Thermal efficiency is the measure of the efficiency and completeness of combustion of the fuel, or, more specifically, the ratio of the output or work done by the working substance in the cylinder in a given time to the input or heat energy of the fuel supplied during the same time. Two kinds of thermal efficiency are generally considered for an engine: indicated thermal efficiency and overall thermal efficiency. Since the work done by the gases in the cylinder is called indicated work, the thermal efficiency determined by its use is often called INDICATED THERMAL EFFICIENCY (ite). If all the potential heat in the fuel could be delivered as 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, the total heat content of the injected fuel can be determined from the heating value, or Btu per pound, of the fuel; and the thermal efficiencies for the engine can then be calculated. From the mechanical equivalent of heat (778 foot-pounds equal 1 Btu and 2545 Btu equal 1 hp-hr), the number of foot-pounds of work contained in the fuel can be computed. If the amount of fuel injected is measured over a period of time, the rate at which the heat is put into the engine can be converted into potential power. Then, if the ihp developed by the engine is calculated, as previously explained, the indicated thermal efficiency can be computed by the following expression:

For example, assume that the same engine used as an example in computing ihp consumes 360 pounds (approximately 50 gallons) of fuel per hour, and that the fuel has a value of 19,200 Btu per pound. What is the ite of the engine? The work done per hour when 1343 ihp are developed is 1343 2545 or 3,417,935 Btu. The heat input for the same time is 360 19,200 or 6,912,000 Btu. Then, by the above expression, the indicated 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) is used. Therefore, overall efficiency (often called brake thermal efficiency) is computed by the following expression:

Converting these factors into the same units (Btu), the expression is written as power output in Btu divided by fuel input in Btu.

For example, if the engine used in the preceding problem delivers 900 bhp (determined by the manufacturer) what is the overall thermal efficiency of the engine?

1 hp-hr = 2545 Btu

900 bhp 2545 Btu per hp-hr =

2,290,500 Btu output per hr

Substituting factors already known, overall thermal efficiency is computed as follows:

Compression ratio influences the thermal efficiency of an engine. Theoretically, the thermal efficiency increases as the compression ratio is increased. The minimum value of a diesel engine compression ratio is determined by the compression required for starting; and this compression is, to a large extent, dependent on the type of fuel used. The maximum value of the compression ratio is not limited by the fuel used, but is limited by the strength of the engine parts and the allowable engine weight per bhp output.