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A dry sump is a lubricating motor oil management method for four-stroke and large two-stroke piston internal combustion engines that uses additional pumps and a secondary reservoir for oil—as compared to a conventional wet sump system, which uses only the main sump (U.S.: oil pan) below the engine and a single pump. Dry sump can also be achieved with a single vacuum pump that creates positive and negative pressures to pull and push the oil out and into the engine. A dry sump engine requires a pressure relieve valve to regulate negative pressure inside the engine, so internal seals are not inverted.
Engines are both lubricated and cooled by oil that circulates throughout the engine, feeding various bearings and other moving parts and then draining, via gravity, into the sump at the base of the engine. In the wet sump system of most production automobile engines, an oil pump collects this oil from the sump and circulates it back through the engine.
In a dry sump, the oil still falls to the base of the engine, but rather than collecting in a reservoir-style oil sump, it falls into a much shallower sump—where one or more scavenge pumps draw it off and transfer it to a (usually external) reservoir, where it is both cooled and deaerated. A pressure pump then draws this oil and circulates it through the engine. Often, dry sump designs mount the pressure pump and scavenge pumps on a common shaft, so that one pulley at the front of the system can run as many pumps as the engine design requires. It is common practice to have one scavenge pump per crankcase section and in the case of a V-type engine an additional scavenge pump to remove oil fed to the valve gear. Therefore, a V-8 engine would have four scavenge pumps and a pressure pump in the pump stack.
A dry sump offers many advantages. The most obvious are increased oil capacity afforded by the remote reservoir, and the capability to mount the engine lower in the vehicle because of the lower sump profile—lowering the overall center of gravity. The external reservoir can also be relocated to another part of the car to improve weight distribution. Increased oil capacity by using a larger external reservoir than would be practical in a wet-sump system cools the oil more and releases entrained gasses from ring blow-by and the action of the crankshaft. Furthermore, dry-sump designs are not susceptible to the oil movement problems from high cornering forces that wet sump systems can suffer. In a wet sump, the force of the vehicle cornering can force the oil to one side of the oil pan, possibly uncovering the oil pump pickup tube and causing a loss of oil pressure.
Because scavenge pumps are typically mounted at the lowest point on the engine, the oil flows into the pump intake by gravity rather than having to be lifted up into the intake of the pump as in a wet sump. Also, the scavenge pumps can be of a design that is more tolerant of entrained gasses than the typical pressure pump, which can lose suction if too much air mixes into the oil. Since the pressure pump is typically lower than the external oil tank, it always has a positive pressure on its suction regardless of cornering forces. Another phenomenon that occurs in high-performance car engines is oil frothing up inside the crank-case due to the very high revs agitating the oil. Lastly, having the pumps external to the engine makes them easier to maintain or replace.
Dry sumps are common on larger diesel engines such as those used for ship propulsion. Many racing cars, high performance sports cars, and aerobatic aircraft also use dry-sump equipped engines because they prevent oil-starvation at high g loads, and because their lower center of gravity positively affects performance.
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Dry sump systems add cost, complexity, and weight. The extra pumps and lines require additional oil and maintenance. Also, the performance-enhancing features of dry sump lubrication can hurt a car's day-to-day driveability. A good example is the classic Mercedes-Benz 300SL, a car that was designed for racing but sold to the general public and used on-road. The car had high oil capacity and a dry sump system to cope with continuous high-speed running while racing. Owners found in general use, however, that the oil never achieved the correct operating temperature because the system was so efficient at cooling the oil. A makeshift solution was devised to deliberately block the oil cooler airflow to boost the oil temperature.
Dry sump motorcycle engines
The advantages of dry sump lubrication are particularly beneficial to motorcycles, which tend to be ridden (driven) more vigorously than other road vehicles. The classic British parallel twin motorcycles such as BSA, Triumph and Norton all used dry sump lubrication. Traditionally, the oil tank was a remote item, but some late-model BSAs and the Meriden Triumphs used "oil-in-the-frame" designs. Although motorcycles such as the Honda CB750 (1969) features a dry-sump engine, modern motorcycles tend to use a wet-sump design. This is understandable with across-the-frame inline four-cylinder engines, since these wide engines must be mounted fairly high in the frame (for ground clearance), so the space below may as well be used for a wet sump. However, narrower engines can be mounted lower and ideally should use dry sump lubrication.
The Yamaha TRX850 270-degree parallel twin motorcycle has a dry sump engine with its oil reservoir not remote, but integral to the engine, sitting atop the gearbox. This design eliminates external oil lines, allowing simpler engine removal and providing faster oil warm up.
Harley-Davidson has used dry sump type lubricating oil systems in their engines since the 1930s.
- Van Valkenburgh,Paul (1976) Race Car Engineering and Mechanics Dodd, Mead & Company, p. 181
- "The iconic SR400, 35 years heritage". Suzuki Press Release, MCNews.com, 04-11-2013.
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