Brake thermal efficiency = 0.23 or 23 percent Reciprocating engines are only about 34 percent thermally  efficient; that is, they transform only about 34 percent of  the total heat potential of the burning fuel into mechanical  energy. The remainder of the heat is lost through the exhaust  gases, the cooling system, and the friction within the engine.  Thermal distribution in a reciprocating engine is illustrated  in  Figure 1-42 . Mechanical efficiency is the ratio that shows how much of  the power developed by the expanding gases in the cylinder  is actually delivered to the output shaft. It is a comparison  between the bhp and the ihp. It can be expressed by the  formula: Mechanical efficiency  = bhp  ihp Mechanical Efficiency Brake horsepower is the useful power delivered to the  propeller shaft. Indicated horsepower is the total hp  developed in the cylinders. The difference between the two  is friction horsepower (fhp), the power lost in overcoming  friction. The factor that has the greatest effect on mechanical  efficiency is the friction within the engine itself. The friction  between moving parts in an engine remains practically  constant throughout an engine’s speed range. Therefore, the  mechanical efficiency of an engine is highest when the engine  is running at the rpm at which maximum bhp is developed.  Mechanical efficiency of the average aircraft reciprocating  engine approaches 90 percent. Volumetric efficiency is a ratio expressed in terms of  percentages. It is a comparison of the volume of fuel/air  charge (corrected for temperature and pressure) inducted  into the cylinders to the total piston displacement of the  engine. Various factors cause departure from a 100 percent  Volumetric Efficiency volumetric efficiency. The pistons of a naturally aspirated  engine displace the same volume each time they travel from  top center to bottom center of the cylinders. The amount of  charge that fills this volume on the intake stroke depends  on the existing pressure and temperature of the surrounding  atmosphere. Therefore, to find the volumetric efficiency of an  engine, standards for atmospheric pressure and temperature  had to be established. The U.S. standard atmosphere was  established in 1958 and provides the necessary pressure and  temperature values to calculate volumetric efficiency. The standard sea level temperature is 59 °F, or 15 °C. At  this temperature, the pressure of one atmosphere is 14.69 lb/ in$^{2}$, and this pressure supports a column of mercury (Hg)  29.92 inches high, or 29.92 "Hg. These standard sea level  conditions determine a standard density, and if the engine  draws in a volume of charge of this density exactly equal  to its piston displacement, it is said to be operating at 100  percent volumetric efficiency. An engine drawing in less  volume than this has a volumetric efficiency lower than 100  percent. An engine equipped with true supercharging (boost  above 30.00 "Hg) may have a volumetric efficiency greater  than 100 percent. The equation for volumetric efficiency is: Volumetric Volume of charge (corrected for Efficiency = temperature and pressure)  Piston displacement Many factors decrease volumetric efficiency, including: •  Part-throttle operation  •  Long intake pipes of small diameter •  Sharp bends in the induction system •  Carburetor air temperature too high •  Cylinder-head temperature too high •  Incomplete scavenging Heat released by combustion 25–30% is converted into useful power 40–45% is carried out with exhaust 5–10% is removed by the oil 15–20% is removed by fins Figure 1-42. Thermal distribution in an engine. 1-36