Semi-dry-sump lubrication

By Kevin Ash

Pictures: BMW Press, Kawasaki Press

BMW’s F800 and the Kawasaki ER-6n/f (and the Versys) are unusual in featuring what’s called a semi-dry-sump lubrication system.

The majority of road bikes use wet sump lubrication, the ‘sump’ being a pan beneath the crankshaft where the oil is stored. A pump picks it up from here and forces it around the engine into the various bearings, spray jets and so on. After the oil has passed through the high pressure part of the system it drains back down into the sump purely under the influence of gravity.

BMW F800 lubrication circuits - Click on imageIt’s simple and inexpensive, but there are disadvantages. The first is the size of the sump. Usually this will have to hold around 4 litres, maybe more. This is quite large – look at a 5 litre oil can to get an idea – and clearly it has to be at the lowest point, so the engine has to sit higher than might be ideal. Under hard acceleration or braking, or when the going is very bumpy, the oil in the sump can slosh around. In extreme cases this can mean the oil pump’s pick up tube becomes open to the air, and air bubbles are passed around the lubrication system, causing a lot of wear and damage. But it also means the oil can wash up against the crankshaft, which usually spins just above the surface of the sump oil. This causes a lot of drag, reducing engine performance as well as causing the oil to become foamy, which degrades its lubrication abilities.

BMW F800 twin, oil is stored on the left behind the crank and under the gearbox. What looks like a sump houses the engine balancing system - Click on imageThe alternative is dry sump lubrication. Instead of storing the oil beneath the engine it’s kept in a separate tank somewhere else on the bike – in the frame on the Aprilia RSV Mille for example, or in the swingarm on air-cooled Buells. This means the engine can be positioned lower in the frame (very useful with naturally tall engines such as the V-twins mentioned), and the problems associated with oil sloshing around are eliminated. It’s easier to increase the oil capacity this way too, which means extended service intervals. The penalty is increased complexity (and hence cost), as you now need a separate tank and two oil pumps. One pump scavenges the oil draining down to the bottom of the engine and feeds it up to the oil tank, while a second, more powerful pump takes oil from the tank and feeds it back into the lubrication system under pressure.

Kawasaki ER-6n/f, note oil pan is offset and not directly beneath the crankshaft - Click on imageSome bikes though use a semi-dry sump system, including many off-road machines as well as the BMW and Kawasaki. What this really means is that the system is to all intents and purposes a dry sump design, with two oil pumps, but the oil tank is still incorporated inside the engine cases. In the F800’s case it’s still stored beneath the engine, but not directly beneath the crankshaft, and in a similar way the ER-6’s oil is stored beneath the gearbox.

It’s a little more complex but by doing it this way the designers have more scope for lowering the engine and making it more compact.

* BMW F800 GS test
* BMW F650 GS test
* Kawasaki ER-6n test
* Kawasaki Versys test

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