compression ratio, accessories on the front of the engine that  cause individual cylinders to run hot, and the design of the  baffling used to deflect airflow around the cylinder. At takeoff  power, the mixture must be rich enough to supply sufficient  fuel to keep the hottest cylinder cool. The induction manifold provides the means of distributing  air, or the fuel/air mixture, to the cylinders. Whether the  manifold handles a fuel/air mixture or air alone depends on  the type of fuel metering system used. On an engine equipped  with a carburetor, the induction manifold distributes a fuel/ air mixture from the carburetor to the cylinders. On a fuel- injection engine, the fuel is delivered to injection nozzles,  one in each cylinder, that provide the proper spray pattern  for efficient burning. Thus, the mixing of fuel and air takes  place at the inlet port to the cylinder. On a fuel-injection  engine the induction manifold handles only air. The induction manifold is an important item because of the  effect it can have on the fuel/air mixture that finally reaches the  cylinder. Fuel is introduced into the airstream by the carburetor  in a liquid form. To become combustible, the fuel must be  vaporized in the air. This vaporization takes place in the  induction manifold, which includes the internal supercharger,  if one is used. Any fuel that does not vaporize clings to the  walls of the intake pipes. Obviously, this affects the effective  fuel/air ratio of the mixture that finally reaches the cylinder  in vapor form. This explains the reason for the apparently  rich mixture required to start a cold engine. In a cold engine,  some of the fuel in the airstream condenses out and clings to  the walls of the manifold. This is in addition to that fuel that  never vaporized in the first place. As the engine warms up,  less fuel is required because less fuel is condensed out of the  airstream and more of the fuel is vaporized, thus giving the  cylinder the required fuel/air mixture for normal combustion. Any leak in the induction system has an effect on the mixture  reaching the cylinders. This is particularly true of a leak at the  cylinder end of an intake pipe. At manifold pressures below  atmospheric pressure, such a leak leans out the mixture. This  occurs because additional air is drawn in from the atmosphere  at the leaky point. The affected cylinder may overheat, fire  intermittently, or even cut out altogether. While considering the operational effect of valve clearance,  keep in mind that all aircraft reciprocating engines of current  design use valve overlap. Valve overlap is when the intake  and exhaust valves are open at the same time. This takes  advantage of the momentum of the entering and exiting gases  to improve the efficiency of getting fuel/air in and exhaust  gases out.  Figure 10-49  shows the pressures at the intake and  exhaust ports under two different sets of operating conditions.  In one case, the engine is operating at a manifold pressure  of 35 "Hg. Barometric pressure (exhaust back pressure) is  29 "Hg. This gives a pressure acting in the direction indicated  by the arrow of differential of 6 "Hg (3 psi).  During the valve overlap period, this pressure differential  forces the fuel/air mixture across the combustion chamber  toward the open exhaust. This flow of fuel/air mixture forces  ahead of it the exhaust gases remaining in the cylinder,  resulting in complete scavenging of the combustion chamber.  This, in turn, permits complete filling of the cylinder with  a fresh charge on the following intake event. This is the  situation in which valve overlap gives increased power. There is a pressure differential in the opposite direction  of 9 "Hg (4.5 psi) when the manifold pressure is below  atmospheric pressure, for example, 20 "Hg. These cause air  or exhaust gases to be drawn into the cylinder through the  exhaust port during valve overlap. In engines with collector rings, this inflow through the  exhaust port at low power settings consists of burned exhaust  gases. These gases are pulled back into the cylinder and mix  with the incoming fuel/air mixture. However, these exhaust  gases are inert; they do not contain oxygen. Therefore, the  fuel/air mixture ratio is not affected much. With open exhaust  stacks, the situation is entirely different. Here, fresh air  containing oxygen is pulled into the cylinders through the  exhaust. This leans out the mixture. Therefore, the carburetor  Operational Effect of Valve Clearance Induction Manifold Intake valve Exhaust valve Impeller Manifold pressure  MAP  Pressure differential  Barometric pressure 35" Hg.  6" Hg.  29" Hg. 20" Hg.  9" Hg.  Figure 10-49.  Effect of valve overlap.  10-34