Camshaft Control (CVVT)

Camshaft Control (CVVT)







When the camshaft is set at the factory, it is aligned with the position of the crankshaft. The position of the camshaft in relation to the crankshaft is designated the camshaft 0 position. During camshaft control (CVVT) the camshaft 0 position is offset. The value of the 0 position offset is called the cam timing. The cam timing is 0 degree if the camshaft is not being controlled.

By controlling the cam timing (the camshaft is deployed from its 0 position) the performance of the engine is increased, the idle speed quality is improved and the emissions are reduced.

In order to detect the cam timing, 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). By comparing these two signals, the control module is able to determine the cam timing (the number of degree the camshaft is from its 0 position).

There are diagnostic for this function. See also Camshaft diagnostics (CVVT).

Camshaft cam timing







The camshaft (A) is divided into four flanks per revolution of the camshaft (flanks 1-4). The angle between flanks 1-2 = 103 degree, flanks 2- 3 = 90 degree, flanks 3-4 = 90 degree and flanks 4-1 = 77 degrees. The angle between the flanks is different, allowing the control module to determine which flank it is detecting and therefore determine which combustion cycle the cylinders are in,

The crankshaft has four reference positions, one for each camshaft flank. 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. For example (may vary between different engine variants): flank 1 at 0 degree, flank 2 at 206 degree, flank 3 at 386 degree and flank 4 at 576 degree (flanks 3 and 4 are detected during the second engine revolution).

The reference positions of the crankshaft coincide exactly with each camshaft flank if the camshaft cam timing is 0 degree. If the cam timing deviates from 0 degree, i.e. the flanks deviate from the reference positions of the crankshaft, the control module calculates the number of crankshaft degree by which the flank is deviating.

The control module detects the reference positions for the camshaft flanks using the signal from the engine speed (RPM) sensor.

Example:
If camshaft flank 1 is detected at 10 degree from the crankshaft reference position for flank 1 (0 degree +10 degree), the cam timing is 10 degree. If the cam timing is the same (10 degree), flank 2 is detected at 216 degree (206 degree + 10 degree), flank 3 at 396 degree (386 degree + 10 degree), and flank 4 at 586 degree (576 degree + 10 degree). The cam timing can be positive (+) or negative (-) depending on whether the flank is detected before or after the different reference positions for the crankshaft.

The engine control module (ECM) controls the camshaft reset valve smoothly in order to regulate the camshaft cam timing. 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.

Also see design.

Control







Control takes place as follows when deploying the camshaft:
1. Oil is forced from the engine lubricating system to the intake port on the reset valve
2. The engine control module (ECM) grounds the valve, the position of the piston in the valve changes and the oil is guided to the continuous variable valve timing (CVVT) unit chamber (A1) via the duct (A2) in the camshaft
3. The continuous variable valve timing (CVVT) unit hub is pressed backwards by the oil pressure. The continuous variable valve timing (CVVT) unit then rotates the hub and the carriers are joined by twisted splines
4. The oil flows to the engine oil pan via the outer ducts on the hub and the reset valve's return hose.

Control takes place as follows when returning the camshaft:
1. Oil is forced from the engine lubricating system to the intake port on the reset valve
2. The engine control module (ECM) breaks the ground connection for the valve. The piston in the valve is then pressed back by a spring. The oil flows to the continuous variable valve timing (CVVT) unit chamber (B1) via a duct (B2) in the camshaft
3. The hub of the continuous variable valve timing (CVVT) unit is forced forward by the oil pressure that is created. The continuous variable valve timing (CVVT) unit will rotate back to the non-deployed position
4. The oil flows to the engine oil pan via the center duct on the hub and the reset valve's return duct.

The above takes place very quickly. The engine control module (ECM) controls the deployment and return of the reset valve continually at high frequency. This results in rapid and exact control. The extent of camshaft control (the change of cam timing) varies depending on the engine variant and the model year.

"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 (turbocharged engines)
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/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)
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 phases 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 to 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 the 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).