Camshaft. Camshaft Functions

The internal combustion engine is a machine that works intermittently. A fresh fuel mix flows through an open intake port and into the cylinder where it is compressed and ignited; it expands and passes through the open exhaust port into the exhaust system. Cam-actuated valves are normally used in four-cycle engines, less often in two- cycle engines, to open and close the ports.

In Wankel and two-cycle engines, the piston itself normally takes care of opening and closing the ports. Other potential embodiments such as rotating or reciprocating sleeves are no longer used in mass production.

Camshaft Functions. The primary function of the camshaft is to open and close the intake and exhaust valves so that gases can be exchanged; these actions are synchronized with the position of the piston and thus with the crankshaft.

Normally the valves are opened by transferring force from the cam to the cam follower, to other actuation elements where required, and ultimately to the valve, opening (or lifting) the valve against the force of the valve spring. During the closing cycle, the valve spring closes the valve. When the follower is in contact with the cam’s base circle (with the cam exerting no lift), the valve spring keeps the valve closed against any gas pressure in the port (turbocharger pressure or exhaust gas counterpressure). During engineering it is particularly important to pay attention to the dynamics of all the peripheral conditions.

Desmodromic systems employed to increase potential engine speed (both the opening and closing phases are cam driven) are rarely used in mass production because of reduced valve train masses in multivalve engines and because improved valve springs have brought about an improvement in performance.

In the four-cycle engine, the camshaft is driven by the crankshaft and rotates at half the crankshaft speed. The valve timing for each individual valve is determined by the geometry and the phase rotation angle of the individual cams, normally separate for intake and exhaust valves and for the cylinders that are located along one or more camshafts. In multivalve engines it is possible to actuate several valves using a single cam with the intervention of linkages or forked levers. In special designs, the valves of multiple cylinders or the intake and exhaust valves are activated by the same cam.

In addition to the movements of the intake and exhaust valves required to control gas flow, the camshaft can also be used to generate the additional valve movements required for engine braking systems used in medium- and heavy-duty utility vehicles. Here existing or additional cams are employed so that engine drag is increased during overrun or coast down; the exhaust valve might, for example, be opened briefly around dead center in the compression stroke.

A further function of the camshaft, in addition to supplying power to auxiliary units (such as vacuum, hydraulic, fuel, or injection pumps), is actuating individual injection pumps in the engine block (pump-line nozzle) or pump nozzles in the cylinder head. Here, in addition to the cams that actuate the valves at the cylinders, further cams are provided to generate the stroke motion in the injection pump(s). Because of the additional loading encountered here, the cams usually have to be considerably more stable in design.

Torque, power output, fuel consumption, and pollutant emissions are influenced decisively by valve timing. The high specific power desired by the customer, smooth torque development, and low fuel consumption and pollutant emissions all across the speed range are difficult to achieve with conventional valve trains (see also the sections on camshaft shifting systems and variable valve actuation).

In every application the valve stroke length, velocity, and acceleration are the products of compromises between the fastest possible opening and closing for the individual valves and the forces and surface pressures created thereby. The friction and friction losses at the camshaft and the valve train as a whole are also important criteria in engineering.

 






Date added: 2024-05-12; views: 84;


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