MAIN COMPONENTS OF AN ENGINE

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Engines may vary considerably but the majority incorporate the components listed

below.

1. Cylinder block and crankcase. The unit forms the principal part of the

engine. It may be cast as a single unit, or the cylinders may be detachable from

the crankcase. A water cooling space is included in the cylinder block. Small

air cooled engines have fins to provide a large surface area for the cooling air.

2. Cylinder head. The head, usually detachable, is bolted to the cylinder block.

In some small two-stroke engines, however the cylinder head is part of the

cylinder and can not be detached. A water cooling space in the head

communicates with the water cooling passages in the cylinder block. Air

cooled types incorporate fins, as in the cylinder block.

2. Sump. This is a light casing fitted to the underside of the crankcase to

contain the lubricating oil. This is normally OMD (oil mineral detergent), or a

commercial equivalent.

3. Piston. The piston is a sliding fit in the cylinder bore. It is fitted with piston

rings to prevent gas leakage past the piston. The top of the piston is known as

the crown. The lower part, which acts as the guide is called the piston skirt.

Compression rings are fitted to maintain a seal between the piston and the

cylinder wall and an oil control (scraper) ring removes excess oil from the

cylinder wall.

4. Connecting rod. The connecting rod is attached to the piston at one end by a

gudgeon pin which is fitted to the piston. A bush in the end of the connecting

rod provides a bearing surface. This is called the small end bearing. The

bottom end of the connecting rod is attached to the crankshaft, with a halved

bearing and bottom cap securely bolted to the connecting rod. This is called

the big end bearing.

5. Crankshaft. The crankshaft is carried in main bearings which are part of the

crankcase casting, with halved bearing shells and bottom caps securely bolted

to the crankcase.

6. Flywheel. Most internal combustion piston engines have a heavy flywheel to

carry the engine over the non-power strokes, particularly in the four-stroke

engine where there is only one power stroke in four on each piston. The

flywheel is attached to the crankshaft, and is outside the crankcase, but

sometimes it is enclosed by the clutch case, or the reversing gear case. The

flywheel usually incorporates a toothed ring into which the engine starter

pinion engages to start the engine.

7. Valves. The valves fitted to all four-stroke engines and some to two-stroke

engines are mushroom-shaped. They have a long stem, over which a spring is

installed to hold the valve on the seating. The valves are usually fitted in the

cylinder head, these are called overhead valves.

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Engine components cont…

8. Camshaft. Each valve is lifted by a cam, which is pear shaped and machined

in an angular position on the camshaft so that it will lift the valve for the

correct period. The camshaft may lie above the cylinder head, when it is called

an overhead camshaft, or alongside the crankshaft with push rods to lift the

valves if they are of the overhead type.

9. Rocker gear. Overhead valves are operated by rockers with one end of each

rocker bearing on the cam or push rod, and the other end on the valve stem.

Side valves are usually placed immediately above the cams so that the rockers

are not required.

10. Tappets. All valve gear is fitted with tappets, a form of set screw and locknut,

which may be fitted to the rockers, the push rods, or the tappet spindle for

side valve engines. The tappets are adjusted to give a slight clearance for

expansion between the tappet and the valve stem. This clearance is always

stated for inlet and exhaust valves of each engine.

OVERHEAD VALVE GEAR

11. Timing gear. The camshaft must be accurately timed to the crankshaft so that

the valves will open when required and for the correct period. The timing gear

consists of gear wheels usually mounted at the front end of the crankcase and

enclosed by a timing case. The gear wheels are marked to show the correct

setting.

12. Fuel pump and injectors. The fuel injection pump may be fitted on the side

of the engine, or an individual fuel injection pump may be fitted alongside

each cylinder. The pump is driven by the timing gear so that fuel is injected

into each cylinder at the correct time. Each cylinder head is fitted with an

injector, which consists of a non-return valve and a nozzle which produces a

fine spray of fuel into the cylinder. The speed of the engine is varied by a

governor which controls the quantity of fuel discharged by the fuel pump.

13. Joints/Gaskets. These are fitted to prevent the leakage of oil, gas or water.

The cylinder head gasket is normally made from copper and compressed fibre,

whilst other joints are of treated paper or metal-to-metal with joining

compound between the mating faces