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Prototyping automobile components

  1. Nov 20, 2013 #1
    I'm creating software based simulations for different components of a car. I've some query regarding this. Also please let me know if what I'm doing is correct or not

    1. Engine component

    It would take the acceleration request and fuel injection(set at the beginning) and would compute output power accordingly. (power = some factor * fuel * request)

    Here my query is if I'm considering that engine is not connected to anything. Its output is a shaft turning at some rpm with some torque. How do I know what torque is it generating and what is the rpm

    2. gear train

    It is combination of gears that connects to shafts. According to the gear ratio, the output torque and rpm would change.

    Here my query is what would happen if the two ends at the time of engaging them are at different rpms or torque (not related by the gear ratio)

    3. car body

    It takes the input torque from engine via gear and performs motion

    Here I'm taking the torque from engine via gear(with gear ratio) and converting it to drive force. I'm subtracting the resistive forces(drag, friction...) and then dividing it with mass to get acceleration

    I've read from some forums that drive speed of wheels may be calculated from rpm of shaft from engine via gears.

    I'm thinking that, the speed of car would start from zero to increase with the acceleration obtained from engine torque and would go till the max value of drive speed from engine rpm. Am I correct? If I'm correct what would happen if the speed goes beyond drive speed found from the engine rpm using the formula(already have it)

    Please remember I need to create each component thinking that are not connected to anything and I cannot depend on numbers from other components for generating torque/power/rpm in engine

    Any formulas/references may be helpful

    Thanks in advance
  2. jcsd
  3. Nov 23, 2013 #2


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    An engine has a power determined by the rate of fuel consumption. Power = torque * RPM. Because the car has inertia, the motor will have to travel at the road speed, (so long as the tyres grip the road). That sets the engine RPM from which you can imply the torque, from known fuel consumption. An engine has a maximum torque curve that depends on RPM.

    It will make a crashing noise as the gears start to mesh. If they mesh it may break a gear, slip a tyre or stall the engine.

    An engine can brake as well as accelerate. If you provide insufficient fuel the torque in the engine output will be negative and the engine will absorb power. The RPM to torque curve specifies the maximum, it does not show the negative torque possibilities.
  4. Nov 24, 2013 #3
    Thanks a ton Baluncore. You've helped me a lot.
  5. Nov 28, 2013 #4
  6. Nov 28, 2013 #5


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    Science Advisor

    Your engine model will have parameters and state variables. Engine RPM is a state variable. It effects many things. You should identify all parameters, state variables and variables needed for your model. The state variables will be needed to communicate between your sub-models as they will fully specify the operation of the system.

    When starting, the difference between engine RPM and road speed through the gearbox will be made up by slip in the clutch. If fuel is insufficient to maintain torque required then RPM will fall.

    I think the clutch should be modelled as a sub-system. That will allow you to slip the clutch or stall the motor if there is a speed mismatch while changing gear. You can then assume the gearbox is a perfect synchromesh box if the differential speed is less than a certain percentage. If outside that band you should be unable to change gear.
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