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Diesel fuel drives your car farther, but watch quality

Saving motor fuel has become a national pastime. Some motorists do it by driving small cars while others turn to the diesel.

The new diesel-driven cars seem to promise less noise, easier cold starts, and fewer oil changes than older-model diesels. Too, acceleration is greatly improved through turbocharging.

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The big payoff with a diesel, however, comes at the fuel pump.

In the US diesel fuel may cost less than regular-grade gasoline, but not always. It depends on whether or not there is a ''gas war'' under way in an area. Also, the gasoline glut has sharply reduced the price of gasoline.

In Europe diesel fuel is far cheaper than gasoline; while in Mexico, diesel fuel sells at the bargain-basement price of 25 cents a gallon.

Diesel fuel, in fact, operates more efficiently than gasoline and provides about 25 percent more mileage than a gasoline-powered car of the same size and weight.

The popularity of the diesel car is easy to understand.

Yet there is another side to the diesel story. The source of one of its primary problems is the fuel itself.

Both gasoline and diesel fuel are extracted from crude oil and the lighter grades of crude, which are easier to refine, have become more difficult to obtain. These are being replaced by lower-quality heavy crude, such as is obtained from the Alaskan pipeline.

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The poorer grades of heavy crude have a higher sulfur content and need more refining to yield the same product quality as can be obtained by starting with a lighter crude oil.

The result is an increasing amount of high-sulfur diesel being sold.

High-sulfur fuel means an earlier demise of a diesel engine because sulfuric acid is produced during combustion. The sulphuric acid then attacks the bearings , cylinder walls, piston rings, and exhaust valves. Slowly, almost imperceptively at first, the engine deteriorates.

The first symptoms of engine damage are oil consumption and blue exhaust smoke. The higher level of sulfur in the fuel requires more frequent oil changes to minimize the detrimental effect of any acid-forming compounds.

The high use of diesel fuel also raises a flag about the possible harmful effect of diesel-driven autos to the environment.

Diesel emissions could have a harmful effect on the atmosphere, some people assert. Indeed, diesel-exhaust controls may soon be required on cars just as catalytic converters are standard equipment on gasoline-powered cars. Ironically , it was the Environmental Protection Agency which mandated that automobiles provide increasingly higher mileage, thus causing the accelerated move to diesels.

Diesel fuel, an oil-base petroleum product that foams when pumped, can be tracked into an auto when servicing a car at the pumps. An unwary driver is liable to track fuel onto the carpeting where the film will attract dirt and provide a lingering, pungent smell.

Fuel contaminated with water also has more serious consequences to a diesel engine.

Water rapidly causes wear inside the injection system, which operates at close tolerances. A bad tankful of fuel can cause an expensive repair bill. Diesel engines now have a special water filter. However, unless it is checked and drained when required, the water can damage engine parts.

GM uses a detector that signals when water is present in the fuel. Some diesel-powered cars have a water-separation system located in the fuel tank as well as a water-in-fuel indicator light.

Diesels are sometimes hard to start in cold weather, especially when the temperature falls toward zero. Special heating systems, of course, are available to minimize the problem. Gasoline engines don't act up until the temperature is 15 or 20 degrees lower.

In 1982 GM alone expects to build a million diesel units. By 1985 about 20 percent of all new cars will have diesel power.

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