SECTION 2

Principles of operation

DESCRIPTION

This section provides general information on two cycle outboard motor engines. Included are discussions of the principles on which internal combustion engines operate and the special principles which apply to two cycles, spark-ignition engines of the type commonly used on outboard motors. The principles of carburetion, ignition, power transfer, coating and lubrication are briefly reviewed. Details of service and eftersyn procedures are found in other sections of this manual.

INTERNAL COMBUSTION ENGINES

Almost all outboard motors use an internal-combustion engine to provide the power that turns the propeller. An internal combustion engine is one in which the fuel is burned inside the engine; a charge of fuel is introduced into a firing chamber within the engine and ignited. By mechanical means, the heat energy released by the burning fuel is converted to torque developed in a shaft. The torque may then be transferred to wheels, gears, pulleys or propellers and used for a variety of purposes.

The device most commonly used in converting fuel energy into useful torque is an arrangement of cylinder, piston, connecting rod and crankshaft. The cylinder functions as a firing chamber. The top and sides of the cylinder are made strong enough to contain an explosion of the selected fuel. The bottom of the cylinder is closed by the piston which is free to slide up and down. The piston, in turn, is connected by a rod to the crankshaft which is rigidly retained in the same structure that holds the cylinder. The piston-crankshaft connection is offset from the axis of the crankshaft so that when the piston moves up and down the crankshaft is rotated. Ignition of the fuel charge is timed to occur when the piston is at the top of its rise or stroke (see Figures 2-1 and 2-2).

Internal-combustion engine fuel is compounded to burn very rapidly; once ignited, it literally explodes. The force generated by the exploding fuel is contained by the top and sides of the chamber and the piston is pushed downward. The downward motion turns the crankshaft. A flywheel, connected to the crankshaft, provides sufficient momentum to return the piston to the top once it has reached the bottom of its travel.

There are several different types of internal-combustion engines; all employ a similar principle; differences for the most part are in how the explosion in the firing chamber is induced. In diesel engines, for example, the power explosion results from compression alone - the fuel mixture is squeezed between the cylinder top and the piston until the heat generated by compression is sufficient to raise the temperature of the mixture to the ignition point. In "limited pressure" engines, a device which superheats the fuel-air mixture and raises it to ignition temperature is employed. Still another engine type uses two different fuels. A small quantity of easily exploded fuel is injected into the firing chamber at a critical point in the compression process. This pilot fuel acts much like a blasting cap; when it explodes it causes the remainder of the fuel-air mixture to ignite. In spark-ignition engines, the type commonly used in outboard motors, a regulated volume of a fuel and air mixture is introduced into the firing chamber, compressed and then ignited by an electrical spark. In all internal-combustion engines it is the explosion of the compressed fuel-air mixture that generates the motive power.

Each type of internal-combustion engine has certain advantages over other types. The spark-ignition type used in small marine engines is characterized by low first cost, light weight, easy starting, a relatively wide range of load and speed combinations, high mechanical efficiency, and reasonably low fuel consumption at top speeds and full throttle. These features make it especially useful for this application.

STROKES AND CYCLES

Regardless of how the ignition is induced, internal-combustion engines are classified as either four-cycle or two-cycle engines. This means that some engines employ four piston strokes to complete a power cycle and some only two. The "four" and the "two" refer to the number of piston strokes required to fill the cylinder with a fuel charge, compress, ignite and explode the charge, and exhaust the waste gas. A piston stroke is piston travel in one direction only - up, for example, is one stroke; down is another.

Piston travel is limited by the rods, bearings, and wrist pins that connect it to the crankshaft. The length of each piston stroke is determined by the distance which the piston rod connection is offset from the crankshaft center line. This distance is called crankshaft "throw." Total "stroke" is twice the "throw" dimension. Each combination of up-and-down (or in-and-out) piston strokes turns the crankshaft one full revolution. In a four-cycle engine, two crankshaft revolutions or four strokes are required to complete each power cycle. In a two-cycle engine the crankshaft is rotated only once per power cycle (see Figure 2-3).

While four-cycle engines are somewhat more efficient than two-cycle engines and are more easily adapted to a wide variety of load and speed requirements, two-cycle engines offer far more output of power per cubic inch of displacement and deliver twice as many power impulses per crankshaft revolution. Further, they are simpler mechanically and much less costly to build. These characteristics make them ideal for use on outboard motors.

