2 The engine block is the main structure of the engine which helps maintain alignment of internal and external engine components.The engine block is the main structure of an engine which supports and helps maintain alignment of internal and external components. The engine block consists of a cylinder block and a crankcase. See Figure 3‑1. An engine block can be produced as a one‑piece or two‑piece unit. The cylinder block is the engine component which consists of the cylinder bore, cooling fins on air‑cooled engines, and valve train components, depending on the engine design. The cylinder bore is a hole in an engine block that aligns and directs the piston during movement. The bore of an engine is the diameter of the cylinder bore. The stroke of an engine is the linear distance that a piston travels in the cylinder bore from top dead center (TDC) to bottom dead center (BDC).
3 Engine displacement is determined by the bore and stroke of the engine. Top dead center (TDC) is the point at which the piston is closest to the cylinder head. Bottom dead center (BDC) is the point at which the piston is farthest from the cylinder head. Displacement (swept volume) is the volume that a piston displaces in an engine when it travels from TDC to BDC during the same piston stroke. See Figure 3‑2. When bore and stroke are known, the displacement of a single‑cylinder engine is found by applying the formula:D = × B2 × SwhereD = displacement (in cu in.)= constantB2 = bore squared (in in.)S = stroke (in in.)
4 The crankcase breather functions as a check valve to maintain crankcase pressure and to route gases to the carburetor.The crankcase breather is an engine component that relieves crankcase pressure created by the reciprocating motion of the piston during engine operation. See Figure 3‑3. When the piston moves toward TDC, volume in the crankcase increases, resulting in a lower than ambient (existing) pressure in the crankcase. When the piston moves toward BDC, the volume in the crankcase decreases, generating a higher than ambient pressure in the crankcase.
5 Cast aluminum alloy cylinder blocks with cast iron cylinder sleeves combine the light weight of aluminum with the durability of cast iron.Cast aluminum alloy cylinder blocks are lightweight and dissipate heat more rapidly than cast iron cylinder blocks. Cast iron cylinder blocks are heavier and more expensive, but are more resistant to wear and less prone to heat distortion than cast aluminum alloy cylinder blocks. Cast aluminum alloy cylinder blocks with cast iron cylinder sleeves combine the light weight of aluminum with the durability of cast iron. See Figure 3‑4.
6 The head gasket is placed between the cylinder block and cylinder head to seal the combustion chamber and to provide even heat distribution.A cylinder head is a cast aluminum alloy or cast iron engine component fastened to the end of the cylinder block farthest from the crankshaft. A head gasket is the filler material placed between the cylinder block and cylinder head to seal the combustion chamber. Head gaskets are made from soft metals and graphite layered together. Head gaskets allow for even heat distribution between the cylinder block and cylinder head for efficient heat dissipation. See Figure 3‑5.
7 The crankshaft is the main rotating component of the engine and is commonly made of ductile iron. The crankshaft is an engine component that converts the linear (reciprocating) motion of the piston into rotary motion. The crankshaft is the main rotating component of an engine and is commonly made of ductile iron. See Figure 3‑6. Orientation of the crankshaft classifies the engine as a vertical shaft engine or a horizontal shaft engine.
8 The piston acts as the movable end of the combustion chamber and is designed to utilize the forces and heat created during engine operation.Piston features include the piston head, piston pin bore, piston pin, skirt, ring grooves, ring lands, and piston rings. The piston head is the top surface (closest to the cylinder head) of the piston which is subjected to tremendous forces and heat during normal engine operation. See Figure 3‑7. The shape of the piston head is either flat or contoured, depending on engine design. Some engine designs use the piston head as an integral part of the combustion chamber. For example, a dished piston head shape creates a swirling effect to mix the air and fuel more completely as it enters the combustion chamber.
9 Piston rings commonly used on small engines include the compression ring, wiper ring, and oil ring. Piston rings commonly used on small engines include the compression ring, wiper ring, and oil ring. A compression ring is the piston ring located in the ring groove closest to the piston head. The compression ring seals the combustion chamber from any leakage during the combustion process. See Figure 3‑8.
10 A connecting rod is designed to withstand sudden impact stresses from combustion and piston movement.A connecting rod is an engine component that transfers motion from the piston to the crankshaft and functions as a lever arm. Connecting rods are commonly made from cast aluminum alloy and are designed to withstand sudden impact stresses from combustion and piston movement. The small end of the connecting rod connects to the piston with a piston pin. See Figure 3‑9. The piston pin, or wrist pin, provides a pivot point between the piston and connecting rod. Spring clips, or piston pin locks, are used to hold the piston pin in place.
