Engine Components and Construction

12.1 Engine Efficiency Metrics

•  Specific Fuel Consumption (SFC): The weight of fuel burnt (in lbs or kg) per hour for the
• power produced (BHP)25. It is a measure of the engine's fuel efficiency.
•  Thermal Efficiency: The efficiency at which the heat energy from the fuel is converted into
• useful work26. For a piston engine, this is typically only 25-28%27.
•  Volumetric Efficiency: The ratio of the weight of the fuel/air mixture actually drawn into the
• cylinder compared to the weight that could fill the cylinder at standard atmospheric
• pressure28. Supercharging is a method used to increase volumetric efficiency29.
•  Compression Ratio: The ratio of the total volume in the cylinder with the piston at BDC to
• the volume with the piston at TDC30.

12.2 Engine Construction

•  Firing Order: The sequence in which the cylinders fire (e.g., 1-3-4-2 for a four-cylinder
• engine)31. Cylinders do not fire consecutively to reduce vibration and load on the
• crankshaft32.
•  Crankshaft: Converts the reciprocating (up-and-down) motion of the pistons into rotary
• motion to turn the propeller33.
•  Connecting Rod: Links the piston to the crankshaft34.
•  Valve Operating Gear:
• Camshaft: Driven by the crankshaft at half crankshaft speed, it has lobes that operate the
• valves35.
• Valve (Tappet) Clearance: A small gap required between the rocker arm and the valve tip to
• allow for thermal expansion363636363636363636. Too much clearance causes the valve to open late
• and close early; too little can prevent the valve from closing at all37373737.
• Valve Springs: Typically two springs per valve are used to keep the valves closed and to
• prevent 'valve bounce' at high RPM38.