Piston Engines - General — Page 15, Lesson 23

I want to walk you through the practical Otto cycle — the real-world version of the four-stroke engine cycle we've been discussing. The theoretical cycle we looked at earlier is clean and simple, but in practice it proved inefficient, so engineers had to modify the timing of valve openings, valve closings, and ignition. That's what this excerpt is about: why those timings are changed, and what actually happens inside the cylinder. Let me start with the induction stroke. In the theoretical cycle, the inlet valve opens exactly at top dead centre. But in practice, we open the inlet valve before TDC. Why? Because the mixture has inertia — it doesn't instantly start flowing the moment the valve cracks open. If we waited until the piston was already moving down, there'd be a time-lag: the piston would be descending, but the mixture would be slow to catch up. By opening the valve early, we ensure it's fully open early in the induction stroke, so there's no lag between the piston moving down and the mixture flowing in. The inflowing mixture can thus keep up with the descending piston. Now, here's a subtle point. As the induction stroke proceeds, the momentum of the mixture increases. By the end of the stroke, that momentum is so strong that the gases will continue flowing into the cylinder even after the piston has passed bottom dead centre and started moving back up slightly. So we don't close the inlet valve at BDC — we delay its closing until after BDC, when the gas pressure in the cylinder approximately equals the gas pressure in the induction manifold. That's the condition for closing: pressure balance between cylinder and manifold. Then we move to the compression stroke. As the piston moves upward, the inlet valve closes and the gas gets compressed. The key idea here is that by squeezing the gas into a smaller space, the pressure it will exert when burnt is proportionally increased. That's the whole point of compression — more pressure at combustion means more power. But there's an important detail I want you to note. As the gas is compressed, it becomes heated adiabatically — that means heated without heat being added from outside, purely from the work of compression itself. The excerpt gives you a lovely everyday analogy: a bicycle pump warms up in action. Same principle. On top of that adiabatic heating, the gas also gains heat by conduction from its hot surroundings — the cylinder walls, the piston, the head. The result is that the pressure rises to a higher value than you'd expect from the reduction in volume alone. So compression heating is a real, measurable effect, and it pushes the pressure above what simple volume reduction would predict. Let me pull that together. We have two modifications to the theoretical cycle in this excerpt: the inlet valve opens before TDC to overcome mixture inertia, and it closes after BDC to exploit the mixture's momentum. And we have one physical phenomenon to remember: compression heats the gas adiabatically, plus by conduction, raising pressure beyond the volume-reduction expectation. There's a timing diagram — Figure 2.5 — that shows all these practical valve and ignition timings visually. It's worth studying because it lays out exactly when each event happens relative to the piston's position.