Introduction to the Piston Engine and the Otto Cycle

11.1 The Piston Engine

• A stroke is defined as the linear distance the piston moves within the cylinder1. When the
• piston is at the very top of its travel, it is at
• Top Dead Centre (TDC), and when it is at the bottom, it is at Bottom Dead Centre
• (BDC)2. The piston is connected to a crankshaft, and one full stroke from TDC to BDC
• rotates the crankshaft 180°3.
• The piston engine operates on a four-stroke cycle, commonly known as the Otto Cycle. This
• complete cycle requires two full rotations of the crankshaft (720°)4.

11.2 The Four-Stroke Otto Cycle

• The practical Otto cycle is modified from the theoretical version to be more efficient by
• adjusting when the valves open and close5.
• 1. Induction (Intake) Stroke:
• The inlet valve opens, and the piston moves down from TDC to BDC, drawing a fuel/air
• mixture into the cylinder6.
• The cylinder pressure decreases below ambient pressure, and the temperature of the charge
• decreases7.
• To improve efficiency, the inlet valve opens slightly
• before TDC and closes slightly after BDC8888.
• 2. Compression Stroke:
• Both the inlet and exhaust valves are closed9.
• The piston moves up from BDC to TDC, compressing the fuel/air mixture101010.
• This compression increases the pressure and temperature of the charge11.
• 3. Power Stroke:
• Just before the piston reaches TDC on the compression stroke, the mixture is ignited by a
• spark plug12.
• The rapid combustion causes a massive increase in pressure and temperature, forcing the
• piston down from TDC to BDC13. This is the stroke that produces power.
• Because the piston is momentarily stationary at TDC during ignition, combustion occurs at a
• nearly constant volume, which is why the piston engine is known as a 'Constant Volume
• Engine'14.
• 4. Exhaust Stroke:
• The exhaust valve opens, and the piston moves up from BDC to TDC, pushing the burnt
• gases out of the cylinder15.
• The pressure in the cylinder during this stroke is slightly higher than ambient due to 'Exhaust
• Back Pressure'16.

11.3 Valve Timing and Overlap

•  Valve Lead: When a valve opens before its theoretical timing (e.g., intake valve opening
• before TDC)17.
•  Valve Lag: When a valve closes after its theoretical timing (e.g., exhaust valve closing after
• TDC)18.
•  Valve Overlap: The period when both the inlet and exhaust valves are partially open at the
• same time (around TDC at the end of the exhaust stroke)19. This improves the engine's
• volumetric efficiency by using the momentum of the outgoing exhaust gases to help draw in
• the new fuel/air charge20.

11.4 Engine Power and Performance Metrics

•  Power Control: The pilot controls engine power by using the throttle, which is a valve that
• varies the quantity of the fuel/air mixture entering the cylinders21.
•  Indicated Horsepower (IHP): The theoretical power produced within the cylinders22.

•  Friction Horsepower (FHP): The power lost due to friction from moving parts23.
•  Brake Horsepower (BHP): The actual, usable power delivered to the propeller shaft.
• BHP = IHP - FHP24.