Piston Engines - General — Page 34, Lesson 39

I want to walk you through the cylinder head and the valve operating gear of a piston engine. This is the heart of the breathing system — how the engine gets its mixture in and its exhaust out, and how that is timed. Let's start with the cylinder head itself. It is generally made of aluminium alloy, and the reason is heat dissipation. Aluminium conducts heat away quickly, which keeps the head cooler. The head seals one end of the cylinder, and together they form the combustion chamber where the fuel-air mixture is burned. Now, the cylinder head is a busy component. It accommodates the valves, the valve guides, and the sparking plugs, and it supports the valve rocker arms. The valve seats are cut into the cylinder head, and these form gas-tight seals with the valves. The head may be detachable, but more commonly it is screwed and shrunk onto the cylinder — that's a very tight interference fit. Let me define each of these parts precisely. The valve guide guides the valve in a straight path and keeps the valve concentric to its seat. Concentric means the valve stays centred relative to the seat, so it doesn't wobble. Usually the valve guide is pressed into the cylinder head. The valve seat is ground to form a gas-tight seal with the face of the valve. It is cut at various angles — either 30° or 45°. That angle matters because it's what the valve face matches. The valves themselves — inlet and exhaust — open and close the passages for the induction and scavenging of the gases. Induction is bringing the fresh mixture in; scavenging is pushing the burnt gases out. The face of the valve is accurately machined to the same angle as the valve seat. Then the valve and seat are lapped together until a full contact is obtained — that's the grinding-in process that gives you that gas-tight seal. There's a clever detail on the exhaust valve. The exhaust valve stems are sometimes hollow and partly filled with sodium to assist in cooling. Sodium is a metal that melts and sloshes inside the hollow stem, carrying heat from the hot valve head up to the cooler stem. The valves themselves may be flat, trumpet, or mushroom shape. Now the valve springs. They are made of special spring steel, and their job is to ensure the valves remain closed except when operated by the cams. They are helical coil type springs, and the usual practice is to fit two springs to each valve, one inside the other. Why two? It provides a safety factor — if one breaks, the other holds the valve — and it helps to eliminate valve bounce. Valve bounce is when the valve, at high speed, doesn't follow the cam and literally bounces off its seat. The springs are held compressed between the cylinder head and the valve spring cap. The cap is located on the valve stem by split collets — those are the little tapered wedges that lock the cap onto the stem. Now let's move to the valve operating gear. This consists of a camshaft, or camshafts, driven from the crankshaft at half crankshaft speed — regardless of how many cylinders there are, or how they are arranged. That's a fixed rule. The camshaft is designed to have one cam lobe to control the opening of each valve. Why half speed? Because each valve is only required to open and close once per working cycle — that is, once every two revolutions of the crankshaft. In a four-stroke engine, the cycle takes two crankshaft revolutions, so the camshaft turns once. Here's an important point about timing. The angular position of the lobes on the camshaft of an aircraft engine is fixed. That means the amount of valve lead, valve lag, and valve overlap remain constant, irrespective of changing engine speed. Valve lead is opening the valve early; valve lag is closing it late; valve overlap is when both inlet and exhaust are open at the same time. Because the camshaft is driven by the crankshaft, valve opening and closing angles are referred to with respect to crankshaft rotation, not camshaft rotation. So when you see a valve timing diagram, the angles are in crankshaft degrees. Finally, valve clearance. To ensure the valves close fully, there must be a valve clearance — also called tappet clearance. This is a small gap measured between the rocker pad and the valve tip. Why do we need it? The valves are continuously heated by combustion and expand at a greater rate than the rest of the operating mechanism. As the engine heats up, that small gap allows the valve to expand at its own rate without being held open. If there were no clearance, the expanding valve would be pushed off its seat and you'd lose compression and burn the valve. That figure shows you the valve springs and how they're assembled. And the accessory housing figure shows the drive side of the engine. So the whole picture: the camshaft, at half crankshaft speed, pushes the rocker arms, which push the valves open against the springs. The springs close them. The clearance lets the hot valve expand freely. And the fixed lobe positions keep the timing constant. That's the breathing system of the piston engine.