Piston Engines - Lubrication — Page 52, Lesson 62

Let's start with the pressure pump, because that's the heart of the whole lubrication system. The pump itself is built from two deep-toothed spur gears — think of two gears with long, deep teeth meshing together — and they rotate inside a very close-fitting pump casing. That casing is driven through the accessory housing, which is how the engine's own rotation powers the pump. Here's the clever part about how it actually moves oil. Oil is carried around either side of the casing in the space between the gear teeth. As the gears rotate, that oil is trapped in the tooth spaces and physically carried along, and that's what makes it flow. The outlet side of the pump is enclosed, and the engine components that need lubricating provide a restriction to that flow. Because the oil can't just flow freely out, that restriction causes a rise in system pressure. That's the whole principle — the pump delivers a flow, and the resistance to that flow is what creates the pressure. Now, the actual oil pressure you get isn't a fixed number. It depends on three things: the speed of the pump, the temperature of the oil, and the resistance offered by the components. Faster pump speed means more flow and more pressure. Thicker, colder oil resists flow more, so pressure rises. And the resistance from the components themselves — the bearings and clearances — also plays its part. The pump's capacity has to be chosen very carefully. It must be big enough to supply a minimum oil pressure even under the worst possible running conditions — that means low turning speed and high inlet oil temperature, when the oil is thin and the pump is slow. But here's the consequence: under normal running conditions, that same pump is delivering more flow than the engine needs, and that increased flow would tend to cause a dangerously high oil pressure. So the pump is deliberately oversized to guarantee pressure at the worst case, which means it's too big for normal running. That's exactly why we need a Pressure Relief Valve — the PRV. It sits across the inlet and outlet connections of the pump, and its job is to limit the maximum pressure in the system. When the pressure reaches a predetermined figure, the valve opens and returns enough oil to the inlet side of the pump to hold the maximum pressure down. So it's a safety valve that prevents those dangerously high pressures. In operation, the engine will have a range of operating pressures related to engine speed, from idle right up to maximum rpm. Now let's move to the check valve, which is also called a non-return valve or a one-way valve. This one solves a specific problem. The oil tank may be mounted higher than the pressure pump to give a gravity feed. When the engine is stopped and the oil is hot and thin, that gravity pressure is enough to force oil through the clearances in the pressure pump. If that happened, the oil tank would drain into the crankcase and the engine would be flooded with oil. This feature of dry-sump operation is sometimes called over-oiling. To prevent it, a check valve is fitted. It's either a lightly sprung loaded valve, or an electrically-operated shut-off valve — the SOV — and it holds the oil back until the engine is started. Finally, the pressure filter. It's fitted downstream of the pressure pump, before the oil enters the engine, and its job is to remove very small solid particles before the oil passes to the bearing surfaces. But there's a spring-loaded relief valve fitted to bypass the filter element when the oil is cold, or if the element becomes blocked. That bypass is critical — it also protects the engine if the pressure pump breaks up, so the oil can still get through even if the filter is blocked or the pump fails. So to tie it all together: the spur gear pump creates flow, the restriction creates pressure, the PRV caps that pressure, the check valve stops gravity draining the tank when the engine is off, and the pressure filter cleans the oil before it reaches the bearings.