MMcfall said:
New guy to the forum but just wanted to add something to consider - From an engineering standpoint a foot pound of torque is a foot pound of torque no matter if it is from a gas, diesel, or turbine engine. I think the big difference is the rapidity that torque is applied. It would be interesting to see dyno charts of gas, and diesel drag engines and see how fast each one puts on the torque.
And don't forget the periodicity of the torque. That is, torque is generated in pulses. The *measured* torque of an engine is an average of the torque applied to the crank.
Using Greg's engine numbers, in an engine making 1000HP at 4200 RPM, each piston produces around 8. 75 power strokes per second. In an engine making 1000HP at 8000 RPM, each piston produces around 16. 67 power strokes per second. The slower engine produces a lot more torque per power stroke at a greater interval, which means much more shock stress on the drivetrain components.
Let's look at it slightly differently. The slower engine produces 1250 lb-ft of torque, while the faster engine produces 656 lb-ft of torque. Assuming the engine is running steady state, this means that each engine generates that torque during every 2 revolutions. Also assume that, at any given moment, only one piston is producing the majority of the torque. On average, each piston in the slower engine produces 1250 lb-ft of torque on the crank, while in the faster engine, each piston produces, on average, 656. Here, I'm going to pick a factor: to maintain an *average* torque, the peak cylinder torque must be much higher; I suspect each piston's peak torque may be around 80% higher than average. That means the slow engine is producing 2250 lb-ft of peak torque per cylinder, while the fast engine is producing 1180 lb-ft of peak torque per cylinder.
Math says, if I computed correctly, that the slow engine is producing 280 'pulses' per second of 2250 lb-ft of peak cylinder torque, while the fast engine is producing 533 'pulses' per second of 1180 lb-ft of peak cylinder torque.
Seems to me the slow engine produces a lot more shock stress and allows more 'unload' time between stresses than does the fast engine.
Now consider the difference between SI and CI engines. Typically, the CI engine produces power during a longer stroke; hence its superior torque capacity. But this reduces the peak torque; let's say it's near the average. The diesel engine is producing a smoother, more constant, torque to the drivetrain, while the spark engine produces a pulsy torque. An analogy might be that the diesel is like using a lever to press-fit a bearing onto a shaft, while the spark engine is like using an air hammer to install the bearing.
The point of this mental exercise is to show that diesels seem to produce a different 'quality' of torque than do spark engines. Perhaps diesel drivetrain components don't need to be as hard as their spark engine counterparts; they don't suffer as much shock stress. And perhaps they do need to be as hard, to stand up to sudden loading shocks found in motorsports. But, diesel components do need to be tougher so they can handle the more constant work loaded on them.
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