THE TWO-CYCLE ENGINE

COMPRESSION RATIOS

Proper compression is important to the effective operation of any internal-combustion engine. The degree of compression is usually expressed as a compression ratio. The compression ratio is calculated as the total volume of the firing chamber (with the piston at the bottom of the stroke) divided by the clearance volume (the cylinder volume above the piston at the top of the stroke). Compression ratios in spark ignition engines range between 4:1 and 12:1 and are limited by combustion "knock." Combustion knock occurs when the compression ratio becomes too high and a portion of the fuel-air mixture is ignited by the compression heat alone before the engine reaches the top of the stroke. This premature firing or "knock" results in a noticeable loss of power.

The compression ratio, as calculated above, is the "nominal" ratio. The actual ratio in four-cycle engines is appreciably smaller than the nominal ratio because of delayed intake or exhaust valve closings. In two-cycle engines the compression ratio is established by dividing the volume of the firing chamber above the exhaust port opening by the clearance volume; thus the nominal and actual compression ratios are identical. Most two-cycle outboard engines are designed to have a nominal compression ratio somewhere between 4.5 to 1 and 6 to 1 (see Figure 2-10).

DISPLACEMENT

Since the horsepower output of an engine varies with the volume of the fuel-air mixture burned, engines are sometimes rated in terms of "displacement." Displacement is the volume measurement of piston travel. In four-cycle engines, it may be calculated by multiplying the area of the cylinder bore by the length of the piston stroke. On two cycle engines, which have induction and exhaust ports in the cylinder wall, displacement is computed by using the distance from the top of the highest port to the top dead center of piston travel. Displacement in any engine takes all cylinders into account. With the throttle wide open, a one-cylinder engine of 30 cubic inches displacement sucks in 30 cubic inches of air-fuel mixture with each complete crankshaft revolution; a two-cylinder engine with identical bore and stroke dimension would have a displacement of twice that amount, or 60 cubic inches.

POWER AND RPM

A given engine will develop zero horsepower at zero rpm. It will, however, begin to deliver power the instant it starts and will increase its output until its rated maximum has been obtained. This maximum will be reached within an rpm range established by the characteristics of motor design and construction (see Figure 2-11).

Each power explosion in an engine releases a certain amount of the energy stored in the fuel. As the power explosions increase in frequency more and more power is released in a given amount of time. A single cylinder, two-stroke (two-cycle) engine delivers one power impulse or stroke per each revolution. If its stroke is six inches and the engine is operating at 1000 rpm, the combined power stroke travel of the piston will total 500 feet in one minute. At 2000 rpm, the piston will travel 1000 feet each minute, proportionately increasing the power delivered. The development of power is simply a matter of increasing the number of power impulses per minute; the greater the rpm the greater the total power developed. At high speeds, however, breathing and internal resistance begin to affect engine performance.

Breathing is a term employed to describe the passage of fuel vapour through the engine - from induction or intake through compression and combustion to exhaust. At cranking speeds with the carburetor shutter full open, a single cylinder engine of 10 cubic inch displacement should draw or take in 10 cubic inches of fuel vapour with each upward stroke of the piston. The amount actually taken in is less than capacity, however, since fuel vapour inertia and friction must be overcome with each stroke. Inertia is the natural tendency of matter to remain motionless or in motion until energy is applied to move it or stop it. The passage of fuel vapour through the engine is interrupted by each power cycle and the vapour must be set in motion once again by engine action. Friction between the incoming fuel vapour and the walls of the intake manifold, the cylinder and the transfer channels also hinder the passage of the charge. Both vapour inertia and friction affect engine performance since both require time and energy to overcome. Breathing time, for example, is controlled by the rate at which the piston uncovers and covers the transfer parts. The time available for charging the crankcase and for the proper transfer of fuel vapour to the cylinder and combustion chamber varies with the rpm at which the engine is turning; it becomes less as the rpm increases. Exhaust action is similarly affected.

As long as the engine is able to breathe effectively, power will increase with increasing rpm. A point is eventually reached, however, when any further increase in rpm does not allow enough time to overcome fuel vapour inertia and friction and still effectively charge the cylinder or crankcase. The faster the engine is made to run, the less time is available to complete the events of the cycle-intake, compression, combustion (power) and exhaust. Consequently, as breathing becomes restricted, power begins to fall off (see Figures 2-12 thru 2-14).

To overcome this breathing difficulty, some engines employ blowers or superchargers to assist the flow of fuel vapour through the induction system. This improves breathing in the higher speed ranges and results in a greater power output. Although this method is not ordinarily applied to standard outboard engines, it is frequently used on racing equipment.

