Camshaft Control (CVVT)
When the camshaft (A) is set at the factory, it is aligned with the position of the crankshaft (B). The position of the camshaft in relation to the crankshaft is designated the camshaft 0 position . During camshaft (CVVT) control, the 0 position of the camshaft is advanced so that the opening and closing of the intake and exhaust valves can be changed to match the camshaft. Engine performance can be increased, idle quality increased and emissions reduced by regulating the camshaft timing changes.
There are diagnostics for this function. Also refer to: CAMSHAFT DIAGNOSTICS (CVVT)
Detecting the position of the camshaft (camshaft timing changes)
In order to detect the position of the camshaft (camshaft timing) in relation to the crankshaft, the engine control module (ECM) uses the signals from the engine speed (RPM) sensor (the position of the crankshaft) and from the camshaft position (CMP) sensor (the position of the camshaft). The control module uses these two signals to determine the position of the camshaft in relation to the position of the crankshaft.
Detecting the camshaft flanks
The camshaft is divided into four flanks per revolution of the camshaft (flanks 1-4). The flanks are detected by the camshaft position (CMP) sensor. The signal affects the shape of the camshaft rotor. Also refer to: DESIGN
The teeth on the camshaft pulse wheel for camshaft flanks 1 and 4 are shorter than the other teeth. This enables the control module to determine the position of the camshaft. The control module is able to establish which combustion cycle the cylinders are in.
Detecting the reference positions of the camshaft
The crankshaft has four reference positions, one for each camshaft flank. The reference positions are predetermined points on the flywheel. The reference positions are detected using the signal from the engine speed (RPM) sensor. Also refer to: DESIGN
The camshaft turns at half the speed of the crankshaft. This means that two reference positions are detected for each turn of the crankshaft. Therefore two engine revolutions are required to detect all flanks on the camshaft.
The positions on the flywheel are designated °CA (Crank angle). 0°CA = Top dead center cylinder 1.
Flanks 3 and 4 are detected during the second revolution of the engine.
Detecting the position of the camshaft in relation to the position of the crankshaft
Each camshaft flank aligns with pre-defined positions on the crankshaft when the camshaft is in its 0 position. These positions on the crankshaft are called reference positions for the flanks.
The illustration shows how the signals relate to each other when the camshaft is in its 0 position (the camshaft is not deployed).
A: Engine speed (RPM) sensor signal.
B: Camshaft position (CMP) sensor signal. From high to low signal when the teeth on the camshaft pulley leave the camshaft position (CMP) sensor.
C: Low engine speed (RPM) sensor signal because of the holes in the flywheel/carrier plate.
D: Top dead center (TDC) cylinder 1, 0°CA (84°CA after hole "C" in the flywheel/carrier plate).
1: Detection of flank 1, reference position 47°CA "D1".
2: Detection of flank 2, reference position 227°CA "D2".
3: Detection of flank 3, reference position 407°CA "D3".
4: Detection of flank 4, reference position 587°CA "D4".
If the flanks do not correspond to the reference positions on the crankshaft when the camshaft is in the 0 position (not deployed), the engine control module (ECM) will store the difference. There may be a difference from the camshaft 0 position if the timing belt is incorrectly seated or the camshaft are not correctly set for example. A mechanically damaged camshaft reset valve may prevent the camshaft moving to the 0 position when the engine control module (ECM) stores the adaptation value for the deviation of the camshaft. This may result in high deviation and a diagnostic trouble code (DTC) being stored.
Deviations can be read out in the diagnostic tool.
Regulating the camshaft position
The engine control module (ECM) controls the camshaft reset valve steplessly. The valve controls the flow of engine oil to the continuous variable valve timing (CVVT) unit which is affected by the oil pressure that builds up. This allows the CVVT unit to change the position of the camshaft. Also see Control, CVVT unit and. Refer to DESIGN .
When deploying the camshaft by for example 10°CA (the engine control module (ECM) deploys the camshaft), the detection of the camshaft flanks will be offset by 10°CA from the reference positions for the crankshaft.
The illustration shows how the signals relate to each other when the camshaft is controlled (deployed camshaft), D1 - D4 is reduced.
The engine control module (ECM) is then able to calculate the °CA (crankshaft degrees from top dead center (TDC)) that the intake valve opens and the exhaust valve closes for each cylinder. This is because the opening and closing angles are fixed and predefined in relation to the flanks on the camshaft rotor.
Intake valve's closing angle and exhaust valve's opening angle can be read off in the diagnostic tool.
The following applies when the camshafts are in their 0 position (no deployment) :
- the opening angle of the intake valve 27°CA
- the closing angle of the exhaust valve is 26.5°CA.
The following applies when the camshaft is fully deployed :
- the opening angle of the intake valve is - (minus) 23°CA
- the closing angle of the intake valve is - (minus) 3.5°CA.
The total of the closing angle of the intake valve - (minus) the opening of the exhaust valve = the valve overlap.
Control, CVVT unit
HINT: The illustration is a view of the CVVT unit from the side and from the rear.
