外文翻译:Piston Engine OperationCustomer’s ConcernInternal combustion engines come in a variety of sizes and power ratings . They supply the mechanical energy needed to put compact , mid-size , and full-size vehicles in motion . Four-cylinder engines power smaller vehicles and provide better fuel economy than larger engines . More powerful six-and eight-cylinder engines are used to power most medium-and full-size vehicles . Ten-and twelve-cylinder engines provide high-performance power for sport models and workhorse power for heavy-duty pickup trucks .Engine construction may differ by manufacturer ,but all piston engines operate on the same principles . A good understanding of these principles will be important in your career as an automotive technician .Technician’s ChallengeAs an automotive technician , you need to find answers to these questions:1 . How does an internal combustion engine produce power ? How does it convert heat into motion ? How is fuel burned inside the engine ?2 . What are the basics of engine construction ？What parts make up an engine ? What is a cylinder , piston , crankshaft , and camshaft ?3 . What is the purpose of the air induction , fuel , ignition , lubricating ,cooling , and exhaust systems ? How do they work together ?Objectives:●Describe the function of the pistons .●Describe the purpose of the connecting rods and crankshaft .●Diagnose abnormal exhaust color , odor , and sound ；determine necessaryaction .Types of EnginesAutomotive engines are internal combustion engines . See Fig. 1-1 . An engine is a machine that turns heat energy into mechanical energy . An internal combustion engine burns fuel internally . The heat produced from burning a fuel creates the power that moves the vehicle .Most automotive engines are called reciprocating engines because their pistons move up and down inside the cylinders , See Fig. 1-2. A piston is a cylindrical plugthat fits inside the cylinder , It receives and transmits motion as a result of pressure changes applied to it .There are two types of internal combustion piston engines : spark-ignition (gasoline) and compression ignition (diesel) .Internal combustion piston engines differ in :●The type of fuel they use .●The way ignition of the air/fuel mixture occurs .Spark-Ignition EngineMost spark-ignition engines run on liquid fuels , such as gasoline , alcohol , or a gasoline/alcohol blend . Some spark-ignition engines run on gaseous fuels , such as propane or natural gas .Air and fuel enter the engine cylinders to create a combustible mixture . The pistons compress (squeeze) the mixture to about one-eighth of its original volume . The ignition system produces a spark at the spark plug, igniting the compressed mixture . As the mixture burns, temperature and pressure increase in the cylinder , The high pressure forces the piston down in the cylinder . This causes the crankshaft to rotate . Gears and shafts carry this motion to the wheels that drive the vehicle . Compression-ignition EngineA diesel ( compression ignition ) engine runs on a light fuel oil similar to kerosene . In this type of engine , the piston compresses only air . Compressing air to about one-twentieth of its original volume raises its temperature to 1,000°F （538°C）or higher . The fuel is injected ( sprayed ) into the cylinder , where it is ignited by the heated air . As the mixture burns , the pressure forces the piston down in the cylinder .Engine ConstructionSpark and compression-ignition engines are similar in construction . Both have engine blocks and cylinder heads . Both have pistons that move up and down in the cylinders . The cylinders , or cylinder bores , are machined openings through the engine block . A cylinder head covers the top of the cylinders . The bottom of each cylinder is open . The pistons are connected through this opening to the crankshaft .The two travel limits for a piston are defined as top dead center ( TDC ) and bottom dead center ( BDC ) . A piston stroke takes place when the piston moves from TDC to BDC or from BDC to TDC .see fig 1-3The Engine BlockThe engine block , also called the cylinder block , is a precision metal casting . See Fig.1-4.The block contains the :●Cylinders , or cylinder bores .●Pistons and connecting rod assemblies .●Camshaft , for engines that do not have an overhead camshaft design.●Crankshaft.Figure 1-5 shows the events that take place in the cylinder of a spark-ignition engine . The piston has completed its intake stroke . It is at its lower limit of travel , bottom dead center . See Fig. 1-5(a). The space above the piston contains the air/fuel mixture .Next , the piston moves up the cylinder toward top dead center . See Fig.1-5(b). This motion compresses the mixture . As the piston nears top dead center , an electric spark ignites the mixture . The mixture burns rapidly . This creates heat and high pressure that push the piston down in the cylinder . See Fig. 1-5( c ).This downward movement creates power . At the bottom of the power stroke , the piston begins the exhaust stroke and moves up in the cylinder .The exhaust valves open , and the burned gases are pushed from the cylinder . See Fig. 1-5( d ).Piston and Piston Rings Figure 1-6 shows a piston and piston rings . Pistons are usually made of an aluminum alloy , which is aluminum mixed with other metals . They are slightly smaller than the cylinders so that they can move up and down freely .The small gap between the piston and cylinder wall is known as piston clearance . See Fig. 1-7 . Piston clearance provides the sliding fit . If not properly sealed , this gap allows some of the compressed air/fuel mixture and combustion gases to leak past the piston . This leakage is called blowby . Blowby reduces power , wastes fuel , and pollutes the air . The piston rings seal the gap between the piston and the cylinder wall . Each ring fits into ring grooves cut into the piston . There are two types of piston rings:●Compression rings form a sliding seal between the piston and the cylinder wall .They reduce or control blowby of combustion gases .