Engine Fundamentals PDF

Engine Fundamentals The engine is the “power plant" that produces the energy needed to propel the vehicle and operate other systems. It converts the heat energy of a fuel(gasoline,diesel oil)into movement. If you know how an engine is constructed and how it is designed to operate, you will be bette...

Engine Fundamentals The engine is the “power plant" that produces the energy needed to propel the vehicle and operate other systems. It converts the heat energy of a fuel(gasoline,diesel oil)into movement. If you know how an engine is constructed and how it is designed to operate, you will be better prepared to diagnose problems when it malfunctions. Engine Operation The engine is the source of power for the vehicle. For this reason, it is also called a power plant. An energy source,or fuel (usually gasoline or diesel oil), is burned inside the engine's combustion chamber(hollow area between the top of the piston and the bottom of the cylinder head) to produce heat.The heat causes expansion(enlargement) of the gasses in the engine.The burning and expansion of the gasses in the combustion chamber produces pressure. The engine piston, connecting rod, and crankshaft convert this pressure into motion for moving the car and operating its other systems. Below shows how an engine converts fuel into a useful form of energy. Combustion pressure forces the piston down. By linking the piston to the crankshaft, an engine can produce a powerful spinning motion. The rotating crankshaft can be used to drive gears,chains and sprockets; belts and sprockets; and other drive mechanisms. A crankshaft converts the downward thrust of the piston into useful rotating motion.The rotating motion can be used to operate the drive mechanism. Piston Travel (TDC, BDC) The distance the piston can travel up or down in the cylinder is limited by the crankshaft. When the piston is at the highest point in the cylinder, it is at top dead center (TDC). When the piston slides to its lowest point in the cylinder, it is at the bottom dead center(BDC). Piston Stroke Piston stroke is the distance the piston slides up or down from TDC to BDC. This takes one-half turn of the crankshaft. The crank rotates 180° during one piston stroke. Four-Stroke Cycle The four-stroke cycle requires four piston strokes to complete one cycle(complete series of events). Everyfour strokes, the engine produces one power stroke(useful energy). Almost all automobiles use four-stroke-cycle engines. The intake stroke of a gasoline engine draws fuel and air into the engine. The intake valve is open and the exhaust valve is closed.The piston slides down and forms a low-pressure area, or vacuum, in the cylinder Outside air pressure then pushes the air-fuel mixture into the Cylinder. The compression stroke squeezes the air-fuel mixture to prepare it for combustion(burning).The mixture is more combustible when pressurized. During this stroke, the piston slides up with both valves closed. The power stroke burns the air-fuel mixture and pushes the piston down with tremendous force. This is the only stroke that does not consume energy—it produces energy. When the spark plug fires(gasoline engine), it ignites the air-fuel mixture.Since both valves are still closed, pressure forms on the top of the piston. The piston is forced down, spinning the crankshaft. The exhaust stroke removes the burned gasses from the engine and readies the cylinder for a fresh charge of air and fuel. During this stroke, the piston moves up.The intake valve is closed and the exhaust valve is open.The burned gasses are pushed out the exhaust port and into the exhaust system. The crankshaft must rotate two complete revolutions to complete the four-stroke cycle. With the engine running, this series of events happens over and over very quickly. Engine Bottom End The term engine bottom end generally refers to the block, crankshaft, connecting rods, pistons and related components. Another name for the engine bottom end is the short block. It is an assembled engine block with the cylinder heads, intake manifold, exhaust manifold and other external parts removed Engine Block The engine block, also called cylinder block, forms the main body of the engine. Other parts bolt to or fit inside the block The cylinders, also known as the cylinder bores are large, round holes machined through the block from top to bottom.The pistons fit into the cylinders of the engine block. The cylinders are slightly larger than the pistons. This lets the pistons slide up and down freely. The deck, or deck surface,is the top of the block surrounding the cylinders. It is machined perfectly flat. The cylinder head bolts to the deck. Oil and coolant passages through the deck surface allow lubrication and cooling of the cylinder head parts. Water jackets are coolant passages through the block. They allow a solution of water and antifreeze to cool the Cylinders. Core plugs, or freeze plugs, are round,metal plugs on the outside of the block. They seal holes left in the block after casting(manufacturing in a foundry). The plugs prevent coolant leakage out of the water jackets Some new engines do not have freeze plugs The main bearing bores are holes machined in the bottom of the block to hold the crankshaft. Removable bearing inserts fit into these bores. Main caps bolt to the bottom of the block and hold the crankshaft and main bearing inserts in place. Two or four large bolts normally secure each cap to the block. The caps and the block together form the main bearing bores. The crankcase is the lowest portion of the block.The