Four-stroke engines are very common in most aircraft  categories and are becoming more common in WSC  aircraft.  [Figure 4-7]  Four-stroke engines have a number  of advantages, including reliability, fuel economy, longer  engine life, and higher horsepower ranges. are pushed out of the cylinder and through the exhaust pipe  system to the atmosphere. Some exhaust systems have an exhaust gas temperature  probe. This probe transmits an electric signal to an instrument  in front of the pilot. This instrument reads the signal and  provides the exhaust gas temperature (EGT) of the gases at  the exhaust manifold. This temperature varies with power and  with the mixture (ratio of fuel to air entering the cylinders),  and is used to make sure the fuel/air mixture is within  specifications. When there is a problem with carburetion,  the EGT gauge will normally be the fi rst notification for a  pilot.  [Figure 4-8] In two-stroke engines, the exhaust system increases the fuel  economy and power of the engine. The two-stroke exhaust  system is an integral part of any two-stroke engine design,  often controlling peak power output, the torque curve, and  even the revolutions per minute (RPM) limit of the engine. These advantages are countered by a higher acquisition cost,  lower power-to-weight ratios, and a higher overall weight.  The increased weight and cost are the result of additional  components (e.g., camshaft, valves, complex head to house  the valve train) incorporated in a four-stoke engine. Engine exhaust systems vent the burned combustion gases  overboard, reduce engine noise, and (in the case of two- stroke engines) help keep the fresh fuel/oil/air mixture in the  cylinders. An exhaust system has exhaust piping attached  to the cylinders, as well as a muffler. The exhaust gases  The exhaust system must be tuned to produce a back pressure  wave at the exhaust port to act as an exhaust valve as shown  in  Figure 4-6E.  When hot spent gases are vented out of the  exhaust port, they are moving fast enough to set up a high  pressure wave. The momentum of that wave down the exhaust  pipe diffuser lowers the pressure behind it. That low pressure  is used to help suck out all of the residual, hot, burnt gas  from the power stroke and at the same time help pull a fresh  fuel/air charge into the cylinder. This is called scavenging  and is an important function of a tuned two-stroke exhaust  system.  [Figure 4-6H] Exhaust Systems Two-Stroke Tuned Exhaust Systems Four-Stroke Engines 1. Intake 2. Compression 3. Power 4. Exhaust Intake valve Exhaust valve Piston Spark plug Crankshaft Connecting rod EGT Probes Tuned Exhaust System Exhaust Silencer Figure 4-8. Two-stroke tuned exhaust system with EGT probes  installed where the exhaust enters the exhaust system. Figure 4-7.  The cycles in a four-stroke engine. 4-6