Engineering: Engine Valve Systems

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Questions and Answers

What is the purpose of having a pre-set gap tappet clearance in valve systems?

To allow for thermal expansion of components

What happens if the tappet clearance is too large?

The valves open late and close early, reducing power and increasing noise

What is the consequence of having a tappet clearance that is too small?

The valves are prevented from closing properly, resulting in a loss of compression

How does the camshaft drive-chain sprocket ratio compare to the crankshaft sprocket in an overhead valve (OHV) system?

The camshaft sprocket has twice as many teeth as the crankshaft sprocket Signup and view all the answers

What is the advantage of an overhead-cam (OHC) engine over an OHV system?

The OHC engine has fewer parts to operate the valves Signup and view all the answers

In an OHC engine, what is the role of the 'fingers'?

They act directly on the valve stems to open and close the valves Signup and view all the answers

What is the purpose of using intermediate sprockets and two shorter drive chains in some OHC engine designs?

To overcome the problem of 'whipping' in a long camshaft drive-chain Signup and view all the answers

In an OHV system, what component is responsible for opening the valves?

All of the above components work together to open the valves Signup and view all the answers

What is the purpose of the valve springs in an engine valve system?

To close the valves after they have been opened by the camshaft Signup and view all the answers

How does the rotational speed of the camshaft compare to the crankshaft in a typical engine design?

The camshaft rotates at half the speed of the crankshaft Signup and view all the answers

What is the main purpose of the inlet and exhaust valves in an engine?

To control the intake of fuel and the release of exhaust gases Signup and view all the answers

How are the opening and closing of the valves controlled in an engine?

By pear-shaped lobes called cams on a rotating camshaft Signup and view all the answers

What is the purpose of the tappets and pushrods in the engine's valve system?

To transmit the motion from the camshaft to the rocker arms Signup and view all the answers

What is the role of the rocker arms in the engine's valve system?

To pivot and push the valves down against the pressure of their springs Signup and view all the answers

What is the main advantage of the overhead-cam (OHC) system compared to the overhead-valve (OHV) system?

The OHC system can operate at higher speeds and produces more power Signup and view all the answers

What is the purpose of the valve springs in the engine's valve system?

To hold the valves in the raised (closed) position Signup and view all the answers

What is the main difference between the overhead-valve (OHV) and overhead-cam (OHC) systems in terms of the number of moving parts?

The OHC system has fewer moving parts than the OHV system Signup and view all the answers

What is the primary function of the camshaft in the engine's valve system?

To open and close the valves at precise moments Signup and view all the answers

How is the camshaft driven in an engine's valve system?

It is driven by a chain, belt, or set of gears from the crankshaft Signup and view all the answers

Study Notes

Engine Systems

There must be some free play in the operating gear to allow for thermal expansion and ensure the valve closes completely.

Tappet Clearance

A pre-set gap (tappet clearance) between the valve stem and rocker arm/cam is essential to allow for expansion.
Tappet clearances vary widely on different cars.
Faulty adjustment can have serious effects on engine performance.

Incorrect Tappet Clearance

If the gap is too large, valves open late and close early, reducing power and increasing engine noise.
If the gap is too small, valves may not close properly, leading to a loss of compression.

Self-Adjusting Tappets

Some engines have self-adjusting tappets, which are hydraulically operated by the engine oil pressure.

Overhead Valve System (OHV)

The OHV system uses pushrods, with the crankshaft adjacent and parallel to the crankshaft in the cylinder block.
The camshaft rotates at half engine speed due to the camshaft drive-chain sprocket having twice as many teeth as the crankshaft sprocket.

Overhead Camshaft (OHC) Engine

The OHC engine has fewer parts to operate the valves, with cams acting directly on bucket tappets or short levers (fingers).
The system eliminates the need for pushrods and rocker arms, reducing weight and mechanical complexity.

Camshaft Drive

A long chain is often used to drive the camshaft, but can 'whip' if not properly tensioned.
Some designs use intermediate sprockets and two shorter drive chains to overcome this issue.

Valve Operation

The inlet valve allows the air-fuel mixture into the cylinder, while the exhaust valve allows spent gases to escape.
The valves open and close at precise moments to allow efficient engine operation at all speeds.

Camshaft

The camshaft is driven by a chain, belt, or gears from the crankshaft.
Pear-shaped lobes (cams) on the camshaft control valve operation.

Pushrod Operation

Pushrods extend from tappets above each cam, meeting rocker arms that bear against the valve stem.
As the pushrod rises on the cam, it pivots the rocker arm, which pushes the valve down (open) against the valve spring.
As the cam lobe rotates further, the valve spring closes the valve.