more work to compress to 20P.
The gas engine has expended 9P on compression while the diesel
expended 19P.
After ignation the pressure
goes up another 10P, so the gas engine is at 20P while the
diesel is at 30P. Now on expansion the gas engine expands 10x
to do (20-20/10)P=18P work. The diesel expands (30-30/20)P=
28.5P work. Subtracting the original work of compression the
gas engine gained a net of 9P while the diesel gained 9.5P
work. Not a big difference but as you keep increasing the
compression ration the amount of work extracted converges to
the amount the gas provided, 10P. |
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| auto.howstuffworks.com/diesel3.htm Diesel fuel evaporates much more slowly than gasoline -- its boiling point is actually higher than the boiling point of water. You w ill often hear diesel fuel referred to as "diesel oil" because it is so oily. refining to create diesel fuel, which is why it is generally cheaper than gasoline. This, combined with the improved efficiency of diesel engines, explai ns why diesel engines get better mileage than equivalent gasoline engine s For more information on diesel engines and related topics, check out the links on the next page. |
| auto.howstuffworks.com/diesel1.htm DaimlerChrysler Atego six-cylinder diesel engine The main differences between the gasoline engine and the diesel engine ar e: * A gasoline engine intakes a mixture of gas and air, compresses it and ignites the mixture with a spark. A diesel engine takes in just air, compresses it and then injects fuel into the compressed air. The heat of the compressed air lights the fuel spontaneously. The higher compression ratio of the diesel engine leads to better efficiency. fuel injection, in which the fuel is injected just prior to the intake stroke (outside the cylinder). Diesel engines use direct fuel injection -- the diesel fuel is injected directly into the cylinder. The following animation shows the diesel cycle in action. Image courtesy Baris Mengutay Note that the diesel engine has no spark plug, that it intakes air and co mpresses it, and that it then injects the fuel directly into the combust ion chamber (direct injection). It is the heat of the compressed air tha t lights the fuel in a diesel engine. |