Electronic Valve Timing Control

Basic Description

The valves in an internal combustion engine open and close to allow the fuel-air mixture into the cylinder prior to combustion and the exhaust gasses out of the cylinder after combustion. In most engines, the valves are opened by lobes connected to the camshaft. The shape of these lobes determines the lift, timing and duration of each valve opening. In an engine with fixed valve timing, the timing is not optimal for all engine speeds. For example, if a camshaft is designed to operate the valves for optimal timing at low RPMs, then at higher RPMs each cylinder would be deprived of sufficient fuel and air mixture, limiting the engine power output. Conversely, if optimized for high RPMs, the vehicle would experience rough idling at low RPMs. There are several methods for changing the valve timing, such as using multiple camshafts or eliminating the camshaft altogether and controlling the valve timing with electronic, hydraulic or pneumatic actuators. Variable valve timing can significantly increase both the power and fuel efficiency of an internal combustion engine.

Electronic valve control (EVC) systems attempt to optimize the valve timing over the entire range of possible engine speeds. Most existing systems manipulate the valve timing using a computer controlled actuator attached to the camshaft. Sometimes two camshafts are used, one to control the intake valves and the other to control the exhaust valves. The camshaft may have two sets of lobes, where one set is designed for low and the other for high RPMs. As the camshaft rotates, the lobes push open the spring-loaded valves, which are then closed by the force of springs. An electronic control unit (ECU) selects which set of lobes to use based on the engine speed. Another approach to variable valve timing employs a cam phasing mechanism to monitor and adjust the rotation of the camshaft relative to the rotation of the crankshaft.

There are other mechanical/electrical methods that come one step closer to eliminating the camshaft altogether. Three of these methods are electro-pneumatic valves (EPV), electro-hydraulic valves (EHV), and electromagnetic valves (EMV). EPV and EHV use electric solenoids to control the flow of compressed air or hydraulic fluid to the valves at the appropriate time. Existing EPV and EHV systems generally eliminate the spring mechanism but still employ the cam. Electromagnetic valves control the valve opening directly with a solenoid. EMV systems have (so far) failed to succeed in the mass consumer market because of the expense and the power required to operate the actuator. EPV systems are commonly found on Formula One engines and EHV systems are being employed by the engines on all new BMW vehicles.

Sensors

RPM sensor, throttle position sensor, fluid pressure sensor.

Actuators

Hydraulic or pneumatic valve actuator, piezoelectric valve actuator, camshaft position pin actuator

Data Communications

Control Unit Communication: Typically Control Area Network (CAN) Bus System

Manufacturers

Bosch, Delphi, Denso, Valeo

For More Information

[1] How Camshafts Work, Karim Nice, Howstuffworks.com, Dec. 13, 2000.

[2] Camshafts Part III: Variable Valve Timing, Abdul Rehman, StreetRacersOnline.com, Aug. 27, 2004.

[3] Camshaft, Wikipedia.

[4] Variable Cam Timing, Wikipedia.

[5] e-Valve System, Valeo brochure describing electromagnetic valve control. [pdf]

[6] EU Project Could See Electromagnetic Valve Actuators Replace Camshafts, Azom.com, Oct. 13, 2004.