The first experiments with phase changes began in the era of steam. As for conventional automotive internal combustion engines, the principle of variable valve timing (VVT) was first patented by Fiat in the late 1960s. In mass production, the first model with the VVT was the 1980 Alfa Romeo Spider. Since then, many manufacturers have used the VVT principle in their designs. A variety of options from different manufacturers gave rise to many different designations of such systems – VTEC, VVT-i.
Engines without VVTs have unregulated camshafts, so the lift height, opening time and valve phases in them are fixed once when installing the crankshaft and camshafts. Variable valve timing can improve engine performance, improve fuel economy and reduce emissions by optimizing engine performance, taking into account operating conditions and load.
VVT makes it possible, depending on speed, load and temperature, to achieve greater valve overlap, to change the lift height, opening time and phase. For example, at low speed and under load, the phases are advancing in order to improve the response to the opening of the throttle and increase the torque. At high speeds, the phases can shift towards the delay to reduce emissions and increase power.
VVT types – cam-phasing
Various VVT implementation options are available on the market, the most common of which are the so-called “cam-phasing”, where the phases change due to the camshaft turning relative to its pulley or sprocket with a built-in VVT regulator.
The earliest designs were simple – they worked only on the intake camshaft and only in two positions (on / off) – “discrete” adjustment, carried out at certain speeds (as, for example, in the BMW Single VANOS). Most discrete VVTs are mechanically controlled using a spring-loaded helical gear in the controller..
Later versions use electronic control and hydraulic drive. Such systems (“continuous operation”) can be used on one or both camshafts. In them, the electromagnetic valve regulates the supply of oil under pressure to the regulator (integrated into the pulley or sprocket), where it acts on the blades connected to the camshaft and causes the shaft to rotate relative to the pulley. In addition to the ability to adjust the phases at any speed, continuous VVTs provide a smooth transition between modes compared to discrete systems.
VVT types – cam-changing
Some manufacturers (Honda Variable Valve Timing and Lift Electronic Control – VTEC) use a separate cam for each valve on the intake and exhaust camshafts (VTEC used only the intake on the early SOHC engines). The pin pressure holds the oil pressure, which connects different rocker arms – for long travel and long opening, and for low travel and short opening of valves. Although such adjustment is essentially discrete, this mechanism has gained wide popularity and deservedly high reputation due to a significant increase in power when using it..
VVT types – cam-phasing + cam-changing
Systems are becoming more complex and expensive – combining phase control and turning and changing cams can increase engine efficiency, increase power and make the exhaust cleaner.
One of the manufacturers who developed this system is the Porsche with Variocam Plus. Regulators are mounted on intake camshafts, which also have cams of two different profiles, based on adjustable hydraulic pushers with separate internal and external segments. The inner segment of the pusher is in contact with the central cam of the camshaft (which provides a lower lift height and a longer valve opening). When a higher lift is required, the inner and outer segments are connected together by a pin driven by an electronically controlled hydraulic solenoid valve. After that, external camshaft cams come into operation, providing a greater rise and duration of valve opening. The functions of changing phases and changing cams are individually controlled by the engine control unit (ECM), taking into account the main parameters – speed, selected gear, accelerator pedal position, engine oil temperature, coolant temperature.
In 2009, Fiat introduced its innovative MultiAir system, where the exhaust valves are driven from the camshaft in a traditional mechanical way, and the inlet pressure from the cam is transmitted through an electro-hydraulic system with a plunger, an electromagnetic valve and purely hydraulic pushers. At full load, the force is transmitted to the valves as if there was a rigid connection with the cam. At partial loads, the valve relieves the pressure on the parts of the plunger stroke, ensuring the supply of the strictly required amount of air to the cylinders (which, among other things, eliminates the throttle). As a result, losses are reduced, efficiency is improved, and engine performance is close to the performance of motors of a significantly larger displacement.
Currently, manufacturers are researching and developing new internal combustion engines without camshafts, which means the end of the era of gas distribution systems and phase change systems in our usual form.
On the one hand, for services this means a reduction in the list of standard service work (such as replacing the timing belt), on the other hand, the engines as a whole will become much more complicated, requiring new equipment and tools.
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