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Homework Help: Exhaust pipe diameter question

  1. Apr 1, 2017 #1
    1. The problem statement, all variables and given/known data

    The engine turns at 3000 rpm at this speed. The engine capacity has a displacement of 1.2 litres: this means that 1.2 litres of exhaust gas is expelled from the motor (into the exhaust manifold) every two* revolutions. Further assume that the gas leaves the exhaust pipe with the same volume** that it leaves the engine. What exhaust pipe diameter is required to obtain the given exhaust speed?

    2. Relevant equations
    Remember the class problem about drag-race cars with exhaust thrust? Is there a serious propulsion effect for normal cars with horizontal exhaust pipes? To get quantitative, we need some chemistry. Here is an approximate reaction for burning hydrocarbons in air:

    (CH2)n + 1.5*n O2 + 6n N2 -> n CO2 + n H2O + 6n N2 (n is an integer)

    The (CH2)n approximates the fuel, and everything on the right is the exhaust. The atomic masses of hydrogen, carbon, nitrogen and oxygen are in the ratio 1:12:14:16. (Hint: how many kg of exhaust for one kg of fuel?)

    A car uses 4.0 kg of (CH2)n hydrocarbon fuel per 100 km while it travels at 100 k.p.h. in a straight line at constant speed. The gas comes out of the (horizontal) exhaust pipe at a speed of v = 50 m.s-1, measured with respect to the car.

    What is the magnitude of the force the exhaust gas exerts on the car? (Hint: the exhaust gasses are accelerated in the frame of the car, so ignore the momentum of the incoming gas.)

    3. The attempt at a solution
    I got V = 0.6R (R = revolutions), dV/dR = 0.6 And R = 50t, dR/dt= 50 dV/dt=30 I don't know what to do after this.
    Last edited: Apr 1, 2017
  2. jcsd
  3. Apr 2, 2017 #2


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    Science Advisor
    Homework Helper
    Gold Member

    There seem to be two parts to the question, the first being:
    But to answer it you need to use this piece of information provided in the second part:
    Where the second part is:
    How can you relate the given value of v to your dV/dt and the diameter of the pipe?
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