Piston Engines - General — Page 29, Lesson 38

I want to walk you through the rest of the piston engine's anatomy. We've already covered the piston and the compression rings, so now let's look at the rings that sit lower on the piston — the scraper rings, also called oil control rings. Their job is twofold. First, they prevent excess oil from passing up into the combustion chamber. Second, they spread the oil evenly around the cylinder bore. Think of them as the wiper that meters the oil film on the cylinder wall — not too much, not too little. They're designed so the bearing face — the surface that actually contacts the cylinder wall — is reduced in area. That reduction concentrates the load, so the bearing pressure is consequently increased. That higher pressure is what lets the ring scrape and control the oil film properly. Now, the material. These rings are generally made of a special grade of cast iron, and they're sprung against the cylinder walls — meaning they're manufactured with an inherent tension that keeps them pressed outward against the bore. Cast iron is chosen for two very specific reasons. First, it has the ability to retain its elasticity when heated. That matters because the ring operates in a hot environment and must keep its spring tension. Second, it has self-lubricating qualities due to the graphitic content of the metal — graphite acts as a solid lubricant. This is desirable because during the power stroke, the cylinder walls are exposed to the hot combustion gases, and the thin film of oil on those walls is burned away. So when the oil film is gone, the graphite in the cast iron keeps the ring from scuffing against the dry wall. Here's a diagnostic point you'll see in the field. Piston rings that are worn, or stuck in their grooves, will cause excessive blue smoke to be ejected from the exhaust pipe. Blue smoke is the signature of burning oil — oil is getting past the rings into the combustion chamber and being consumed. Now let's move up to the cylinder itself. The cylinder barrel, or block, is made of alloy steel. It has two jobs: it resists the pressure of combustion, and it provides a working surface for the piston. The cylinders are usually secured to the crankcase by studs and nuts. One end of the cylinder is sealed by the cylinder head, and the movable piston seals the other end. So you have a fixed seal at the head and a moving seal at the piston — that's the combustion chamber boundary. Look at Figure 2.14 — it labels the full assembly: the cylinder head, the cylinder barrel, the valves, valve guides, valve springs, camshafts, and the spark plug. That's the complete top end of the engine. Now, the thermal problem. About 30% of the heat generated during combustion is transferred to the cylinders. That's a huge amount of heat that must be removed, or the cylinder will overheat and fail. There are two cooling methods used. Liquid cooling has a jacket around the cylinders — a water-filled passage that allows a liquid to flow around them and carry the heat away. Air-cooled engines, on the other hand, have fins machined onto the cylinder. Those fins increase the surface area in contact with the air, and that increased area is what's used to dissipate the heat. So the cooling method determines the external shape of the cylinder — a smooth jacketed barrel for liquid cooling, or finned for air cooling. That's the cylinder and ring system complete. When you see blue smoke, you know the rings are failing; when you see fins, you know it's air-cooled; when you see a jacket, liquid-cooled.