●Oil rings , or oil-control rings , scrape excess oil from the cylinder wall and returnit to the crankcase .Crankshaft The reciprocating motion of the pistons must be changed to rotary motion . Rotary motion is what turns the vehicle’s drive wheels . The connecting rodsand the crankshaft make this conversion possible . A piston pin connecting rod each piston to the small end of the connecting rod. The connecting rod connects the piston to the crankshaft. See Fig. 1-8.The rod cap and rod bolts attach the connecting rod to the connecting rod journal. The journal holds a split bearing ( two halves ) , or connecting rod bearing , in place in the cap and rod . See Fig. 1-9. A slight clearance allows the connecting rod journal to turn inside the bearing . Oil fills this clearance to lubricate the bearing and prevent metal-to-metal contact. As the crankshaft turns , the connecting rod journal moves in a circle .As the piston moves up and down in the cylinder its connecting rod journal moves in a circle around the centerline of the crankshaft. On the down stroke the centerline of the crankshaft. On the down stroke the connecting rod moves to one side, as its lower end follows the movement of the crankshaft rod journal. As the piston reaches BDC, the connecting rod journal continues to move up the connecting rod pushes the piston up on the next stroke. In this way, the crankshaft changes the reciprocating motion of the piston to rotary motion at the drivetrain .The Cylinder HeadFigure 1-10 shows a cylinder head . The cylinder head is bolted to the top of the engine block . The cylinder head contains the :●Intake valves ,exhaust valves , and connecting parts .●Camshaft for engines with overhead camshaft design●Combustion chamber ( the upper portion of the cylinder located in the head ) .Hot Gases Are Really CoolA diesel engine would have no power without hot air to ignite the fuel within its cylinders . When a diesel engine piston moves up on the compression stroke , it compresses the air above it , This causes the air pressure and temperature increases so much that a light spray of fuel ignites as soon as it mixes with the highly compressed air in the top of the cylinder !Increasing the pressure for a gas ( air ) trapped in a container increases the temperature . Increasing the temperature increases the pressure . Gas pressure also decreases with decreasing temperature .Safety First: Personal Protection Use eye protection . Wear gloves and clothing with long sleeves .Caution Do not heat an empty bottle in the microwave . Always put some waterinside the bottle . Never heat the bottle with the top sealed .Exploring Temperature and PressureMeets NATEF Science Standards for understanding the relationship between pressure and temperature and the effect of how adding heat causes vaporization .Here’s a simple experiment that you may want to try at home .It’s just the reverse of what happens to the air within an engine’s cylinder on the compression stroke .1 . Remove the lid from the plastic bottle and rinse it thoroughly.2 . Add 4 tablespoons of water to the bottle . Heat it a microwave oven for 1 minute .3 . Remove the warm bottle from the oven. Screw on the lid tightly.4 . Run cold water over the bottle for a minute or so , Observe what happens . Results and Analysis As you probably observed , when the warm plastic bottle is cooled , it collapses . Can you explain why ?Materials and Equipment(1)Small plastic bottle with a screw-on top(2)Microwave oven and sink or water hose(3) 4 tablespoons of waterFig. 1-9 Crankshaft with one piston and connecting rod assembly . This shows how the piston attaches through the connecting rod to the rod journal on the crankshaft .What is the function of the crankshaft ?Each cylinder has one or more intake valves and exhaust valves . See Fig.1-11. The intake valve controls the flow of the air/fuel mixture into the cylinder . The exhaust valve controls the flow of exhaust gas from the cylinder , The valves fit in the intake and exhaust ports of the cylinder head .Most cylinders have two ports , or holes , in the combustion chamber area of the cylinder head . One port is the intake port ; the other is the exhaust port . The air/fuel mixture enters the cylinder through the intake port . Burned gases leave the cylinder through the exhaust port . Some engines have multiple intake and exhaust ports and valves .When a valve closes , it seals tightly against the valve seat . A valve seat is the surface against which the valve face comes in contact to provide a seal against leakage . In the closed position , the valve moves off its seat ,the port is open . The air/fuel mixture or exhaust gas can then pass through the port .The timing of valve opening and closing will vary with engine design . The intake valve opens before the intake stroke begins and closes after it ends . Theexhaust valve opens before the exhaust stroke begins and closes after it ends , This valve overlap improves engine “breathing,”or the flow of air/fuel mixture and exhaust gases into and out of the cylinders.第一章活塞式发动机的工作客户的关注内燃发动机在各种尺寸和额定功率上的要求。