CHAPTER 3

Internal combustion engines convert potential chemical energy in the form of heat derived from a fuel into mechanical energy.

Approximately 30% of the energy released in an internal combustion engine is converted into work.

1. Engine block2. Cylinder head3. Crankshaft4. Piston5. Piston rings6. Connecting rod7. Bearings8. Flywheel9. Valve train

Is the main structure of an engine. Supports and helps maintain

alignment of internal & external components.

Consist of cylinder block & a crankcase

Can be one-piece or two-piece unit Consist of cylinder bore, cooling

fins and valve train components

Cylinder bore is a hole in an engine block that aligns and directs the piston during movement. (Bore is the diameter of the cylinder bore)

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)

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 is the volume that a piston displaces in an engine when it travels from TDC to BDC during a same piston stroke

Displacement of a single-cylinder engine can be found by applying the following formula.

D = 0.7854 x B² x S

D = displacement in cu. Inches0.7854 = consantB² = bore squared in inchesS = Stroke in inches

Example: What is the displacement of a single-cylinder engine that has a 2.5” bore and a 2” stroke.

D = 0.7854 x B² x SD = 0.7854 x (2.5 X 2.5) x 2

D = 0.7854 x 6.25 x 2D = 9.8175 cu inches

Cooling fin is an integral thin cast strip designed to provide efficient air circulation and dissipation of heat away from the engine cylinder block into the air stream. The flywheel act as fan blades to provide air circulations.

Crankcase is an engine component that houses and supports the crankshaft. In 4-stroke engines it also acts as an oil reservoir for lubrication.

Sump is a removable part of a vertical shaft engine crankcase that serves as an oil reservoir and provide access to internal parts.

Crankcase cover is a removable part of a horizontal shaft engine that provides access to internal components.

Crankcase breather relieves crankcase pressure created by the reciprocating motion of the piston during engine operation. It maintain the crankcase pressure less than atmospheric pressure of 14.7 psi at sea level.

Cylinder head is a cast aluminum alloy or cast iron engine component fastened to the end of the cylinder block farthest from the crankshaft.

Cylinder head gasket is the filler material placed between the cylinder block and cylinder head to seal the combustion chamber. It allows heat to be evenly distributed.

Two-stroke engines combines the cylinder head and the cylinder block into a jug. A jug is when the block and the head are cast as a single unit.

Overhead Valve (OHV) is an engine that has valves and related components located in the cylinder head.

L-head engine is an engine that has valves and related components located in the cylinder block.

Direct Overhead Valve (DOV) is an engine that has valves and related components located within the cylinder heads, above the combustion chambers and drives the valves in a more direct manner compared to OHV and L-head engines.

Crankshaft is made of ductile iron that converts the linear motion of the piston into rotary motion. The crankshaft is the main rotating component of an engine.

Orientation of the crankshaft classified the engine as a vertical or horizontal shaft engine

Crankpin journal is a precision ground surface that provides a rotating pivot point to attach connecting rod to the crankshaft.

The throw is the measurement from the center of the crankshaft to the center of the crankpin journal which is used to determine the stroke of the engine. The throw is equal to one-half the stroke.

Magneto (bearing) journal is a precision ground surface within which the crankshaft rotates. The journal mates with the bearing surface of the engine block. The end of the shaft is tapered to allow for the flywheel.

PTO or crankgear journal is an extension of the crankshaft that allows an engine to transmit power to an application.

A piston is a cylindrical engine component that slides back and forth in the cylinder bore by forces produced during the combustion process. Commonly made of a cast aluminum alloy for excellent and lightweight thermal conductivity.

Piston head is the top surface of the piston which is subjected to tremendous forces and heat during normal engine operation.

See Figure 3-7 – Different head shape.

Location of the following:Piston pin borepiston pinskirtring groovering lands

Piston rings is an expandable split ring used to provide a seal between the piston and the cylinder wall. Piston rings are made from cast iron.

Piston rings commonly used on small engine are-Compression ring –closest to piston head-Wiper (scraper) ring- wipes cylinder wall clean from excess oil-Oil ring – lubricate and wipe excessive oil from cylinder wall

Connecting rod is an engine component that transfers motion from the piston to the crankshaft and functions as a lever arm.

Piston pin(wrist pin) provides a pivot point between on the crankshaft.

Rod cap is the removable section of the a two piece connecting rod that provides a bearing surface for the crankpin journal.

Bearing is a component used to reduce friction and to maintain clearance between stationary and rotating components of the engine.

Bearings subjected to radial, axial or a combination of both.Radial load is a load apply perpendicular to shaft.Axial load is a load applied parallel to shaft

Bearings are classified as friction and antifriction.

Friction bearings consists of a fixed, non-moving bearing surface that provides low-friction support surface for rotating or sliding surfaces.

Antifriction bearings contains moving elements to provide low friction support

Main bearings supports and provides a low friction bearing surface for the crankshaft. See Figure 3-11

Friction bearings are commonly made from nonferrous metals such as bronze, aluminum, and babbitt.

A nonferrous metal is metal that does not contain iron.

Bronze consists of brass and zincAluminum consists of zinc or copperBabbitt consists of copper, lead, and

tin or lead and tin.

Flywheel is a cast iron, aluminum, or zinc disk that is mounted at one end of the crankshaft(magneto journal side) to provide inertia for the engine.

Inertia is not a force, it is a property of matter.

Valve train includes components required to control the flow of gases into and out of the combustion chamber.