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A diesel car is different -- both the driving and the upkeep

Fuel-saving diesel automobiles are attracting more and more drivers all the time. As a result, carmakers are putting diesel engines into new car lines continually.

Yet living with a diesel engine takes some adjustment for the motorist, experts on the operation of the engine report. For one thing, starting a diesel engines generally takes a little longer because they are ignited by high compression rather than a spark.

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As Donald Reilly, an assistant staff engineer at Chevrolet, points out: ''Diesel engines ignite because the ambient air temperature is raised in a confined space when the fuel is sprayed in the cylinder. It's like having a quart jar, where the volume is reduced to one-twenty-first of the normal volume.''

A glow plug, or electric element, helps to get the car started. A light on the dashboard tells when to engage the starter. Depending on the temperature, it takes a few seconds for the cylinder chamber to warm up enough for a start. Some cars start faster than others.

There are two classes of diesel engines: direct injection and the more popular, indirect injection designs, which are quieter but not as fuel-efficient , Reilly says.

The GM engineer says diesels do not require tuneups because they do not have spark plugs that need to be cleaned and timed. He also emphasizes the importance of keeping the fuel system clean.

''Gasoline engines are much more tolerant of dirt, while diesel engines are more sensitive,'' he says. As a result, filter changes are important, and the air cleaner should be changed regularly.

Keeping the fuel clean and water-free is also important because moisture that condenses from the fuel into the system could cause rust and costly repairs. That's why some cars have a signal light on the dash to indicate the presence of water in the system. They also offer a mechanism to eliminate the water.

For cars that do not have this feature, a water-purging system is available from a number of companies.

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Thomas A. Mooney, general manager of CR Industries, says a system like his company's Scavenger III reduces water content in diesel fuel to six parts per million to keep the fuel injection system free from rust.

''The big difference with a diesel engine is in caring for it properly,'' says a spokesman for Mercedes-Benz of North America Inc. About 75 percent of Mercedes production is diesel. In a Mercedes diesel-engine automobile, oil changes should only be made every 5,000 miles, according to the spokesman.

Other companies recommend much more frequent changes of engine oil.

Charles Lewis, in charge of service training for Volkswagen of America Inc., recommends draining the fuel filter every 7,500 miles. On a VW it can be done by the owner with a 13-mm wrench and a screwdriver, he adds. Mr. Lewis also recommends watching the quality of the fuel put into the tank.

Keep an eye on the cetane number of the fuel you buy. The cetane number is an indication of how well the fuel burns and should be about 45 for a diesel-engine automobile. The lower the number the more resistance to ignition of the fuel under pressure.

Also keep an eye on the power point number of a fuel. Look for fuel with a power point of 10 degrees F. or better, suggests Mr. Lewis. The power point refers to the viscosity of the fuel. A fuel with a power point of 10 degrees F. will flow freely at that temperature, an important point because diesel fuel thickens like molasses in cold weather.

The cloud point refers to the wax crystals which cloud the fuel at lower temperatures and should be about 20 degrees F. or higher. Below that number the car won't run well.

A diesel-engine automobile is no great mystery, but it does take a little getting used to.

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