In addition to the breathing problem internal counter-resistance affects engine performance at high speeds. Power is required to start and stop the piston rod assembly at the end of each stroke (twice per revolution); power is used to shear the lubricating oil film; power is required to overcome resistance presented by the bearings, bushings, gears, etc.; power is also consumed in inducting, compressing and transferring the fuel vapour charge prior to ignition. It takes power to operate the magneto and to drive the generator or alternator. The faster the engine runs, the greater the effect of these internal counter forces becomes. Ultimately the amount of power available to drive the boat is sharply reduced. Up to a certain point - at the top of the rpm range or just beyond it - an engine develops power more than sufficient to overcome the effects of internal resistance. Beyond the critical point, however, the effects of counter- resistance and breathing difficulties increases rapidly until the engine is at least using all its power simply to drive itself.

LEAF VALVES

The system which controls the intake of the fuel-air mixture in a modern two- cycle outboard engine is simple and effective. It consists of a set of leaf valves - sometimes called reed valves - which serve the same purpose as the intake valves on a four-cycle engine. The leaf valves are mounted between the carburetor and the crankcase. The valves are thin, flexible metal strips which are made to tit smoothly over openings in a heavier intake plate or box. Differences in pressures on the two sides of the box or plate openings causes the leaf or reed to be forced away from the supporting surface or against it as the case may bee in a two-cycle engine, the leaf valve is mounted so that it opens into the crankcase and closes toward the carburetor.

When the piston in a two-cycle engine is on the upstroke, it creates a partial vacuum in the crankcase. Since the air-fuel mixture in the carburetor manifold is at atmospheric pressure, the leaf controlling the connecting passage between carburetor and crankcase is forced away from its opening and a charge of the fuel-air mixture is admitted to the crankcase. When the piston begins its down stroke, it compresses the crankcase charge to something greater than atmospheric pressure. This forces the leaf against the passage opening, closing it and sealing off the crankcase from the carburetor (see Figure 2-18).

Since the reed must be thin and flexible and since the opening and closing cycle may occur in excess of 4000 times per minute with the engine running at normal operating speed, a back-up plate is placed behind the leaf to limit its flexing. The back-up plate permits the leaf to open just far enough to be effective.

CARBURETORS

In a completely filled closed container or in a vacuum, gasoline will not burn; the presence of oxygen is required to make combustion possible. Gasoline, however, is a volatile substance; it evaporates quickly, rapidly mixing with air. Oxygen, which is needed to make the gasoline burn, is a basic component of normal air, comprising about 21% of the atmosphere. It is therefore in plentiful supply. When gasoline is combined with air to form a vapour, the mixture becomes highly inflammable. When ignited, it burns with almost explosive effect. For an internal combustion engine to develop all possible power, however, combustion chamber explosions must be sudden and complete; all fuel in the cylinder must be burned in the shortest possible time. Slow combustion caused by too lean a mixture (too much air) or incomplete combustion caused by too rich a mixture (too much gas) will severely reduce an engine's power output. To obtain the sharpest, strongest explosion, the fuel and air must be correctly proportioned and thoroughly mixed. It is the function of the carburetor to accomplish this.

The correct proportions of air to fuel range from 12:1 to 18:1 by weight, the lower ratio indicating the richer mixture, the higher ratio a leaner mixture. Mixtures of different proportions are required for different purposes. Idling calls for a relatively rich mixture; lean, maximum- economy mixture is desirable for normal load conditions; high speeds require a rich, maximum-power mixture. The carburetor on an internal- combustion engine is so designed as to accommodate these various service conditions and to deliver the correct proportion of fuel and air to the engine at all times.[ALT] + [ ← ] lukker det store billede  

Carburettors on small outboard engines are relatively simple devices. Essentially, the carburetor is a metering device. In most carburettors, a small chamber holds a limited quantity of fuel. A float valve in this chamber regulates the amount of fuel admitted to the carburetor from the fuel tank supply. Needle valves connected to the float chamber permit a precisely limited amount of fuel to flow to jets which open into the carburetor throat. The upstroke action of the pistons inside the engine creates a suction which draws air through this throat. At a particular point the throat air stream is restricted by what is called a Venturi.

The passage of the air stream through the Venturi has the effect of reducing the air pressure at that point and fuel is actually sucked into the air stream from the jet nozzles (see Figures 2-19 and 2-20).

As it is rushed along to the firing chamber, the fuel is swirled about and vaporized. A shutter or butterfly in the throat regulates the amount of air drawn through the carburetor. Opening or closing the shutter increases or reduces the effect of the Venturi action. As the shutter is opened and more air is drawn through the Venturi, pressure in the throat drops and more fuel is sucked into the air stream. When the shutter is closed less air passes the Venturi and air pressure in the throat rises, reducing the suction effect. In this way the fuel-air ratio is maintained at a relatively constant level. The shutter also serves to control engine speed (see Figures 2-21 and 2-22).