crankshaft rotates inside the crankcase. Crankshaft The crankshaft harnesses the tremendous force produced by the downward thrust of the pistons. It changes the up-and-down motion of the pistons into a rotating motion.The crankshaft fits into the bottom of the engine Block, The crankshaft main journals are surfaces that are precisely machined and polished. They fit into the block's main bearings. The crankshaft rod journals are also machined and polished surfaces,but they are offset from the main journals.The connecting rods bolt to the rod journals. With the engine running,the rod journals circle around the centerline of the crankshaft. Counterweights are formed on the crankshaft to prevent vibration. These weights counteract the weight of the connecting rods, pistons,rings, and rod journal offset. See picture below. Flywheel A flywheel is a large metal disk mounted on the rear of the crankshaft. A flywheel can have several functions: ​ The flywheel connects the engine crankshaft to the transmission or transaxle. Either the manual clutch or the automatic transmission torque converter bolts to the flywheel. ​ The flywheel for a vehicle with a manual transmission is very heavy and can help smooth the engine operation. ​ The flywheel generally contains a large ring gear, which is used to start the engine. A small gear on the starting motor engages the flywheel ring gear and turns the flywheel. Connecting Rod The connecting rod fastens the piston to the crankshaft. It transfers piston movement and combustion pressure to the crankshaft rod journals.The connecting rod also causes piston movement during the non-power-producing strokes(intake,compression, and exhaust). The connecting rod, small end or top end, fits around the piston pin. Also called the upper end, it contains a one-piece bushing.The bushing is pressed into the rod small end. The connecting rod I-beam is the center section of the rod. The I-beam shape provides a very high strength-to-weight ratio. It prevents the rod from bending, twisting, and breaking. The connecting rod cap bolts to the bottom of the connecting rod body. It can be removed for disassembly of the engine. The connecting rod big end, or lower end, is a hole machined in the rod body and cap.The connecting rod bearing fits into the big end. Connecting rod bolts and nuts clamp the rod cap and rod together. They are special high tensile strength fasteners. Some rods use cap screws without a nut.The cap screws thread into the rod itself. This design reduces rod weight. Connecting Rod Bearings The connecting rod bearings ride on the crankshaft rod journals. They fit between the connecting rods and the crankshaft. The rod bearings are also removable inserts. Rod bearing clearance is the small space between the rod bearing and crankshaft journal. As with main bearing clearance, it allows oil to enter the bearing.The oil prevents metal-to-metal contact that would wear out the crank and bearings. Piston The piston transfers the pressure of combustion(expanding gas) to the connecting rod and crankshaft. It must also hold the piston rings and piston pin while operating in the cylinder. The piston head is the top of the piston and is exposed to the heat and pressure of combustion. This area must be thick enough to withstand these forces.It must also be shaped to match and work with the shape of the combustion chamber for complete combustion. Piston ring grooves are slots machined in the piston for the piston rings. The upper two grooves hold the compression rings. The lower piston groove holds the oil ring. Piston oil holes in the bottom ring groove allow the oil to pass through the piston and onto the cylinder wall.The oil then drains back into the crankcase. Piston Pin The piston pin, also called wrist pin, allows the piston to swing on the connecting rod. The pin fits through the hole in the piston and the connecting rod small end. Piston Clearance Piston clearance is the amount of space between the sides of the piston and the cylinder wall. Clearance allows a lubricating film of oil to form between the piston and the cylinder. It also allows for expansion when the piston heats up. The piston must always be free to slide up and down in the cylinder block. Piston Rings The piston rings seal the clearance between the outside of the piston and the cylinder wall.They must keep combustion pressure from entering the crankcase. They must also keep oil from entering the combustion chambers. The Piston Pin allows the connecting rod to swing in the piston Engine Top End The term engine top end generally refers to the cylinder heads, valves,camshaft and other related components.These parts work together to control the flow of air and fuel into the engine cylinders. They also control the flow of exhaust out of the engine. Cylinder Head The cylinder head bolts to the deck of the cylinder block. It covers and encloses the top of the cylinders. Combustion chambers are small pockets formed in the cylinder heads.The combustion chambers are located directly over the cylinders. Combustion occurs in these areas of the cylinder head. Spark plugs(gasoline engine) or injectors(diesel engine) protrude through holes and into the combustion chambers. combustion chamber. Intake and exhaust ports are cast into the cylinder head.The intake ports route air(diesel engine) or air and fuel(gasoline engine) into the combustion chambers. The exhaust port routes burned gasses out of the engine. Valve guides are small holes machined through the cylinder head for the valves.The valves fit into and slide in these guides. Valve seats are