1: Timing belt pulley
2: Lock pin with spring
3: Rotor
4: Rotor wings
A1: Chamber A
B1: Chamber B
The continuous variable valve timing (CVVT) unit allows the position of the camshaft to be adjusted relative to the crankshaft.
The camshaft is secured to the rotor (3). The rotor (and with it the camshaft) rotates in relation to the timing belt pulley (1) within set angles.
When the camshaft is in its 0 position, the timing belt pulley and the rotor are locked together by the lock pin (2). A spring-loaded lock pin slides into a hole on the inside of the end of the timing belt pulley to secure it.
Camshaft reset valve
5: Piston with slits
6: Return spring
A: Channel leading to chamber A1 in the CVVT unit
B: Channel leading to chamber B1 in the CVVT unit
C: Channel for oil (pressure)
D: Channel for oil (return)
The camshaft reset valve controls the oil flow to the continuous variable valve timing (CVVT) unit. The engine control module (ECM) uses a pulse width modulation (PWM) signal to control the valve. Also refer to: DESIGN
Control takes place as follows when deploying the camshaft
HINT: The illustration is a view of the CVVT unit from the side and from the rear.
- The oil is forced from the engine lubrication system (C)
- The valve is grounded by the engine control module (ECM). The oil flows via the slits in the piston (5) to the oil channel (A) in the camshaft
- The oil flows via oil channels in the camshaft to the top of the lock pin (2). If the camshaft is in its 0 position, the lock pin will be forced in by the oil pressure and the rotor releases from the timing belt pulley
- The chamber (A1) fills with oil. The oil pressure will rotate the rotor (3)
- The oil in the chamber (B1) will be forced out of the chamber by the rotation of the rotor. The oil flows to the engine oil pan via the camshaft, channel (D) and the valve.
Control takes place as follows when returning the camshaft
HINT: The illustration is a view of the CVVT unit from the side and from the rear.
- The oil is forced from the engine lubrication system (C)
- The engine control module (ECM) breaks the ground connection for the valve. The piston (5) in the valve springs back (6) and the oil flows via the piston slits in the valve to the oil channel (B) in the camshaft
- The chamber (B1) fills with oil. The oil pressure in the chamber will rotate the rotor
- The rotor (4) reaches its limit position and the lock pin slides into a hole on the inside of the front end of the camshaft pulley
- The oil in the chamber (A1) will be forced out of the chamber by the rotation of the rotor. The oil flows to the engine oil pan via the camshaft, channel (D) and the valve.
The reset valve is controlled by the engine control module (ECM) at high frequency. The frequency changes for deployment and return. This ensures rapid and precise control. The extent of camshaft control (the angle of the camshaft) varies depending on the engine variant.
"Wide Range" concept
Ignition timing
The combustion of the fuel film on the cylinder walls is improved by retarding the ignition.
Retarded ignition reduces the efficiency of the engine and the heat energy which is generated is released with the exhaust gases. This is used to heat the three-way catalytic converters (TWC).
Exhaust camshaft
By opening the exhaust valve late, combustion takes place over a relatively long period. The film of fuel on the cylinder walls combusts, reducing the exhaust emissions.
Intake camshaft
By opening and closing the intake valve late:
- so that there is little or no overlap, a predetermined pressure difference is maintained between the intake manifold and the combustion chamber/cylinder. The lower pressure in the cylinder ensures that all the injected fuel reaches the cylinder. This allows the Engine Control Module (ECM) to calculate and control the fuel quantity required in the combustion phase in advance
- maintains a high and stable pressure in the intake manifold (due to the upwards movement of the piston). Stable high pressure means that the vaporization of the fuel which has condensed on the walls of the intake manifold can be predicted.
Double continuous variable valve timing (CVVT)
The CVVT on both the intake camshaft and the exhaust camshaft means that the valve overlap can be changed to a greater degree than on engines where only one of the camshafts is controlled. Valve overlap is the extent to which the intake and exhaust valves (on the same cylinder) are open at the same time.
The advantages of continuous variable valve timing (CVVT) are used in different operating conditions:
- during cold starting and during the warm-up process when the intake camshaft and exhaust camshaft are set late. This reduces the emissions
- during idle and when the engine is at operating temperature when the exhaust camshaft is set to early and the intake camshaft is set to late. This results in small valve overlap, reducing exhaust gas recirculation (EGR) and ensuring stable idling
- at part load when both the exhaust and intake camshaft are set relatively late, with greater valve overlap. Greater valve overlap results in internal exhaust gas recirculation (EGR) which reduces the release of nitrous oxide. This also limits the incoming fuel/air mixture to the cylinder. As a result, the throttle does not need to reduce the supply of air, thereby reducing "pump losses" and lowering fuel consumption. At higher engine speeds (RPM), the camshafts are set for a smaller valve overlap. The exhaust camshaft is set earlier, the intake camshaft later. This provides an optimum fuel/air mixture to the cylinder. Reduces internal exhaust gas recirculation (EGR).