Induction and Exhaust

Induction and Exhaust
 In internal combustion engines the induction and exhaust processes give importance to the performance and e ciency of the engine. In the two stroke engine the ow is regulated by the piston covering and uncovering ports, but in the four stroke engine the induction and exhaust processes are controlled through valves. The four types of valves used are poppet, sleeve, rotary, and disc valves.

Valves

The most commonly used valve is the poppet valve. The poppet valve has a straight cylinder rod and its end has the shape of a mushroom. The advantages of the poppet valve are that it is cheap, it has good ow properties, good seating, it is easy to lubricate, and it has good heat transfer to the cylinder head. Rotary and disc valves are sometimes used, but contrary to poppet valves, they have heat transfer, lubrication, and clearance problems. The other type of valve is the sleeve valve. The sleeve valve has some advantages over the poppet valve, but its disadvantages discontinued the use of it. The use of sleeve valves was best suited for aerospace engines before the introduction of the gas turbine engine. The advantages of sleeve valves were that they eliminated the \hot spot associated with the poppet valve." (Stone, p.232) Other advantages were that it produced higher outputs and higher e ciencies due to a higher compression ratio, which was due to the use of low octane fuel. The disadvantages of the sleeve valve were the cost and di culty to manufacture, the lubrication and friction between the cylinder wall and sleeve valve, and the heat transfer from the piston through the sleeve and oil lm to the cylinder. 

A camshaft is used in the mechanism that operates the valves. Engines that use overhead poppet valves (ohv) use a camshaft that \is either mounted in the cylinder block, or in the cylinder head." (Stone, p.233) Overhead camshafts (ohc) use chain or toothed belts to provide its drive. Gear drives are also possible to use but they would be expensive, noisy, and cumbersome. The best belts are toothed belts because the rubber damps out torsional vibrations. The other type of valve system is the twin or double overhead camshaft. This mechanism is used when the need for two inlet and two exhaust valves are needed. High performance spark ignition engines or large compression ignition engines use the double overhead camshaft mechanism. One camshaft operates the inlet valves and the other camshaft operates the exhaust valves. The disadvantage of having two camshafts operate four valves is the cost of having a second camshaft, the more involved machining, and the di culty of providing an extra drive. The British Leyland four valve pentroof mechanism got rid of the problem of having two camshafts operating four valves by introducing a single camshaft operating all four valves with the aid of a rocker arm. the mechanism proposed by Leyland.  

In this mechanism the camshaft comes in direct contact with the inlet valves and through a rocker the exhaust valves. The advantages of four valves per combustion chamber result in \larger valve throat areas for gas ow, smaller valve forces, and larger valve seat area." (Stone, p.236) The reason for having smaller valve forces is because a lighter valve with a lighter spring will reduce the hammering e ect on the valve seat.

Valve Timing

Valve timing is characterized by the camshaft and valve mechanism. There are two timing processes. Both of these processes involve inlet ow and exhaust interaction, but the di erence between the two is the time of interaction. In compression ignition engines and conventional spark igniton engines the valve overlap is only 15 of the crank angle but in high performance spark ignition engines the valve overlap is 65o of the crank angle. the valve timing diagrams for small valve overlap and large valve overlap for the four stroke engine. In diagram (a), which corresponds to the small valve overlap, the inlet valve opens 5 before top dead center (btdc) and does not close until 45 after bottom dead center (abdc). The reason for the rapid closure abdc is because the pressure needs to increase rapidly. If the inlet valve closed later in the compression stroke then there would not be enough pressure build up. Also after the piston moves to BDC following the induction stroke the the piston will start the compression stroke. If the inlet valve is open for to long of a time then the air and fuel mixture will be pushed out by the piston face. In compression ignition engines this will make cold starting the engine di cult. The compression stroke will then initiate until hitting top dead center, which will then lead to the power stroke. At about 40 before bottom dead center (bbdc) the exhaust valve will open, so that the combustion products have enough time to leave the chamber. By having the exhaust valve open 40 bbdc, the problem arises of wether or not opening the valve bbdc will cut away from the power stoke and therefore cause the engine to be less e cient. At 40 bbdc the power stroke is almost nished and as a matter of fact only 12 % of the stroke is lost. Another reason for opening the exhaust valve 40 bbdc is because the valve is not fully opened until 120 after starting to open. The cycle repeats again when the piston is 5 btdc.

The valve overlap of the high performance spark ignition engine is best described by diagram (b). The disadvantages of having a large overlap is due to the mixture caused by the exhaust and air and fuel mixture. Since there is a large valve overlap some unburnt fuel will cross ow from the inlet manifold into the exhaust valve and therefore the e ciency will be reduced. Exhaust residuals are a concern in large valve overlap engines. An increase level of residual in the combustion chamber has a signi cant e ect on the combustion process. With a high level of exhaust residual the e ciency of the engine will decrease since the engine will be doing work on both burned and unburned fuel instead of doing on as much unburnt fuel instead of doing work on as much unburnt fuel as possible. The level of residuals increase with a)decreasing absolute inlet manifold pressure, b)reducing compression ratio, c)increasing valve overlap, d)decreasing speed, and e)increasing exhaust back pressure.