The effect of the power explosion in a gasoline engine is in direct proportion to the size of the charge introduced into the firing chamber. A small charge - one containing little fuel and air - produces a weak explosion; maximum power is obtained when an amount of fuel-air mixture equal to piston displacement is inducted into the cylinder, compressed and ignited. To vary the speeds of a gasoline engine, the size of the charge entering the firing chamber must be increased or decreased. Opening or closing the throttle shutter controls the amount of air drawn into the engine; since the amount of fuel taken in is in proportion to the amount of air, changing the position of the throttle shutter regulates engine speed.

A comparatively rich fuel mixture is required for starting a cold engine. To generate this richer mixture, a second shutter commonly called a choke, is built into the throat forward of the gasoline jet, to restrict the flow of air. When this shutter is closed, cylinder action suction causes proportionately more gasoline to flow into the air stream from the jet. This increase in fuel is required only for starting. Once the engine is started, engine temperature rises and the choke is gradually opened. When normal operating temperature is reached, the choke is opened fully and the standard ratio of gasoline and air is allowed to flow through the carburetor.

To obtain greater flexibility of engine speed, a second jet is inserted into the carburetor throat to provide more efficient action at slow and intermediate speeds. This jet is usually placed slightly forward of the throttle shutter plate and is arranged to function when the air velocity over the high speed jet is not sufficient to properly vaporize the gasoline. With the throttle shutter at near closed position, the comparatively high velocity of air over the slow speed jet causes sufficient gasoline to be vaporized for low speed performance. Vaporization at the slow speed jet gradually diminishes as the shutter plate is opened for maximum power and the velocity of air through the carburetor throat becomes great enough to vaporize the gasoline at the high speed jet.

Some outboard engines employ a "down-draft" carburetor. Down draft carburettors do not differ in principle from "side-draft" carburetor which has a horizontal draft throat. A down-draft carburetor is just what the name implies. The air stream is made to flow downward through a vertical throat passage. The vertical air flow is advantageous in certain applications; operational differences, however, are insignificant (see Figure 2-23).

IGNITION

FLYWHEEL MAGNETOS

The flywheel magneto is a self-contained unit, requiring no assistance from dry cells or storage batteries to generate the spark required for starting and operating the engine.

A two-cylinder flywheel magneto consists of (a) an armature plate, on which are mounted two ignition coil assemblies, two condensers and two sets of primary breaker points, (b) a permanent magnet cast into the rim of the engine flywheel and (c) a cam which rotates with the crankshaft and which activates the breaker points. One flywheel magnet and one crankshaft cam is used in a flywheel magneto system; a coil, a condenser and a set of points, wired together into an assembly, are provided for each cylinder (see Figure 2-25).

THE COIL

1. a laminated core of special alloy steel shaped to form opposing poles or "coil heels,"
2. a primary winding of heavy copper wire wound around the core and surrounded by a layer of insulation,
3. a secondary winding of light copper wire wound outside the primary winding,
4. an insulating jacket.

The primary winding, formed of a relatively few turns of heavy wire, is the inner winding of the coil. One end of the primary winding is grounded to the magneto frame and the other end is connected to the breaker plate contact. The function of the primary winding is to provide a brief choking action that opposes the efforts of the magnet to change the direction of magnetism in the laminated core as the south pole replaces the north pole opposite the center leg of the core. When the breaker points open, the primary circuit opens and the choking action ceases. The direction of magnetism in the core changes suddenly, and a very high voltage is induced in the secondary winding.

The secondary winding is formed of thousands of turns of fine wire, wound outside the primary winding. Heavy wire is not necessary because the secondary winding carries little current. The inside end of the secondary winding is grounded with the primary. The outside end is connected to the spark plug wire. The surge of high voltage current induced in the secondary winding by the action of the magnet, primary winding and breakers, is conducted to the spark plug by this wire.

CONDENSER

BREAKER POINTS

HOW THE MAGNETO WORKS

MAGNETO DISTRIBUTOR SYSTEM

SPARK TIMING

For an internal combustion engine to operate at maximum efficiency, all the fuel vapour charge should be burned while the piston is at the dead-center top of its stroke. Combustion of the fuel charge, however, does not occur all at once. Although it is measured in small fractions of a second, there is an appreciable lapse of time between ignition and total combustion. To compensate for this, the spark which begins combustion is timed to occur before the piston reaches the top of its rise. This spark advance distributes the combustion process so that maximum benefit is obtained from the power explosion (see Figure 2-34).

