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When this big digger needs an oil change, you can't just drive it down to the local service station. See more green science pictures.

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An engine has hundreds of metal parts that move against other metal parts. Left unchecked, friction between those moving parts would turn a powerful machine into an inert piece of sculpture.

To prevent this from happening, oil bathes the innards of an engine, coating its parts and allowing them to slip and slide with relatively little resistance. Over time, however, oil gets dirty and loses its effectiveness. Traditional oil filters are meant to clean the lubricating fluid, but even they have their limits. Eventually, both the oil and the filter have to be replaced. This seems like a good solution until you realize that all of that oil has to go somewhere. According to the Environmental Protection Agency, 200 million gallons (757 million liters) of used oil are improperly disposed of each year. That oil can contaminate groundwater, foul beaches and harm wildlife [source: EPA Common Wastes & Materials, "You dump it, you drink it"].

Even beyond the environmental issues, changing oil has other problems. Inefficiency is the biggest problem of all. When oil is allowed to drain, as much as 40 percent of it remains in the engine, clinging stubbornly to surfaces in rams and plumbing [source: Oil Solutions N.Q.]. That means new oil added to the engine mixes with dirty, contaminated liquid left behind after the last changeover. And this puts a vehicle even closer to its next oil change.

Centrifugal oil cleaners were introduced to help with these problems. They take advantage of a well-known force -- centrifugal force -- associated with circular motion. Technically speaking, centrifugal force doesn't exist in inertial frames of reference, but in everyday usage, people understand it to be the force directed away from the center of an object rotating in a circle. We use it to explain how water is spun out of wet clothes in a washing machine or, in the case of centrifugal oil cleaners, how contaminants are separated from oil that is spun at very high revolutions per minute (rpm).

At first, centrifugal oil cleaners were used primarily on diesel engines, especially those of heavy-duty vehicles such as earthmovers and tractors. But now they are enjoying widespread use in many industrial applications. They can either be permanently fitted to an engine or mounted on a trolley, which can be moved near machinery that needs its lube oil cleaned. In principle, both types of cleaners work the same way, but this article will focus on portable centrifugal oil cleaners. Let's start with the basics: the parts and the process.