power stroke. The fuel/air charge in the cylinder can be  compared to a coil spring in that the more it is compressed,  the more work it is potentially capable of doing. inches of mercury ("Hg). Manifold pressure is dependent on  engine speed (throttle setting) and the degree supercharging.  The operation of the supercharger increases the weight of  the charge entering the cylinder. When a true supercharger  is used with the aircraft engine, the manifold pressure may  be considerably higher than the pressure of the outside  atmosphere. The advantage of this condition is that a greater  amount of charge is forced into a given cylinder volume, and  a greater output of horsepower results. The compression ratio of an engine is a comparison of the  volume of space in a cylinder when the piston is at the bottom  of the stroke to the volume of space when the piston is at the  top of the stroke.  [Figure 1-38]  This comparison is expressed  as a ratio, hence the term compression ratio. Compression  ratio is a controlling factor in the maximum horsepower  developed by an engine, but it is limited by present day fuel  grades and the high engine speeds and manifold pressures  required for takeoff. For example, if there are 140 cubic  inches of space in the cylinder when the piston is at the bottom  and there are 20 cubic inches of space when the piston is at  the top of the stroke, the compression ratio would be 140  to 20. If this ratio is expressed in fraction form, it would be  140/20 or 7 to 1, usually represented as 7:1. Compression ratio and manifold pressure determine the  pressure in the cylinder in that portion of the operating cycle  when both valves are closed. The pressure of the charge  before compression is determined by manifold pressure,  while the pressure at the height of compression (just prior  to ignition) is determined by manifold pressure times the  compression ratio. For example, if an engine were operating  at a manifold pressure of 30 "Hg with a compression ratio  of 7:1, the pressure at the instant before ignition would be  approximately 210 "Hg. However, at a manifold pressure of  60 "Hg, the pressure would be 420 "Hg. The limitations placed on compression ratios, manifold  pressure, and the manifold pressure’s effect on compression  pressures has a major effect on engine operation. Manifold  pressure is the average absolute pressure of the air or fuel/ air charge in the intake manifold and is measured in units of  Without going into great detail, it has been shown that  the compression event magnifies the effect of varying the  Figure 1-38. Compression ratio. 1-30 1 2 3 4 5 6 7 Clearance volume Total volume Displacement volume BDC TDC