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A water-cooled engine produces 1589 W of power. Water enters the engine block at 14.61 °C and exits at 29.91 °C. The rate of water flow is 147.3 L/h. What is the engine’s efficiency?

  • physics -

    You know that the mechanical power out is
    W = 1589 J/s

    The mass flow rate of coolant is
    dm/dt = 147.3 l/h = (147.3L/h*10^3 g/L)/3600s/h
    = 40.9 g/s

    The heat energy (rate) out to the cooling water is
    Qout = C*(dm/dt)*deltaT
    = 4.18 J/(°C*g)*40.9 g/s *15.3 °C
    = 2617 J/s

    The heat energy rate in = Qout + Wout
    = 2617 + 1589 = 4206 J/s

    Efficiency = Wout/(Qin) = 1589/4206 = 37.8%

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