INTRODUCTION TO SINGLE-CYLINDER AIR-COOLED ENGINES

THE BASIC ENGINE

Small engines are classified as being either vertical or horizontal shaft types. This identification refers to the position of the crankshaft of the engine.

Most rotary lawmnowers use a vertical shaft engine.

The small engine is made up of several systems. The basic systems are as follows:

1. Fuel and carburetion

2. Ignition

3. Lubrication

4. Cooling

5. Exhaust

These systems all work together to convert air and fuel into mechanical energy. Each must serve its own function and be timed properly to the other systems as well as to the piston-crankshaft assemblies.

The FUEL and CARBURETION system provides the proper air-fuel mixture for the engine to operate under all conditions. Proper operation of this system is dependent upon a supply of clean, fresh fuel in the tank and proper adjustments at the carburetor.

The IGNITION system must provide the voltage necessary to jump the spark plug gap at exactly the right time. For this system to do its job, the spark plug, wires, and all other electrical components must be in good condition and properly adjusted.

The LUBRICATION system delivers oil to all moving parts inside the engine. The oil in the crankcase is either circulated by a pump or splashed through the engine. The lubricating oil seals the rings in the cylinder, removes some heat from the piston, crankshaft, and valve train, cushions the shock experienced by the bearings, cleans particles and dirt from the bearings and the cylinder, and reduces friction. The importance of keeping the engine properly filled with clean oil is apparent.

The COOLING system removes the excess heat from the engine. The flywheel causes air to circulate over the fins of the engine where the excess heat is removed. This system requires periodic service because accumulated dust and grass can cause the engine to overheat.

The EXHAUST system removes the burned gases from the engine and muffles the noise of combustion. On most units the exhaust system consists of a muffler attached to the engine block. A clogged or internally collapsed muffler can cause a loss of power and poor engine performance.

PRINCIPLES OF COMBUSTION

PRINCIPLES OF COMBUSTION

Combustion is defined most simply as burning. Combustion in the internal-eombustion engine refers to burning which takes place inside the engine.

The fuel stored in the fuel tank represents potential energy. This means that while in the tank little or no energy is released. If a drop of the fuel is placed in a pan and ignited, kinetic energy, or energy which lias been released, is formed. The burning fuel gives off heat and light.

Pure fuel (gasoline) will provide heat when burned. If, however, the fuel is mixed properly with air to form a gas, it will give off more heat or burn more efficiently.

If heat is applied to air in a confinement, the air expands. If a balloon is placed over a heat duct, the balloon becomes "larger" because of the "expansion" of the air within. Burning the mixture in the cylinder in like manner expands the gases in the cylinder.

In the internal-combustion engine the fuel is mixed with the air by the carburetor. Approximately four gallons of air are mixed with each tablespoon of gasoline. The mixture enters the engine as a mist and changes to a gas or vapor when it reaches the hot port area of the engine. The mixture enters the engine because the piston creates a partial vacuum (suction) as it moves downward on the intake stroke.

At the end of the intake stroke, the intake valve closes trapping the gases in the cylinder. As the piston moves toward TDC (top dead center), the trapped gas is compressed.

The compressed gas will burn more violently than if it were not compressed. The violent burning creates heat, which causes expansion of the gas in the cylinder. The expanding gas exerts much pressure, which causes the piston to move down in the cylinder.

The gas acting upon the piston is the point at which the chemical energy (burning gas) is converted into mechanical energy. It is the mechanical energy which is used to turn the crankshaft.

Combustion, then, in terms of a piston engine means that an air-fuel mixture is drawn into the cylinder, is changed to a gas by heat, is compressed by the piston, and is ignited to provide heat which expands the gases in the cylinder. Expansion of the gases in the cylinder pushes the piston which transmits the force to the connecting rod and crankshaft.

