is, it is positioned in the cylinder head. There are various designs using direct or indirect mechanisms to convert the rotating cam motion into a reciprocating motion, and then transfer this motion to the valve stems. These designs are proposed to facilitate a close tolerance in operating clearances.
A further development, now employed extensively, is the use of twin or double overhead camshafts (DOHC) ( Fig. 2.100 ). These can use direct or indirect valve actuation and are well suited to multivalve engine designs, including those with variable valve timing.
Figure 2.99 Overhead valves (OHV): 1, rocker shaft bolt; 2, push rod; 3, lubrication passages;
4, exhaust valve; 5, cam bearing journal; 6, cam lobe; 7, hydraulic tappet; 8, turbulence ramp; 9, inlet valve; 10, rocker
Figure 2.100 Double overhead camshaft engine (DOHC): 1 and 2: tensioner wheels; 3, timing
belt; 4, inlet cam; 5, exhaust cam; 6, distributor drive; 7, valve
Key fact
Overhead cam (OHC) refers to the position of the camshaft in the engine.132
2
Automobile mechanical and electrical systemsIndirect, rocker arm-type valve actuators incorporate close tolerance adjusters. Two systems are commonly seen, a rocker shaft and pivot stud, or a rocker arm supported on a pedestal at one end and the valve stem at the other, the cam acting between these two points ( Fig. 2.101 ). A hydraulic pedestal can be used for self-adjustment of the mechanism.
Figure 2.102 shows a typical engine oil lubrication circuit that feeds the self- adjusting followers with pressurized oil to maintain the correct valve clearances. Always refer to manufacturers’ data for the service requirements of the valve train system. Often, special procedures are required when replacing and recommissioning self-adjusting valve mechanisms, and these must be followed to prevent engine damage ( Fig. 2.103 ).
Many engines now employ variable camshaft timing to optimize the inlet valve timing with respect to engine speed and load conditions ( Fig. 2.104 ).
As air enters the engine through the inlet manifold this forms a column of moving air that possesses kinetic energy ( Fig. 2.105 ). The pulsating nature of the engine’s
Figure 2.101 Valve operating mechanisms. Left, 1, cam; 2, adjusting screw in direct acting
rocker. Centre, 1, cam; 2, hydraulic follower. Right, 1, cam; 2, pivot and adjuster; 3, fi nger follower
Figure 2.102 Hydraulic tappets and components: 1, oil to rocker arms, hydraulic tappets;
3, fi lter; 4, crank main bearings; 5, big end bearings; 6, crank driven oil pump; 7, oil under pressure; 8, camshaft
133
Engine systems
2
Figure 2.103 Hydraulic lifters
Figure 2.104 Timing is changed under electronic control: 1, camshaft gear; 2, spring; 3, outer
helical teeth; 4, adapter with ring grooves; 5, oil supply to front chamber via hollow bolt (not shown); 6, oil supply to rear chamber; 7, rear chamber; 8, front chamber; 9, blanking plug; 10, inner helical teeth
134
2
Automobile mechanical and electrical systemsair consumption creates pressure waves in this air column. The energy in these pressure waves can be harnessed to assist in charging the cylinder, increasing the volumetric effi ciency of the engine. In order to do this, the valve opening point must be optimized according to the engine condition, and with variable valve timing this can be achieved to increase engine torque and power at various points in the operating speed range.
There are various technologies available to provide the required phase angle between the cam drive and the camshaft for variable valve timing ( Figs 2.106 and 2.107 ). It can be generated via a hydraulic mechanism in the cam wheel that is controlled via a valve assembly from the engine’s electronic control unit (ECU). Cam wheel actuators can employ a ‘helix’ or pressure differential actuation principle. In addition, some engines have employed valve mechanisms with alternative cam profi les where the engine switches over to a different cam lift profi le at certain engine speeds.
The camshaft-driven gearwheel has twice as many teeth as the crankshaft drive gearwheel. The camshaft is therefore driven at half engine speed. Various drive mechanisms are used.
Figure 2.106 Electrohydraulic variable cam control method: 1, adjusting piston with inner and
outer helical teeth; 2, outer helical teeth connected to camshaft pulley; 3, inner helical teeth connected to the camshaft
Figure 2.107 Variable cam timing. (Source: Ford Media)
Key fact
Variable valve timing (VVT) optimizes the valve opening point to increase engine torque and power at various points in the operating speed range.
135
Engine systems
2
For many engines, a toothed belt is used to drive the camshaft ( Fig. 2.108 ). The belt is manufactured from a durable, synthetic rubber with reinforcing fi bres. The teeth moulded on the inside of the belt mate with the corresponding teeth on the crankshaft and camshaft pulley wheels. The teeth formed on the belt can be trapezoidal or rounded ( Fig. 2.109 ). Note that they are not interchangeable and it is important to fi t the correct type when replacing the belt.
Correct tension of the timing belt is imperative for maximum belt life. The belt is generally tensioned by adjustable tensioner wheels ( Fig. 2.110 ). It is important to note that manufacturer- and engine-specifi c information must be sought when making adjustments in service. The advantages of timing belts are that they are cheap and run quietly.
Drive gears ( Fig. 2.111 ) are not used on many applications but have the advantage of a very positive drive because they have little backlash compared with belts and chains ( Fig. 2.112 ).
Covers are fi tted to enclose moving parts and retain oil ( Fig. 2.113 ). The sump or oil pan is fi tted on the underside and holds the oil capacity. The cover that
Key fact
The advantages of timing belts are that they are cheap and run quietly.
Figure 2.108 Cam drive belt in position on the Duratec engine; not to be confused with the
serpentine alternator drive belt. (Source: Ford Media)
136
2
Automobile mechanical and electrical systemsencloses the valve gear is known as the rocker or cam cover. Generally, it incorporates the oil fi ller cap and part of the engine breather system.
At the front of the engine, there is a casing that retains the crankshaft oil seal. In addition, a cover is fi tted to enclose and protect the camshaft drive belt. At the
Figure 2.110 Belt drive components: 1, cam gear; 2, belt; 3, crank gear; 4, water pump;
5, tensioner
Camshaft gear
Injector pump gear
Crankshaft gear
137
Engine systems
2
rear of the engine, another crankshaft oil seal is fi tted to a casing that is bolted to the engine and located by dowels.
The sump contains baffl es to prevent excessive oil movement or surge. This maintains a good supply of oil around the area of the oil pick-up.
Attached around the engine are auxiliary components and subsystems for lubrication, cooling, ignition, fuel, air, exhaust and electrical systems.