Two and Four Cycle Engine Terminology

Briggs and Stratton Engine

Two and Four Cycle Small Engine Terminology Explained.

If you have looked for parts for your outdoor power equipment or even taken a glimpse into the information regarding your equipment, you may have stumbled upon a few things that just don’t make sense. You’re in good company. I deal with this stuff every day and I still come across the strange and unexplained.

So in an effort to help clear things up a bit, here are a few of the most common terms we tend to use on a daily basis.

Please, feel free to contact us with anything not listed here that you feel bears explanation.

Model number: This is the number most often used to identify the particular type (or model) of a product or machine. When looking for parts diagrams, the model number is often the first thing required. Example: We know you have a Briggs & Stratton brand engine. The Model number will tell us which engine you have.

Serial number: This is the number normally used to specify which version of a certain model you have. Example: A Dixon mower may be a model 3014. The serial number will help to narrow down exactly which 3014 the mower is.

Spec number: The “Spec number” (or specification number) is often associated with Kohler brand engines and are required to identify the exact engine in question.

Carburetor: This engine component regulates the mixture of gasoline and air into a highly combustible combination required for your engine to operate.

Primer bulb: When compressed, this rubber bulb moves gasoline from the fuel tank to the carburetor, effectively “priming” the carburetor.

Fuel line: Conduit used to carry fuel in the fuel tank to and from the carburetor.

Pre-mix: Engines require oil to lubricate internal parts. Two cycle engines without an oil reservoir do not store oil for this purpose. Because of this, the oil is “pre-mixed” into the gasoline. When the gasoline is consumed within the engine, an oil residue is left to coat the moving parts. Pre-mix can be added to gasoline or purchased in products already containing the oil/gas mixture.

Tune up: A tune up is a service conducted on an engine wherein the function of several components of the machine are tested, then adjusted or repaired for optimal performance. A tune up can include, but is not limited to checking, cleaning or replacing the fuel and air filters, the gasoline and oil as well as the spark plug. A tune up may also include cleaning and adjusting the carburetor as well as an inspection of belts, pulleys, and blades.

Mulcher: A mulcher or Mulching Blade is designed to lift and circulate grass clippings beneath the lawn mower deck, resulting in a multiple and finer cut clipping. The clippings are then left to sift between the grass blades, eliminating the need to bag or rake up clippings.

High Lift: a High lift blade is designed to create a greater air flow beneath the lawn mower deck. The increased air flow can cause the blade clippings to be discharged at a higher velocity; which is often ideal for bagging clippings.

Bagger: The bagger is a grass collection receptacle attached to a lawn mower, designed to assist with collecting and disposing of grass clippings. Baggers come in various sizes and can be made of cloth or plastic.

De-thatcher: When grass clippings, leaves and other debris begin to build up in the lawn, this is called “thatch”. A de-thatcher acts as a wire rake to loosen and remove this “thatch” allowing air, light and moisture to reach the soil beneath.

Trimmer: Often referred to as a “Weed Eater” or “Weed Whacker”, the trimmer is a device designed to trim grass and weeds that grow where the average lawn mower cannot reach. The spinning head at the bottom of the elongated shaft turns at a rapid speed. Trimmer heads can contain thin nylon line or can be outfitted with disposable nylon blades which cut the grass and weeds to the desired height.

Self-Propel: Self propel describes a machines ability to propel itself under the power of the engine. Often the self propel function requires the operator to engage a lever or handle to maintain the propulsion of the machine without having to move the machine through physical strength.

Zero Turn: Zero turn describes the turn radius of a zero-turn mower. The Zero Turn mower is operated by two individual hydro-static trans-axles that turn the rear wheels independently from each other. As one wheel turns while the opposite wheel remains motionless, the mower is effectively “spun” in one direction or the other.

Ethanol: A hydropilic alcohol which is added to gasoline to bond with water, allowing it to pass harmlessly through the combustion system. With smaller two and four cycle engines, there have been a multitude of detrimental effects of Ethanol. Many manufacturers and repair centers recommend avoiding Ethanol rich fuels.

Cylinder: The steel cylindrical area at the center of the engine that houses the piston. Combustion within the engine occurs in the cylinder.

Piston: The cylindrical part that slides to and fro through the cylinder. As gasoline ignites, the pressure of the combustion drives the piston away, drawing gasses through the cylinder and out of the exhaust.

Spindle: The spindle is found at the top of the mower deck and houses the components required to spin the lawn mower blade. Often, the spindle will have a pulley attached to the deck belt.

Starter: Machines with electronic ignition also have an electronic starter. The starter is a device attached to the side of the engine which engages the flywheel, starting the engine.

Ignition: The ignition switch (or starter switch) is the key and switch used to start the engine. When the key is turned, the circuit is completed, allowing electricity from the batter to engage the starter.

Air Filter: Designed to prevent particulates from entering your engine. A blocked air filter can result in catastrophic failure of your engine.

Fuel Filter: Prevents particles in the fuel or fuel tank from entering the cylinder.

Oil Filter: Prevents debris and contaminates in your engines oil from entering the cylinder.

Choke: Many two cycle engines offer a “choke” function. Engaging the “choke” temporarily closes the air supply to the carburetor, allowing only pure fuel to enter.

Valves: Valves control the flow of fuel vapor into the combustion chamber and the flow of exhaust gases leaving the engine. Valve design for the four-stroke small engine includes one intake valve and one exhaust valve per cylinder. In L-head engines, the valves are located to one side of the cylinder. The valve stems run through the cylinder block, parallel to the piston. In overhead valve engines, the valves are located in a cylinder head that is much larger than that found in an L-head engine. Overhead valves are pushed open by pivoting rocker arms, operated by push rods. The push rods, in turn, are pushed toward the rocker arms by the tappets. The slightly more complex design yields greater power.

– Bill Brown

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Small Engine Terminology
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Small Engine Terminology
Learn the correct words and phrases to use with talking about a small engine.
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About Bill Brown 12 Articles
Bill is the head of content creation for the LawnMowerPros Blog and DIY section. He’s been in the Outdoor Power Equipment Industry for years and he’s still learning new things everyday. You can often find him creating featured articles, DIY guides, videos, graphics and much more.