It is important to note that the air-fuel mixture does not explode in the engine. Instead, it burns rapidly. If the engine is timed improperly or if the air-fuel mixture is incorrect, the mixture may "explode" or burn too fast. Operating the engine in this condition will produce "knocking" and excess heat which can damage the piston.

ENGINE IDENTIFICATION

The identification of the engine is necessary for locating specifications and for purchasing replacement parts.

The engine is usually identified by manufacturer's identification on the engine. This identification usually includes the model information and in some cases the serial number and type.

The identification plate or information may be located on the engine block or on the flywheel shroud. In some cases the number will be stamped on the shroud and no identification tag will be used.

Copy this identification information for your engine below for future reference when referring to specifications or ordering parts.

Manufacturer____________ Model No____________

Serial No____________ Type____________

FOUR-STROKE CYCLE THEORY AND INTAKE STROKE

"Four-stroke cycle" means that it requires four strokes of the piston to complete one cycle.

Each time the piston moves from the top of the cylinder to the bottom of the cylinder it completes one stroke. Similarly, each time the piston moves from the bottom of the cylinder to the top of the cylinder it completes one stroke.

The term "cycle" means completion of four movements or strokes of the piston before it repeats a stroke.

To complete one stroke either from bottom dead center (BDC) to top dead center (TDC) or from TDC to BDC involves moving the crankshaft one-half revolution, or 180°. Since four strokes are required to complete one cycle, it can be seen that the crankshaft completes two revolutions or 720° of rotation during one cycle.

In one cycle the intake valve opens one time and the exhaust valve opens one time. This means that the camshaft, which causes the valves to open, completes but one revolution per cycle.

INTAKE STROKE

During the intake stroke the piston is moved from TDC to BDC. This causes a partial vacuum or suction inside the cylinder. Atmospheric pressure (outside air) rushes toward the partial vacuum in the cylinder. It moves through the air cleaner where dirt is filtered out. From the air cleaner it rushes through the carburetor where it "picks up" some gasoline. This air-fuel mixture then enters the cylinder through the open intake valve.

Near the end of the intake stroke the intake valve closes. When the valve is tightly closed, the air-fuel mixture is trapped in the cylinder. At this point the intake stroke is completed and the piston is at BDC (bottom dead center).

COMPRESSION STROKE

During the intake stroke an air-fuel mixture was "drawn" into the cylinder by the piston moving from TDC to BDC.

The second stroke in the cycle is the compression stroke. The piston is moved from BDC to TDC by the rotating crankshaft. Note that both valves remain closed during this stroke. This means that there is no way that the air-fuel mixture can escape—it is trapped in the cylinder. As the piston moves toward TDC, the air-fuel mixture is compressed. This compression takes place because as the piston moves toward TDC the volume of the cylinder decreases.

By compressing the air-fuel mixture, more pressure is created when it is burned.

POWER STROKE

The air-fuel mixture was compressed as the piston moved from BDC to TDC during the compression stroke.

Just before the piston reached TDC on the compression stroke, the ignition system created enough voltage to jump the spark plug gap. When the spark jumped the gap of the spark plug, the power stroke began.

The spark ignites the air-fuel mixture. As the mixture burns the gases expand. The expansion of the gases increases the pressure inside the cylinder. Since both valves are closed, there is no means whereby the pressure can leak out. The increasing pressure is exerted on all parts of the inside of the cylinder. The piston is the only part which can move. The pressure forces the piston to move toward BDC. As the piston moves, the force is transmitted to the crankshaft by the connecting rod.

The power stroke is the only stroke in the cycle which produces usable energy. The power stroke causes the crankshaft to revolve which makes the engine "run."

EXHAUST STROKE

EXHAUST STROKE

The power stroke moved the piston from TDC to BDC. Near BDC the expansion of gases stopped, which ended the power stroke.

Near BDC of the power stroke the camshaft and valve train cause the exhaust valve to open. As the piston moves from BDC toward TDC, the burned gases are forced out through the open exhaust valve and muffler. This is the exhaust stroke. Near the end of the exhaust stroke the exhaust valve closes and the intake valve opens. This action signals the beginning of another cycle.

