Internal Combustion (Four Stroke) Engine

Note

The animated illustration of a four stroke internal combustion engine in this article is a simplified schematic illustration rather than a true-to-life one. The emphasis is on the basic principles of operation.

Inventor

The four stroke internal combustion engine was invented by Nicolaus August Otto (1831 to 1891). The four stroke cycle is sometimes referred to as the Otto cycle. For details regarding Otto and the development if his engine click here (University of Leipzig website).

The Four Strokes

The animated illustration demonstrates the four strokes of the four stroke internal combustion engine. The graph at the bottom of the illustration shows the cylinder head pressure as the engine goes through its four strokes. The horizontal purple line represents atmospheric pressure.

The four strokes are:

Intake: The piston moves downwards with the intake valve (right-hand valve) open. A mixture of air and fuel is drawn into the cylinder from the carburettor because the pressure within the cylinder is below atmospheric pressure.

Compression: The piston moves upwards with both valves shut. The fuel-air mixture is compressed and at the end of the stroke, occupies only the small space within the cylinder head.

Power: Both valves are kept shut during this stroke. At the beginning of this stroke (ignition), the fuel-air mixture is ignited by the spark plug. The flame spreads rapidly and the mixture burns to completion (combustion). This produces a considerable amount of heat that raises the temperature and pressure of the gases in the cylinder head. The resulting high pressure of the gasses produces a force on the piston that drives it downwards (expansion). By the time that the cylinder has reached its lowest position combustion has completed and the cylinder is full of exhaust gasses. 

Exhaust: During this stroke the piston moves upwards with the exhaust valve (left-hand valve) open. This drives the exhaust gasses from the cylinder.

Air and Fuel System

Air is sucked in through the throttle and carburettor via the air intake as shown in the animated illustration. Its flow is controlled by the throttle valve. The suction occurs during the intake stroke when the piston is moving down and the intake valve (the valve shown on the right in the animated illustration) is open.

The throttle valve is used for controlling the speed (power output) of the engine. It is a disc that can be rotated so as to either un-impede the air flow (full throttle) or block the airway preventing air flow. The throttle valve is operated by hand through the use of a control lever (not shown). 

After passing through the throttle valve the air passes through the carburettor. The carburettor has a narrow air passage that causes the air flow to increase in speed. At the point where air flow reached its maximum speed a hollow needle feeds liquid fuel into the air stream. This causes the liquid fuel to break up into microscopically fine droplets producing an aerosol. This mixture of air and fuel is highly combustible.

Piston, Cylinder, Cams, and Valves and Spark Plug

The engine block is a metal casting (shown in light grey in the animated illustration). The piston (orange) moves, as shown, up and down within the cylinder and drives the crank via the piston rod. Piston rings (not shown) prevent gasses from escaping. The cavity below the cylinder is the crankcase which contains oil (not shown) that provides lubrication. 

There are two valves: intake valve (on the right) and the exhaust valve (on the left). These valves are operated by the two cams. The cams are geared to the drive shaft so that the drive shaft must make two complete revolutions for each revolution of a cam.

Note that the crank, the small (middle) gear and the flywheel are all on the drive shaft through which mechanical power is delivered to whatever the engine is being used for.

The Electrical System

The spark plug has a narrow gap through which an ionising current passes resulting in a high temperature spark that causes the fuel-air mixture to ignite.

Like the cams the contactor is driven at the half the speed of the drive shaft. The contactor completes the circuit to the ignition coil just after the intake valve has closed and the the piston is starting to move downwards.

The ignition coil is actually a type of electrical transformer. The low voltage current in the primary coil (few turns) results in a very high voltage in the secondary coil (many turns).

The Flywheel

The flywheel is a large diameter metal wheel with a thick, heavy rim. It evens out the flow mechanical energy transmitted via the drive shaft. It also keeps the engine turning - the piston is supplying power only only 25 percent of the time.