indicated horsepower gives no indication of how much actual  power is delivered to the propeller shaft for doing useful  work. However, it is related to actual pressures that occur in  the cylinder and can be used as a measure of these pressures. To compute the friction loss and net power output, the  indicated horsepower of a cylinder may be thought of as  two separate powers, each producing a different effect. The  first power overcomes internal friction, and the horsepower  thus consumed is known as friction horsepower. The  second power, known as brake horsepower, produces useful  work at the propeller. That portion of IMEP that produces  brake horsepower is called brake mean effective pressure  (BMEP). The remaining pressure used to overcome internal  friction is called friction mean effective pressure (FMEP).  [Figure 1-41]  IMEP is a useful expression of total cylinder  power output, but is not a real physical quantity; likewise,  FMEP and BMEP are theoretical but useful expressions of  friction losses and net power output. Although BMEP and FMEP have no real existence in the  cylinder, they provide a convenient means of representing  pressure limits or rating engine performance throughout its  entire operating range. There is an operating relationship  between IMEP, BMEP, and FMEP. One of the basic limitations placed on engine operation is  the pressure developed in the cylinder during combustion.  In the discussion of compression ratios and indicated mean  effective pressure, it was found that, within limits, increased  pressure resulted in increased power. It was also noted that if  the cylinder pressure were not controlled within close limits,  it would impose dangerous internal loads that might result in  engine failure. Therefore, it is important to have a means of  determining these cylinder pressures as a protective measure  and for efficient application of power. If the bhp is known, the BMEP can be computed by means  of the following equation: BMEP =  bhp × 33,000  LANK Given: bhp = 1,000  Stroke = 6 in Bore = 5.5 in rpm = 3,000 Number of cycles = 12 Find BMEP: Find length of stroke (in ft): L = 6 in = .5 ft Find area of cylinder bore: A =  1 ⁄ 4 πrD 2 A = ¼ × 3.1416 × 5.5 in × 5.5 in A = 23.76 in 2 Figure 1-41. Power and pressure. FMEP BMEP TC BC IMEP Power stroke Actual pressure Friction Impeller Accessories BHP IMEP FHP 1-34