11 Bearings and bearing surfaces are subjected to radial, axial (thrust), or a combination of radial and axial loads.A bearing is a component used to reduce friction and to maintain clearance between stationary and rotating components of an engine. Bearings, or bearing surfaces, are located on the crankshaft, connecting rod, and camshaft, and also in the cylinder block. Bearings can be subjected to radial, axial (thrust), or a combination of radial and axial loads. A radial load is a load applied perpendicular to the shaft. An axial load is a load applied parallel to the shaft. Bearings are classified as friction or antifriction bearings. See Figure 3‑10.
12 Small engines commonly have two main bearings to provide a low-friction bearing surface on each end of the crankshaft.The crankshaft is supported by main bearings. A main bearing is a bearing that supports and provides a low‑friction bearing surface for the crankshaft. Small engines commonly have two main bearings, one at each end of the crankshaft. See Figure 3‑11. Small engines with three or more cylinders may require more than two main bearings to provide additional support to the crankshaft. Main bearings are mounted in the crankcase and can be either friction or antifriction bearings. Antifriction bearings used for main bearings increase the radial and axial load capacity of the engine design.
13 Rod bearings provide a low-friction pivot point between the connecting rod and the crankshaft and the connecting rod and piston.A rod bearing is a bearing that provides a low‑friction pivot point between the connecting rod and the crankshaft and the connecting rod and piston. The large end of the connecting rod is connected to the crankpin journal. The small end of the connecting rod is connected to the piston pin. Rod bearings are friction bearings (integrally machined, sleeve, or split‑sleeve) or antifriction bearings. Most connecting rods for small engines use integrally machined friction bearings. See Figure 3‑12.
14 The flywheel supplies inertia to dampen acceleration forces caused by combustion intervals in an engine.The flywheel is a cast iron, aluminum, or zinc disk that is mounted at one end of the crankshaft to provide inertia for the engine. Inertia is the property of matter by which any physical body persists in its state of rest or uniform motion until acted upon by an external force. Inertia is not a force, it is a property of matter. During the operation of a reciprocating engine, combustion occurs at distinct intervals. The flywheel supplies the inertia required to prevent loss of engine speed and possible stoppage of crankshaft rotation between combustion intervals. See Figure 3‑13.
15 The intake event occurs when the air-fuel mixture is introduced into the combustion chamber as the piston moves from TDC to BDC.The intake event is an engine operation event in which the air‑fuel mixture, or just air in diesel engines, is introduced to fill the combustion chamber. The intake event occurs when the piston moves from TDC to BDC and the intake valve is open. See Figure 3‑14. The movement of the piston toward BDC creates a low pressure in the cylinder. Ambient atmospheric pressure forces the air‑fuel mixture through the open intake valve into the cylinder to fill the low pressure area created by the piston movement. The cylinder continues to fill slightly past BDC as the air‑fuel mixture continues to flow from motion and inertia while the piston begins to change direction. The intake valve remains open a few degrees of crankshaft rotation after BDC. The number of degrees the intake valve remains open after BDC depends on engine design. The intake valve then closes and the air‑fuel mixture is sealed inside the cylinder.
16 The compression event is an engine operation event in which the trapped air-fuel mixture is compressed to form the charge.The compression event is an engine operation event in which the trapped air‑fuel mixture, or just air in diesel engines, is compressed inside the cylinder. The combustion chamber is sealed to form the charge. The charge is the volume of compressed air‑fuel mixture trapped inside the combustion chamber ready for ignition. Compressing the air‑fuel mixture allows more energy to be released when the charge is ignited. Intake and exhaust valves must be closed to ensure that the cylinder is sealed to provide compression. Compression is the process of reducing or squeezing a charge from a large volume to a smaller volume in the combustion chamber. See Figure 3‑15. The flywheel helps to maintain the momentum necessary to compress the charge.
17 The compression ratio of an engine is a comparison of the volume of the combustion chamber with the piston at BDC and TDC.The compression ratio of an engine is a comparison of the volume of the combustion chamber with the piston at BDC to the volume of the combustion chamber with the piston at TDC. See Figure 3‑16. This area, combined with the design and style of combustion chamber, determines the compression ratio. Gasoline engines commonly have a compression ratio ranging from 6:1–8.5:1. Diesel engines commonly have a compression ratio ranging from 14:1–25:1. The higher the compression ratio, the more fuel‑efficient the engine. A higher compression ratio normally provides a substantial gain in combustion pressure or force on the piston. However, higher compression ratios increase operator effort required to start the engine. Some small engines feature a system to relieve pressure during the compression stroke to reduce operator effort required when starting the engine.
