Piston Engines - General — Page 34, Lesson 41

Let’s pick up with the valve clearance, because that’s where the last part left off. I want you to picture the rocker arm pressing down on the valve tip. The clearance is the tiny gap between the rocker pad and the valve tip when the valve is supposed to be shut. We measure that gap with feeler gauges, and there’s a provision on the rocker arm to adjust it. Now, here’s the key relationship. If the clearance is too big, the valve opens late and closes early. If it’s too small, the opposite happens: the valve opens early and closes late. And if it’s really too small, the valve may never close at all. That’s when you get a phenomenon called “popping back into the carburettor.” The same thing can happen if an inlet valve is sticking in its guide. Some engines avoid this entirely by using hydraulic tappets. These are self-adjusting, they operate with no clearance, and therefore there’s no tappet noise. A hydraulic tappet is made of two main parts, one sliding inside the other. Oil is supplied under pressure, and that pressure causes the tappet to lengthen and take up any clearance while the engine is running. Now let’s move to the sump. The sump is a casing attached to the base of the crankcase. Its job is to collect the lubricating oil after it has passed through the engine. In some lubricating systems, the sump also acts as the oil reservoir, meaning all the oil is contained within it. There’s a filter housed in the sump that traps any debris in the oil, preventing damage to the oil pumps. Next, the carburettor. The carburettor meters the air entering the engine and adds the required amount of fuel as a fine spray, under all conditions of engine running. For an aircraft engine, the correct mixture must be supplied regardless of altitude or attitude of the aircraft. Some engines use an injector instead of a carburettor. These are attached to the base of the crankcase, and metal pipes connect the outlet from the carburettor or injector to the cylinders. That pipe system is called the induction manifold. After combustion, the waste gases are carried away from the cylinders by the exhaust system. The exhaust consists of steel pipes connected to each cylinder. The pipes from each cylinder usually connect up and go into one or two pipes, which then carry the hot gases outside the aircraft to atmosphere. Finally, the accessory housing, also called the wheelcase. For the engine to operate, supporting systems are needed, and they may need power to drive them. Oil pumps, fuel pumps, superchargers, and magneto ignition systems are all fitted to the accessory housing and driven via gears by the crankshaft. The housing casing is bolted to the rear of the crankcase, enclosing the gear train and providing mounting pads for the ancillary equipment. A starter motor can be connected to the housing to initially rotate the crankshaft and start the cycle of operation. The accessory housing can also provide the drive to power aircraft systems such as electrical generation, hydraulics, and pneumatic systems. So to tie it together: the valve clearance affects valve timing, hydraulic tappets eliminate that clearance automatically, the sump collects and filters the oil, the carburettor or injector feeds the mixture through the induction manifold, the exhaust carries away the waste gases, and the accessory housing drives all the supporting systems off the crankshaft.