Spark advance is measured in degrees of crankshaft travel. Top dead center of the piston stroke is considered zero degrees. A spark advance of 35^o, for example, indicates that ignition occurs when the crankshaft is 35⁰ short of top dead center. The optimum degree of advance for any engine depends on the fuel-air mixture, combustion chamber design, location of the spark plugs and a number of other factors. Low speeds require relatively little spark advance. High speeds require more. Racing speeds still more. Since the volume of the fuel charge is constant and the speed at which ignition proceeds varies little, a greater degree of advance must be allowed for proper ignition as piston speeds increase with higher rpm's. In outboard engines, a linkage between the carburetor and the magneto automatically controls the degree of advance; as the fuel air mixture is enriched during acceleration, the degree of spark advance increases. When the throttle is closed and the engine is slowed, the spark is retarded.

SPARK PLUG

FUEL PUMPS, FUEL LINES, AND FUEL TANKS

A few very small outboard engines utilize a gravity feed system for fuel. In such systems, the fuel tank is an integral part of the engine. Most engines, however, utilize a non-pressurized system which includes a separate fuel tank, a fuel pump mounted on the engine and a single fuel line.

The standard fuel tank is not pressurized. Venting is accomplished by spring loaded valves. When the fuel line is uncoupled, the valves rest against the seats closing the vent and preventing fuel flow. When the fuel line coupling is attached, the two small steel plungers closed depressed; one forces the vent valve off its Beat to vent the tank, the second opens the fuel passage to the pump. A secondary vent valve, operated by fuel pump vacuum, prevents loss of vapour or fuel through the vent.

In the non-pressurized system, a pump assembly is attached to the intake manifold immediately below the carburetor. The pump is a diaphragm displacement type operated by crankcase impulses. A flexible tube leads from the diaphragm side of the pump to the lower crankcase chamber. Inside the pump are a spring loaded diaphragm and two spring loaded valves - one for intake and one for discharge. The alternate suction and compression generated in the crankcase by piston action causes the diaphragm to pulsate. On the upward stroke of the piston, the diaphragm flexes inward, creating a suction which causes liquid fuel to be drawn into the pump through the inlet valve. On the downward stroke of the piston, crankcase pressure causes the diaphragm to flex in the opposite direction. The inlet valve is forced against its seat to prevent fuel from returning to the tank; the outlet valve is simultaneously forced open and fuel flows directly into the carburetor float chamber (see Figures 2-36 thru 2-38).

A priming bulb, equipped with check valves, is incorporated into the fuel line. To prime the system it is necessary to compress the bulb several times until resistance is felt. The bulb end of the fuel line should be attached at the fuel tank end. An arrow indicates direction of fuel travel (see Figure 2-39).

Where permanently installed tanks of greater capacity closed to be employed, details of the installation should conform to the current fire protection standards- for motor craft issued by the National Fire Protection Association.

Permanent tanks should be fitted with a coarse screen at the outlet baffles and if a flat bottom tank is used, a sump or trough should be provided to trap fuel when the tank level is low, thus avoiding running dry in a rolling sea.

LOWER UNIT

1. the exhaust system,
2. the cooling system, and
3. the driveshaft, propeller shaft and shifting mechanisms.

EXHAUST SYSTEM

WATER PUMP

LUBRICATION

The internal moving parts in most outboard engines are lubricated by droplets of oil taken into the crankcase with the fuel. Lubricating oil is pre-mixed with the gasoline used and is drawn through the carburetor into the engine. Inside the crankcase enough of the oil settles on the cylinder walls, the crankshaft journals and piston rod bearing surfaces to provide adequate lubrication. Eventually, most of the oil is burned in the combustion chamber along with the gasoline.

In some engines an excess of the oil-gas mixture may accumulate in the crankcase at low speeds. On these engines, a very small orifice connects the crankcase to the atmosphere. This orifice is closed by a valve during the intake stroke of the piston. On the down stroke of the piston the valve opens permitting the puddled gas-oil mixture to be blown out through the orifice. An oil stick, caused by the escaping gas and oil may sometimes be observed behind the boat when the engine has been operated at low speeds for some time, as, for example, when trolling. The loss is very small and does not appreciably affect either the fuel consumption rate or the lubrication system.

Lubrication of other mechanical elements is accomplished by conventional means. Fittings and plugs are provided where access to gearcases or cavities are required. Lubrication charts have been provided at the end of Section 4 of this manual and additional information on lubrication requirements may be found in sections of the manual devoted to specific assemblies.