For proper combustion (burning), it is very important
that the valves, rings, and other components do not allow
pressure leakage out of the combustion chamber. Leakage
during the compression stroke may prevent the mixture
from igniting and burning on the power stroke.
Power Stroke
The air-fuel mixture is ignited and burned during the
power stroke to produce gas expansion, pressure, and a
powerful downward piston movement. See Figure 1-16C.
Both valves are still closed. The spark plug fires and the
fuel mixture begins to burn.
As the mixture burns, it expands and builds pressure in
the combustion chamber. Since the piston is the only part
that can move, it is thrust downward with several tons of
force. This downward thrust pushes on the connecting rod
and crankshaft forcing the crankshaft to turn. The power
stroke is the only stroke that does not consume (use) energy.
Exhaust Stroke
The exhaust stroke pushes the burned gasses out of
the cylinder and into the vehicle’s exhaust system. See
Figure 1-16D. The intake valve remains closed, but the
exhaust valve is open. Since the piston is now moving up,
the burned gasses are pushed out of the exhaust port to
ready the cylinder for another intake stroke.
Valve Train
The valve train operates the engine valves. It times
valve opening and closing to produce the four-stroke
cycle. Basic valve train parts are shown in Figure 1-17.
The camshaft opens the valves and allows the valve
springs to close the valves at the proper times. The
camshaft has a series of lobes (egg-shaped bumps) that act
on the valves or valve train to slide the valve down in its
guide. See Figure 1-18.
Chapter 1 Review of Engine Operation 17
Rocker arm
pushes
down
Valve
pushed
off seat
Cam lobe
touches
lifter
Rocker arm pushed
up by spring
Spring
Valve closed
on seat
Lifter
(tappet)
Lobe away
from lifter
Push rod
Valve open Valve closed
Pivot point
Figure 1-17. The camshaft operates the valve train.When the cam lobe rotates into a lifter, the valve is opened.When a lobe moves
out of a lifter, the valve spring closes the valve. (Ford)
Camshaft
Cam lobes
Figure 1-18. The lobes on the camshaft act on the valves or
valve train to open and close the valves. As shown here, the
camshaft fits into the cylinder head on many engines. This lets
it operate directly on valves without using push rods.
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