18 During the ignition event, atmospheric oxygen and fuel vapor in the charge are consumed by the progressing flame front.The ignition (combustion) event is an engine operation event in which the charge is ignited and rapidly oxidized through a chemical reaction to release heat energy. See Figure 3‑17. Combustion is the rapid, oxidizing chemical reaction in which a fuel chemically combines with oxygen in the atmosphere and releases energy in the form of heat.
19 During the power event, hot expanding gases force the piston head away from the cylinder head. The power event is an engine operation event in which hot expanding gases force the piston head away from the cylinder head. Piston force and subsequent motion are transferred through the connecting rod to apply torque to the crankshaft. The torque applied initiates crankshaft rotation. See Figure 3‑18. The amount of torque produced is determined by the pressure on the piston, the size of the piston, and the throw of the engine. During the power event, both valves must be closed.
20 During the exhaust event, piston movement evacuates exhaust gases to the atmosphere. As the piston reaches BDC during the power event, combustion is complete and the cylinder is filled with exhaust gases. See Figure 3‑19. The exhaust valve opens, and inertia of the flywheel and other moving parts push the piston back to TDC, forcing the exhaust gases past the open exhaust valve. At the end of the exhaust stroke, the piston is at TDC and one operating cycle has been completed.
21 Valve overlap is the period between the exhaust event and the intake event when the piston nears TDC.Valve overlap is the period during engine operation when both intake and exhaust valves are open at the same time. Valve overlap occurs when the piston nears TDC between the exhaust event and the intake event. Duration of valve overlap is between 10°–20° of crankshaft rotation, depending on the engine design. See Figure 3‑20. The intake valve is opened during the exhaust event just before TDC, initiating the flow of a new charge into the combustion chamber.
22 A two-stroke cycle engine completes five events in one operating cycle. A two‑stroke cycle engine is an internal combustion engine that utilizes two distinct piston strokes to complete one operating cycle of the engine. The crankshaft turns only one revolution for each complete operating cycle, providing twice as many power strokes in the same number of crankshaft rotations as a four‑stroke cycle engine. The valving system in a two‑stroke cycle engine requires fewer parts, making it lighter in weight than a four‑stroke cycle engine. Weight reduction is especially desirable in applications such as chainsaws, leaf blowers, and other hand‑held outdoor power equipment. Like a four‑stroke cycle engine, a two‑stroke cycle engine completes five events in one operating cycle. Some events occur concurrently, such as ignition/power and exhaust/intake. See Figure 3‑21.
23 Two-stroke valves are widely used in the outdoor power equipment industry for hand-held equipment applications such as chain saws, trimmers, and leaf blowers.Two‑stroke cycle engines are widely used in outdoor power equipment applications such as chain saws, trimmers, and leaf blowers. Two‑stroke cycle engines have specific characteristics that provide advantages and disadvantages for different applications. See Figure 3‑22. Two‑stroke cycle engines ignite the air‑fuel mixture once every revolution of the crankshaft, while four‑stroke cycle engines ignite the air‑fuel mixture once every other revolution.
24 Valves seal the combustion chamber to control the flow of air-fuel mixture into the cylinder and exhaust gases out of the cylinder.Four‑stroke cycle engines control the flow of gases in the cylinder with valves. A valve is an engine component that opens or closes at precise times to allow the flow of air‑fuel mixture into the cylinder and to allow the flow of exhaust gases from the cylinder. Valve features include the valve head, margin, valve face, valve seat, valve stem, valve neck, and retainer groove. See Figure 3‑23. The valve head is the large end of the valve that contains the margin and the valve face. The margin is the surface of a valve joining the valve face and the top surface of the valve head. The valve face is the machined surface of a valve that mates with the valve seat to seal the combustion chamber.
25 Valve location determines whether an engine is an L-head or OHV engine. The location of valves determines the type of head design and the necessary components for the valve train. Valves are located in the cylinder block or in the cylinder head. In an L‑head engine, the valves are located in the cylinder block to one side of the cylinder. Small engines are commonly L‑head engines. See Figure 3‑24.
26 Timing marks on the cam gear and crankgear indicate the proper gear teeth mesh required to prevent damage to engine components.The opening and closing of the valve is precisely timed by the camshaft to control the air‑fuel mixture entering the cylinder and the exhaust gases leaving the cylinder. The camshaft is an engine component that includes the cam gear, cam lobes, and bearing surfaces. The cam gear is the portion of the camshaft that meshes with the crankgear. The camshaft is driven by the crankgear and is usually constructed as one unit. See Figure 3‑25.
