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Home , Combustion chamber, Crank (mechanism), Cylinder (engine), Exhaust system, Intake, Small engine

This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved.

CHAPTER5 Two-Cycle and Four-Cycle

After studying this chapter, you will be able to: Two strokes occur during each revolution of the M Figure 5-1. Sequence of events in a four- cycle , requiring two revolutions of the Describe four-stroke cycle engine operation crankshaft. Therefore, a four-stroke cycle requires and one power stroke out of four. and explain the purpose of each stroke. two revolutions of the crankshaft. Figure 5-1 illus- M Explain the concept of timing. trates each of the four strokes taking place in proper M Compare the lubrication system in a four-cycle sequence. As the moves upward, the air-fuel mix- engine to the system in a two-cycle engine. ture is compressed into a smaller space. This M Describe two-stroke cycle engine operation and stroke increases the of for two reasons: explain the principles of two-cycle operation. 1. When atoms that make up tiny molecules of M List the advantages and disadvantages of two- Figure 5-1A shows the piston traveling air and fuel are squeezed closer together, heat cycle and four-cycle engines. downward in the on the intake stroke. As energy is created. Each molecule of fuel is piston moves down, the volume of space above it heated very close to its flash point (point at Small Engine Identification is increased. This creates a partial vacuum that which fuel will ignite spontaneously). When A basic design feature that aids in small draws the air-fuel mixture through the intake valve combustion does occur, it is practically engine identification is the number of piston port and into the cylinder. instantaneous and complete for the entire air- strokes required to complete one operating With the intake valve open during the intake fuel mixture. (power) cycle. A four-stroke cycle engine, for stroke, atmospheric outside the engine 2. The force of combustion is increased because air through the . This gives a large Figure 5-2. The shape of the valve smoothes the flow of tightly packed molecules are highly activated example, requires four strokes per cycle; a two- gases around it. Note how the flow follows the fillet, speed- boost to the air-fuel induction process. With nature stroke cycle engine requires two. ing entry or expulsion. (Cedar Rapids Engineering Co.) and are striving to move apart. This energy, A stroke of the piston is its movement in the balancing unequal in this manner, it fol- combined with expanding energy of combus- cylinder from one end of its travel to the other. Each lows that the larger the diameter of the cylinder tion, provides tremendous force against the stroke of the piston, then, is either toward the rotat- and the longer the stroke of the piston, the greater The intake are not subjected to as high piston. ing crankshaft or away from it. Each stroke is iden- the volume of air entering the cylinder on the as the exhaust valve. The incoming tified by the job it performs (intake, exhaust, etc.). intake stroke. air-fuel mixture tends to cool the intake valve dur- It is possible to run an engine on Bear in mind that the intake valve, Figure 5-2, ing operation. uncompressed mixtures, but power loss Four-Stroke Cycle Engine performs several key functions. These key functions produces a very inefficient engine. are as follows: Compression stroke In a four-stroke cycle engine (called a four- 1. It must open at the correct instant to permit cycle), four strokes are needed to complete the intake of air-fuel mixture. The compression stroke is created by the Power stroke operating cycle. The four strokes are as follows: 2. It must close at the correct time and seal dur- piston moving upward in the cylinder. See • intake stroke ing compression. Figure 5-1B. Compression is a squeezing action During the power stroke, both valves remain • compression stroke 3. Its shape must be streamlined, so the flow of while both valves are closed. On this stroke, the in the closed position. See Figure 5-1C. As the • power stroke gases into will not be valves are tightly sealed and the piston rings piston compresses the charge and reaches the top • exhaust stroke obstructed. prevent leakage past the piston. of the cylinder, an electrical spark jumps the gap

