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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 external pumps and a secondary external reservoir for oil, as compared to a conventional wet sump system.
Four-stroke engines are both lubricated and cooled by oil which is circulated throughout the engine feeding the various bearings and other moving parts of the engine; and thereafter allowed to drain to the sump at the base of the engine under gravity. In most production automobiles, which use a wet sump system, this oil is simply collected in a 3 to 10 litres (0.66 to 2.2 imp gal; 0.79 to 2.6 US gal) capacity pan at the base of the engine, known as the oil sump (or oil pan in American English), where it is circulated back through the engine by an oil pump drawing, or "scavenging" from the sump's reservoir. This pump is mounted internally to the engine and is typically driven by the camshaft.
In a dry sump, the oil still falls to the base of the engine, but rather than being allowed to collect in a reservoir style oil sump it falls into a much shallower sump where it is removed by one or more externally mounted scavenge pumps and is pumped into an external reservoir where it is both cooled and deaerated. These pumps are typically belt-driven from the front or back of the crankshaft. Oil is then drawn from this reservoir by the pressure pump and circulated through the engine. Typical dry sump systems have the pressure pump and scavenge pumps mounted on a common shaft so that one pulley at the front of the system can run as many pumps as required by the design of the engine. 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 being fed to the valve gear. Therefore, a V-8 engine would have five scavenge pumps and a pressure pump in the pump "stack".
A dry sump offers many advantages, the most obvious of which are increased oil capacity afforded by the remote reservoir and the capability to mount the engine lower in the vehicle because of the thinner profile of the sump — and thereby 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 allows the oil to cool and release entrained gasses from ring blow-by and the action of the crankshaft. Furthermore, dry sump designs are not susceptible to the oil movement problems that wet sump systems can suffer due to high cornering forces. If the oil in a wet sump is forced to one side in the oil pan the oil pump pickup tube can be temporarily uncovered leading to a loss of oil pressure. Because the scavenge pumps are typically mounted at the lowest point on the engine the oil flows into the pump suction by gravity rather than having to be lifted up into the suction of the pump like a wet sump does. Also the scavenge pumps can be of a different design that is more tolerant of entrained gasses than the typical pressure pump which can lose suction if too much air is mixed 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 allows them to be maintained or replaced more easily.
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.
From a performance perspective, the dry sump is a superior solution. Dry sump systems add cost and complexity, however. The extra pumps and lines require additional oil, causing maintenance costs to rise accordingly. 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.
The advantages (above) of a 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 the ground-breaking Honda CB750 of 1969 had a dry-sump motor, modern motorcycles tend to use a wet-sump design. This is understandable with across-the-frame inline four-cylinder engines, since these wide motors need to be mounted fairly high in the frame (for ground clearance), so the space beneath the engine may as well be used for a wet sump. But narrower engines can be mounted lower and ideally should use dry sump lubrication.
It is notable that the Yamaha TRX850 270-degree parallel twin motorcycle has a dry sump engine. The TRX has an unusual feature in that the oil tank is not remote, but is integral to the engine, sitting atop the gearbox. This design eradicates external oil lines, allowing simpler engine removal and providing faster oil warm up.
Harley-Davidson has also used dry sump type lubricating oil systems in their engines since the 1930s.
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