Chapter 9, Problem 9.61P

A mixture of air and a fine spray of gasoline at ambient (outside air) temperature is fed to a set of piston fitted cylinders in an automobile engine. Sparks ignite the combustible mixtures in one cylinder after another, and the consequent rapid increase in temperature in the cylinders causes the combustion products to expand and drive the pistons. The back-and-forth motion of the pistons is converted to rotary motion of a crank shaft, motion that in turn is transmitted through a system of shafts and gears to propel the car.

Consider a car driving on a day when the ambient temperature is 298 K and suppose that the rate of heat loss from the engine to the outside air is given by the formula

   $-{\stackrel{\dot{}}{Q}}_1\left(\frac{kJ}{h}\right)\approx \frac{15\times {10}^6}{T_a\left(K\right)}$

where T_ais the ambient temperature.

1. Take gasoline to be a liquid with a specific gravity of 0.70 and a higher heating value of 49.0 kJ/g, assume complete combustion and that the combustion products leaving the engine are at 298 K, and calculate the minimum feed rate of gasoline (gal/h) required to produce l00 hp of shaft work.
2. If the exhaust gases are well above 298 K (which they are), is the work delivered by the pistons more or less than the value determined in Part (a)? Explain.

If the ambient temperature is much lower than 298 K, the work delivered by the pistons would decrease. Give two reasons.