Piston Engines - General — Page 29, Lesson 33

Let’s start with the compression ratio, because it’s the single most important number that defines how a piston engine behaves. The compression ratio is the ratio of the total volume enclosed in the cylinder when the piston is at BDC, to the volume at the end of the compression stroke when the piston is at TDC. BDC is bottom dead centre — the piston is at the very bottom of its travel, so the cylinder holds its maximum volume. TDC is top dead centre — the piston is at the very top, so the volume is at its minimum. So the formula is: Compression Ratio equals Total Volume divided by Clearance Volume. Now, what are those two volumes? The total volume is the swept volume plus the clearance volume. The swept volume is the volume the piston actually sweeps as it moves from BDC to TDC. The clearance volume is the space that remains above the piston when it’s at TDC — the space in the combustion chamber that never gets swept. Let me run the example from the text. Suppose the swept volume is 1300 cc, and the clearance volume is 200 cc. Total volume equals swept plus clearance, so 1300 plus 200 gives 1500 cc. Then compression ratio equals total volume divided by clearance volume, so 1500 divided by 200, which gives 7.5 to 1. Now here’s the important consequence. An increase in compression ratio results in better fuel utilisation — that means greater thermal efficiency — and a higher mean effective pressure, provided the correct fuel is used. Mean effective pressure is essentially the average pressure that does useful work on the piston. But there’s a trade-off: this comes at the expense of higher loading on the moving parts, because the working pressure inside the cylinder is increased. So a higher compression ratio gives you more efficiency and more power, but it stresses the engine more. Now let’s move to engine construction. The main components were stated in the introduction, and now we go into more detail on the mechanical components and their function. We start with the crankcase. The crankcase is usually made in two halves, and that’s to make installation and removal of the crankshaft easier. It houses the main bearings for the crankshaft, it supports the cylinders, and it provides mounting faces and spigots for the attachment of the other main engine casings. A spigot is a small projection or boss that locates one part onto another. It’s generally made of light alloy. It forms a sealed chamber for the lubricating oil, and it’s provided with the means of attaching the engine to its mounting frame in the aircraft. And there’s one more critical detail: a vent to atmosphere is normally provided, so that gas pressure build-up in the crankcase is avoided. That’s important — if pressure built up inside the crankcase, it could force oil out past the seals, so the vent keeps the internal pressure from rising. That figure shows the crankcase with its vent. So to summarise: the crankcase is the structural backbone of the engine — it holds the crankshaft, supports the cylinders, seals in the oil, and mounts the whole engine to the airframe, and it breathes to atmosphere to prevent pressure build-up.