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July 2010 Willie's Workbench

Posted in Features on July 1, 2010 Comment (0)
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Willie's Workbench Oil Pumps Last month, I talked a bit about water pumps, so this month I'm going to talk a bit about what may be considered the heart of an engine: the oil pump. Not only does the oil pump move the life blood of an engine throughout its internal passages in order to provide a protective lubricating film between moving parts, but it also absorbs heat that is then dissipated through the oil pan. The oil is also used to maintain proper valve clearance, and in some engines even valve timing.

There is no real way for an individual to measure the efficiency of an oil pump other than by a gauge that measures oil pressure. However, an oil pump really doesn't produce pressure but volume. The restriction to flow within an engine is what develops the pressure. For a better understanding, let's take a garden hose as an example: Water coming out of a un-nozzled hose will perhaps squirt out a foot or so at, say, 40 psi. Put a nozzle on the end of the hose, which restricts flow due to its smaller size, and the discharge pressure may jump to, say, 60 to 80 psi and have a range of perhaps 18 feet.

These restrictions are usually referred to as clearances. For instance, the clearance on crankshaft main bearings or connecting rod bearings may be 0.002 to 0.003 inch. That's the distance between the bearing shell and the crankshaft. The more the clearance, the greater the oil flow, and the lower the oil pressure. That's why when the engine components get worn, the oil pressure diminishes. The higher the engine rpm, the higher the loads that are being put on the components, and the oil film may not be strong enough to prevent metal-to-metal contact. This is one benefit to the rotary design that the oil pump provides. The faster the engine turns, the faster the oil pump turns, thus increasing oil pressure.

There is a general rule of thumb that says that an engine needs 10 pounds of oil pressure for each 1,000 rpm. The oil "weight" can also have a direct effect on oil pressure, too. Heavier oil takes more effort to pump throughout the engine, which again takes more horsepower. The heavier-weight oil will increase oil pressure but takes longer to flow through the engine. In fact if the clearances are tight, such as low as 0.0015 inch or even less, the oil may not provide proper lubrication

Excessive oil pressure or excessive volume is not good. Why? Because it takes horsepower to turn that pump. When the pressure exceeds the setting of the pressure relief spring, that oil is dumped back into the pan. The more you pump the oil, the hotter it gets. There is even some thought that higher-than-needed oil pressure can erode bearing material. With excessive pressure, you take the chance of blowing out the gasket on the oil filter or breaking the seam on it. Oh, and just so I don't get a letter telling me this: it's not just the pump's oil pressure that maintains the oil film between the bearing and the crankshaft. It's an action called a "hydraulic wedge," that is developed when the crankshaft turns, that forces the loaded area away from the bearing.

There are multiple types of oil pumps being used, but all work on the same principle, with the most common being sometimes called a "wet sump" system. Meshing gears within a housing rotate, pulling in oil from the oil pan, forcing it throughout the engine, and eventually returning it to the oil pan. Generally, these are driven off the camshaft by a shaft connected to the distributor. These types of pumps run at half engine speed, and generally can be found either inside the engine (within the oil pan) or on an accessory cover (on the front of the engine). Some late-model engines mount the oil pump directly off the front of the crankshaft, thereby running at a speed equal to the engine and producing more volume at lower speeds.

There is one other type of pump-actually, an entirely different type of system. It's used by some aircraft, as well as some types of race vehicles that experience high G-forces where the oil may be forced away from the pickup. The oil pump is driven off an accessory belt, and actually is two or even more pumps. One pulls the oil out of a very small oil pan from several locations and pushes it into a specially designed storage tank that may hold as much as 20 quarts. Oil is then pulled from the tank and pressure-fed into the engine. This method always ensures that the engine is getting oil, as well as preventing the crankshaft from splashing in the oil. It is referred to as a "dry sump" system.

Clearances within an oil pump are quite close and are the only part of the engine that does not initially receive filtered oil. However, the oil does pass first through the filter before traveling to the rest of the engine.

There is one other type of oiling system that adds a crutch to the wet sump system called an "accumulator"-a special auxiliary oil tank that holds two to three quarts. Cylindrical in shape, it has one chamber for oil and another divided by a movable piston and seal that holds pressurized air. An electric valve triggered by an oil pressure switch connects the tank via a hose to a special adapter between the engine and oil filter. When the ignition key is turned to "On," the valve opens and pressurizes the oil passageways. When the engine is started and the engine's oil pump develops pressure above that on the pressurized side of the accumulator, the additional three quarts of oil are forced back into the storage side of the accumulator.

The first advantage to this system is the elimination of "no-oil-pressure" start-ups. However, the biggest advantage, especially to a vehicle that may be in a position where the oil is pulled away from the pickup tube, is that as soon as oil pressure drops below the setting of the oil pressure switch, pressurized oil from the accumulator is forced into the engine. Granted, three quarts flowing from the accumulator through the engine may not last more than a couple of minutes at slow speeds, and only a few seconds at high engine speeds, but it just may be long enough to prevent engine damage. For example: You're on a steep angular descent and suddenly a front tire drops into a hole and the back end comes up, almost pitching you over. All the oil in the pan's rear sump come rushing forward, thus uncovering the oil pickup tube. The oil pump would then be sucking air, not oil, and damage to the engine could occur. I consider it sort of an insurance policy against damage.

The oil pump, like the water pump we discussed last month, is a simple but very vital part of your engine.


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