NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Small engine technology                 
 Copyright © 1994-2007, Samuel M. Goldwasser. All Rights Reserved. Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying. I may be contacted via the Sci.Electronics.Repair FAQ (www.repairfaq.org) Email Links Page.

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Internal Combustion Engines

Small engine technology

If you have some idea of how your automobile engine operates - or a Model T Ford for that matter - then you know the basic operating principles of your small engine as well. In fact, your Craftsman Eager 1 has a lot more in common with a Model T than a Honda Accord. However, strip off, the electronics, pollution control devices, and engine powered accessories, and the basic mechanical construction is very similar, though the lawn mower engine is not manufactured to quite the same tolerances and with the same quality materials as an automobile engine.

Nearly all small engines up to 20 HP or so are single cylinder affairs - one piston, one spark plug, no distributor, forced air cooled - about as simple and straightforward as it gets.

If you have never been under the hood of your automobile, then the description in the following sections may be of some help.

The next chapter: "Engine Diagrams" provides an explanation of each of the 4 strokes of a 4 stroke engine. However, if you cannot get the hang of my fabulous ASCII graphics, check out the following site:

The How Stuff Works Web site has some really nice introductory material (with graphics) on a variety of topics relating to technology in the modern world. Of relevance to this document is an article on "How Car Engines Work" which is really mostly about the basic principles but WITH some real animated graphics!

Types of engines

Unless otherwise noted, most of the descriptions and procedures in this document apply to both 4 stroke and 2 stroke engines. However, there are fundamental differences in the proper fuel and oil that is used with each type.

The small 4 stroke engine has a separate oil sump just like the engine in an automobile. Therefore, gasoline and oil are separate. Oil changes are also required.

WARNING: a new lawn mower or other piece of yard equipment will very likely be shipped without oil or just a minimal oil fill. Check it first and add oil if necessary. Running an engine without oil for a few minutes can cause serious - or terminal - damage. Even if your mower was assembled by the store where you bought it, don't assume they filled it with oil and tried it out!

The 2 stroke engine requires that the gas and special oil be mixed prior to use in specific proportions. Leave out the oil - or get your gas cans mixed up - and you will quickly ruin a 2 stroke engine due to lack of lubrication if plain gas is used by mistake. Clearly label the gas cans for each type and instruct anyone using them in the proper fueling technique.

Portable tools like chain saws, weed whackers, and backpack type leaf blowers use 2 stroke engines as these need to operate in a variety of positions.

Stationary or wheel-about equipment including most lawn mowers, rototillers, shredders, backup electric generators, and large blower/vacs, use 4 stroke engines.

Another distinction is that engines smaller than about 2 horsepower are generally 2 stroke while those larger than 2 horsepower are generally 4 stroke but there are exceptions. Lawnboy lawn mowers tend to have 2 stroke engines and there are some types of equipment with very small 4 stroke engines. Of course, if your engine has a cap marked 'oil' then it is a 4 stroke.

Larger pieces of yard equipment like riding mowers and lawn tractors use 4 stroke engines that are really very similar in most respects to their smaller cousins - much more so than to the engine in your automobile, for example. Similar servicing procedures apply. In fact, if you read the respective chapters in any of the engine repair books for engines (listed in the section: References) under 5 horsepower and those between 5 and about 20 horsepower, the only significant differences will be in the size of the various engine parts!

Parts of a 4 stroke engine

You may be surprised at the large number of individual parts which comprise the engine even on a $100 mower. The following description is for a typical single cylinder 4 stroke engine as would be found on most rotary mowers, rototillers, shredders, backup electric generators, larger snow throwers and leaf blowers, and even modest size riding mowers and lawn tractors:

Engine operating principles

These are internal combustion engines which means that the burning of the fuel-air mixture itself powers the engine. External combustion engines use the heat from combustion to expand or boil a working fluid as in a steam engine. Other examples of internal combustion engines are the rotary Wankel engine and gas turbines (jet engines).

The type of engine in your lawn mower or automobile operates on what is called the 'Otto' cycle (if you care). A complete 'cycle' is needed to supply one power impulse to the output shaft. All engines must provide the following regardless of whether they are 2 stroke or 4 stroke, rotary, or turbines (though turbines or jet engines operate in a continuous rather than pulsed manner):

For the following, refer to the section: The four strokes of a four stroke engine in living ASCII art.

  1. Intake stroke. Air is mixed with fuel (gasoline for piston engines) and drawn into the combustion engine.

    4 stroke: The air-fuel mixture is sucked into the cylinder through the open intake valve as the piston moves downward on the intake stroke.

  2. Compression stroke. The air-fuel mixture is squeezed into a smaller space. This heats it to some extent and prepares it to be burnt. (Note: in a diesel engine, this heating alone causes the mixture to ignite and there is no spark needed). Compression ratios for small engines are typically low compared to automobile engines.

    4 stroke: Both valves are closed. The piston moves upwards thus reducing the space above it and compressing the air-fuel mixture.

  3. Power stroke. The compressed air-fuel mixture is ignited at a precise time by the ignition system (spark).

    4 stroke: Both valves are closed. The heat produced by the rapidly burning gases to expand and drive the piston downward and because it is connected to the crankshaft, drives the load as well.

  4. Exhaust exhaust. The burnt combustion products are driven out of the cylinder. These consist of carbon dioxide, carbon monoxide, water vapor, oxides of nitrogen, some unburnt hydrocarbons, and numerous other mostly harmful compounds.

    4 stroke: The exhaust valve is open. The piston moves upward and pushes the exhaust gasses out through the muffler. A relatively massive flywheel attached to the crankshaft provides the inertia to allow the engine to coast through the non-power strokes (1-3). However, this is not always enough by itself - the blade on a rotary lawn mower is often required as well and a rotary mower may not start easily if at all without the blade in place and tightened securely.

    Note that the terms '2 cycle' or '4 cycle' are often used incorrectly when what is meant is 2 stroke or 4 stroke. The cycle is the entire sequence of events including intake, compression, power, and exhaust. The complete cycle for a 4 stroke engine is two complete revolutions of the crankshaft. The complete cycle for a 2 stroke engine is one rotation of the crankshaft. This means that a 2 stroke engine produces a power stroke on every rotation of the crankshaft while a 4 stroke engine does this only on every other one. Thus, a 2 stroke engine will be more powerful than a similar size 4 stroke engine. However, on the down side, 2 stroke engines tend to be less efficient in fuel utilization and pollute much more than 4 stroke engines.

    Bearings and bushings

    The shafts of rotating parts normally are mounted in such a way that friction is minimized - to the extent needed for the application. A bearing is any such joint with more specific terms used to describe the typical types found in lawn mowers - or small motors, automobile engines, or 100 MW turbines.

    The bearings to be concerned with in a lawn mower or small engine are:

    A variety of bearing types are available. For most inexpensive rotary lawn mowers, plain bearings are most popular due to their simplicity and low cost.

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