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Guide: Technology – Motors and Drive Everything about engine lubrication
Wet sump lubrication
Husquarna spray oil lubrication
Dry sump lubrication
Oil reserves

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Guide: Technology – engine lubrication

Guide: Technology – Motors and Drive
Everything about engine lubrication

To ensure engine lubrication, a sufficient amount of oil must be stored. There are different methods of creating this oil reservoir.

Werner Koch

01/21/2010

It was not as easy as the designers had thought of the engine lubrication for their four-stroke cross machines in the early 1980s. Back then, the Swedish technicians from Husqvarna planted an air-cooled cylinder with a four-stroke head on the housing of the large two-stroke engine. The oil thrown into the crankshaft drive should suffice for lubrication; a pump was dispensed with. Although the shafts running in roller bearings and the valve train passed this test to a certain extent unscathed, the thermally stressed areas overheated because the extremely small oil volume did not guarantee a sufficient cooling effect. This more than meager, albeit very light and simple system of spray oil lubrication has not caught on in any motorcycle engine.

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Simple, light and reliable: wet sump lubrication is standard in engine construction today

The advantages of this system: You only need an oil pump and apart from the oil cooler no other external hose connections are required. The complete oil circuit takes place inside the motor housing.

If the oil pan is large enough, the oil can cool down sufficiently. In addition, with an ample oil reservoir with a large surface area, there is enough time for the gas and air bubbles contained in the oil to escape.

Wet sump lubrication

Drawing: archive

Wet sump lubrication sketch.

All that is required here is a pressure pump that sucks the oil out of the large and deeply set oil pan (yellow) and presses it to the lubrication points with a pressure of around 3.5 bar (red circuit). The oil is stored at the bottom of the engine, which significantly increases the overall height. If the oil pan and thus the oil level were installed too high, the crankshaft and clutch would transform the lubricant into a foamy emulsion of air and oil, which of course would drastically impair the cooling and lubricating effect.

While on motorcycles when cornering, the oil hardly escapes to the side due to the inclined position, in four-wheeled vehicles it is pushed to the outer wall of the oil pan due to the high centrifugal forces, which can cause a breakdown of the oil circuit and thus the lubrication in long curves due to insufficient oil level.

With motorcycles, it becomes critical when the driver does long wheelies in a strong position and the oil shifts from the oil pan to the rear of the engine housing. In contrast to the early 1970s, when motorcycle engines often had to struggle with insufficient lubrication of the distant camshaft bearings (1) after a cold start, nowadays powerful oil pumps and sophisticated lubrication systems also reliably supply these bearing points with oil.

Husquarna spray oil lubrication

Drawing: archive

The spray oil lubrication developed by Husquarna, which, however, did not catch on.

The four-stroke single-cylinder from Husqvarna were based in the early 1980s on the light and simple two-stroke engines, on the housing of which a four-stroke was simply mounted. To make lubrication as simple and easy as possible, the oil was conveyed from the crankcase to the timing chain (2) via a membrane (1) (Husqvarna Reed Activated Lubrication). This dredged the lubricant more or less reliably upwards in order to lubricate camshafts and rocker arms. The problem with this: The small amount of oil in the engine had to be changed extremely often, because the oil could not be pressed through a fine-pored paper filter due to the lack of a pressure pump and it became soiled extremely quickly. Which even destroyed the rolling bearings of the connecting rod and crankshaft, which were inherently resistant. In addition, the cooling effect was miserable, which especially drove the air-cooled enduro and cross engines into heat collapse in tough off-road use with low airflow but high load.

Dry sump lubrication

Drawing: archive

Sketch of dry sump lubrication.

The schematic representation clearly shows the advantages and disadvantages of this system. In direct comparison to wet sump lubrication, the lower overall height of the oil pan is clearly visible, which improves the ground clearance. A great advantage for enduros or tall engines (Harley-Davidson V2). Thanks to the compact oil tank, which is often provided with baffles, the oil is reliably available for lubrication even with high G-forces (acceleration, braking).

A second pump is required to suck up the oil from the small oil pan and transport it to the oil tank. When entering the oil tank, it is advantageous if the oil jet is swirled and thereby loosens air bubbles from the oil.

The return pump (1) must have a significantly higher delivery rate than the pressure pump (2). This is the only way to ensure that the oil tank is sufficiently refilled in all speed ranges and that the pressure pump cannot suck in air. If the oil tank is placed above the engine housing, the oil seeps out of the tank via the oil pressure pump into the engine when the engine is idle for a long time, which is why the driver’s manuals describe a specific procedure for checking the oil level.

Oil reserves

KTM

Without external lines and placed directly in front of the crankcase, the oil tank on the KTM RC8 moves towards the center of gravity.

In order to accommodate the lubricant in wet sump lubrication, the engine designers came up with a lot. Not only Yamaha used the frame tubes as reservoirs on the XT-500/550/600 series, for example. The big advantage here: the relatively thin front central pipe bunkered the oil in this long shaft so compactly that it fed the oil to the pressure pump even in the event of a fall or extremely steep ascents. The oil level was checked by a dipstick between the steering head and the tank (photo below).

Erik Buell not only used the large frame profiles as a fuel tank, but also the swing arm as an oil reservoir. Flexible lines compensated for the rotary movements. The large surface of the aluminum swing arm, surrounded by the wind, provided a welcome cooling for the oil.

Guide: Technology – engine lubrication

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Guide: Technology – engine lubrication