round,machined surfaces in the combustion chamber port openings. When a valve is closed, it seals against the valve seat. The combustion chamber is formed in the cylinder head. Valve ports enter the cylinder head, also note the spark plug tip and valve seats. Valve Train The engine valve train consists of the valves and the parts that operate them. These include the camshaft, lifters, pushrods, rocker arms, valves and valve spring assemblies. The valve train must open and close the valves at the correct time. Camshaft The camshaft has lobes that open each valve. It can be located in the engine block or in the cylinder head. The cam lobes are egg-shaped protrusions (bumps)machined on the camshaft. One cam lobe is provided for each engine valve. Assuming the engine has two valves per cylinder, a 4-cylinder engine camshaft would have eight cam lobes; a 6-cylinder, twelve lobes; etc. The camshaft sometimes has a drive gear for operating the distributor and oil pump. A gear on the ignition system distributors may mesh with this gear. An eccentric(oval) may be machined on the camshaft to operate a mechanical(engine-driven) fuel pump. This feature can be found on camshafts used in older engines or some diesel engines. It is similar to a cam lobe but is more Round. Camshaft Journals are precisely machined and polished surfaces for the cam bearings. Like the crankshaft, the camshaft rotates on its journals. Oil separates the cam bearings and cam journals. Valve Lifters A valve lifter, also called a tappet, usually rides on the cam lobes and transfers motion to the rest of the valve train. The lifters can be located in the engine block or cylinder head. They fit into lifter bores, which are machined holes in the block or head When the cam lobe moves toward the lifter, the lifter is pushed up in its bore.This opens the valve. Then, when the lobe rotates away from the lifter the lifter is pushed down in its bore by the valve spring. This keeps the lifter in constant contact with the camshaft. Push Rods Push rods transfer motion between the lifters and the rocker arms. They are needed when the camshaft is located in the cylinder block. They are not needed when the camshaft is in the cylinder head. A push rod is a hollow metal tube with a ball or socket formed on each end. One end of the push rod fits into the lifter. The other end fits against the rocker arm. In this way when the lifter slides up, the push rod moves the rocker arm. Rocker Arms Rocker arms can be used to transfer motion to the valves. They mount on top of the cylinder head. A pivot mechanism allows the rockers to rock back and forth, opening and closing the valves. Valves Engine valves open and close the ports in the cylinder head. Until recently, only two valves were used per cylinder: one intake valve and one exhaust valve. To improve efficiency, many newer model engines are equipped with four valves per cylinder: two intake valves and two exhaust valves. Valve springs keep the valves normally closed in a cylinder head. The intake valve is larger than the exhaust valve. It controls the flow of the fuel mixture(gasoline engine) or air(diesel) into the combustion chamber. The intake valve fits into the port leading from the intake manifold. The exhaust valve controls the flow of exhaust gasses out of the cylinder. It is smaller than the intake valve. The exhaust valve fits into the port leading to the exhaust manifold. Intake Manifold The intake manifold bolts to the side of the cylinder head or heads. On newer-model engines, the fuel injectors and the throttle body mount on the intake manifold. On older engines, the carburetor is mounted on the top of the manifold. The intake manifold contains runners(passages) going to each cylinder head port. Air and fuel (gasoline engine)are routed through these runners, Exhaust Manifold The exhaust manifold also bolts to the cylinder head; however, it fastens over the exhaust ports to carry burned gasses into the exhaust system. During the exhaust strokes, hot gasses blow into this manifold before entering the rest of the exhaust system. An exhaust manifold can be made of heavy cast iron or lightweight aluminum or stainless steel tubing. The trend is toward lighter designs. Valve Cover The valve cover, also called the rocker cover, is a thin metal or plastic cover over the top of the cylinder head. It simply keeps valve train oil spray from leaking out of the engine. Engine Front End The engine front end operates the engine camshaft and sometimes the oil pump, distributor, engine sensors and diesel injection pump. Basically, the engine front end consists of a drive mechanism for the camshaft and other devices, a front cover, an oil seal, and a crankshaft damper Camshaft Drive A camshaft drive is needed to turn the camshaft at one-half engine speed. A belt and sprockets, gears, or a chain and sprockets can be used to turn the camshaft. These parts are often called the timing belt, timing gears, or timing chain because they time the camshaft with the crankshaft. The camshaft is designed to turn at one-half engine speed. As a result, each valve will open only once for every two crankshaft revolutions. For instance, the intake valve must open only on the intake stroke, not the compression, power, or exhaust strokes. To do this, the camshaft gear or sprocket is twice as big as the gear or sprocket on the crankshaft. Crank Damper A crank damper is a heavy wheel on the crankshaft snout.It is mounted in rubber and helps prevent crankshaft vibration and damage.This damper is also called the harmonic balancer or vibration damper.