Car engine power 44 kW. What is the efficiency of this engine, if at a speed of 100 km / h

Car engine power 44 kW. What is the efficiency of this engine, if at a speed of 100 km / h it consumes 14 liters of gasoline per 100 km of track?

Efficiency of the engine η = A useful: A expended.
1) To find the work spent – A spent, we will use the fact that a car engine consumes gasoline with a volume of V = 14 liters per 100 km.
The mass of consumed gasoline can be found through the formula: m = ρV, where the density is ρ = 710 kg / cubic meter, and the volume is V = 14 liters = 14 cubic dm = 0.014 cubic meters. We get the mass m = 9.94 kg. To calculate the amount of heat released during fuel combustion, we use the formula: Q = q • m, where q = 46 MJ / kg is the specific heat of combustion of gasoline. Q = (46 MJ / kg) • 9.94 kg. Hence, Q = 457.24 MJ, then A expended = Q = 457.24 MJ
2) To find useful work performed on the path S = 100 km = 100000 m – A useful, we use the fact that the power of the car engine N = 44 kW = 44000 W and the car moves at a speed v = 100 km / h = (100: 3.6) m / s. And useful = N • t = (N • S): v, since t = S: v. We combine the formulas: η = ((N • S): v): Q. Substitute the values ​​of physical quantities in the combined formula and make calculations: η = (44000 W • 100000 m: (100: 3.6) m / s): 457240000 J = 0.3464 ≈ 35%.
Answer: The efficiency of the motor is η ≈ 35%.