Valves or poppet valves control¬†the inflow and outflow of air inside the¬†cylinder for burning fuel and powering the¬†vehicle. Valves are operated by the camshafts¬†placed above them. Valves sit in the¬†cylinder head above the piston and seal the¬†combustion chamber (internal space where¬†fuel is burned) from the atmosphere when¬†the engine is not in operation. Air paths are¬†bored out inside the cylinder head for each¬†cylinder, which originate from the valve¬†seating and connect to a manifold or¬†common collector box.
Now for the seamless operation of the¬†engine, we need at least a couple of valves,¬†namely the inlet and exhaust valve. For¬†example, when the piston is moving¬†downwards in the suction stroke, it creates¬†vacuum inside the cylinder and the inlet valve¬†opens inwards to let the fresh air in. When¬†the piston moves upwards and compresses¬†the air inside the cylinder (compression¬†stroke), both the valves are closed. The fuel is¬†burnt with the compressed air, and the piston¬†is pushed down in the cylinder (power¬†stroke). When the piston comes up again, the¬†exhaust gases created due to combustion are¬†expelled by the opening of the exhaust valves¬†(exhaust stroke) and the cycle repeats. This is¬†the example of the simplest form of valve¬†arrangement, a 2 valve/cylinder arrangement,¬†as most prevalent in 100cc 4-stroke bikes.
When the displacement or the rpm of¬†the engine increases, more air is required to¬†burn more fuel, hence bigger valve area.¬†This can be achieved in two ways ‚ÄĒ¬†a large¬†inlet and exhaust valve, or two small inlet¬†and exhaust valves pair. The latter is a 4-valve/cylinder arrangement and widely¬†used by the manufacturers for the reason¬†that two small valves have greater surface¬†area than a large valve for the same space¬†occupied, and also a small valve carries less¬†inertia than a large valve, thus better for¬†high-speed operation.
Manufacturers have even used 3-valve/cylinder (TVS Flame) and 5-valve/cylinder¬†(Ferrari F355) arrangements, but these are less¬†conventional owing to the limited gain they¬†deliver for specific performance parameters.
As discussed earlier, valves are¬†opened by camshafts, but what closes them¬†back and keeps them in contact with the¬†camshaft are the ‚Äėsprings‚Äô. Almost all¬†passenger cars and bikes use metallic wire¬†springs for closing the valves, simplicity in¬†design and reliability being their trait. But the¬†trait changes when the rpm increases,¬†especially in racing engines, where the¬†metallic springs are unable to retract the¬†valve as quickly as the frequency of piston¬†strokes and we are left wanting for more revs.
There are alternatives though. Ducati¬†has a unique Desmodromic valve-train¬†which has an innovative arrangement of¬†closing the valves with a mechanical link,¬†rather than springs. Hence, as the rpm¬†increases, the valve movement keeps up with¬†the piston strokes. Formula 1 and most¬†Moto GP engines employ pneumatic valve¬†springs for the rapid closing of valves at¬†those very high revs.
Much R&D has gone in to make the¬†springs perform better under adverse¬†conditions, hence high-performance road¬†cars and bikes still prefer springs over Desmodromic or pneumatic valve springs¬†because of the high cost involved in these¬†options. One very promising development¬†that surfaced lately is electromagnetic¬†valve springs, i.e. the opening and closing¬†of the valves is precisely controlled by electricity¬†and magnets with the total absence¬†of any mechanical link. But this is still a¬†distant dream before it comes to the production¬†level.
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