A dry sump system is a method to manage the lubricating motor oil in four-stroke and large two-stroke reciprocating internal combustion engines. The dry sump system uses two or more oil pumps and a separate oil reservoir, as opposed to a conventional wet sump system, which uses only the main sump (U.S.: oil pan) below the engine and a single pump. A dry sump engine requires a pressure relief valve to regulate negative pressure inside the engine so that internal seals are not inverted.
Dry sump lubrication is common on larger diesel engines such as those used in ships, as well as gasoline engines used in racing cars, aerobatic aircraft, high-performance personal watercraft, and motorcycles. Dry sumps may be chosen for these applications due to increased reliability, oil capacity, reduction of oil starvation under high g-loads, or other technical or performance reasons. Dry sumps may be unsuitable for some applications, usually due to their increased cost, complexity, or bulk.
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 nearly all production automobile engines, oil that is not actively circulating is in the sump. A pump collects oil from the sump and directly circulates it back through the engine. In a dry-sump system, oil still collects at the base of the engine, but into a much smaller sump, where one or more scavenge pumps draw it away and transfer it to a (usually external) reservoir, where it is both cooled and de-aerated before being recirculated through the engine by a pressure pump. The reservoir is usually tall, narrow, and designed with the oil outlet at the very bottom and baffles above the outlet, for uninhibited oil supply even during sloshing.
Dry sump pump operation consists of a pressure stage and a scavenging stage. Although the term "stages" is commonly used to describe the work of the multiple pumps, they typically run in parallel rather than in series as might be implied by the term. The pressure stage draws oil from the bottom of the reservoir and passes it through the filter and into the engine itself. An adjustable pressure regulator ensures that oil pressure is kept stable at different engine speeds. The dry-sump system requires at least two pumps â one pressure and one scavenge â and sometimes as many as four or five scavenge pumps are used to minimize the amount of oil in the sump. The pressure pump and scavenge pumps are frequently mounted on a common crankshaft, so that a single pulley at the front of the system can run as many oil pumps as the engine design requires. It is common practice to have one scavenge pump per crankcase section; however, in the case of inverted engines (typically aircraft engines), it is necessary to employ separate scavenge pumps for each cylinder bank. Therefore, an inverted V engine would have a minimum of two scavenge pumps and one pressure pump.
Dry sump systems may optionally be designed to keep the engine's crankcase at lower than atmospheric pressure (i.e. a partial vacuum) by sealing the crankcase and allowing the scavenge pumps to draw out both oil and gases. Pressure equilibrium in such an engine will be reached when the rate of gases entering the crankcase (blow-by gases past the piston rings, but also air leaks and oil vapor) equals the rate of gas removal from the scavenge pump capacity beyond what is required to remove just the oil. Alternatively, the crankcase may be kept near atmospheric pressure by venting it to the oil reservoir, which in turn is vented into the engine's air intake or to outside air.
A dry-sump system offers many advantages over a wet-sump. The primary advantages include:
Dry-sump engines have several disadvantages compared to wet-sump engines, including;
Dry sumps are common on larger diesel engines such as those used for ship propulsion, largely due to increased reliability and serviceability. They are also commonly used in racing cars and aerobatic aircraft due to problems with g-forces, reliable oil supply, power output, and vehicle handling. The C6, C7, and C8 generations of the Chevrolet Corvette Z06 use dry sump lubrication, which necessitates an initial oil change after .
Dry sump lubrication is particularly useful for motorcycles, which tend to be operated more vigorously than other road vehicles. Although many motorcycles such as the Honda CB750 (1969) feature dry sump engines, modern motorcycle engines tend to use a wet sump design (particularly transverse inline-four engines, which are wider and 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 should ideally use dry-sump lubrication.
Several motorcycle models that use dry sumps include: