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 Engine Exhaust Systems 

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). 

## Engine Warming 

## Two-Stroke Engine Warming 

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. 

