Variable Valve Timing

Variable valve timing is a method to improve performance, fuel economy and to reduce emissions by varying the valve lift timing. Inlet valve is used to allow the combustible mixture to enter the combustion chamber. Varying the inlet valve lift timing can lead to a significant change in the performance of the engine.

Engine requires large amount of air at high speeds, therefore a shorter inlet valve lift duration doesn't let the required amount of air to enter the combustion chamber and this affects the performance. On the other hand, at lower speeds if the valves are open for a prolonged time then the fuel would escape unburnt leading to higher emission levels.

Various adjustments that can be implemented:

1. Earlier closing of inlet valve: This is an effective way of increasing the fuel economy. At low load conditions, the amount of combustible mixture required is less, therefore closing the inlet valve earlier allows lesser amount of combustible mixture to enter the combustion chamber. The temperature inside the combustion chamber is significantly reduced, thereby reducing nitric oxide emissions by 25%. But it can increase hydrocarbon emissions. It can reduce pumping losses up to 40%.


2. Late closing of inlet valve: Closing the inlet valve late allows some of the combustible mixture to flow back to the inlet manifold, thus increasing the pressure inside the inlet manifold. The next stroke will allow the combustible mixture to flow in at a higher pressure, thereby reducing pumping losses by 40% at low load. It also reduces nitric oxide emissions by 25%.


3. Earlier opening of intake valve: This is related to valve overlapping where few amount of exhaust gases enter the inlet manifold via the inlet valve and it is cooled. In the next inlet stroke, the exhaust gases enter the combustion chamber, thus helping in reducing the temperature and results in lesser nitric oxide emissions. This is favourable for low load conditions.


4. Earlier closing of exhaust valve: Closing the exhaust valve early can trap few amount of exhaust gases inside. This allows lesser amount of air-fuel mixture to enter inside the cylinder in the next stroke, hence increasing fuel efficiency.


5. Late closing of exhaust valve: Closing the exhaust valve late allows the entire exhaust gases to escape to the atmosphere, hence allowing more air-fuel charge in the intake stroke.

HONDA VTEC:

Honda engines are known for its VTEC (Variable Valve Timing and Lift Electronic Control). It is a combination of mechanical and electrical system allowing the engine to have multiple camshafts. The camshafts are provided with lobes of varying sizes that can alter the timing of valve operation at different speeds.

The basic function of a VTEC system is to allow the inlet valves to open for a longer duration at higher speeds. This will allow more air to flow into the engine, thereby the engine can burn more fuel and generate more power. Rocker arms are used to open and close the valves. The rotation of cam moves the rocker arm and the rocker arm operates the valve. Rocker arm shafts are provided with an oil supply line which comes from the solenoid. The solenoid varies the oil pressure based on the signals from the ECU (Electronic Control Unit). There are 3 cam lobes provided for one cylinder. One big lobe at the centre and 2 smaller lobes set apart by the bigger lobe. Similarly, there are 3 rocker arms, two for the smaller cam lobes and one for the bigger lobe. the oil supply line is connected to the centre rocker arm.

At lower speeds, the rocker arms are operated by the 2 smaller lobes to allow less air-fuel mixture to enter the cylinder. At higher rpm, the solenoid increases the oil pressure and there are pins in the centre rocker arm that are pushed outside and these pins sit inside a slot provided in the other 2 rocker arms. The pins lock the other 2 rocker arms and now all three rocker arms work as a single unit. The rocker arm unit is operated by the bigger lobe and hence allowing more air-fuel mixture to enter the cylinder.