Powered Parachute Flying Handbook

Chapter 4 — Powerplants

Four-Stroke Engines

Four-stroke engines are very common in most aircraft 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 horsepower ranges.

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, etc., incorporated in a four-stoke 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, 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 fuelair 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 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 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 twostroke exhaust system is an integral part of any twostroke 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 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.

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 particular RPM range. The more a certain RPM range is emphasized, the less effective the engine will operate at other RPMs. 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 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 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 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 metals expand at different rates as they heat up. If you heat up steel and aluminum, you will find that 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 cylinder 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. That 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 engine crankshaft. This is an area where things 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 you heat up the engine two quickly, the aluminum case will out-expand those steel bearings and the crank will cause the bearings to start spinning along with it. If those steel bearings start spinning, they can ruin the soft aluminum walls of the case, which is very expensive.

If heat is slowly added to an engine, all the 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 your POH; however, a good rule of thumb is to initially start the engine at idle RPM, get it operating smoothly, and then warm the engine at 3,000 RPM for 5 minutes.

Once the engine is warmed up and the powered parachute is flying, it is still possible to cool down the engine too much. This will happen 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 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 first 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 because of the added heat and the cylinder is cooling with the cold radiator water resulting in a piston seizure. To prevent this, slowly add power well before you get close to the ground where you will need power. This will give the system a chance to gradually open the thermostat and warm up the radiator water.

Just as it takes a while for the engine crankcase and bearings to warm up, it also takes those steel parts a long time to cool down. If you land, refuel and want to take off again quickly, there is no need to warm up again for 5 minutes. The lower end of the engine will stay warmed up after being shut down for short periods. An engine restart is an example where it would be appropriate to warm the engine up until the gauges reach operating temperatures. The lower end of the engine is warm and now you only need to be concerned with preventing the pistons from seizing.

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