Craftsman 143.991200 Specifications download pdf (Page 61)

CHAPTER 9 INTERNAL ENGINE AND CYLINDER

OPERATION

4-CYCLE ENGINE THEORY

All 4-cycle engines require four piston strokes or

cycles to complete one power cycle. The flywheel or

the equipment load on the crankshaft provides the

inertia to keep the engine running smoothly between

power strokes.

The engine camshaft gear is twice as large as the

mating gear on the crankshaft to allow proper engine

valve timing for each cycle. The crankshaft makes

two revolutions for every camshaft revolution.

1. INTAKE. The intake valve is open and the

exhaust valve is closed. The piston is

traveling downward creating a low pressure

area, drawing the air-fuel mixture from the

carburetor into the cylinder area above the

piston (diag. 1).

2. COMPRESSION. As the piston reaches

Bottom Dead Center (BDC) the intake valve

closes. The piston then rises, compressing

the air-fuel mixture trapped in the combustion

chamber due to both valves being closed

(diag. 2).

3. POWER. During this piston stroke both valves

remain closed. As the piston reaches the

Before Top Dead Center (BTDC) ignition

point, the spark plug fires, igniting the air-fuel

mixture. In the time it takes to ignite all the

available fuel, the piston has moved to Top

Dead Center (TDC) ready to take the full

combustive force of the fuel for maximum

power during downward piston travel. The

expanding gases force the piston down (diag.

3).

4. EXHAUST. The exhaust valve opens. As the

piston starts to the top of the cylinder, the

exhaust gases are forced out (diag. 4).

After the piston reaches Top Dead Center (TDC), the

four cycle process will begin again as the piston

moves downward and the intake valve opens.

GENERAL INFORMATION

This chapter covers the cylinder block, piston and rod assemblies, cylinder head, crankshaft, camshaft, valve

train, breather, cylinder cover, flywheel, counterbalance systems, and lubrication systems. The governors and

the governor systems are covered in Chapter 4.

All Tecumseh engines covered in this manual are four cycle engines with the valves in the engine head. The

crankshaft position is designated as either horizontal or vertical as the engine rests on its base. The engines

identified by decals or model as XL (Extra Life) or XL/C (Extra Life / Commercial ) are made using aluminum

alloy diecast around a cast iron cylinder liner. However, not all engines with cast iron cylinder liners are identified

as XL or XL/C.

Main Menu

1 2 ... 56 57 58 59 60 61 62 63 64 65 66 ... 93 94

Comments to this Manuals

No comments

Craftsman 143.991200 Specifications Page 61

Comments to this Manuals