Valve Gear Design

Multiple Valves:

Four valves (2 intake and two exhaust valves) per cylinder are now common on the  SI engines. The use of multiple valves:

• Increases valve flow area compared to two valves
• Spark plug can be placed centrally reducing the maximum flame travel distance hence  faster and less knocking tendency combustion,
•   Only one intake valve may be used at low speeds and low loads  to obtain high intake mixture velocity giving high turbulence and swirl  that improves combustion at light loads. At high speeds and high loads both valves are open.
• The two intake valves may be provided with different valve timings and different valve lifts to obtain higher volumetric efficiency.
• The two valves may use different port designs to obtain the desired fluid motion in the cyliunder

Variable Valve Timings and Lift

• A large valve overlap is required to obtain high volumetric efficiency at high engine speeds to make use of the ram effect
• At low engine speeds due to backflow of residual gases in the intake system a low valve overlap is desired.
• With increase in valve overlap, residual gas fraction at low speeds increases giving lower NO_x emissions, but HC emissions increase.
• Ideally the valve timings should vary with the engine speed.
• The intake gas flow velocity through the valves governs the air motion and turbulence in the engine cylinder and hence the rate of combustion and performance For fixed intake valve lift, intake airflow and turbulence would  be reduced at low engine loads and speeds, which may not be adequate for good combustion.  Hence a variable valve lift is required that would depend on engine load and speed for good combustion and engine performance.

Variable valve timing control allows for continuously variable camshaft phasing over the entire power range of the engine. Honda has developed electronically controlled variable valve timing and lift (VTEC) system. Toyota’s system is known as VVTi (intelligently controlled variable valve timing). Other manufacturers have developed their own designs of varable valve actuation. The NO_x emissions with fixed and typically with variable valve timings are compared on Fig. 5.5. Reductions of 30 to 70 % in NOx with variable valve timing are observed. Change in HC emissions between the fixed and variable valve timings is however, small. The CO emissions however, are a function of air-fuel ratio.