Inlet valve is used for control flow of fuel air mixture into the cylinder on engine and outlet valve is used for control flow of exhast gases form cylider to enviorment.
The valve which allows mixture into the cylinder is the inlet valve; the
one through which the spent gases escape is the exhaust valve. They are
designed to open and close at precise moments, to allow the engine to run efficiently at all speeds.
How valves operated ?
The operation is controlled by pear-shaped lobes, called cams, on a
rotating shaft, the camshaft, driven by a chain, a belt, or a set of
gears from the crankshaft.
Where the camshaft is mounted in the engine block, small metal
cylinders tappets sit in channels above each cam, and from the tappets
metal pushrod extend up into the cylinder head. The top of each pushrod
meets a rocker arm which bears against the stem of a valve, which is
held in a raised (closed) position by a strong coiled spring the valve
spring.
As the pushrod rises on the cam it pivots the rocker arm, which
pushes the valve down (open) against the pressure of its spring. As the
cam lobe rotates further, the valve spring acts to close the valve. This
is called an overhead-valve (OHV) system.
Camshaft with pushrods:
The overhead valve system (OHV) system, operated by
pushrods, has the crankshaft adjacent and parallel to the crankshaft in
the cylinder block.
As the crankshaft rotates, each valve is opened by means of a tappet,
pushrod and rocker arm. The valve is closed by spring pressure.
The camshaft drive-chain sprocket has twice as many teeth as the
crankshaft sprocket, so that the camshaft rotates at half engine speed.
Some engines have no pushrods; the valves are operated more directly
by single or double camshafts in the cylinder head itself the
overhead-cam system.
Engine with overhead camshaft:
An overhead-cam (OHC) engine needs fewer parts to
operate the valves. The cams act directly on bucket tappets or on short
levers - known as fingers - which in turn act directly on the valve
stems.
The system dispenses with the extra weight and mechanical complexity of pushrods and rocker arms.
A long chain is frequently used to drive the camshaft from a sprocket
on the crankshaft, but such a long chain tends to 'whip'. The problem
is overcome in some designs by fitting intermediate sprockets and two
shorter drive chains, kept under tension.
Another method uses a non-stretch oil-resistant, toothed rubber drive
belt which engages with toothed sprockets on the camshaft and
crankshaft.
As there are fewer moving parts between the camshaft and the valve,
the overhead-cam (OHC) method is more efficient and produces more power
for a given engine capacity than an engine with pushrods, because it can
operate at higher speeds. With either system, there must be some free
play in the operating gear, so that the valve can still close completely
when parts have expanded through heat.
A pre-set gap tappet clearance is essential between the valve stem
and the rocker arm or cam, to allow for expansion. Tappet clearances
vary widely on different cars, and faulty adjustment can have serious
effects.
If the gap is too large, the valves open late and close early, reducing power and increasing engine noise.
Too small a clearance prevents the valves from closing properly, with a consequent loss of compression.
Some engines have self-adjusting tappets, which are hydraulically operated by the engine oil pressure.
Valve timing:
To see how valve-timing works in a 4-stroke engine cycle, let’s show
piston motion as a circle. In this simple cycle, each stroke is shown as
a semi-circle.
This intake valve opens at top dead center, and closes at bottom
dead center. The blue line shows that period and it matches the intake
stroke.
The exhaust valve opens at bottom dead center, then closes at
top dead center before the new air-fuel mixture enters the cylinder.
In practice, the intake valve usually opens earlier than top dead center, and stays open a little past bottom dead center.
The exhaust valve opens a little before bottom dead center, and stays open a little past top dead center.
When the valves actually open and close, can be measured by
angles. To make these angles easier to read, let’s use a spiral instead
of a circle.
This intake valve opens 12° before the piston reaches top dead center.
And it closes 40° after bottom dead center.
The exhaust valve opens 47° before bottom dead center - and
stays open - until 21° past top dead center. This gives exhaust gases
more time to leave.
By the time the piston is at 47° before bottom dead center on
the power stroke, combustion pressures have dropped considerably and
little power is lost by letting the exhaust gases have more time to
exit.
When an intake valve opens before top dead center and the exhaust valve opens before bottom dead center, it is called lead.
When an intake valve closes after bottom dead center, and the exhaust valve closes after top dead center, it is called lag.
On the exhaust stroke, the intake and exhaust valve are open at
the same time for a few degrees around top dead center. This is called
valve overlap. On this engine, it is 33°.
Different engines use different timings. Manufacturer specifications contain the exact information.
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By:Prayag nao An internal expanding brake consists of two shoes S 1 and S 2 . The outer surface of the shoes are lined with some friction material (usually with Ferodo) to increase the coefficient of friction and to prevent wearing away of the metal. Each shoe is pivoted at one end about a fixed fulcrum O 1 and O 2 and made to contact a cam at the other end. When the cam rotates, the shoes are pushed outwards against the rim of the drum. The friction between the shoes and the drum produces the braking torque and hence reduces the speed of the drum. The shoes are normally held in off position by a spring . The drum encloses the entire mechanism to keep out dust and moisture. This type of brake is commonly used in motor cars and light trucks. We shall now consider the forces acting on such a brake, when the drum rotates in the anticlockwise direction . It may be noted that for the anticlockwise direction, the left hand shoe is known as leading or primary shoe
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