Powered Parachute Flying Handbook

This chapter covers the engines found on most powered parachutes and includes the exhaust, ignition, fuel, lubrication, cooling, propeller, gearbox, induction, charging, and fuel systems. Reciprocating engine operating theory is covered, for both two-stroke and four-stroke engines.

The powered parachute engine and propeller, often referred to as a powerplant, work in combination to produce thrust. The powerplant propels the powered parachute and charges the electrical system that supports PPC operation.

The engine is one of the key components of a powered parachute and should be maintained according to both the engine and airframe manufacturer recommendations. Preflight information, along with maintenance schedules and procedures, can be found in the Pilot’s Operating Handbook (POH) and/or maintenance references from the manufacturers.

Engine inspections and maintenance must be performed and documented in a logbook. You should review this logbook before flying an unfamiliar powered parachute.

Reciprocating Engines

Most powered parachutes are designed with reciprocating engines. Two common means of classifying reciprocating engines are:

1. By the number of piston strokes needed to complete a cycle: two-stroke or four-stroke; and
2. By the method of cooling: liquid or air-cooled.

Refer to Chapter 5 of the Pilot’s Handbook of Aeronautical Knowledge for a comprehensive review of how reciprocating four-stroke engines operate.

Two-Stroke Engines

Two-stroke engines are commonly used in powered parachutes. Two-stroke aviation engines evolved from two-stroke snowmobile and watercraft engines, the difference being that an aircraft engine is optimized for reliability with dual ignition often installed for each cylinder. Two-stroke engines are popular because they have fewer components than four-stroke engines which makes them less expensive to manufacture, and lighter, thus increasing their power-toweight ratio.

Two-stroke engines require that oil be mixed into the fuel to lubricate the engine, instead of being held in a sump and having a separate recirculating system like a four-stroke engine. Details on two-stroke oil mixing are covered later under the “Lubrication” section.

One stroke as the piston moves up is intake and compression, the second stroke as the piston moves down is power and exhaust. The two-stroke engine performs the same functions as a four-stroke engine in half the strokes.

A wide range of valve systems are found on two cycle engines, for the purpose of opening and closing ports in the cylinder to let fuel in and exhaust out at the proper time, similar to the intake and exhaust valves on a four-stroke engine. One-way pressure valves, called spring, reed, or poppet valves, open when the pressure drops within the crankcase, pulling the fuel from the carburetor into the crankcase. [Figure 4-1]

Mechanical rotary valves are driven off the engine, rotate to provide an opening at the precise time, and can be on the intake and exhaust ports. [Figure 4-2]

Piston porting does not use any valves. The fuel inlet port is opened and closed by the piston position as it moves up and down in the cylinder. This is called a “piston ported inlet” and will be used in the Two- Stroke Process description that follows. [Figure 4-3]

Two-Stroke Process

The two-stroke process begins with the fuel entering the engine and concludes as it exits as exhaust. [Figure 4-3]

Crankcase Vacuum Intake Stroke—Piston Moving up: Figure 4-3 a to b

The upward stroke of the piston [Figure 4-3a] creates a vacuum in the crankcase and pulls the fuel/ air/oil mixture into the crankcase through the intake valve system from the carburetor. [Figure 4-3b] This can be a pressure-actuated reed valve, a rotary valve, or a third ported inlet system where the lower piston skirt provides an opening for the fuel/air/oil mixture to flow in when the piston is reaching its highest point Top Dead Center (TDC). At this point, the greatest portion of the fuel/air/oil mixture has filled the crankcase.

Crankcase Compression Stroke—Piston Moving down: Figure 4-3 b to c

During the downward stroke, the pressure valve is forced closed by the increased crankcase pressure, the mechanical rotary valve closes, or the piston closes off the fuel/air oil mixture intake port. The fuel mixture is then compressed in the crankcase during the downward stroke of the piston.

Crankcase Transfer/Exhaust—Piston at lowest: Figure 4-3 d

When the piston is near the bottom of its stroke, the transfer port opening from the crankcase to the combustion chamber is exposed, and the high pressure fuel/air mixture in the crankcase transfers around the piston into the main cylinder.

This fresh fuel/air/oil mixture pushes out the exhaust (called scavenging) as the piston is at its lowest point and the exhaust port is open. Some of the fresh fuel/ air/oil mixture can escape out the exhaust port resulting in the higher fuel use of the two stroke engine.

Cylinder start of Compression Stroke—Piston initially Moving up: Figure 4-3 e

As the piston starts to move up, covering the transfer port, the tuned exhaust bounces a pressure wave at the precise time across the exhaust port (more on this in the exhaust system discussion) to minimize the fuel/air/oil mixture from escaping out the exhaust port.

Cylinder Compression Stroke—Piston Moving Up: Figure 4-3 e to f

The piston then rises, and compresses the fuel mixture in the combustion chamber. During this piston compression process, the crankcase vacuum intake process is happening simultaneously, as described earlier. This is why four processes can happen in two strokes.

Cylinder Power Stroke—Piston Moving Down: Figure 4-3 f to g

At the top of the stroke, the spark plug ignites the fuel mixture and drives the piston down as the power stroke of the engine.

Cylinder Power Stroke—Piston Moving Down: Figure 4-3 g to h

As the piston passes the exhaust port, the exhaust starts to exit the combustion chamber. As the piston continues down, the transfer port opens and the swirling motion of the air/fuel/oil mixture pushes the exhaust out the exhaust port.

Piston Reverses Direction From Down Stroke to Up Stroke: Figure 4-3 h to a

As the piston reverses direction from the down stroke to the up stroke the process is complete.

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