4-CYCLE OVERHEAD VALVE ENGINES - Small Engine Discount

CHAPTER 9 INTERNAL ENGINE AND CYLINDER
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.
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.
1
2
3
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