14-01-2013, 04:37 PM
Valve and Valve Trains
1Valve and Valve.ppt (Size: 876.5 KB / Downloads: 63)
Purpose of Valves
The valves and valve-operating mechanism by which an internal combustion engine takes air or fuel-air mixture into the cylinders and discharges combustion products to the exhaust.
The intake and exhaust valves are operated by the valve trains. These are overhead camshaft with bucket tappets or rocker arms, and camshaft in the block with pushrods
Valves may be arranged in various ways in the cylinder block
Overhead Valve(OHV)/Pushrod Engines
Have camshaft in the cylinder block and valves in the cylinder head
Have higher compression ratio than L-head engines
Locating the valves directly over the piston permits the clearance volume to be smaller
Some overhead valve engines have valve relief cut into the piston head to provide space to open without striking the piston
Multivalve Engine
Some engines can have three, four, five and even six valves per cylinder to improve engine breathing and increase output power
It improve the volumetric efficiency of the engine
Driving the Camshaft
The camshaft in the block is driven either by timing gears or by sprocket and timing chain.
Sprocket are like gears, except that the teeth are shaped to fit into the chain links
Timing chain and sprocket usually run more quitely than gears
Camshaft Timing
Camshaft timing is the relationship between the camshaft and the crankshaft. The camshaft is driven by the crankshaft
There are four stroke to complete cycle action in four stroke engine. So the crankshaft must turn two times to turn the camshaft once, which open each valve once.
This 1:2 gear ratio is achieved by making the camshaft gear or sprocket twice as large as crankshaft gear or sprocket
Valves
The intake valve is usually larger than the exhaust valve because when the intake valve is open , the only force moving air-fuel mixture into the cylinder at atmospheric pressure, but when exhaust valve opens it is still have high pressure in the engine cylinder.
A smaller exhaust valve provides enough space for high pressure exhaust gases to get out of the cylinder