The Outdoor Power Equipment Institute, Inc.

Homeowners Guide
To Low Emission Engines

Glossary Of Small Engine Terms

The following comprehensive listing of terms relate to small, off-road engines primarily used on today's clean-running outdoor power equipment. Refer to this glossary to improve your knowledge of how engines work. You may gain extra points with your local servicing dealer's engine service technician when you can talk "engine talk." Review these terms and be the smartest equipment user on your block!

2-Cycle Engines: Air cooled 2-cycle engines differ from 4-cycles by having one power stroke for every revolution of the engine versus every other stroke on 4-cycles.

4-Cycle Engine: All 4-cycle engines require four piston strokes or cycles to complete one power cycle. The flywheel on one end serves several functions, the first of which are engine cooling and inertia. The other functions are holding the external magnets for ignition and internal magnets for charging if equipped.

Air Cleaner: The air cleaner is a device used to eliminate dust and dirt from the air supply. Filtered air is necessary to assure that abrasive particles are removed before entering the combustion chamber. Dirt allowed into the engine will quickly wear the internal components and shorten the life of the engine.

Camshaft: Lobes push the lifters at the proper time to allow air and fuel in and exhaust out of the cylinder. Teeth on the camshaft gear time the camshaft movement to the crankshaft movement.

Carburetion System: The carburetion system used on most engines is a float style. This type of carburetion makes for easy starting by having a ready fuel supply in the fuel bowl.

Choke Style: This system has a manual choke level on the carburetor or the choke is in the control and operated remotely by a speed control cable. This system offers the great starting of float style, and is used on all tractors and some horizontal engines.

Choke with Primer: By combining full choking and one-to-two pushes of the primer, the snow blower customer will get easy starting in winter's subzero temperatures.

Compression Relief: The main purpose is to provide the end user with the least possible pulling effort during starting. The system operates by momentarily opening one of the valves just prior to Top Dead Center on the compression stroke. The system most commonly used today on 4-cycles is mechanical. This type disengages after the engine is started. When disengaged, the engine creates full power.

Connecting Rod: The link between the piston (piston pin) and the crankshaft.

Counterbalance Systems: Some engines may be equipped with a counterbalance system. This system uses a single or double shaft that is driven off the crankshaft. This shaft's function is to counteract the imbalance caused by the counterweights on the crankshaft and combustion forces. This system makes the engine as smooth running as many twin cylinder engines.

Crankcase Breather: This is a one way check valve that allows air out and prevents air from coming in. It allows the engine to develop a vacuum in the crankcase during operation to reduce oil consumption.

Crankshaft: Converts the up and down piston movement to the rotational force (torque) through an offset crankpin or rod journal.

Cylinder Block: Houses the piston, crankshaft, and along with the cylinder cover, all the internal components. The block is a one piece diecast aluminum alloy cylinder casting which may have a cast iron liner.

Cylinder Cover: Provides the bearing surface for the power take off (PTO) end of the crankshaft and camshaft. This bearing may be a bushing, a ball bearing or the cover material itself.

Cylinder Head: A one piece aluminum alloy casting that is bolted to the top of the cylinder block. The overhead design contains all of the valve train except for the camshaft, lifters and push rods.

Exhaust or Scavenge Phase: The burned gases must be cleared out of the combustion chamber and replaced by a fresh charge of fuel-air mixture through the intake ports. The exhaust passes out through the exhaust ports into the outside air.

Exhaust Ports: Allow the burned gases to pass out of the combustion chamber.

Flywheel: Provides inertia to keep the engine running smoothly between power strokes. By attaching a fan to the flywheel air flow is created and directed over the cooling fins. Hence the term Air Cooled Engine.

Ignition System: Electronic CDI (Capacitor Discharge Ignition). This system eliminated ignition points and their need for regular service. The benefit to the operator is low speed firing of the spark plug. This system requires approximately 1/3 the pulling speed of competitive inductive ignition systems. Which translates to 1/3 less effort for the end user.

Lubrication: 2-cycle engines are lubricated by a gas oil mix. The correct mix ratio of oil and gas combines with air and the venturi and enters the crankcase. During engine operation, the oil clings to all the internal moving parts for lubrication.

Oil Dipper: Used on horizontal shaft engines, is attached to the connecting rod. As the crankshaft rotates, the dipper picks up oil on the downward stroke and distributes it throughout the crankcase area.

Oil Pump (vertical shaft only): Consists of a steel plunger and a nylon or aluminum housing that rides an offset on the camshaft. The rotary pump consists of a lobed gear in a rotating ring that is driven by the camshaft. Both of these systems pump oil under pressure to all critical areas.

Overhead Engines: Overhead valve engines use the same cycles listed above with a primary change in valve location to the cylinder head. This increase in combustion efficiency which typically gives a 20% HP increase over the same displacement L-head engine and a 20% fuel efficiency improvement.

Paper-Type Filter Element: Paper type air filter elements can only be serviced by replacement. Do not attempt to clean a paper filter element.

Piston: Transmits the force of the burning and expanding gases through the connecting rod to the crankshaft.

Piston Rings: Provide the seal between the cylinder wall and the piston. The rings keep the combustion pressures from entering the crankcase and also wipe the oil off the cylinder wall and return it to the pump.

Polyurethane-Type Filter Element: This type of air filter can be serviced when restricted with dust or dirt. Wash the filter or pre-cleaner in a liquid detergent and water solution until all the dirt is removed. Rinse in clear water to remove the detergent solution. Squeeze the element (do not twist) to remove the excess water. Wrap the element in a clean cloth and squeeze it (do not twist) until completely dry.

Ports: Openings in the cylinder allow gases to pass into and out of the combustion chamber. The ports are opened or closed by the upward and downward movement of the piston.

Primer Type (Also known as manual fuel injection): This style has a primer button that pushes a metered amount of fuel out of the main nozzle. This requires 3-5 complete pushes of the button, waiting 3 seconds between each prime. After the engine is warmed up, no priming is needed for restart. These carburetors are used on rotary mower engines and some small horsepower engines.

Reed Valve: A reed valve is activated by crankcase pressure or vacuum. A decrease in crankcase pressure opens the reed, allowing the fuel air and oil mixture to enter the crankcase. Increased crankcase pressure closes the valve, preventing the escape of the fuel-air and oil mixture back through the carburetor.

Third Port: Sometimes called the piston port, a third port is for entry of the fuel-air and oil mixture to the crankcase. From the crankcase the fuel-air mixture enters the combustion chamber through the intake ports. The third port is controlled by the piston skirt.

Valves: Allow air-fuel mixture to enter the cylinder and exhaust gases to exit. The valves provide a positive seal when closed.

Valve Lifters: Maintain contact on the camshaft, move the push rods to pivot the rocker arms and move the valves.

Valve Springs: Return the valves to the closed position and must be strong enough to maintain valve lifter and cam lobe contact. The valve retainers lock the spring to the valve stem.