Under pressure: Engine lubricationJune 27, 2018
If a fluid is considered incompressible, how does any hydraulic system create pressure? An engine’s oiling system bows to the laws of hydraulics. Pressure is created by limiting or restricting flow.
A garden hose with no nozzle has a high rate of flow but little to no pressure. With the nozzle, discharge pressure increases but flow drops. A restriction is required to increase pressure.
Pressure pushes the oil through the intricate engine passages and creates an oil wedge (clearance) between the crankshaft journal and the bearing. The oil wedge (or film) differs from the main and rod bearing clearance —- it is much thinner.
Oil system pressure is the cumulative effect of the bearing clearances (both rod and main), the viscosity (thickness) of the oil, and the length, diameter and surface finish of the oil passages (galleries), along with the output of the pump.
Most engines employ one of two oil pump designs; a gear -type or a rotor-type. The gear-type is comprised of the pump body and two gears meshed together inside. When one of the gears is driven, the other rotates in the opposite direction. Oil is carried around the outside of each gear and forced into the space between the gear teeth and the body, thus producing pressure at the outlet.
With the rotor-type, the drive rotor and the driven rotor are meshed inside the pump body. The drive rotor shaft is made eccentric in relation to the body, so when the drive rotor turns, the driven rotor also turns but the space between the rotors changes. Oil enters thru the inlet side where the space between the rotors is large and is sent to the other side, where the space is smaller. This diminishing space causes the pressure to be developed and oil forced thru the outlet.
Oil pumps are classified as positive displacement. This means that maximum pressure must be limited with a pressure-relief valve. If a pressure relief valve were not used, system pressure would become dangerously high.
The oil pressure relief valve consists of a valve or ball and coil spring. The relief valve opens as delivery pressure rises in relation to engine rpm. Oil will then be bypassed from the outlet side into the inlet side to maintain a specified pressure. A stiffer spring will result in higher oil pressure. However, it will not increase the volume of oil delivered.
Pump volume is determined by the length of the gears, pressure by the spring.
Pressure or flow?
A minimum industry standard for oil pressure is 10 psi for every 1,000 rpm. An engine that idles at 700 rpm needs a minimum pressure of 7 psi and should increase as engine speed goes higher. At 3,000-rpm the engine requires a minimum oil pressure of 30 psi.
A pump that is delivering 80 psi takes more power to run then one creating 40 psi. In many instances the only good the higher pressure does is to please the owner by seeing the needle on the gauge over to the right. To effectively lubricate an engine you want sufficient pressure but with a high flow of oil. A high volume oiling system not a high- pressure one.
Oil is also used as a coolant; the greater the volume, the more heat that is removed from critical engine parts. It also allows for fresh oil to be readily supplied to the oil wedge. The goal is to have enough pressure to maintain the wedge while flooding the bearings with lubrication.
If you bought piece of equipment from someone that had the engine rebuilt and you have no idea about the bearing
clearance, and it is more than 100 horsepower, set the pump for 15 psi (hot) at idle with 10W-40 oil and then follow that curve so at 3,000 rpm the engine sees about 45 to 50 psi.
For a lower horsepower engine, the 10psi/1,000 rpm is fine.
If the bearings are worn the engine will require a higher pump output to maintain a nominal value.