Table of Contents
What is VVT | Variable Valve Timing ( VVT ) Diagram
VARIABLE VALVE TIMING (VVT)
Fixed valve timing has been a standard feature of all engines until relatively recent times. The valves opened and closed at a fixed period in relation to crankshaft rotation at all engine speeds and loads. When the inlet and exhaust valve timing is fixed, the timing is not suitable for all engine operating speeds and loads. So, there is increase in use of mechanisms to provide variable valve timing (VVT). The variable valve timing systems alter the valve timing to suit engine speed and load conditions. Although systems are purely mechanical-based systems, most modern systems make use of the electronic engine management system to regulate the mechanical actuation of changes to the valve timing.
A petrol engine has to operate at varying engine speeds from idle, typically 750 rpm through to high speeds up to 7000 rpm and above. So, the engine will have valve timing more suitable for one particular engine speed which is generally at around 50% to 60% of the engine’s maximum speed. Therefore, the variable valve timing results the increased combustion efficiency at the selected engine speed.
Variable valve timing optimises the overlap period during engine operation. It allows different overlap periods to be used at different engine speeds. Practically, VVT systems are used to enable good power or torque to be achieved over the whole engine speed range but the added benefit of valve overlap is to facilitate the mixing of some exhaust gas with the fresh charge of air. Therefore, the combustion temperature and NOx emission are reduced.
(i) Slow engine speeds:
At slow engine speeds, if the inlet valve timing is altered so that it is retarded or opened late, all of the exhaust gas will be expelled through the exhaust valve and the cylinder will fill with fresh uncontaminated mixture. So, a fresh mixture will provide a good bum of the gas during next power stroke. Therefore, the engine is stable when it is at idle.
(ii) High engine speeds:
At high speed, if the inlet valve timing is advanced, the fresh mixture can be drawn into the cylinder by the depression caused by the flow of exhaust gas through the exhaust valve. The flow of exhaust gas will improve the gas flow. through the cylinder at higher engine speeds.
(iii) Valve opening period:
The valve opening period is dictated by the profile of the cam lobe which is constant with conventional valve operating gear. When the engine is at low to medium speeds, the valve opening period allows sufficient fresh mixture into the cylinder to provide good engine performance. At higher engine speeds, the volumetric efficiency of an engine will increase if the opening period of the valve is increased. It is possible to increase the valve opening period by increasing the valve lift. Increasing the valve lift at high engine speeds provides an increase in the volume of fresh mixture drawn into the cylinder. So, it results an increase in engine power.
Advantages of Variable Valve Timing
1. It allows to recirculate internal exhaust gas.
2. Increased torque can be obtained.
3. It ensures better fuel economy.
4. It reduces nitrogen oxide.
5. Hydrocarbon emissions can be controlled.
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