The sequence of strokes-intake, compression, power, and exhaust-continues to repeat so long as the engine is running.

The important points of the four-stroke cycle operation are as follows:

1. Each time the piston moves from one end of the cylinder to the other end a stroke is completed.

2. Four strokes-intake, compression, power, and exhaust—are required to complete one cycle of a piston.

3. To complete one cycle the crankshaft must make two complete revolutions and the camshaft must make one revolution.

4. The engine receives power to the crankshaft only on the power stroke. The crankshaft "coasts" through the intake, compression, and exhaust strokes.

The fact that the engine crankshaft must "coast" through three strokes requires a flywheel. Once in motion, the flywheel tends to continue to turn. The heavier the flywheel, the greater its tendency to continue to turn after the power causing it to turn is eliminated.

The flywheel on the small single-cylinder engine is relatively heavy because there is only one piston delivering power to the crankshaft. On many rotary-type lawnmowers the blade of the mower also helps the flywheel to maintain rotation of the crankshaft. Many of these types of engines run roughly if the mower blade is not bolted to the crankshaft.

TWO-STROKE CYCLE THEORY

Two-stroke cycle means that two strokes of the piston complete one cycle. In other words, the piston must move from TDC to BDC and then back to TDC in order to complete a cycle. The two-stroke cycle engine completes a cycle each time the crankshaft completes one complete revolution.

Figure A in the chart shows the piston moving down from TDC. The air-fuel mixture has been compressed (or squeezed) above the piston, and the ignition system has created a spark at the spark plug. The spark has jumped the gap at the spark plug igniting the air-fuel mixture.

As the gases burn (A), they expand, thereby pushing the piston. As the piston moves down, the air-fuel mixture (B) (below the piston) in the crankcase is compressed.

At the end of the down stroke (Figure B) the compressed gases in the crankcase (D) flow up to the cylinder through the intake port. The exhaust gases are released from the cylinder at the end of the power stroke when the piston has passed the exhaust port (uncovering the port) which allows them to escape. The incoming intake gases help clean the remaining exhaust gases from the cylinder (C). Note that at BDC both ports are open.

When the piston begins the up stroke, it covers both the intake and exhaust ports, thereby sealing the cylinder and trapping the intake gases. As the piston continues to move upward, the intake gases are compressed.

Figure C shows that while the gases on top of the piston are being compressed (E), a partial vacuum is being created below the piston (F). The crankcase of the two-cycle engine is small so that this partial vacuum can be created. Air rushes in through the carburetor, picks up fuel, and enters the crankcase through the reed valve.

The reed valve is a one-way valve-it allows air-fuel mixtures to enter the crankcase but will not let them out once they have entered.

When the piston reaches TDC, ignition takes place and the cycle begins again.

The two-stroke cycle piston engine has activity on both the top and the bottom of the piston. Gases are compressed on both sides; however, those compressed in the crankcase do not burn. Instead, they move through the inlet port to the top side of the piston.

Most two-stroke cycle engines have no reservoir of oil in the crankcase. Lubricating oil is mixed with the fuel. When the air-fuel mixture enters the crankcase, the fuel and air form a vapor while the oil remains in droplets which cling to the components inside the crankcase. These droplets of oil maintain an oil film on the inside of the crankcase and all the moving parts. This film provides the necessary lubrication for the engine.

Adapted from:

SMALL GAS ENGINES

James A. Gray and Richard W. Barrow

School of Technology Indiana State Univetsitv

Prentice-Hall, Inc., Englewood Cliffs, New Jersey

© 1976 by Prentice-Hall, Inc. Englewood Cliffs, New Jersey

All rights reserved. No pari of this book may be reproduced in any form or by any means without permission in writing from the publisher.

Drawings by Robert F. MacFarlane