A dry sump is a lubricating oil management method for four-stroke and large two-stroke piston internal combustion engines that uses a secondary external reservoir for oil, as compared to a conventional wet sump system.
Four-stroke engines are lubricated by oil which is pumped into various bearings and thereafter allowed to drain to the base of the engine. In most production cars, which use a wet sump system, this oil is simply collected in a three to seven litre capacity pan at the base of the engine, known as the oil pan (or "sump"), where it is pumped back up to the bearings by the oil pump, internal to the engine. In a dry sump, the oil still falls to the base of the engine, but rather than being collected into an oil pan, it is pumped into another reservoir by one or more scavenger pumps, run by belts from the front or back of the crankshaft. Oil is then pumped from this reservoir to the bearings of the engine by the pressure pump. Typical dry sump systems have the pressure pump and scavenger pumps "stacked up", so that one pulley at the front of the system can run as many pumps as desired, just by adding another to the back of the stack.
A dry sump offers many advantages, namely increased oil capacity and a lower center of gravity for the engine. Because the reservoir is external, the oil pan can be much smaller in a dry sump system, allowing the engine to be placed lower in the vehicle; in addition, the external reservoir can be as large as desired, whereas a larger oil pan raises the engine even further. Increased oil capacity by using a larger external reservoir leads to cooler oil. Furthermore, dry sump designs are not susceptible to the oil starvation problems wet sump systems suffer from if the oil sloshes in the oil pan, temporarily uncovering the oil pump pickup tube. Having the pumps external to the engine allows them to be maintained or replaced more easily, as well.
Dry sumps are common on larger diesel engines such as those used for ship propulsion. Many racing cars, sports cars, and aerobatic aircraft also utilize dry-sump equipped engines because they prevent oil-starvation at high g loads and because their lower center of gravity positively affects performance.
On the downside, dry sump systems add cost and complexity, and the extra pumps and lines require more oil, so maintenance costs go up as well.
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 in line 4-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 Motorcycle Corp. has also used Dry Sump type lubricating Oil systems in their engines since the 1930's.
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