Dry sump

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, the oil that's not actively circulating is stored in the sump, which is large enough for this purpose. A pump collects oil from the sump and directly circulates it back through the engine. In a dry-sump system, the oil still falls to the base of the engine, but into a much shallower 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 sump in a dry-sump system is not actually dry; it is still wet from oil draining from the engine. The reservoir is usually tall and narrow and specially designed with internal baffles, and an oil outlet (supply) at the very bottom for uninhibited oil supply even during sloshing.

A dry-sump system requires at least two pumps - one pressure and one scavenge - and sometimes as many as four or five scavenge pumps are used to minimize the amount of oil in the engine. Dry-sump designs frequently mount the pressure pump and scavenge pumps on a common crankshaft, 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, however in the case of inverted engines (aircraft engine) 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 a pressure pump in the pump stack. Although the term "stages" is commonly used to describe multiple scavenge pumps, they typically run in parallel, rather than in series as the term might imply. The pressure stage draws oil from the bottom of the reservoir, and along with an adjustable pressure regulator, supplies the oil under pressure through the filter and into the engine. If an oil cooler is used usually it is mounted inline between the scavenge outlets and the tank. The dry-sump pump is usually driven by a Gilmer or High Torque Drive (HTD) timing belt and pulleys, off the front of the crankshaft, at approximately one half crank speed.

Dry sump systems may optionally be designed to keep the engine's crankcase at lower than atmospheric pressure (vacuum), by sealing the crankcase and allowing the scavenge pumps to draw out both oil and gases.[1][2] An equilibrium pressure 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's 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:[1][3][4]

• Prevention of the engine experiencing oil starvation during high g-loads when oil sloshes, which improves engine reliability. Most engines can be damaged by even brief periods of oil starvation. This is the reason why dry-sumps were invented, and is particularly valuable in racing cars, high performance sports cars, and aerobatic aircraft that regularly experience high accelerations. Oil slosh occurs in dry-sump systems too, but it is much easier to design a remote reservoir to tolerate high amounts of slosh, by being tall and narrow, and having large baffles.
• Increased oil capacity by using a large external reservoir, which would be impractical in a wet-sump system.
• Improvements to vehicle handling and stability. The vehicle's center of gravity can be lowered by mounting the (typically very heavy) engine lower in the chassis due to a shallow sump profile. A vehicle's overall weight distribution can be modified by locating the external oil reservoir away from the engine.
• Improved oil temperature control. This is due to increased oil volume providing resistance to heat saturation, the positioning of the oil reservoir away from the hot engine, and the ability to include cooling capabilities between the scavenger pumps and oil reservoir and also within the reservoir itself.
• Improved oil quality. When oil sloshes against the crankshaft and other high-speed spinning parts, it causes a "hurricane that whips the oil in a wet-sump engine into an aerated froth like a milkshake in a blender".[5] Aerated oil protects engine components far less effectively. A dry-sump system minimizes oil aeration, and also de-aerates oil far more effectively by pumping it first into a remote reservoir.
• Increased engine power. In a wet-sump engine, oil slosh against spinning parts causes substantial viscous drag which creates parasitic power loss.[5][6] A dry-sump system removes oil from the crankcase, along with the possibility of such viscous drag. More complex dry-sump systems may scavenge oil from other areas where oil may pool, such as in the valvetrain. Power can be further increased if the dry-sump system is designed to create a vacuum inside the crankcase, which reduces air drag (or 'windage') on moving parts as well.
• Improved pump efficiency to maintain oil supply to the engine. Since 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. Furthermore, scavenge pumps can be of a design that is more tolerant of entrapped 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.[7]
• Having the pumps external to the engine makes them easier to maintain or replace.

Dry-sump engines have several disadvantages compared to wet-sump engines, including;[1] [8] [9][10]

• Dry-sump systems add cost, complexity, and weight.
• The extra pumps and lines in dry-sump engines require additional oil and maintenance.
• 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 large external reservoir and pumps can be tricky to position around the engine and within the engine bay due to their size.
• As wrist pins and pistons rely for lubrication and cooling respectively, on the oil being splashed around in the crankcase, these parts might have inadequate oiling if too much oil is pulled away by the pump. Installing piston oilers can circumvent this issue, but do so with additional cost and complexity for the engine.[10]
• Inadequate upper valvetrain lubrication can also become an issue if too much oil vapor is being pulled out from the area, especially with multi-staged pumps.[10]

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 Chevrolet Corvette Z06 has a dry sump engine which requires initial oil change after 500 miles.

• Wet sump

Wikimedia Commons has media related to Dry sump lubrication.

• "Why do some engines use a dry sump oil system?". HowStuffWorks. Retrieved 2006-11-16.
• "Technical Explanation The Dry Sump System". ARE. Retrieved 2016-08-03.
• "Wet Sump vs Dry Sump - Engine Oil Systems". YouTube.com. EngineeringExplained. 2017-01-04. Retrieved 2017-01-04.