About Gasoline Engines

The two types of small gasoline engines most commonly found in gardeners’ tool storage areas are four-cycle engines (used on lawn mowers, rotary tillers, and other large equipment) and two-cycle engines (used on chainsaws, gas-powered blowers, and weed cutters).

The “cycles” refer to how the engine is powered. In the four-cycle engine, power is produced during one of every four strokes of the piston; in the two-cycle engine, power is produced on every down stroke. The other major difference between the two engines is the fuel system. In the four-cycle engine, the oil and gas are kept separate; in the two-cycle engine, they are mixed together. However, both types of engines operate on the same basic principles, which are outlined below.

Both two-cycle and four-cycle engines in garden equipment are cooled by air rather than by water and they usually have only one piston (although some four-cycle engines have two). Some equipment comes with a battery and a starting mechanism, but most small engines are started by pulling a rope. This action turns the engine over—that is, the piston goes up and down and sets a whole chain of events in motion.


As the engine begins turning over, the flywheel moves. Attached to one point of the flywheel are one or more magnets. These pass very close to a coil mounted inside the engine, creating a magnetic force.

Within the coil there are two sets of copper wire windings: a thick, inner “primary” wire, which has about 100-200 turns, and a thin, outer “secondary” wire, which has about 10,000-20,000 turns.

The primary wire leads to a set of breaker points and to a condenser. The secondary wire leads to the spark plug. When the starter rope is pulled, it moves the crankshaft, which has a teardrop-shaped lobe or a flat spot on it. This cam on the crankshaft opens and closes the breaker points. When the points are closed, the electrical current, set up by the turning flywheel passing near the coil, flows through the primary wire. However, just before a spark is needed, the breaker points are opened, breaking the circuit of the current in the primary wire, causing the magnetic field to reverse and collapse.

This creates a very high voltage which is picked up by the thin secondary wire. The condenser absorbs the current in the primary current so that it does not jump the gap in the breaker points. The high-voltage secondary current is then channeled through the thin secondary wire and the spark plug wire to the spark plug center electrode. The unstable current seeks a ground, and as it jumps to the spark plug’s ground electrode, it causes a spark.

This electrical process is repeated with each revolution of the flywheel. The faster the flywheel turns, the faster the spark is ignited in the combustion chamber, and the more fuel is required to keep up this speed.


As the engine begins turning over with the pull of the starter rope, air is sucked through the air filter, down an air horn, and through the carburetor. At the same time, gasoline is drawn into the carburetor from the fuel tank. The air and fuel are mixed very precisely by the carburetor as they are drawn into the combustion chamber between the spark plug and the top of the piston.

At this point, the spark leaps across the gap on the spark plug electrodes and ignites the mixture of fuel and air in the combustion chamber. The mixture explodes, creating a force that drives the piston down.

The piston is linked by the connecting rod to the crankshaft (the driving mechanism of the engine), which is turned by the up-and-down motion of the piston. The rotating crankshaft is linked to the “business end” of the power equipment (e.g., the blade on a lawn mower or the chain on a saw). This link can either be direct (i.e., the equipment is attached right to the shaft) or indirect (i.e., the equipment is connected via a belt, chain, or gear).

Despite the similar principles behind the operation of the two-cycle and four-cycle engines, each kind has variations. To spot variations in engine design from one piece of equipment to another or from one manufacturer to another, examine your owner’s manual and the engine itself.

It is in the fuel system that the basic difference between the two engines is found. This system consists of a gas tank, a gas line, and the carburetor where the fuel is mixed with air and pumped into the combustion chamber. There are three types of fuel systems: the gravity-feed system, which is used when the gas tank is located above the carburetor; the fuel pump, which is used when the gas tank is located below the carburetor; and the diaphragm, or suction feed, system, which is used on equipment that is held at a variety of angles while in operation.