Why Use Variable Valve Timing?
- Increased Volumetric Efficiency
- Reduction of Pumping Losses
- Lower Parts Count
- More efficient
Variable Valve Timing or VVT is common on most engines and is responsible for the elimination of many EGR valves as well as increased performance and fuel economy on many engines. Most systems are lube oil activated and use a control solenoid as well as the camshaft sensor, crankshaft sensor, and PCM for control. Newer systems operate off rotational torque from the engine. Some DTCs set by these systems can be confusing to a technician and the OE diagnostics often leave much to be desired. This article will focus on the different type of VVT systems and there operation.
The PCM calculates and determines the desired camshaft position. It will continually update the VCT solenoid duty cycle until desired positioning is achieved. When the VCT solenoid is energized, engine oil is allowed to flow to the VCT actuator assembly which advances or retards the cam timing. One half of the VCT actuator is coupled to the camshaft and tlie other half is connected to the timing chain. Oil chambers between the two halves couple the camshaft to the timing chain. When the flow of oil is shifted from one side of the chamber to the other, the differential change in oil pressure forces the camshaft to rotate in either an advance or retard position depending on the oil flow. A difference between the desired and actual camshaft position represents a position error in the PCM’s VCT control loop. For the VVT system to operate properly, the engine oil must be clean and at the proper level. The correct viscosity and type of oil must be used and the oil filter must have a drain back valve if the vehicle was equipped with one originally. Adequate oil pressure is required to operate the VVT system.
Fixed Timing Limitations
With a fixed camshaft engineers had to balance between idle quality and performance on the one side and low emission and fuel economy on the other. As a result, none of these goals was achieved completely. Variable valve timing allows the engine to obtain a smooth idle while achieving the rest of the goals. Modern VVT systems combined with technologies like electronic throttle control and direct fuel injection allow smaller engines to produce high horsepower and torque at lower RPM.
Idle Valve Timing
In the (idle valve timing) camshaft timing chart at the end of the article the left hand trace represents exhaust valve opening while intake valve opening is on the right. For smooth idle operation the VVT system reduces overlap (where both the exhaust and intake valves are open.) Smooth idle requires that the exhaust and intake valves are not both open due to the low air velocity. At low engine speeds the time the valves remain open is longer. If the exhaust valve is open it will cause pulsing into intake.
Performance Valve Timing
For increased performance the exhaust cam is retarded a small amount to promote engine breathing. Higher engine speeds mean shorter valve open times and higher air velocity. The increased velocity pushes more exhaust out of the cylinder. Increased Volumetric Efficiency: The retarded exhaust valve timing increases Volumetric Efficiency. The exhaust valve is still open when the intake opens. Outgoing exhaust pulse creates a low pressure zone behind the valve. This increases the pressure differential between the intake port and the combustion chamber resulting in better cylinder filling. Remember, this can’t be done at idle due to low air speeds.
EGR Function Valve Timing
To provide an EGR function, the exhaust cam is fully retarded adding lots of overlap. This causes exhaust gas to remain trapped in the cylinder. This ability allows a reduction in hardware and service issues from carbon. Exhaust Gas Retention: Opening the exhaust valve later retains more exhaust pressure in the cylinder. This causes pushback to the intake charge and exhaust Gas RETENTION. Trapped Exhaust Charge: This provides EGR without hardware, even distribution, no problems with clogged ports, and more precise EGR control.
Types Of Variable Valve Timing
Exhaust Only: The exhaust camshaft is retarded at part throttle. This delays the exhaust valve closing which allows some exhaust gas into the intake stroke, which has an EGR effect. Th is also delays exhaust valve opening which increases the expansion stroke.
The intake camshaft is advanced at part throttle and WOT. This opens the intake valve sooner and allows some exhaust gas into the intake stroke, which has an EGR effect. This also closes the intake valve sooner which increases the compression stroke. On a cold engine opening the intake valve sooner will also warm the intake charge and assist in reducing start up emissions.
Both the intake and exhaust camshafts are retarded at various points in the throttle range. This will increase the EGR effect and improves fuel economy by reducing pumping losses.
The exhaust camshaft is retarded and the intake valve is advanced independent of each other. This maximizes the EGR effect, and further reduces pumping losses for maximum efficiency.