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Dynamic Diesels

Driven by economic and environmental concerns, diesel-engine cars are gaining traction in Europe. Will the mania for compression ignition make a comeback in America?
Paul A. Eisenstein
From the Print Edition:
Tom Selleck, Nov/Dec 2007

(continued from page 1)

Like the gasoline power trains in most U.S. cars, diesels are part of the bigger family known as internal combustion engines. A mixture of fuel and air, pumped into a cylinder and then ignited, creates the explosive energy that drives a piston and, ultimately, turns the wheels. With gas, that combustive mixture is touched off with a spark. In a diesel, ignition occurs when the mix is compressed to the point at which it fires spontaneously. The science is all about controlling the eruption within a compression-ignition engine.

There's much debate over who came up with the concept first. In 1890, two British engineers, Herbert Akroyd Stuart and Charles Richard Binney, obtained a patent for "Improvements in Engines Operated by the Explosion of Mixtures of Combustible Vapour or Gas and Air." A year later, they had a working prototype running in Bletchley, England. But historians generally credit—as the name would suggest—German Rudolf Diesel, who got his own patent in 1892, then took another five years to get his invention to work in the laboratory. The technology found quick application in the industrial world. By 1898, the brewer Anheuser-Busch had installed a design by Rudolf Diesel in St. Louis. Soon, the engines were powering everything from generators to U-boats. Surprisingly, it took until the 1920s to fit a diesel into a truck, and Citro&oml;n only launched the Rosalie, the world's first diesel-powered passenger car, in 1933.

Technological Revolution
For decades, astoundingly few real changes were made to the technology. Until relatively recently, the engines relied on mechanical pumps and injectors that led to wide variations in the amount of fuel supplied to each cylinder. Fuel and air didn't always mix well, leaving loads of unburnt hydrocarbons, along with foul-smelling sulfur compounds that spewed out of the exhaust.

Over the last decade or so, change has come at a rapid pace, driven largely by demand in Europe, where motorists are now paying as much as $6 a gallon for gasoline. With the advent of lower-sulfur fuels, manufacturers were able to tinker with more sophisticated ways to stabilize the combustion process, improving mileage and lowering emissions, while also adding the means to enhance performance.

Start with Common Rail technology, which can compress fuel to as much as 29,000 pounds a square inch, then deliver it into a cylinder through the latest in computer-controlled injectors. The newest of these rely on piezoelectric crystals, rather than magnetic solenoids, to further improve accuracy. At the front of the engine, there's the turbocharger, which boosts the pressure, or "charge" of the air flowing into the engine. The latest innovations not only yield dramatic improvements in horsepower and torque—the twisting power that we associate with a car's fast takeoff—but eliminate "turbo lag," that annoying hesitation when you stomp on the accelerator. At the back end, particulate filters and other, electronically regulated systems scrub out most of the soot and other noxious exhaust products.

And since the amount of global-warming carbon dioxide an engine produces is inextricably linked to its fuel consumption, diesels have an inherent advantage over less efficient gasoline power trains. That's why the diesel is the darling of the European environmental movement.

So why do so many U.S. "greenies" see diesel technology as public enemy number one? There are two culprits: inherently high levels of NOx, otherwise known as nitrogen oxides, and particulates, those micro-fine bits of soot that studies have linked to serious respiratory ailments. Over the years, federal regulators have steadily ratcheted up their Clean Air rules, something that has, until recently, been difficult for diesels to meet. Then there are the even more stringent California standards, which, until now, diesels simply couldn't overcome. So most current models, such as the Mercedes E320 CDI, cannot be sold in California and a number of other states, such as New York, that have adopted its stricter guidelines. That accounts for more than a quarter of the potential American market, leading many carmakers to simply abandon diesel technology.

But that's about to change, with the widespread availability of ultra-low-sulfur diesel fuel, and the new emissions systems that this permits. Earlier this year, the Environmental Protection Agency approved the automotive use of urea. A simple, nitrogen-based liquid, similar to ammonia, urea is the fix the diesel has been waiting for. Injected, in microscopic amounts, into the exhaust stream, it can all but eliminate smog-causing NOx. And other sophisticated control systems are on line to cut particulates down to size as well.

Urea-based Selective Catalytic Reduction, or SCR, systems will be used on larger diesel engines, typically about 2.0 liters and up, by manufacturers who include Volkswagen and Mercedes, which has dubbed the technology Bluetec. Honda, VW and several other makers have an alternative for small diesels. In a sophisticated sleight of hand, they convert NOx to ammonia, which, in turn, breaks down more nitrogen oxide, much like urea.

Diesel Comeback
Current diesel technology already carries a premium over gasoline power, typically in the range of several thousand dollars. Tomorrow's environmentally friendly diesels could double that penalty again. But considering the current cost of fuel, the money saved by an increase in mileage will quickly make up for the higher price tag at the dealership. It's notable that diesels deliver that mileage in real-world applications, unlike hybrids, which work best only in the worst stop-and-go environments, such as Los Angeles and New York and its inner suburbs.


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