Torque: Understanding Gear Ratios & Cruise Control

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Discussion Overview

The discussion revolves around understanding torque, gear ratios, and the functioning of cruise control in vehicle simulations. Participants explore the relationship between engine RPM, horsepower, torque, and how these factors influence vehicle acceleration and speed across different gears.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant calculates torque to the wheel using engine RPM and horsepower but struggles to understand why higher gears, with smaller ratios, allow for higher speeds despite lower torque.
  • Another participant explains that power in the drivetrain remains constant, and as speed changes, torque must also change, emphasizing the relationship between power, torque, and speed.
  • A participant points out that the power output of an engine varies with throttle position, suggesting that torque at the wheel may not necessarily be lower at higher RPMs if the throttle is wide open.
  • There is a discussion about the non-linear relationship between throttle position and power output, with one participant confirming that power can vary significantly at a constant RPM depending on throttle position.
  • Mathematical relationships between power, torque, and RPM are presented, highlighting that less power is often required to maintain certain speeds.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between throttle position and power output, with some suggesting a linear relationship while others indicate it may not be strictly linear. The discussion remains unresolved regarding the exact dynamics of torque and power at varying RPMs and throttle positions.

Contextual Notes

Participants acknowledge that the power output of an engine is not constant at a given RPM and that various factors, such as throttle position, influence the actual power available for acceleration. There are also references to the complexities of gear ratios and their impact on vehicle performance.

Who May Find This Useful

This discussion may be useful for individuals interested in vehicle dynamics, simulation modeling, and the relationships between engine performance parameters in automotive engineering.

Peetch
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I am working on a vehicle simulation and I have calculated the torque to the wheel using engine RPM and the corresponding amount of HP. Then I multiplied that torque by the gear ratios but I do not understand how at a higher gear the ratio is smaller but it is suppose to go faster than the torque provided at a lower gear. I think it has something to do with MOI and the AA but I do not get it please help. Also how does cruse control work if you set your engine to 3000 RPM's and the same amount of torque is being applied to the wheel wouldn't you continue to accelerate?
 
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The power in the drive train at any given moment is the same. The power leaving the engine and going into the gearbox will be the same power that leaves the gearbox (ignore losses in the gearbox), and goes into the drive shaft, but because the speed is changed the torque must change,(power is proportional to torque times speed) same thing happens through the diff.
I think that you are slightly confused about power speed curves for a engine the power quoted at a certain speed is the maximum power, you can run at that speed at a lower power without any problem.
 
Peetch said:
I am working on a vehicle simulation and I have calculated the torque to the wheel using engine RPM and the corresponding amount of HP. Then I multiplied that torque by the gear ratios but I do not understand how at a higher gear the ratio is smaller but it is suppose to go faster than the torque provided at a lower gear. I think it has something to do with MOI and the AA but I do not get it please help. Also how does cruse control work if you set your engine to 3000 RPM's and the same amount of torque is being applied to the wheel wouldn't you continue to accelerate?
You've made one error and applied it twice. The power output of an engine is not always the same at a given RPM. It varies with throttle position. So applying that back into your first question, torque at the wheel need not be lower at higher rpm. It is if your foot is on the floor and the throttle wide-open, though. So what does that tell you? It tells you your acceleration is better at low speed partly because you are in a lower gear.
 
The power output of an engine is not always the same at a given RPM.

Ok so i can be at 3000 RPM's with the throttle fully opened and the engine's power would be ~100 HP but if I am at 3000 RPM's and the throttle only halfway open then the engine's power would be ~50 HP?
 
I'm sure the relationship between throttle position and power isn't that linear, but yes, you can be at 3000 rpm and practically anywhere between 0-100 hp.
 
Power (watts) = torque (Newton-meters) x 2π RPM/60

HP = power (watts)/746

An engine could generate 100 HP at 3,000 RPM, but to drive at say 35 MPH (15 meters per second), usually less than 10 HP (7,500 watts) is required.

Bob S
 

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