Tuesday, May 17, 2011

Cont. Drives for DOHC and Variable valve timing

Operation of the timing unit
The timing chain rotates sprocket on the timing unit and this rotation is carried through to the camshaft by the internal gears of the unit. The gears provide a connection between the sprocket and the camshaft, but they do not rotate.
In the retarded position, oil pressure is blocked off by the Solenoid valve and piston is held in the forward position by the spring.
In the advanced position, the Solenoid valve is opened so that the engine of flows into the advance unit. Pressure of oil against the ring piston moves it and the ring gear rearwards. This turns the camshaft in relation to the sprocket, so that the camshaft and timing is advanced.
The function of Advance and retard are:

1.     Camshaft advance 
when the piston moves the ring gear, helical teeth slide on each other and so the ring gear causes the sleeve to turn. This moves the camshaft in forward (advance) direction in relation to the housing and sprocket. This occurs within the advance unit while it is rotating.

§         Camshaft advance occurs the because of the helical shape of the gear teeth.

2.      Camshaft retard
When the solenoid is operated to relieve the oil pressure in the unit, the oil drains away and the spring force against the piston moves the ring gear forward. Again the gear teeth slide on each other, but this time to retard the camshaft.
The solenoid valve is controlled by the engine’s electronic control module (ECM). At a higher engine speeds, for example above 2000 RPM, ECM opens the valve to allow the engine oil flow into the advance unit. At lower speeds, the solenoid valve Closes off the oil supply and allows oil to drain from the unit.

The engine control module (ECM) of microcomputer that is responsible for monitoring and controlling the number of engine functions.

Variable valve timing in DOHC
Fig. 2.40
With double overhead-camshafts, an advance device is fitted to the intake camshaft but not to the exhaust camshaft. This is because altering the intake valve timing has a much greater benefit than altering the exhaust timing. Figure 2.40 shows schematic arrangement of two camshafts of an engine and a control system. The camshafts are driven by a noticed Timing belt.  The intake-camshaft pulley has a variable timing unit and the exhaust camshaft has a normal pulley.
Components of the system shown are:
  1. The variable valve timing unit.
  2. An oil control valve.
  3. Electronic sensors.
  4. An engine electronic control module.

Operation of the timing system
Engine oil under pressure is directed to the oil control valve. This is a shuttle valve that is operated by an electronic solenoid.  The Solenoid moves the valve, as required, to direct oil to the variable valve-timing unit. And alter the camshaft timing. The Solenoid of the Control valve is operated by the engine’s electronic control module (ECM).
            The ECM receives signals from a number of sources, including the sensor on the camshaft, which registers engine RPM, and the sensor on the camshaft, which detects piston TDC, input from the sensors is processed by the ECM, which determines how much advance or retard the intake camshaft needs for the particular operating conditions.  Output signals from the ECM go to the oil control valve.

§         Variable valve-timing systems rely on correct lubricating oil viscosity for their operation.  Servicing at the recommended intervals with the recommended grade of oil is critical to maximum performance. The use of oil additives is not recommended.

Hydraulic system
Fig. 2.41
The variable valve timing unit has a rotor with in its housing. The housing carries the timing pulley and the rotor is mounted to the end of the camshaft.  Movement of the rotor in relation to the housing will advance or retard camshaft timing shown in fig. 2.41.
The rotor is shaped to fit into the housing and fitted with seals to form hydraulic chambers.
The spool valve and the sections through the timing unit are shown. In the diagram, the spool valve has been moved by the solenoid to direct oil to the advance side of rotor. This has turned the rotor in the housing fully advance the camshaft.
To retard the camshaft, the spool valve is moved in opposite direction. Pressure oil is directed to the retard side of the rotor and drained away from the advance side.
The camshaft does not have to be fully advanced or retarded. It can be held in any intermediate position by the spool valve directing pressure oil to both sides of the rotor. The ECM determines the camshaft timing for the particular operating conditions.
When the engine is stopped, the rotor is held in the retarded position by a spring loaded pin. When the engine is started, engine oil pressure disengages the pin so that the rotor can be turned in the housing.

See Variable valve opening & diagram

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