Camshafts and Related Parts

The primary function of the camshaft in a four-stroke engine is to:

Drive the crankshaft Open and close the engine valves in the correct sequence and timing Control oil pressure Ignite the fuel–air mixture directly

In most four-stroke engines, the camshaft rotates at:

The same speed as the crankshaft Half the speed of the crankshaft Twice the speed of the crankshaft A random speed independent of the crankshaft

Camshaft lobes are shaped to control:

Oil flow through the galleries How far and how long a valve opens (lift and duration) Piston diameter Combustion chamber size

A common camshaft drive method on overhead valve engines is:

Friction between shafts Timing chain and sprockets connecting crankshaft and camshaft Electric motor drive Hydraulic pressure alone

Timing belts are used on many engines to:

Drive accessories only Drive the camshaft(s) from the crankshaft with precise synchronization Control fuel injection timing only Drive the oil pump internally

The ILM explains that valve timing must be correct so that:

Valves remain closed at all times Valves open and close at appropriate points in relation to piston position for efficient filling and scavenging Only exhaust valves open during intake Valves contact pistons for maximum compression

Camshaft bearings are used to:

Support the crankshaft Support the camshaft in the block or head and allow smooth rotation Support the pistons Support the flywheel

In an overhead valve (OHV) engine, valve motion is typically transferred from camshaft to valves using:

Only rocker arms directly Lifters (tappets), pushrods and rocker arms Timing chains directly on the valves Hydraulic pumps

Hydraulic valve lifters are designed to:

Reduce oil pressure Automatically maintain zero valve lash by using engine oil pressure internally Hold valves open at all times Replace the need for camshaft bearings

Mechanical (solid) lifters differ from hydraulic lifters because they:

Have internal oil chambers Require periodic adjustment of valve clearance (lash) Are filled with coolant Cannot be used in overhead cam engines

In overhead cam (OHC) engines, camshafts are located:

In the engine block beside the crankshaft In the cylinder head above the valves In the oil pan Inside the flywheel

The term 'camshaft phasing' or 'variable valve timing (VVT)' in the ILM refers to systems that:

Change crankshaft speed Change the relationship between camshaft and crankshaft to alter valve timing for performance and emissions Change piston stroke Change coolant temperature

Camshaft position sensors provide information used to:

Control alternator output Help the engine control module time fuel injection and ignition events Control cooling fan speed only Set brake balance

Valve overlap is defined as the period when:

Both valves are closed Intake and exhaust valves are open at the same time near top dead centre between exhaust and intake strokes Only intake valves open Only exhaust valves open

Timing chain guides and tensioners are needed to:

Increase noise Control slack and maintain proper chain tension and tracking Prevent oil from reaching the chain Support the pistons

Camshaft duration, as used in the ILM, refers to:

Crankshaft angle over which the valve is held open Total engine run time Oil change interval Time required to machine the camshaft

High-performance camshafts may have increased lift and duration, which:

Improves low-speed torque and fuel economy only Can increase high-speed power but may reduce idle quality and low-speed performance Eliminate the need for valves Remove the need for springs

Pushrods in an OHV engine are designed to:

Carry coolant to the head Transfer motion from lifters to rocker arms Support the crankshaft Hold piston rings in place

Rocker arms pivot in order to:

Move pistons back and forth Change motion direction and apply cam/lifter movement to open valves Drive the timing chain Control oil pump speed

Rocker arm ratio is defined as the ratio between:

Valve stem diameter and rocker length Distance from pivot to valve tip and from pivot to pushrod, affecting valve lift Camshaft speed and crankshaft speed Number of cylinders and number of valves

According to the ILM, worn camshaft lobes may cause:

Increased valve lift and performance Reduced valve lift, misfire, rough idle and loss of power Improved emissions Higher oil pressure

Timing belt replacement intervals are important because a failed belt on an interference engine can:

Improve compression Cause valve-to-piston contact and severe engine damage Increase oil pressure Improve fuel economy

Camshaft end play is controlled by:

Head gasket thickness Thrust plates, flanges or retainer designs specified by the manufacturer Valve spring pressure only Crankshaft balance

In dual overhead cam (DOHC) engines, there are:

Two camshafts total for all cylinders Two camshafts per cylinder bank, typically one for intake and one for exhaust valves No camshafts at all Camshafts located in the crankcase

From a parts perspective, camshaft-related components include:

Only timing belts Camshafts, lifters/tappets, pushrods, rocker arms, timing chains, sprockets, guides and tensioners Only connecting rods Only pistons

Variable valve timing units are often mounted:

On the crankshaft rear flange On the camshaft sprockets or cam phasers at the front of the engine Inside the oil pan On the flywheel

Valve float occurs when:

Valves close too quickly Valve springs cannot control the valves at high engine speed, causing them to lose contact with the cam profile Valves are fully seated Valve guides are excessively tight

Camshaft timing marks are provided so that:

Paint colour can be matched Crankshaft and camshaft can be aligned correctly during assembly Oil pressure can be adjusted Fuel pressure can be increased

In engines with separate camshaft bearing shells, the ILM notes that bearings must be:

Interchanged freely between positions Installed with correct orientation and size according to manufacturer specifications Painted before assembly Filed for clearance without measurement

The ILM highlights that noisy valve trains can result from:

Perfect valve adjustment Excessive lash, worn lifters, worn rocker arms or worn cam lobes High-quality oil Correct spring pressure

When supplying timing components, parts technicians must consider:

Only belt length Engine family, belt or chain type, number of camshafts, tensioner style and any VVT components Only coolant type Only exhaust system design

Camshaft thrust loads are created primarily by:

Straight cam lobes Angled cam lobes and helical drive gears that try to move the camshaft endwise Coolant circulation Battery voltage

In some engines, a balance shaft system is driven from the cam drive to:

Increase oil pressure Counteract vibration from engine firing forces Drive fuel pumps only Control emissions directly

The ILM stresses that correct valve train parts supply is important because:

Mismatched parts only affect appearance Incorrect lifters, camshafts or timing sets can lead to rapid wear or catastrophic failure Any cam and lifter set will work together Timing belts never fail