- #1
codyo83
- 2
- 0
Does someone know how much torque a 20hp gasoline engine produce at 3500rpm? An equation that I can use to figure this out would be helpful also.
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Discussion Overview
The discussion centers around the torque output of a 20hp gasoline engine at 3500rpm, exploring the calculations involved, the variability of torque in internal combustion engines, and the implications of peak performance on torque values.
Discussion Character
- Technical explanation
- Mathematical reasoning
- Debate/contested
Main Points Raised
- One participant seeks to determine the torque produced by a 20hp engine at 3500rpm and requests a formula for calculation.
- Another participant provides a formula for calculating torque, using 3800rpm instead of 3500rpm, yielding a torque value of approximately 27.64 lb-ft.
- A different participant presents the formula HP = T * RPM / 5252, calculates torque at 30 lb-ft for the engine at 3500rpm, and explains the derivation of the factor 5252.
- One participant cautions that the calculated torque assumes the engine is producing 20hp at 3500rpm, noting that actual torque may be lower if peak performance occurs at a different RPM.
- Another participant discusses the variability of torque in internal combustion engines, mentioning that instantaneous torque can fluctuate significantly, particularly in single-cylinder engines.
- A later reply provides an example of calculated torque output from a single-cylinder engine, indicating negative torque during certain strokes and contrasting average torque with peak torque values.
- One participant acknowledges the pressure-crank angle curve shared by another, noting the difference between instantaneous maximum torque and average torque, as well as the occurrence of torque reversal.
Areas of Agreement / Disagreement
Participants express differing views on the reliability of torque calculations based on horsepower and RPM, with some emphasizing the variability of torque in engines and the conditions under which the horsepower rating applies. The discussion remains unresolved regarding the exact torque output at 3500rpm.
Contextual Notes
Participants highlight limitations in the calculations, including assumptions about peak performance RPM and the nature of torque fluctuations in internal combustion engines. The discussion also reflects on the complexity of torque behavior in different engine configurations.
- #2
- #3
SteamKing
Staff Emeritus
Science Advisor
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codyo83 said:
HP = T * RPM / 5252, where T is torque in lb-ft
For your motor, HP = 20, RPM = 3500, therefore doing a little algebra, T = 20 * 5252 / 3500 = 30 lb-ft of torque produced by the motor at this output condition.
Since 1 HP = 33,000 lb-ft/min, and 1 revolution of the output shaft = 2 pi radians, then the factor 5252 = 33,000 / (2 pi).
The formula works for any motor where the output is in HP and the speed is in RPM.
- #4
cjl
Science Advisor
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Keep in mind though that this only works if the motor is producing 20hp at 3500rpm. What is colloquially called a "20hp" motor is really a motor which, when operated at its peak performance point, outputs 20hp. If peak performance occurs at some other point than 3500rpm, the torque at 3500rpm will be lower than your calculated number.
- #5
Dr.D
- 2,411
- 723
Bear also in mind the fact that the torque produced by any IC engine fluctuates quite substantially. On a single cylinder engine, the instantaneous torque may actually reverse. So, while the power calculation (based on average torque and average speed) can give you an average torque value, instantaneous values will be all over the place.
- #6
Kozy
- 140
- 7
Dr.D said:
Indeed it does. Here's a calculated torque output from a single cylinder engine showing negative torque on compression and exhaust and exhaust strokes.
The peak torque is showing around 430lbft, however this is actually representative of a single cylinder in a four cylinder engine producing an average torque value of a mere 138lbft.
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- #7
Dr.D
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Kozy, thanks for posting the pressure - crank angle curve. That looks just about right.
It also show how much higher the instantaneous maximum torque is compared to the average torque, as well as showing the torque reversal.
It also show how much higher the instantaneous maximum torque is compared to the average torque, as well as showing the torque reversal.
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