mcconoughey
New member
- Joined
- Aug 17, 2001
- Messages
- 13
I think that Aardvark and Slowzuki got it most nearly right. First, designers of both diesel engines and gasoline engines have tremendous numbers of design options to choose from. These variables make any sweeping general conclusion suspect.
A key route to high torque in many modern diesel engines is the modern turbocharger. Especially with variable inlet vanes it can maintain high cylinder charge rates over a broad speed range. Unlike a gasoline engine, the diesel engine does not face the problem of knock. Or, viewed differently, knocking is what a diesel engine is supposed to do, if one views a knock as the much more rapid combustion of the diesel.
The comment on gearing was correct. What the driver feels when he or she accelerates is the result of torque at the rear wheel. The diameter of the tire and the weight of the vehicle are other key variables. High wheel torque can come from a higher speed engine of lower torque that is geared down more, or a higher torque engine of lower speed that is not as geared down. In real life, a higher rpm engine will seldom be precisely geared down to produce exactly the same wheel torque as a lower rpm engine of higher torque. There are a vast number of variables that would have to match up to give identical results from the driver's viewpoint. Some are: vehicle weight, air flow resistance, number of tires (more friction) etc.
If one knows the engine rpm then either torque or horsepower will perfectly describe the power output. Either one can mathematically be converted to the other. I repeat, if one knows the rpm, then knowing the torque permits calculation of the horsepower at that rpm. The converse is also true.
Many industrial engines run at a slow speed because the designer's intention is to achieve a very long engine life. Thus, one can buy industrial generators in 1800 or 1200 rpm models. The light duty generators running at 3600 rpm have shorter lives. Putting in conservative valve sizes and valve timings limits peak horsepower, but does little to limit peak torque. That is because at a lower rpm the smaller valves and time of opening do little to prevent full cylinder charging.
Engine design is fascinating and most colleges have one or several engine design textbooks. They make fascinating reading. The best general introduction that I know of is by John Heywood, a longtime MIT professor and part time water color painter.
A key route to high torque in many modern diesel engines is the modern turbocharger. Especially with variable inlet vanes it can maintain high cylinder charge rates over a broad speed range. Unlike a gasoline engine, the diesel engine does not face the problem of knock. Or, viewed differently, knocking is what a diesel engine is supposed to do, if one views a knock as the much more rapid combustion of the diesel.
The comment on gearing was correct. What the driver feels when he or she accelerates is the result of torque at the rear wheel. The diameter of the tire and the weight of the vehicle are other key variables. High wheel torque can come from a higher speed engine of lower torque that is geared down more, or a higher torque engine of lower speed that is not as geared down. In real life, a higher rpm engine will seldom be precisely geared down to produce exactly the same wheel torque as a lower rpm engine of higher torque. There are a vast number of variables that would have to match up to give identical results from the driver's viewpoint. Some are: vehicle weight, air flow resistance, number of tires (more friction) etc.
If one knows the engine rpm then either torque or horsepower will perfectly describe the power output. Either one can mathematically be converted to the other. I repeat, if one knows the rpm, then knowing the torque permits calculation of the horsepower at that rpm. The converse is also true.
Many industrial engines run at a slow speed because the designer's intention is to achieve a very long engine life. Thus, one can buy industrial generators in 1800 or 1200 rpm models. The light duty generators running at 3600 rpm have shorter lives. Putting in conservative valve sizes and valve timings limits peak horsepower, but does little to limit peak torque. That is because at a lower rpm the smaller valves and time of opening do little to prevent full cylinder charging.
Engine design is fascinating and most colleges have one or several engine design textbooks. They make fascinating reading. The best general introduction that I know of is by John Heywood, a longtime MIT professor and part time water color painter.