Can Engine Oil Be Stored Outside The Engine?

A colleague brought his Yamaha MT-01 to the office recently, and I was quite intrigued by a large canister to the left of the engine (seen in the image below). Upon closer inspection, I found that this canister had lines leading to and from the engine’s oil pan, and was actually a dry sump oil reservoir. We’ve all read the term ‘wet sump’ when looking up our bikes’ lubrication systems, and there are a few bikes out there like the MT that even use a dry sump. So what exactly are the two systems and how are they different?

Wet Sump Lubrication

Typical engine architecture consists of a cylinder head up top, a cylinder block in the middle and a crankcase at the bottom. Underneath the crankcase is a deep oil pan where the engine oil is stored. This oil is in constant circulation when the engine is running - an oil pump collects the engine oil from the oil pan and sends it to all the important areas that need lubrication. Parts like the crankshaft and camshaft bearings are sprayed by jets. After being sprayed, the oil then drips back down to the oil pan and the whole process starts over. This is the basic design and working of a wet sump lubrication system, which is the most widespread kind seen in motorcycles.

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Dry Sump Lubrication

A dry sump lubrication system is quite similar in many ways to a wet sump design. The oil is still circulated by a pump and sent to all the important areas, but rather than having a deep oil pan below the crankcase, the engine oil is instead stored in an external reservoir. After being sprayed, the oil drips down to a much shallower oil pan from where it is sucked away by a scavenge pump and sent to an external reservoir. This is the ‘can’ seen on the MT-01 above. The primary oil pump then draws the oil from this reservoir and sends it to the various jets to be sprayed.

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So Which Is Better?

The biggest advantage of dry sump systems is their shallower oil pan which results in a shorter (in terms of height) engine which can be mounted lower. Since the engine is usually one of the heaviest components in any vehicle, having a shorter engine which can be mounted lower greatly lowers the overall centre of gravity of the vehicle. This is extremely advantageous in cars, where having a lower centre of gravity greatly improves handling characteristics. In motorcycles, though mass centralisation is becoming an ever more important factor, it isn’t quite as important to have your weight very low down as it is in cars. In fact, having a very low centre of gravity can result in a bike that feels heavy to steer and unwilling to lean, as Honda found out with its upside-down 1984 NSR500 grand prix bike.

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A dry sump system also adds complexity and weight, in the form of an extra scavenge pump, oil reservoir and lines to and from the engine. For this reason, dry sump systems have been few and far between when it comes to motorcycles. Aprilia employed the design on its RSV Mille, RST1000, SL1000 and ETV1000. Honda’s NX650, XR500R, XR600R, XR650R and XR650L four-stroke dirt bikes and the Suzuki DR-Z400 use a very innovative dry sump design, housing their engine oil in the frame tubing. Closer to home, Royal Enfield engines built prior to 2007 used a dry sump positive plunger design.