piston has passed TDC and has started the intake stroke (valve  lag). This timing is called valve overlap and is designed to  aid in cooling the cylinder internally by circulating the cool  incoming fuel/air mixture, to increase the amount of the fuel/ air mixture induced into the cylinder, and to aid in scavenging  the byproducts of combustion from the cylinder. timing of the valve and ignition events is always specified in  degrees of crankshaft travel. It should be remembered that a  certain amount of crankshaft travel is required to open a valve  fully; therefore, the specified timing represents the start of  opening rather than the full-open position of the valve. An  example valve timing chart can be seen in  Figure 1-37 . During the intake stroke, the piston is pulled downward in  the cylinder by the rotation of the crankshaft. This reduces  the pressure in the cylinder and causes air under atmospheric  pressure to flow through the carburetor, which meters the  correct amount of fuel. The fuel/air mixture passes through  the intake pipes and intake valves into the cylinders. The  quantity or weight of the fuel/air charge depends upon the  degree of throttle opening. The intake valve is timed to close about 50° to 75° past BDC  on the compression stroke, depending upon the specific  engine, to allow the momentum of the incoming gases  to charge the cylinder more completely. Because of the  comparatively large volume of the cylinder above the piston  when the piston is near BDC, the slight upward travel of the  piston during this time does not have a great effect on the  incoming flow of gases. This late timing can be carried too  far because the gases may be forced back through the intake  valve and defeat the purpose of the late closing. The intake valve is opened considerably before the piston  reaches TDC on the exhaust stroke, in order to induce a  greater quantity of the fuel/air charge into the cylinder and  thus increase the horsepower. The distance the valve may be  opened before TDC, however, is limited by several factors,  such as the possibility that hot gases remaining in the cylinder  from the previous cycle may flash back into the intake pipe  and the induction system. After the intake valve is closed, the continued upward travel  of the piston compresses the fuel/air mixture to obtain the  desired burning and expansion characteristics. The charge is  fired by means of an electric spark as the piston approaches  TDC. The time of ignition varies from 20° to 35° before TDC,  depending upon the requirements of the specific engine to  ensure complete combustion of the charge by the time the  piston is slightly past the TDC position. In all high-power aircraft engines, both the intake and the  exhaust valves are off the valve seats at TDC at the start  of the intake stroke. As mentioned above, the intake valve  opens before TDC on the exhaust stroke (valve lead), and the  closing of the exhaust valve is delayed considerably after the  Many factors affect ignition timing, and the engine  manufacturer has expended considerable time in research and  testing to determine the best setting. All engines incorporate  devices for adjusting the ignition timing, and it is most  important that the ignition system be timed according to the  engine manufacturer’s recommendations. As the piston moves through the TDC position at the end  of the compression stroke and starts down on the power  stroke, it is pushed downward by the rapid expansion of  the burning gases within the cylinder head with a force that  can be greater than 15 tons (30,000 psi) at maximum power  output of the engine. The temperature of these burning gases  may be between 3,000° and 4,000 °F. As the piston is forced  downward during the power stroke by the pressure of the  burning gases exerted upon it, the downward movement of  the connecting rod is changed to rotary movement by the  crankshaft. Then, the rotary movement is transmitted to the  propeller shaft to drive the propeller. As the burning gases  are expanded, the temperature drops to within safe limits  before the exhaust gases flow out through the exhaust port. The timing of the exhaust valve opening is determined by,  among other considerations, the desirability of using as much  Power Stroke Compression Stroke Intake Stroke 1-27 Figure 1-37. Valve timing chart. V a $_{l}$v e $_{ }$o v e r l a p 7 5 ° b o th v a l v e s o p e n E x h a u s t  s t r o k e C o m p r e s s i o n s t r o k e I n t a k e s t r o k e$^{P}$ o w e r s t r o ke $_{I}$g n i t i o n BTC = before top center ATC = after top center ABC = after bottom center BBC = before bottom center Finish cycle 25° ATC  Exhaust valve closed Exhaust valve opens 30° BBC Start cycle 50° BTC  Intake valve opens Intake valve closes 30° ABC One complete actual cycle of a four-stroke cycle reciprocating engine 