Simple Tech: Valves and valve springs explained

Shashank Singh  /
20 Jun 2016 14:54:08 IST

Valves or poppet valves controlthe inflow and outflow of air inside thecylinder for burning fuel and powering thevehicle. Valves are operated by the camshaftsplaced above them. Valves sit in thecylinder head above the piston and seal thecombustion chamber (internal space wherefuel is burned) from the atmosphere whenthe engine is not in operation. Air paths arebored out inside the cylinder head for eachcylinder, which originate from the valveseating and connect to a manifold orcommon collector box.

Simple tech valve

Now for the seamless operation of theengine, we need at least a couple of valves,namely the inlet and exhaust valve. Forexample, when the piston is movingdownwards in the suction stroke, it createsvacuum inside the cylinder and the inlet valveopens inwards to let the fresh air in. Whenthe piston moves upwards and compressesthe air inside the cylinder (compressionstroke), both the valves are closed. The fuel isburnt with the compressed air, and the pistonis pushed down in the cylinder (powerstroke). When the piston comes up again, theexhaust gases created due to combustion areexpelled by the opening of the exhaust valves(exhaust stroke) and the cycle repeats. This isthe example of the simplest form of valvearrangement, a 2 valve/cylinder arrangement,as most prevalent in 100cc 4-stroke bikes.

When the displacement or the rpm ofthe engine increases, more air is required toburn more fuel, hence bigger valve area.This can be achieved in two ways "a largeinlet and exhaust valve, or two small inletand exhaust valves pair. The latter is a 4-valve/cylinder arrangement and widelyused by the manufacturers for the reasonthat two small valves have greater surfacearea than a large valve for the same spaceoccupied, and also a small valve carries lessinertia than a large valve, thus better forhigh-speed operation.

Manufacturers have even used 3-valve/cylinder (TVS Flame) and 5-valve/cylinder(Ferrari F355) arrangements, but these are lessconventional owing to the limited gain theydeliver for specific performance parameters.

As discussed earlier, valves areopened by camshafts, but what closes themback and keeps them in contact with thecamshaft are the 'springs'. Almost allpassenger cars and bikes use metallic wiresprings for closing the valves, simplicity indesign and reliability being their trait. But thetrait changes when the rpm increases,especially in racing engines, where themetallic springs are unable to retract thevalve as quickly as the frequency of pistonstrokes and we are left wanting for more revs.

There are alternatives though. Ducatihas a unique Desmodromic valve-trainwhich has an innovative arrangement ofclosing the valves with a mechanical link,rather than springs. Hence, as the rpmincreases, the valve movement keeps up withthe piston strokes. Formula 1 and mostMoto GP engines employ pneumatic valvesprings for the rapid closing of valves atthose very high revs.

Much R&D has gone in to make thesprings perform better under adverseconditions, hence high-performance roadcars and bikes still prefer springs over Desmodromic or pneumatic valve springsbecause of the high cost involved in theseoptions. One very promising developmentthat surfaced lately is electromagneticvalve springs, i.e. the opening and closingof the valves is precisely controlled by electricityand magnets with the total absenceof any mechanical link. But this is still adistant dream before it comes to the productionlevel.

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Tags:  | simple tech  | Valves  | valve springs  | valvetrain  | inlet  | intake  | exhaust  | automonitor

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