List at least three methods of variable valve control and de

List at least three methods of variable valve control and describe each. ANS. Why is the volumetric efficiency higher in constant speed engines? ANS. Why is tuning of the intake manifold important? ANS. Why have 2 or 3 intake valves rather than just one large one if the valve area is the same in either case? ANS.

Solution

6. Methods of vaiable valve control: Controlling when valves open and how long they will be kept ope.

i) Cam Phasing: This advances or retards the valve opening by a device called variator. But the duration and lift cannot be varied.

ii) Cam changing: Here the profile of the cam is changed by using two sets of profiles. This allows varying the lift and duration of valve opening.

iii) Camless: The valves are opened and closed through electro magnetic device, hydraulic / pneumatic device and stepper motor.

7. Volumetric efficiency inversely varies with speed as speed is in the denominator. In constant speed engines the mass flow rate of air increases with load and as other parameters are same, volumetric efficiency is higher than varying speed engines.

8. Tuning of exhaust system improves efficiency by using precise geometry to reflect pressure from the exhaust valve. As the efficiency of the engine is determined by the geometry of exhaust geometry, it is important to tune the exhaust system.

9.Multiple intake valves increase the coverage of valves over the combustion chamber, allowing faster breathing thus enhance power at high rev. Secondly,they allow the spark plug to be positioned in the center of combustion chamber, enabling quicker flame propagation, more even and more efficient burning. Thirdly, using more but smaller valves instead of two large valves means lower mass for each valve. This prevent the valves \"float\" from its designed position at very high rev, thus enabling the engine to rev higher and make more power as a result.