Question 6.2: Fuel Consumption Rate of a Car A car engine with a power out...

The power output and the efficiency of a car engine are given.
The rate of fuel consumption of the car is to be determined.
Assumptions The power output of the car is constant.
Analysis A schematic of the car engine is given in Fig. 6–17. The car engine is powered by converting 24 percent of the chemical energy released during the combustion process to work. The amount of energy input required to produce a power output of 65 hp is determined from the definition of thermal efficiency to be

\dot{Q}_{H}=\frac{W_{\text {net,out }}}{\eta_{\text {th }}}=\frac{65 hp }{0.24}\left(\frac{2545 Btu / h }{1 hp }\right)=689,270 Btu / h

To supply energy at this rate, the engine must burn fuel at a rate of

\dot{m}=\frac{689,270 Btu / h }{19,000 Btu / lbm }=36.3 lbm / h

since 19,000 Btu of thermal energy is released for each lbm of fuel burned. Discussion Note that if the thermal efficiency of the car could be doubled, the rate of fuel consumption would be reduced by half.