81 82 Small Gas Engines between the electrodes of the . This with different engines. However, if the timing engine oil in the . Several methods are ignites the air-fuel mixture, and the force of the marks on the crankshaft and , used to feed the oil to the correct locations. The explosion (violent burning action) forces the pis- sprockets, or pulleys are aligned, the two most common methods are the splash system ton downward. will take care of itself. and the system. Some engines employ one Actually, the full charge does not burn at or the other; others use a combination of both. once. The flame progresses outward from the Engineers also specify the point at The multiple vee cylinder engine shown in spark plug, spreading combustion and providing which the spark must occur. Chapter 9 Figure 5-6 utilizes a combination splash and pres- even pressure over the piston face throughout the of this text explores this in more detail. sure lubrication system. The pump picks up the oil power stroke. from the crankcase and circulates some oil The entire fuel charge must ignite and expand Figure 5-5 shows one complete operating through the filter and directly back to the in an incredibly short period of time. Most engines cycle of a four-cycle engine. Beginning at point A, crankcase. This keeps a clean supply available. ° have the spark timed to ignite the fuel slightly the intake valve opens 10 before TDC and stays Oil is also pumped through a spray nozzle ° ° before the piston reaches top dead center (TDC) open through 235 . The exhaust valve closes 30 aimed at the crankshaft. As the shaft rotates, it of the compression stroke. This provides a little after TDC. Valve overlap occurs when both valves deflects the oil toward other moving parts. In addi- more time for the mixture to burn and accumulate Figurerange from5-3. 1200°FExhaust (649°C) valve temperatureto 2500°F (1371°C) may due are open at the same time. tion, the splash finger on the bearing cap dips into its expanding force. to the hot gases surrounding it. The pressure of com- During the compression stroke, the intake the crankcase oil and splashes it on various inter- ° Basically, the amount of power produced by bustion may valve closes and ignition occurs 30 before TDC. nal surfaces. inch. (Briggs be& Strattonas high Corp.)as 500 pounds per square ° the power stroke depends on the volume of the air- The power stroke continues through 120 past Part of the engine oil is pumped through a ° fuel mixture in the cylinder and the compression TDC. The exhaust valve opens 60 before BDC tube to lubricate the assembly above the ° ° ratio of the engine. The is the Valve timing and stays open through 270 . During the last 40 , engine. Oil holes are provided in the connecting proportionate difference in volume of cylinder and the intake valve is also open and the second cycle rod for lubricating the bearings and piston pin. combustion chamber at bottom dead center and at Valve timing is measured in degrees of - has begun. Obviously, the oil in a four-cycle engine must top dead center. If the compression ratio is too shaft rotation. The point at which the valves open be drained periodically and replaced with clean high, the fuel may be heated to its flash point dur- or close before or after the piston is at top dead Lubrication oil. Also worth noting, four-cycle engines must be ing the compression stroke and ignite too early. center (TDC) or bottom dead center (BDC) varies operated in an upright position or the oil will flow Lubrication of the four-cycle engine is pro- away from the pump or splash finger, preventing Exhaust stroke vided by placing the correct quantity and grade of lubrication. After the piston has completed the power stroke, the burned gases must be removed from the Two-Stroke Cycle Engine cylinder before introducing a fresh charge. This takes place during the exhaust stroke. The exhaust The two-stroke cycle engine (commonly valve opens and the rising piston pushes the called two-cycle) performs the same cycle of exhaust gases from the cylinder. See Figure 5-1D. events as the four-cycle engine. The main differ- The exhaust valve has to function much like the ence is that intake, compression, power, and intake valve. When closed, the valve must seal. exhaust functions take place during only two When open, it must allow a streamlined flow of strokes of the piston. The two strokes occur during exhaust gases out through the port. See Figure 5-2. each revolution of the crankshaft. Therefore, it The removal of gases from the cylinder is called takes only one revolution of the shaft to complete . a two-stroke cycle. The passageway that carries away exhaust A two-cycle engine has several advantages gases is referred to as the or over a four-cycle unit. It is much simpler in design exhaust port. Like the intake manifold, the exhaust than the four-cycle engine because the conven- manifold must be designed for smooth flow of tional camshaft, valves, and are unneces- gases. sary. See Figure 5-7. The heat absorbed by the exhaust valve must Additionally, a two-cycle engine is smaller be controlled or the valve will deteriorate rapidly. and lighter than a four-cycle engine of equivalent Figureexact number 5-5. The of degrees four-stroke each cycle valve diagram is open shows or closed the See Figure 5-3. Some valve heat is carried away . Unlike the four-stroke cycle engine, Figureincredibly 5-4. shortThe period exhaust (1/50 valve sec. must at 3600 cool rpm). during Heat an is exhaustingand the time gases spark to ignition create occurs.a partial Note vacuum that bothin the valves cylin- by conduction through the valve stem to the guide. conducted from the valve through the seat to the cylin- derare andopen help (overlap) draw a through mixture an of arcfuel ofinto 40 the cylinder. the two-cycle engine will get adequate lubrication However, the hottest part of the valve, the valve der block. Some heat travels down the stem and to the °, permitting even when operated at extreme angles. It receives head, transfers heat through the to the valve guide. its lubrication as fuel mixed with oil is passed cylinder block. See Figure 5-4. through the engine.

