04-10-2005, 03:49 PM
Discussion time?
An internal combustion engine is a heat engine. Efficiency of a heat engine is simply the work output divided by the heat input. In this case the heat input is the energy content of the fuel. You may also measure this efficincy by taking the work output divided by the mass of fuel input.
The efficiency you are speaking of is volumentric efficiency. This has to deal with the theoretical maxium amount of air that can be pumped thru an engine. The max volumetic efficnecy occurrs at WOT, but VE is NOT how an engines effiency is measure, it's thermo, as mentioned above.
Peak thermal effiency almost never occurs at WOT.
Diesel are more effiency mainly for 2 reasons. First, they have substancially higher compression ratio. Higher CR = higher efficiency (this is primary reason). Second, diesels control power output by meter FUEL not AIR. Gasoline engines use a throttle plate to control output. To insure proper combustion, an nominal air/fuel ratio of 14.7:1 must be attained (notice nominal0.
In a diesel, there is no throttle (like you mentioned). Natually the engine has greatter volumetric efficiency, but this isn't the primary reason that diesels rock. Instead of having the maintain that stoicmetric ratio, a diesel just injects enough fuel to maintained the desired RPM set by the governer (ultimetly set by the operator). Essentially, under no load conditions a diesel engine is running extremely lean the exhaust consists of mostly air. This is why a diesel will take FOREVER to obtain operating temperature if you just let it sit and idle.
You also mentioned diesel fuel has a higher energy content than gas, which is true, but this has nothing to do with thermal efficiency, only with MPG.
Lastly, there is mechanical efficiency, but that does not factor into this discussion.
An internal combustion engine is a heat engine. Efficiency of a heat engine is simply the work output divided by the heat input. In this case the heat input is the energy content of the fuel. You may also measure this efficincy by taking the work output divided by the mass of fuel input.
The efficiency you are speaking of is volumentric efficiency. This has to deal with the theoretical maxium amount of air that can be pumped thru an engine. The max volumetic efficnecy occurrs at WOT, but VE is NOT how an engines effiency is measure, it's thermo, as mentioned above.
Peak thermal effiency almost never occurs at WOT.
Diesel are more effiency mainly for 2 reasons. First, they have substancially higher compression ratio. Higher CR = higher efficiency (this is primary reason). Second, diesels control power output by meter FUEL not AIR. Gasoline engines use a throttle plate to control output. To insure proper combustion, an nominal air/fuel ratio of 14.7:1 must be attained (notice nominal0.
In a diesel, there is no throttle (like you mentioned). Natually the engine has greatter volumetric efficiency, but this isn't the primary reason that diesels rock. Instead of having the maintain that stoicmetric ratio, a diesel just injects enough fuel to maintained the desired RPM set by the governer (ultimetly set by the operator). Essentially, under no load conditions a diesel engine is running extremely lean the exhaust consists of mostly air. This is why a diesel will take FOREVER to obtain operating temperature if you just let it sit and idle.
You also mentioned diesel fuel has a higher energy content than gas, which is true, but this has nothing to do with thermal efficiency, only with MPG.
Lastly, there is mechanical efficiency, but that does not factor into this discussion.
I feel more like i do now than I did before.