WSC aircraft are designed with reciprocating engines. [Figures 4-1 through 4-3] Two common means of classifying reciprocating engines are the:
- Number of piston strokes needed to complete a cycle—two or four.
- Method of cooling—liquid or air.
Refer to the Pilot’s Handbook of Aeronautical Knowledge for a comprehensive review of how reciprocating four-stroke engines operate.
Two-stroke engines are commonly used in WSC aircraft. Twostroke 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 the power-to-weight ratio.
Two-stroke engines require that oil be mixed into the fuel to lubricate the engine, instead of being held in a sump and requiring a separate pressurized recirculating system like that of a four-stroke engine. Details on two-stroke oil mixing are covered in the lubrication section. One stroke as the piston moves up is intake and compression, while 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 number of strokes.
A wide range of valve systems are found on two-stroke engines for the purpose of opening and closing ports in the cylinder to let fuel in and exhaust out at the proper time. This is 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 4-4]
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 4-5]
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 is used in the two-stroke process description that follows.
The two-stroke process begins with the fuel entering the engine and concludes as it exits as exhaust.
Crankcase Vacuum Intake Stroke—Piston Moving Up
The upward stroke of the piston [Figure 4-6A] 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 4-6B] This can be a pressure-actuated reed valve, a rotary valve, or a 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 of top dead center (TDC). At this point, the greatest portion of the fuel/oil/air mixture has filled the crankcase. [Figure 4-6B]
Crankcase Compression Stroke—Piston Moving Down
During the downward stroke, the pressure valve is forcibly closed by the increased crankcase pressure, the mechanical rotary valve closes, or the piston closes off the fuel/air oil mixture intake port as shown. The fuel/oil/air mixture is then compressed in the crankcase during the downward stroke of the piston. [Figures 4-6B to 4-6D]
Crankcase Transfer/Exhaust—Piston at Lowest
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/oil/air 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/oil/air mixture can escape through the exhaust port, resulting in the higher fuel use of the two-stroke engine. [Figure 4-6D]
Cylinder Start of Compression Stroke—Piston Initially Moving Up
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 to minimize the fuel/air/oil mixture escaping through the exhaust port. [Figure 4-6E]
Cylinder Compression Stroke—Piston Moving Up
The piston then rises and compresses the fuel mixture in the combustion chamber. [Figure 4-6E to 4-6F] 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. [Figures 4-6B and 4-6C]
Cylinder Power Stroke—Initial Piston Moving Down
At the top of the stroke, the spark plug ignites the fuel/oil/air mixture and drives the piston down as the power stroke of the engine. [Figures 4-6F and 4-6G]
Cylinder Power Stroke—Final Piston Moving Down
As the piston passes the exhaust port, the exhaust exits the combustion chamber. As the piston continues down, the transfer port opens and the swirling motion of the fuel/ oil/air mixture pushes the exhaust out of the exhaust port. [Figures 4-6H]
Piston Reverses Direction From Down Stroke to Up Stroke
As the piston reverses direction from the downstroke to the upstroke, the process is complete. [Figures 4-6H and 4-6A]
Four-stroke engines are very common in most aircraft categories and are becoming more common in WSC aircraft. [Figure 4-7] 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) incorporated in a four-stoke engine.