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. Called pulse charging, it is another important function  of the exhaust system.  [Figure 4-6E] Tuned exhaust systems are typically tuned to a particular  rpm range. The more a certain rpm range is emphasized, the  less effective the engine will operate at other rpm. Vehicles  like motorcycles take advantage of this with the use of  transmissions. Motorcycle 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 rpm from idle to full speed, a  reason that simply putting a snowmobile engine on a WSC  does not work well. Overall, the two-stroke exhaust system for a WSC aircraft 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 appropriate location. Exhaust  silencers can be added to reduce noise, but additional weight,  cost, and slight power reduction are the byproducts. Four-stroke engines are not as sensitive as two-stroke engines  because they have exhaust valves and, therefore, do not  need the precision pulse tuned exhaust system. However,  directing the exhaust out appropriately and reducing  the noise are important considerations. Again, using the  manufacturer’s recommended configurations is required  for S-LSA and recommended for Experimental Light-Sport  Aircraft (E-LSA). Two-stroke engines must be warmed because different metals  expand at different rates as they are heated. When heating  steel and aluminum, the aluminum 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. The cylinders have steel walls that expand slowly, compared  to aluminum pistons that expand quickly. If an engine is  revved too quickly during takeoff before warming up, a lot of  heat is generated on top of the piston. This quickly expands  the piston, which can then seize in the cylinder. A piston  seizure will stop the engine abruptly.  The second area of concern is lower in the engine around  the crankshaft. This is an area where parts may get too loose  with heat, rather than seizing up. Additionally, the crankcase  has steel bearings set into the aluminum which need to  expand together or the bearings could slip. Many two-stroke  engines have steel bearings that normally hug the walls of  the aluminum engine case. The crank spins within the donuts  of those steel bearings. If the engine heats too quickly, the aluminum case out- expands those steel bearings and the crank causes the bearings  to start spinning along with it. If those steel bearings start  spinning, it can ruin the soft aluminum walls of the case,  which is very expensive. If heat is slowly added to an engine,  all parts will expand more evenly. This is done through a  proper warm-up procedure. Many two-stroke engines are  best warmed up by running the engine at a set rpm for a set  amount of time. Follow the instructions in the POH; however,  a good rule of thumb is to start the engine initially at idle  rpm, get it operating smoothly at 2,500 rpm for 2 minutes  for initial warm up, and then warm the engine at 3,000 rpm  for 5 minutes. The cylinder head temperature or coolant  temperature must be up to the manufacturer’s recommended  temperatures before takeoff. This may require running the  engine at higher rpm to reach required temperatures on some  engines. Once the engine is warmed up and the aircraft is flying, it is  still possible to cool down the engine too much. This happens  when the engine is idled back for an extended period of time.  Even though the engine is running, it is not generating as  much heat as the cooling system is efficiently dumping engine  heat into the atmosphere. An immediate power application  with a cooled engine can seize the engine just as if the engine  had not been warmed in the fi rst place. In water-cooled engines, on a long descent at idle, the  coolant cools until the thermostat closes and the engine is not  circulating the radiator fluid through the engine. The engine  temperature remains at this thermostat closed temperature  while the radiator coolant continues to cool further. If full  throttle is applied, the thermostat can open, allowing a blast  of coolant into the warm engine. The piston is expanding due  to the added heat, and the cylinder is cooling with the cold  radiator water, resulting in piston seizure. To prevent this,  slowly add power well before getting close to the ground  where power is needed. This gives the system a chance to open  the thermostat gradually and warm up the radiator water. Just as it takes time for the engine crankcase and bearings  to warm up, it also takes those steel parts a long time to cool  down. If a pilot lands, refuels, and wants to take off again  quickly, there is no need to warm up again for 5 minutes.  The lower end of the engine stays warmed up after being  shut down for short periods. Engine Warming Two-Stroke Engine Warming Four-Stroke Engine Exhaust Systems 4-7