Piston Engines - General — Page 29, Lesson 36

Let’s start with the firing interval, because that’s the idea that ties the whole cylinder arrangement together. I mentioned earlier that more cylinders give more power strokes, and the reason is that each cylinder fires once every 720° of crankshaft rotation — that’s two full revolutions of the crank, which is the complete cycle for a four-stroke engine. The firing interval is simply the angle of crankshaft rotation between successive power strokes. You find it by dividing 720° by the number of cylinders. So a four-cylinder engine has a firing interval of 180°, and a six-cylinder engine has 120°. Four cylinders is generally regarded as the minimum number to give a reasonable firing interval. Now, the firing order is different from the firing interval. The firing interval tells you the spacing in degrees; the firing order tells you the sequence in which the individual cylinders fire, and that’s determined by the crankshaft and cylinder arrangement. A typical four-cylinder engine might have a firing order of 1-3-4-2. Notice the cylinders do not fire consecutively — that’s deliberate, because firing them consecutively would increase the load and vibration on the crankshaft. And there’s a note worth remembering: the Lycoming firing order is 1-3-2-4. Let’s move to the connecting rods. Their job is to transmit the forces of combustion to the crankshaft, converting the linear movement of the pistons into rotary movement of the crankshaft. A connecting rod is usually made of H-section high tensile steel — the H shape gives you lightness combined with the strength needed to withstand the compressive and tensile loads imposed as the piston changes direction. The rod connects to the crank-pin of the crankshaft through a large circular bearing at what we call the Big End of the rod. Now the pistons. They’re generally made of aluminium alloy, and the piston forms a sliding plug in the cylinder. It transmits the force of the expanding gases, via the connecting rod, to the crankshaft. On the piston, bosses are formed to house the gudgeon pin, which fastens the piston to the Small End of the connecting rod. Circumferential grooves are machined in the piston to accommodate the piston rings. The piston rings have a critical sealing job. They prevent pressure leakage past the piston in one direction and oil leakage in the other. A number of rings can be fitted, and their arrangement varies from engine to engine, but it’s similar to what I’ll describe now. The compression rings prevent gas leakage into the crankcase. They’re fitted into grooves cut into the upper portion of the piston. Here’s the clever part of the mechanism: gas passing down between the piston and the cylinder wall forces a compression ring down in its groove and outwards against the cylinder wall — so the gas pressure itself energises the seal. A small amount of gas will pass the top ring, which is why a second, and sometimes a third, compression ring is fitted. That figure shows the piston and its associated components, so you can see the rings sitting in their grooves and how the gudgeon pin ties the piston to the small end of the rod.