Converting engine power into a force

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Discussion Overview

The discussion centers on calculating the top speed of a car by translating engine power into the force exerted by the wheels. Participants explore the relationships between power, torque, force, and various factors affecting vehicle dynamics, including mass, drag, and friction. The scope includes theoretical and practical considerations relevant to automotive physics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant seeks a method to convert engine power into forward force, considering the effects of mass, drag, and friction.
  • Another participant states that instantaneous power can be expressed as force multiplied by velocity, suggesting a relationship between these quantities.
  • A different participant questions how to convert engine torque to wheel force, proposing a formula involving torque and wheel radius but expressing uncertainty about its validity.
  • One participant provides a formula for calculating horsepower from force and speed in English units, mentioning the need to account for drivetrain and rolling friction losses, as well as aerodynamic drag.
  • There is a discussion about the relationship between wheel torque and engine torque, including considerations of gear ratios and efficiency factors.

Areas of Agreement / Disagreement

Participants express various approaches to the problem, with no consensus on the best method to convert engine power into force. Multiple competing views and uncertainties remain regarding the calculations and assumptions involved.

Contextual Notes

Participants note the importance of estimating drivetrain losses and aerodynamic drag, but specific assumptions and definitions are not fully resolved. The discussion reflects a range of perspectives on how to approach the calculations.

Who May Find This Useful

Individuals interested in automotive engineering, physics of motion, or those looking to understand the dynamics of vehicle performance may find this discussion relevant.

batmx3man
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I am wanting to accurately calculate the top speed of a car (using it's power, mass, drag, and friction). I started by suming up the forces, but I cannot think of a way to translate the engines power into the force pushing the car forward.
 
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power is energy/time. energy or work, is force*distance. so instantaneous power is force*distance/time, which is force*velocity.
 
Prob. not enough detail. How do I convert the torque of power the engine is sending to the wheel into a force. I thought about t=rF => F=t/r with r being the wheel radius, but that doesn't make sense. So, I need a way to translate the torque/ power the engine is sending to the wheel into a force that the wheels are exerting (sp?) against the ground to move the car forward. Maybe I am going about this problem in the wrong way idk.
 
Power = force times speed.

In English units to calculate horsepower given force and speed:

power/hp = force (lbs) x speed (mph) / 375 (conversion factor).

You can ignore the gearing factor, since it's assumed that you gear the car so peak power occurs at top speed, so the rpm, torques, ... don't need to be known. You'll need to estimate drivetrain and rolling friction losses and reduce the peak power by this amount, usually 15% loss is a good estimate for cars with manual trannys. The next problem is calculating aerodynamic drag, which will be related to cross sectional area times coefficient of drag times speed2.

How do I convert the torque of power the engine is sending to the wheel into a force?
Driven wheel force = wheel torque divided by effective radius of tire. Wheel torque = engine torque times overall gear ratio (times effeciency factor, 85% is a reasonable esitmate).
 
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