Chapter 5 Two-Cycle and Four-Cycle Engines 83 84 Small Gas Engines Installing the correct mixture of fuel and oil is fuel in proper proportion before being placed in running. The oil eventually burns in the combustion from going straight out the exhaust port, which is a critical factor in maintaining a two-cycle engine the . chamber and is exhausted with other gases. located directly across from the intake port. in good working condition. The prescribed type In this way, there is clean oil continuously Two-cycle engines are popular in lawn mow- Cross-scavenged engines usually employ and grade of engine oil must be mixed with the supplied to all moving parts while the engine is ers, snowmobiles, chain saws, string trimmers, reed valves or a , which is attached to leaf blowers, and other high-rpm applications. the . See Figure 5-8B. These valves hold the incoming charge in the crankcase so it can be Governor Oil filler compressed while the piston moves downward in Variations in design the cylinder. With this design, the piston acts as a valve in opening and closing intake, exhaust, and Two basic types of two-cycle engines are in transfer ports. The transfer port permits passage of general use. They are the cross-scavenged and the fuel from the crankcase to the cylinder. loop-scavenged designs. See Figure 5-8. The loop-scavenged engine does not have to The cross-scavenged engine has a special deflect the incoming gases, so it has a relatively contour on the piston head, which acts as a baffle flat or slightly domed piston, as shown in to deflect the air-fuel charge upward in the cylin- Figure 5-8C. The fuel transfer ports in loop- der. See Figure 5-8A. This prevents the charge scavenged engines are shaped and located so that

Oil holes Spray nozzle

Dipstick

Splash Oil finger pump

Figuresplash 5-6.systemTwo and common the pressurized methods system. of supplying The engine lubrication shown in employsfour-cycle both engines methods. are the The - splash finger churns oil into a mist that makes its way into oil holes and other parts. The pump directs oil to remote parts and sprays some on critical parts. (Wisconsin Motors Corp.) -

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Figurehave a 5-8.contouredBasically, baffle two-cycle on top of enginesthe piston are to either direct cross-scavenged the air-fuel mixture or loop-scavenged.upward into the cylinder Cross-scavenged while exhaust engines gases are being expelled. Loop-scavenged engines have flat or domed with more than one transfer port. Figureences are5-7. listedThe in number the chart of atmoving the end parts of thisin a chapter.four-cycle (Lawn-Boy engine is Power greater Equipment, than in a two-cycle Gale Products) engine. Other differ- Note the three styles of crankcase intake valves. (Kohler Co.)

Chapter 5 Two-Cycle and Four-Cycle Engines 85 86 Small Gas Engines the incoming air-fuel mixture swirls. This con- is timed to ignite the air-fuel mixture when the pis- trolled flow of gas helps force exhaust gases out and ton reaches TDC. See Figure 5-10B. permits a new charge of air and fuel to enter. On some small engines, spark occurs almost at TDC during starting, then automatically advances so Principles of operation that it occurs earlier. This is done to get better effi- ciency from the force of combustion at higher speeds. The location of the ports in a two-cycle Peak combustion pressure is applied against engine is essential to correct timing of the intake, the piston top immediately after TDC. Driving transfer, and exhaust functions. The cutaway downward with maximum force, the piston trans- cylinder in Figure 5-9A shows the exhaust port at mits straight line motion through the connecting the highest point, the transfer port next, and the rod to create rotary motion of the crankshaft. See intake port at the lowest point. Some engines, par- Figure 5-10C. ticularly loop-scavenged engines, have more than one transfer port. See Figure 5-9B. Exhaust Intake into crankcase Several things happen during the exhaust As the piston moves upward in the cylinder of phase. See Figure 5-10C. As the piston moves to a two-cycle engine, crankcase pressure drops and expose the exhaust port, most of the burned gases the intake port is exposed. Because atmospheric are expelled. Complete exhausting of gases from pressure is greater than the crankcase pressure, air the cylinder and combustion chamber takes place rushes through the carburetor and into the crankcase when the transfer ports are opened and the new to equalize the pressures. See Figure 5-10A. air-fuel charge rushes in. While passing through the carburetor, the intake air pulls a charge of fuel and oil along with Fuel transfer it. This charge remains in the crankcase to lubri- cate ball and needle bearings until the piston opens Figure 5-10C and Figure 5-10D show the the transfer port on the downstroke. piston moving downward, compressing the air- fuel charge in the crankcase. When the piston trav- Ignition-power els far enough on the downstroke, the transfer port is opened and the compressed air-fuel charge rush- As the piston travels upward, it also com- es through the port and into the cylinder. The new presses the air-fuel charge brought into the cylin- charge cools the combustion area and pushes engine. Compression and intake occur simultaneously, then ignition occurs. Exhaust precedes der during the previous cycle to about one-tenth of (scavenges) the exhaust gases out of the cylinder. Figure 5-10. These illustrations show the sequence of events that take place in a two-cycle its original volume. See Figure 5-10A. The spark This completes one cycle of operation. the transfer of fuel during the lower portion of the power stroke. The piston functions as the only valve in the engine. (Rupp Industries, Inc.)

