Engine Fundamentals: Four-Stroke Cycle and Component Identification

The Four-Stroke Cycle

Every spark-ignition gasoline engine used in passenger vehicles today operates on the four-stroke cycle, also called the Otto cycle. Understanding this cycle is the foundation of every diagnostic and overhaul task covered in ASE A1 (Engine Repair). The four strokes are intake, compression, power, and exhaust. Each stroke is one full downward or upward travel of the piston inside the cylinder.

Intake stroke: The piston moves downward from top dead center (TDC) to bottom dead center (BDC). The intake valve opens and the exhaust valve stays closed. The downward piston motion creates low pressure (vacuum) inside the cylinder, drawing the air-fuel mixture (port injection) or air alone (direct injection) through the intake port.

Compression stroke: Both valves are closed. The piston moves upward from BDC to TDC, compressing the charge into the combustion chamber. Compression raises the temperature and pressure of the mixture, which increases the energy released during combustion. Typical gasoline engine static compression ratios range from about 8:1 to 12:1.

Power stroke (combustion stroke): Near TDC, the spark plug fires. The burning mixture expands rapidly, forcing the piston downward with great force. This is the only stroke that produces work; the other three strokes consume energy stored in the crankshaft's rotating mass (flywheel). Both valves remain closed during this stroke.

Exhaust stroke: The exhaust valve opens and the piston moves back up from BDC to TDC, pushing burned gases out through the exhaust port. At TDC the exhaust valve closes, the intake valve opens, and the cycle repeats.

In a multi-cylinder engine, the crankshaft is designed so that the power strokes of individual cylinders overlap, providing smooth, continuous power delivery. A four-cylinder engine fires once every 180° of crankshaft rotation; a V8 fires once every 90°.

Major Engine Components and Their Functions

Engine block: The main structural casting that houses the cylinders, coolant passages, and oil galleries. It is the foundation to which all other components attach.

Pistons: Cylindrical components that slide inside the cylinder bores. They transfer combustion pressure to the connecting rods. Pistons also must seal the combustion chamber and transfer heat to the cylinder walls.

Piston rings: Thin metal rings seated in grooves around the piston. Compression rings seal combustion gases above the piston; the oil control ring scrapes excess oil from the cylinder wall and returns it to the sump.

Connecting rods: Link each piston to the crankshaft. They convert the piston's reciprocating (up-and-down) motion into the crankshaft's rotary motion.

Crankshaft: A forged or cast shaft with offset journals (crank pins) where connecting rods attach. It converts reciprocating motion to rotation and drives the flywheel, timing system, and accessory drives.

Camshaft: A shaft with precision-ground lobes that open and close the intake and exhaust valves at the correct time relative to piston position. Camshaft position is timed to crankshaft position by the timing system.

Valves, valve springs, and valve train: Poppet valves open and close the intake and exhaust ports. Valve springs return valves to their closed (seated) position. The valve train includes the components between the camshaft and the valve stem: lifters (followers), pushrods (in pushrod engines), rocker arms, and — in overhead-cam designs — camshaft lobes acting directly on followers or rocker arms.

Cylinder head: The casting that closes the top of the cylinders. It contains the combustion chambers, intake and exhaust ports, valve seats and guides, and (on OHC engines) the camshaft(s).

Timing components: A timing chain, belt, or gear set that links the crankshaft to the camshaft(s), ensuring valve events occur at the correct crank angle. The crankshaft turns twice for every one full camshaft rotation in a four-stroke engine.

Gaskets and seals: The head gasket seals combustion pressure and coolant/oil passages between block and head. Crankshaft seals (front and rear) prevent oil leakage at the ends of the crankshaft. Valve stem seals prevent oil from entering the ports.

Pushrod vs. Overhead-Cam Designs

In a pushrod (OHV) engine, the camshaft sits in the block. Lobes push lifters, which push pushrods, which actuate rocker arms to open the valves. This design is compact and robust and is still widely used in American V8 applications.

In an overhead-cam (OHC) engine, the camshaft is mounted in the cylinder head, directly above the valves. A single-overhead-cam (SOHC) engine uses one camshaft per bank; a dual-overhead-cam (DOHC) engine uses separate camshafts for intake and exhaust valves. OHC designs typically allow higher engine speeds and more flexible valve timing.

Safety in the Engine Lab

Before handling any disassembled engine or running engine display:

• PPE: Safety glasses and nitrile or leather gloves are required at all times in the lab. Steel-toed footwear is required.
• Lifting: Engine assemblies are heavy. Use an engine stand, hoist, or team lift. Never place fingers under an unsupported engine.
• Sharp edges: Machined engine components have sharp casting edges. Handle with care.
• Fluids: Even "used" display engines may contain residual oil. Avoid skin contact; wash hands before eating. Dispose of fluids per your shop's hazardous-waste procedure.
• Rotating/running engines: Never reach into or lean over a running engine. Secure loose clothing and tie back long hair before approaching any running equipment.
• Housekeeping: Wipe up oil spills immediately to prevent slip hazards.