## Four-Stroke Engines 

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. 

## Exhaust Systems 

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. 

## Two-Stroke Tuned Exhaust Systems 

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 

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. 

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. 

## Four-Stroke Engine Exhaust Systems 

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). 

## Two-Stroke Engine Warming 

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. 

