to each other, the cycle efficiency is actually com- puted by measuring the temperature. The specific heat of the mixture in the cylinder is either known or  assumed,  and  when  combined  with  the temperature, the heat can be calculated at any instant. Thermal  Efficiency Thermal efficiency is the measure of the effi- ciency and completeness of combustion of the fuel, or, more specifically, the ratio of the out- put 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  con- sidered 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 IN- DICATED 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 deter- mined 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  effi- ciency  can  be  computed  by  the  following expression: ite  = hp × 2545 Btu per hr per hp Rate of heat input of fuel in Btu per hr × 100 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: Overall thermal efficiency = bhp Heat input of fuel ×  100 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: Overall thermal efficiency = 2,290,500 6,912,000 =  0.331,  or  33.1% Chapter  5—ENGINE  PERFORMANCE  AND  EFFICIENCY 5-7