27 Valving systems on two-stroke cycle engines require fewer parts and are less complicated than four-stroke cycle engine valving systems.Two‑stroke cycle engine valving systems require fewer parts than four‑stroke cycle engine valving systems. Engine components such as the camshaft and tappets are not required. This results in less engine size and weight. Engine components used for controlling the air‑fuel mixture and exhaust gases are also less complicated. Two‑stroke cycle engines commonly use a reed valve valving system, a three‑port valving system, or a rotary valve valving system. See Figure 3‑26.
28 Diesel engines use an injection pump to deliver pressurized fuel to the cylinder at precise intervals.The injection pump is a diesel engine component that provides pressurized fuel to the cylinder at precise intervals. See Figure 3‑27. The injection pump is similar to the distributor cap used on multiple cylinder spark‑ignition engines. Instead of providing a spark to a specific cylinder at the precise time, the injection pump provides high‑pressure fuel to the cylinder at the precise time. One injection pump may supply multiple cylinders, or a separate injection pump may be required for each cylinder. To be fed into the cylinder, fuel pressure must exceed the pressurized air in the cylinder. The injection pump is mechanically timed to piston movement and is usually driven by the crankshaft or camshaft.
29 The injector is hydraulically activated by the pressurized fuel delivered from the injection pump. The injector is a diesel engine component that functions as an ON/OFF valve to introduce fuel into the cylinder. The injector is located in the cylinder head. See Figure 3‑28. Engines that have multiple cylinders require an injector for each cylinder, similar to spark plugs for multiple cylinder gasoline engines.
30 Heat in the glow plug is created by resistance to current passed through a heating coil. A glow plug is a diesel engine component that preheats air inside the combustion chamber to facilitate ignition of the charge. Diesel fuel is more difficult to ignite at lower temperatures than gasoline. The glow plug is located in the cylinder head. Heat is created by electrical resistance to current passed through a heating coil. The heating coil begins to glow and heat the air inside the cylinder. See Figure 3‑29.
31 Load is increased or decreased by adding or removing water from the impeller housing of a water dynamometer.The water dynamometer is a dynamometer used to measure engine torque using load produced by a water pump. See Figure 3‑30. The function of a water dynamometer is similar to the function of a common water pump. The output shaft of the engine drives an impeller located inside the water dynamometer. Load is increased or decreased by adding or removing water from the impeller housing. As water is added, it moves to the outside surfaces of the impeller housing, increasing the load. A load cell measures the twisting force exerted on the outside of the impeller housing. Water dynamometers are most commonly used in industry.
32 The electric dynamometer measures brake horsepower by converting mechanical energy into electrical energy.An electric dynamometer is a dynamometer used to measure brake horsepower by converting mechanical energy into electrical energy. See Figure 3‑31. The output shaft of the engine drives an electric generator that is connected to a load bank. Load is increased until the engine reaches wide open throttle (WOT) at the rated rpm. The electric power produced is consumed by the load bank and measured in watts (W).
33 The eddy current dynamometer measures engine torque using load from the magnetic field produced by current in eddy current coils.The eddy current dynamometer is a dynamometer used to measure engine torque using load produced by a magnetic field. See Figure 3‑32. The eddy current dynamometer consists of a disk that is driven by the engine being tested. The disk turns in a magnetic field controlled by varying the current through a series of coils located on both sides of the disk. Increasing the current in the coils causes an increased magnetic field, resulting in a greater load on the engine. The increased load on the engine results in more torque and pressure applied to the load cell.
34 The prony brake dynamometer measures engine torque using an adjustable brake that exerts pressure on a spring scale.The prony brake dynamometer is a dynamometer for measuring engine torque using a brake that exerts pressure on a spring scale. An adjustable brake with an extension arm is mounted in contact with the flywheel. The end of the extension arm is connected to a spring scale. The engine is operated at a set speed while the adjustable brake is tightened to increase the load. As the load increases, the governor system keeps the engine at a constant speed by opening the throttle. Load is increased until reaching WOT. At WOT a reading is taken from the spring scale and is used to determine torque produced. See Figure 3‑33. The prony brake dynamometer has been largely replaced by the electric dynamometer and water dynamometer.
35 Engine horsepower decreases 3 1/2% for each 1000′ above sea level. Air density affects engine output. Denser air allows for a greater charge, resulting in greater engine output. Air density is most affected by altitude, temperature, and humidity. Higher altitudes have less oxygen in the air or thinner air. Thinner air results in a smaller charge inside the cylinder. Engine horsepower decreases 3 1/2% for each 1000′ above sea level. See Figure 3‑34. For example, a 10 HP engine operating at 10,000′ produces only 6.5 HP. See Appendix.