Chapter 10 Emission Control and Exhaust System Fundamentals 187 Copyright by Goodheart-Willcox Co., Inc. When leaner mixtures became necessary, valve overlap was reduced to prevent exhaust dilution and rough idle. These “milder” camshafts reduce engine power to increase smoothness. However, some engines are now designed with greater overlap. By drawing some exhaust gases into the cylinder on the intake stroke, the EGR valve can be eliminated. This also allows the engine to develop more power. Variable valve timing devices are also used for these purposes. Variable Valve Timing Many vehicles use variable valve timing to increase performance and reduce emissions. At higher engine speeds, airflow is sluggish in relation to piston speed. To get more air into the engine at high speeds, the valves are opened sooner in relation to the position of the piston. This advancement allows more time for incoming air to enter the combustion chamber. At lower speeds, the valve timing is returned to its normal position for smooth operation. Some systems use a hydraulic or electromechanical actuator located inside of the camshaft sprocket to advance valve timing. Other sys- tems use separate cam lobes activated at high engine speeds. Dual overhead cam (DOHC) engines may use separate actuators for the intake and exhaust camshafts. Figure 10-7 shows a typical variable valve timing actuator and a typi- cal control system. On most engines, the intake camshaft advances from the default (not activated) setting and the exhaust camshaft retards from the default setting. On some overhead camshaft engines, the variable valve timing system can vary valve lift and duration as well as valve timing. These systems are controlled by the ECM, usually in response to changes in engine speed and load. Mechanisms are installed on the valve rocker arms or fol- lower in the head. The mechanism may switch to alternate lobes or vary the lift and duration at the camshaft lobe. A variable valve timing system that alternates lobes to vary lift is shown in Figure 10-8. In this design, the low-speed lobe normally opens and closes the valve, Figure 10-8A. This lobe has a lift and duration curve that allows for smooth idle and low-speed operation. Note the pad and shaft that is moved by the high-speed lobe. The pad and shaft move up and down, but have no effect on the rocker arm. In Figure 10-8B, hydraulic pressure has engaged the rocker arm pin with the pad and shaft. This forms a solid connection between the high-speed lobe and the rocker arm and the high-speed lobe operates the valves. The low-speed lobe no longer contacts the rocker arm. The lift and duration of the high-speed lobe are greater than that of the low-speed lobe. Some systems use a series of gears and levers driven by a motor to affect the movement of the rocker arm. This system can also be used to control engine speed. The motor turns a worm gear that rotates a larger gear. This gear is attached by a linkage to the rocker arm. The vehicle ECM controls the motor. The design of the worm gear prevents the valve train from moving the motor. Some variable valve timing systems are used in con- junction with the ECM to control engine speed. Instead of controlling the throttle valve with a drive-by-wire system, the ECM responds to changes in accelerator pedal position by varying the opening and closing of the intake valves. At idle, the valves are only slightly opened. When the accel- erator pedal is pressed, the valve opening is increased. A throttle valve is not needed. Figure 10-4. Cylinder heads with four valves per cylinder permit better flow through the combustion chamber than heads with two valves per cylinder. (Acura) Camshafts Spark plug Valves Piston Figure 10-5. Some low-displacement engines have three valves per cylinder. This improves flow at a lower cost than four valves per cylinder. Three valves per cylinder are also used in cases when the combustion chamber is too small to fit four valves.