to each other, the cycle efficiency is actually com-puted by measuring the temperature. The specificheat of the mixture in the cylinder is either knownor assumed, and when combined with thetemperature, the heat can be calculated at anyinstant.Thermal EfficiencyThermal efficiency is the measure of the effi-ciency and completeness of combustion of thefuel, or, more specifically, the ratio of the out-put or work done by the working substance in thecylinder in a given time to the input or heat energyof the fuel supplied during the same time. Twokinds of thermal efficiency are generally con-sidered for an engine: indicated thermal efficiencyand overall thermal efficiency.Since the work done by the gases in thecylinder is called indicated work, the thermalefficiency determined by its use is often called IN-DICATED THERMAL EFFICIENCY (ite). If allthe potential heat in the fuel could be deliveredas work, the thermal efficiency would be 100%.Because of the various losses, however, this per-cent is not possible in actual installations.If the amount of fuel injected is known, thetotal heat content of the injected fuel can be deter-mined from the heating value, or Btu per pound,of the fuel; and the thermal efficiencies for theengine can then be calculated. From themechanical equivalent of heat (778 foot-poundsequal 1 Btu and 2545 Btu equal 1 hp-hr), thenumber of foot-pounds of work contained in thefuel can be computed. If the amount of fuelinjected is measured over a period of time, therate at which the heat is put into the engine canbe converted into potential power. Then, if theihp developed by the engine is calculated, aspreviously explained, the indicated thermal effi-ciency can be computed by the followingexpression:ite =hp × 2545 Btu per hr per hpRate of heat input of fuel in Btu per hr× 100For example, assume that the same engineused as an example in computing ihp consumes360 pounds (approximately 50 gallons) of fuel perhour, and that the fuel has a value of 19,200 Btuper pound. What is the ite of the engine?The work done per hour when 1343 ihp aredeveloped is 1343 × 2545 or 3,417,935 Btu. Theheat input for the same time is 360 × 19,200 or6,912,000 Btu. Then, by the above expression, theindicated thermal efficiency is as follows:The other type of thermal efficiency—OVERALL THERMAL EFFICIENCY—considered for an engine is a ratio similar to ite,except that the useful or shaft work (bhp) isused. Therefore, overall efficiency (often calledbrake thermal efficiency) is computed by thefollowing expression:Overall thermal efficiency =bhpHeat input of fuel× 100Converting these factors into the same units(Btu), the expression is written as power outputin Btu divided by fuel input in Btu.For example, if the engine used in thepreceding problem delivers 900 bhp (determinedby the manufacturer) what is the overall thermalefficiency of the engine?1 hp-hr = 2545 Btu900 bhp × 2545 Btu per hp-hr =2,290,500 Btu output per hrSubstituting factors already known, overallthermal efficiency is computed as follows:Overall thermal efficiency =2,290,5006,912,000= 0.331, or 33.1%Chapter 5—ENGINE PERFORMANCE AND EFFICIENCY5-7

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