Scavenging and tuning device, which amplifies the sound to speed up scav- enging. The sound waves are reflected back into the When properly designed, the megaphone to develop back pressure, which pre- scavenges all exhaust gases from the combustion vents the incoming air-fuel mixture from leaving chamber. The system allows the new fuel charge with the exhaust gases. Compare this device with to move in more rapidly for cleaner and more straight operation shown in Figure 5-12. complete combustion. For best efficiency, the fuel charge should be Rotary disc valve engine held in the cylinder momentarily while the exhaust port is open. This helps prevent fuel from being Figure 5-13 illustrates a two-cycle engine drawn out of the cylinder with exhaust gases. equipped with a rotary disc valve. The intake port is located directly in the crankcase, allowing room Figureports of 5-9. a loop-scavengedA cutaway cylinder engine. block A—Due shows to thethe cutaway,location ofonly the one intake, of two exhaust, transfer and ports transfer Some well-engineered exhaust systems use the is shown. B—The section is revolved 90 energy of sound waves from the exhaust gases for for additional transfer ports that promote better ° to show both ports. proper tuning. Figure 5-11 shows a megaphone-like fuel transfer and scavenging.

88 Small Gas Engines Chapter 5 Two-Cycle and Four-Cycle Engines 87 Figureengines. 5-11. ExhaustPressure sound wavespulse exhaust reflected tuning back isinto an the effective manifold way create of increasing a back pressure power and that efficiency stops the in fuel two-cycle mixture from leaving the cylinder before the piston closes the port. This system requires precise engineering. (Kohler Co.)

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FigureIn this 5-12.illustration,A straight center pipe of sound may sound is too louderfar and and lacks more amplification powerful than to havetuned any exhaust, beneficial but actually effect on is farengine. less efficient.

Reed valve engine In operation, the reed is opened by atmos- pheric pressure during the intake stroke. It is The reed valve engine permits fuel intake closed by the springiness of the and the directly into the crankcase. See Figure 5-14. The compression in the crankcase on the power stroke. reed is made of thin, flexible spring , which is Figure 5-16A illustrates the air-fuel mixture These diagrams show how a rotary valve operates in a two-cycle engine. Since the rotary disc is part of the crankshaft, fastened at one end. See Figure 5-15. The oppo- entering the crankcase. Figure 5-16B shows how site end covers the intake port. The reed stop is the reed valve is closed by crankcase pressure. thick and inflexible. It prevents the reed from There are many reed valve designs. Some typ-

opening too far and becoming permanently bent. ical configurations are illustrated in Figure 5-17. the port is open only when the hole in the disc and the crankcase port are in alignment. (Evinrude Motors) Figure 5-13.

Chapter 5 Two-Cycle and Four-Cycle Engines 89 90 Small Gas Engines AB

Figureatmospheric 5-16. pressureReed valve to force action. air-fuel A—Vacuum mixture inthrough the crankcase, the port opening.formed by B—Downward the upward movingpiston movement piston, causes com- presses the fuel mixture in the crankcase to a pressure greater than . The springiness of the reed and crankcase pressure act together to close the port. AB C Figureupon the 5-14. differenceA popular between method atmospheric of crankcase pressure valving and is crankcasea reed valve pressure designed to tobe fit opened. into the The crankcase default wall.position It relies of the reed valve is the closed position.

the engine is intended. It cannot be said that one type of engine is better than another without con- sidering every aspect of its application. The chart in Figure 5-18 lists the differences between two- and four-cycle engines.

Summary

The stroke of a piston is its movement in the cylinder from one end of its travel to another. Four-stroke cycle engines need four strokes to complete the operating cycle: intake, compression, power, and exhaust. Lubrication of four-cycle engines is generally provided by a splash system or a pump system. In a two-stroke cycle engine, the intake, com- Figurespring steel5-15. reedThis covering side view the of entry a reed hole. valve The showsreed stop the pression, power, and exhaust functions take place Figurethe reed 5-17. openingSeveral distance forms is of controlled reed valves. by the A—Single stop. C—Twin reed, closed reed. D—Tripleposition. B—Single reed. E—Another reed, open form position. of triple Note reed. how controls the distance the reed may open. This prevents during two strokes of the piston. Two-cycle F—Single reed. G—Multiple reed. H—Four reed. I—Multiple reed. permanent distortion and failure of the reed to return engines have many advantages over four-cycle snugly against the port during crankcase compression. units. They do not have conventional valves, - pets, or a camshaft, so they are simpler in design. Two-cycle engines are also smaller and lighter Four-Cycle Engine vs. than four-cycle engines of equivalent horsepower. Two-Cycle Engine The two-cycle engine receives its lubrication as a fuel-oil mixture is passed through the engine. The advantages and disadvantages of any Therefore, it will receive adequate lubrication engine are directly related to the purpose for which even when operated at extreme angles.

