Continuous Variable Valve Timing - Traditional Internal Combustion Engines

Traditional Internal Combustion Engines

Traditional Internal Combustion Engines are designed to optimize efficiency and performance, while minimizing emissions, under a wide variety of driving conditions. The way this is traditionally accomplished is relatively simple: the driver controls the volume of air allowed into the engine by way of a throttle plate linked to the gas pedal. Electronic sensors in the intake stream measure the intake air volume and temperature, and the Powertrain Control Module (PCM) determines the required fuel amount, as well as the appropriate spark timing to allow the air/fuel mixture to burn optimally and maximize output. Generally, fuel volume and spark timing are pulled off a fuel map – a computer program which looks at various inputs from engine sensors. The intake and exhaust valves are fixed, meaning they are driven by the engine crankshaft, and they open and close at set intervals during the rotational cycle, independent of engine load conditions. When an engine is designed, the inherent inflexibility in the timing of the valve events results in a compromise. Simply, engineers must design the engine to run optimally under a wide variety of driving conditions, but perfectly under none. During most driving, the engine is operating at part load and fuel economy is the priority. At part load, the air-fuel ratio is kept stoichiometric to enable efficient operation of the catalytic converter”.

One of the major inefficiencies of the Internal Combustion Engine is the loss of rotational energy caused by the intake stroke. As the piston travels downward during the intake stroke, air is drawn into the cylinder via engine vacuum. The pressure is below atmospheric, which means the engine has to do work to take in the air – at the expense of volumetric efficiency. “These pumping losses depend on the opening and closing of the throttle valve. The losses are high when the throttle valve tends to close and are low at wide open throttle. Thus the pumping losses are inversely proportional with engine load”. If intake air volume were to be controlled by the valvetrain, rather than a throttle plate, the pumping losses could be reduced. “The inlet-valve timing is the most important parameter for optimizing the engine volumetric efficiency, whereas the exhaust-valve timing controls the RGF which reduces exhaust NO emissions. For the engine to operate efficiently and effectively over its entire operating range and conditions, the valve event should be able to vary with speed and load anywhere on the engine map”.

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