In internal combustion engines, particularly for spark ignition (SI ) engines, valve events and their timings have a major influence on the engine's overall efficiency and its exhaust emissions. Because the conventional SI engine has fixed timing and synchronization between the camshaft and crankshaft, a compromise results between engine efficiency, performance, and its maximum power. By using variable valve timing ( VVT ) technology it is possible to control the valve lift, phase, and valve timing at any point on the engine map, with the result of enhancing the overall engine performance. To get full benefits from VVT, various types of mechanisms have been proposed and designed. Some of these mechanisms are in production and have shown significant benefits in improving engine performance. During the last two decades, remarkable developments have been seen in the field of VVT. This paper reviews the literature in the technology of intake and exhaust philosophies of VVT and their effects on the pressure-volume (PV ) cycle of the engine. A single-cylinder engine is simulated by the GT-Power software. The effects of different VVT philosophies from the simulations are analysed and compared to those of the literature reviewed.Keywords: variable valve timing ( VVT ), variable valve actuation ( VVA), valve overlap, exhaust gas recirculation (EGR)
INTRODUCTIONResidual gas fraction ( RGF ) is controlled by the valve overlap and cannot be changed for various speeds and loads. The fixed-valve events of conventional cam-A major goal of engine manufacturers is to minimize specific fuel consumption and emissions from engines.controlled engines compromises the engine for better performance under all operating conditions. The inlet-valve One solution is by the independent actuation of the inlet and exhaust valves at any position of the piston, with timing is the most important parameter for optimizing the engine volumetric efficiency, whereas the exhaustno more need for a camshaft.A major disadvantage of conventional spark ignition valve timing controls the RGF, which reduces exhaust NO x emission. For the engine to operate efficiently and (SI ) engines results from the energy losses during the inhaling of the sub-atmospheric gases during the suction effectively over its entire operating range and conditions, the valve events should be able to vary with speed and stroke and the expelling of exhaust gases to the atmosphere during the exhaust stroke. These pumping losses depend load anywhere on the engine map. The present investigation undertakes the assessment of on the opening and closing position of the throttle valve. The losses are high when the throttle valve tends to close intake and exhaust philosophies for VVT. and are low at wide-open throttle. Thus, the pumping losses are inversely proportional with the engine load. Without a throttle valve, control of the air-fuel mixture 2 VALVE EVENTS OF CONVENTIONAL SI can be realized by variation of the intake valve-opening ENGINES period; therefore, variable valve timing ( VVT ) h...