Chapter 5 Two-Cycle and Four-Cycle Engines 91 92 Small Gas Engines Characteristics Four-Cycle Engine Two-Cycle Engine 11. The baffle on a contoured piston is for (equal hp) (equal hp) Know These ______. One Cylinder One Cylinder a. creating turbulent flow of gases Te r m s b. slowing the air-fuel mixture entering the 1. Number of major combustion chamber moving parts Nine Three stroke bottom dead center c. directing the flow of air-fuel mixture four-stroke cycle valve overlap upward in the cylinder 2. Power strokes One every two revolutions One every revolution intake stroke two-stroke engine d. directing oil evenly to the cylinder walls of crankshaft of crankshaft compression stroke cross-scavenged 12. The ______type of two- power stroke loop-scavenged cycle engine requires a contoured piston. 3. Running Cooler running Hotter running top dead center rotary disc valve 13. In a properly system, ______exhaust stroke reed valve engine ______prevent the air-fuel mixture from 4. Overall engine size Larger Smaller scavenging reed stop leaving with the exhaust. valve timing 14. What advantage is there in having the intake 5. Engine weight Heavier construction Lighter in weight port lead directly into the crankcase? 15. Time during the four-stroke cycle when both 6. size equal hp Larger Smaller valves are open is called ______. Chapter 5 16. A four-cycle engine accelerates slower than a two-cycle engine, because ______. 7. Fuel and oil No mixture required Must be premixed Review Questions a. there is only one power stroke in four b. the flywheel is heavier to carry the engine 8. Fuel consumption Fewer gallons per hour More gallons per hour Answer the following questions on a separate through three nonpower strokes sheet of paper. c. there are more moving parts to be driven 9. Oil consumption Oil recirculates and Oil is burned with fuel 1. Name the four strokes of a four-cycle engine by the engine stays in engine in proper order. d. All of the above. 2. Name three important intake valve functions. 10. Sound Generally quiet Louder in operation 3. Explain why a four-cycle engine runs cooler than a two-cycle engine. 11. Operation Smoother More erratic 4. Why is there a difference in temperature Suggested between the intake and exhaust valves? Activities 12. Acceleration Slower Very quick 5. The exhaust valve is cooled mainly by ______. 1. Look up additional information about internal 13. General maintenance Greater Less a. radiation c. convection b. conduction d. air-fuel circulation combustion engine development. Names to look up: Christian Huygens, Philip Lebon, 14. Initial cost Greater Less 6. How does compression increase engine power? 7. The compression ratio must be limited in gaso- Samuel Brown, William Barnett, Pierre Lenoir, Beau DeRochas, Dr. N. A. Otto, 15. Versatility of Limited slope operation Lubrication not affected line spark ignition engines, because ______. a. there is no power advantage after com- Atkinson, Gottlieb Daimler, Priestman and operation (Receives less lubrication at any angle of operation Hall, Herbert Akroyd Stuart, Rudolph Diesel. when tilted) pressing the fuel to a certain point b. the engine becomes too difficult to start 2. Begin a collection of engine repair and ser- vice manuals. 16. General operating Less efficient More efficient c. mechanically it is not possible to increase efficiency (hp/wt. the compression ratio 3. Using a worn out engine, cut away portions ratio) d. the heat of compression will ignite the air- that will make the working parts visible while fuel mixture too soon still enabling them to move. Report on the 17. Pull starting Two crankshaft rotations One revolution produces 8. What are the two methods employed for operation and timing of each part. After required to produce one an ignition phase lubricating four-cycle engines? further study, replace the spark plug of the ignition phase 9. What are the two types of scavenging systems cutaway engine with a small lightbulb used in two-cycle engines? switched on and off by the ignition switching 18. Flywheel Requires heavier flywheel Lighter flywheel 10. Why can two-cycle engines be run in any system to simulate ignition. to carry engine through position? 4. Make a bulletin board display that illustrates three nonpower strokes the principles of two- and four-cycle engines.

Figure 5-18. This chart lists the differences between two-stroke and four-stroke cycle engines.

94 Small Gas Engines Chapter 5 Two-Cycle and Four-Cycle Engines 93