Four-Stroke Engines Exhaust Systems Four-Stroke Engine Exhaust Systems Two-Stroke Tuned Exhaust Systems Two-Stroke Engine Warming Four-stroke engines are very common in most air- craft categories, and are becoming more common in  powered parachutes. [Figure 4-4] Four-stroke engines  have a number of advantages, including reliability,  fuel economy, longer engine life, and higher horse- power ranges. These advantages are countered by a higher acquisi- tion 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, etc.,  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- air mixture in the cylinders. An exhaust system has  exhaust piping attached to the cylinders, as well as  a muffler. The exhaust gases are pushed out of the  cylinder and through the exhaust pipe system to the  atmosphere. Some exhaust systems have an exhaust gas tempera- ture 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 tempera- ture (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 specifica - tions. When there is a problem with carburetion, the  EGT gauge will normally be the first notification for  a pilot. 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 RPM limit of  the engine. The exhaust system must be tuned to produce a back  pressure wave to act as an exhaust valve. 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 pres- sure is used to help suck out all of the residual, hot,  burnt gas from the power stroke and at the same time  Two-stroke engines must be warmed up because met- als expand at different rates as they heat up. If you  heat up steel and aluminum, you will find that the alu - minum parts expand faster than the steel parts. This  becomes a problem in two different areas of many  two-stroke engines. The first place is in the cylinders  of the engine.  Four-stroke engines are not as sensitive as two-stroke  engines because they have exhaust valves and there- fore do not need the precision pulse tuned exhaust sys- tem. However, directing the exhaust out appropriately  and reducing the noise are important considerations.  Again, using the manufacturer’s recommended con- figurations is required for Special Light Sport Aircraft  (S-LSA) and recommended for Experimental Light  Sport Aircraft (E-LSA). Overall, the two-stroke exhaust system for a PPC is  a specific design and must be matched to the engine  to operate properly and obtain the rated power. It also  reduces noise and directs the exhaust to an appropri- ate location. Exhaust silencers can be added to reduce  noise but additional weight, cost, and slight power re- duction are the byproducts. 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. The design of the exhaust converging section causes  a returning pressure wave to push the fresh fuel-air  charge back into the exhaust port before the cylinder  closes off that port. That is called pulse-charging and  is another important function of the exhaust system.  Tuned exhaust systems are typically tuned to a par- ticular RPM range. The more a certain RPM range  is emphasized, the less effective the engine will op- erate at other RPMs. Vehicles like motorcycles take  advantage of this with the use of transmissions. Mo- torcycle exhaust pipe builders can optimize a certain  RPM range and then the driver shifts gears to stay in  that range. Aircraft, with no transmission, do not have  this ability. On an aircraft, an exhaust pipe has to be designed to  operate over a broad range of RPMs from idle to full  speed. This is part of the reason that simply putting a  snowmobile engine on a powered parachute doesn’t  work well.  4-4