3to 11hp 4-cycle l-head engines - Small Engine Suppliers

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 block. 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. Engine blocks of
the heavy frame series (HH, VH) are made of cast iron. All other engines use aluminum alloy for the cylinder block
along with pistons that are chromium plated.
OPERATION
4-CYCLE ENGINE THEORY
All 4-cycle engines require four piston strokes to complete
one power cycle. The flywheel on one end of the
crankshaft provides the inertia to keep the engine running
smoothly between power strokes.
The camshaft gear is twice as large as the mating gear
on the crankshaft so as 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 (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
stroke process will begin again as the piston moves
downward and the intake valve opens.
INTAKE
COMPRESSION
1 2
POWER EXHAUST
3 4
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