Piston Engines - General — Page 24, Lesson 24

Let’s start with the word adiabatic, because it sets the scene for everything that follows. Adiabatic means without loss or gain of heat. In other words, a process where no heat energy crosses the boundary into or out of the gas. Now, I want to be honest with you right away: with present technology, it is not actually possible to compress or expand a gas without some gain or loss of heat. So adiabatic is the theoretical ideal we aim for, not something we achieve in practice. Keep that in mind — it’s the yardstick. Now let’s walk through the power stroke. This is the stroke where the engine actually does work. Here’s the sequence. Before the piston reaches TDC — that’s top dead centre, the very top of its travel — on the compression stroke, the gas is ignited by a spark. The piston doesn’t stop at TDC; the momentum of the moving parts carries it past TDC while the flame is still spreading through the mixture. As the flame spreads through the combustion chamber, the intense heat raises the pressure rapidly to a peak value. That peak pressure is reached when combustion is complete, and this coincides with the piston being at about 10° past TDC. So the peak pressure doesn’t happen exactly at TDC — it happens about ten degrees after. Now, here’s the clever part about valve timing. If the exhaust valve is not opened until BDC — bottom dead centre, the bottom of the stroke — then the pressure of the gases still remaining in the cylinder would create a back pressure resisting the upward movement of the piston on the next stroke. So we don’t want that. As the piston descends on the power stroke, the pressure falls rapidly. By 45° of crank angle after TDC, the pressure is approximately half its peak value. And by 90° of crank angle after TDC, most of the energy in the gases has been converted into mechanical energy. So the useful work is essentially done by the time the crank has turned ninety degrees past TDC. That’s why we open the exhaust valve before BDC. If we do that, the residual pressure will start the first stage of exhaust scavenging — that’s the process of clearing the burnt gases out. By the time the piston reaches BDC, there will be no back pressure on it. Now, you might wonder: doesn’t opening the exhaust valve early waste some of the power stroke? The answer is no, and there are two reasons. First, there is only a short distance left for downward movement of the piston after the exhaust valve is opened. Second, relatively little pressure is still being exerted on the piston by the cooled, expanded gases. So the loss of mechanical energy is not significant. Then we come to the exhaust stroke. The piston moves upward, forcing the remaining gases out of the cylinder. And here’s a detail: the exhaust valve is left open after TDC to permit the gases to scavenge the cylinder as completely as possible by their momentum. So the gases don’t just get pushed out — their own momentum helps clear the cylinder. Now, there’s a geometric fact that explains why we fiddle with valve timing at all. About the positions of TDC and BDC, the distance the piston moves is very small compared to the large angular movement of the crankshaft. This is called the Ineffective Crank Angle — and that’s Figure 2.6. Because there is little change in cylinder volume at these times, the weight of charge into the cylinder and the exhaust of the burnt gases can be improved by opening the valves early and closing them late. These changes to the valve timing have three names: Valve Lead, Valve Lag, and Valve Overlap — see Figure 2.5. Let me define each precisely. Valve Lead is when the valve opens before the theoretical opening time. So the inlet valve opens before TDC, and the exhaust valve opens before BDC. Valve Lag is when the valve remains open after the theoretical closing time. So the inlet valve remains open after BDC, and the exhaust valve remains open after TDC. And Valve Overlap is a period when both valves are partially open together. During this overlap period, the action of the exhaust gases flowing out of the cylinder tends to reduce the gas pressure in the cylinder below the gas pressure in the induction manifold. That creates a low-pressure area. The fresh mixture then commences to flow into that area of low pressure, and it assists in displacing the remaining burnt gases. By doing so, it improves the volumetric efficiency of the engine — because it induces a greater weight of charge into the cylinder. So the whole story hangs together: the ineffective crank angle near TDC and BDC is wasted motion, and valve lead, lag, and overlap are the ways we exploit that wasted motion to get more fresh charge in and more burnt gas out. That’s the heart of valve timing in a piston engine.