VW TDI is Overachiever

It's No Surprise Americans are Not Especially Fond of diesel cars.

Past U.S. diesels have been clattery, smoky and slow compared with gasoline counterparts, and U.S. diesel fuel has been sticky, stinky, harder to find and sometimes a lot more expensive than gasoline.

Yet today's quiet, quick and smokeless modern diesels are more popular than gasoline burners throughout most of Europe because they get 25% to 30% better fuel economy, and diesel fuel, thanks to diesel-friendly tax policies, costs about the same as gasoline.

The engines also are winning over Ward's editors, who named two diesels to the 10 Best Engines list this year: BMW AG's 3.0L DOHC I-6 turbodiesel and Volkswagen AG's 2.0L SOHC I-4 turbodiesel.

Oil burners got a boost in the U.S. beginning in late 2007, when the Environmental Protection Agency mandated the phased-in introduction of ultra-low sulfur diesel fuel. Sulfur, a natural part of crude oil, fouls catalytic converters and is a key source of particulates in diesel exhaust. Purging all but trace levels of the mineral enables modern diesels to have much cleaner tailpipe emissions. By 2010, all U.S. diesel fuel must be ultra-low sulfur, which has the added bonus of being less unpleasant to touch and smell.

Thanks to the cleaner fuel, European auto makers now are importing a variety of modern diesel cars and cross/utility vehicles sweet enough to whet Americans' appetites and change perceptions, including VW's over-achieving 2.0L I-4 turbodiesel offered in the compact Jetta TDI sedan and wagon.

This small but broad-shouldered oil-burner generates 140 hp and a brawny 236 lb.-ft. (320 Nm) of torque along with impressive fuel efficiency: 29/40 (8-5.9 L/100 km) city/highway with a 6-speed dual-clutch DSG transmission, and 1 mpg better with a manual transmission.

By comparison, the standard 2.5L gasoline-burning 4-cyl. offers 30 more horses but 25% less torque and 30% less mileage in the 5-speed-manual Jetta sedan. VW's superlative 200-hp 2.0L turbocharged gasoline 4-cyl. also can't come close to matching the diesel in efficiency, with the manual offering a mere 21/29 mpg (11-8 L/100 km).

VW also is quick to point out that independent testing company Automotive Marketing Consultants Inc. has reported 24% better mileage for the Jetta TDI in real-world driving equaling 38/44 mpg (6-5.3 L/100 km).

Mercedes-Benz diesels have made the Ward's 10 Best Engines rankings a number of times in recent years, but a truly affordable diesel has not held the honor since a VW Passat TDI made the list in 1997.

What differentiates this all-new engine from its predecessor, the 1.9L diesel I-4 that VW last offered here in 2006? Four valves per cylinder vs. two, a new combustion-chamber design, a high-pressure 29,000-psi (2,000-bar) common-rail fuel system, a variable-vane turbocharger and — very important for U.S. emissions — two separate exhaust gas recirculation systems working in combination.

“For high-speed, high-load, we use low-pressure EGR in addition to a high-pressure system to get more exhaust gas back into the combustion process to comply with U.S. emissions standards,” says VW of America Engineering and Environmental Director Norbert Krause. “Volkswagen is first to do that, and it is key to having very good oxides of nitrogen emissions,” he says.

Compliance with long-range European and U.S. emissions standards and 50-state U.S. emissions compliance was the highest priority when this engine's design team set to work in 2004. “For the European market,” Krause says, “we had to comply with Euro 4 standards with a view toward 2009 Euro 5 and 2014 Euro 6.”

And in North America, where this delightful engine debuted last fall in '09 Jettas and soon will be offered in the VW Rabbit and Audi A3 platform mates, it had to meet federal Tier 2, Bin 5 standards and at least California LEV 2 rules for it to be sold in all 50 states. “We comply with the ULEV (ultra low emissions vehicle) standard,” Krause says. The federal Tier 2 program consists of different bins, and this engine meets Bin 5 standards, which is similar. Believe me, it's a real challenge.”

Because larger engines burn more fuel and, therefore, generate more exhaust, and U.S. emissions are measured by volume, it is somewhat less challenging for a 2.0L 4-cyl. to comply with the standards than a larger-displacement V-6 such as those offered by BMW and Mercedes. Is that why VW alone so far has been able to meet ultra-tough U.S. standards without using a urea injection exhaust aftertreatment system?

“Exactly,” Krause responds. “It's a question of both engine size and vehicle size. If we had a 2.0L engine in an Audi A6, for example, we definitely would need the urea with a Selective Catalytic Reduction converter system. The SCR can handle all ranges of engine displacement. But it adds cost and complexity, and the customer has to become part of the emissions-control system because he has to refill the urea (tank).”

Krause adds that an advanced piezo-injector fuel-delivery system contributes to reducing both emissions and noise. “The high-pressure direct injection, the cylinder head pressure control and the piezo injectors together reduce the noise significantly. We also have multiple injections, instead of a single injection, over a period of milliseconds, which reduces noise during the combustion process.”

Even without an SCR converter, there is a lot of emissions hardware in the diesel Jetta's exhaust system: an oxidation catalytic converter that reduces hydrocarbons and carbon monoxide, a particulate trap to reduce particulate matter, plus a NOx storage catalytic converter (or “lean NOx trap”) to reduce emissions. Is it difficult for value-oriented VW to absorb all that cost?

“We have to have a competitive price,” Krause says. “It cannot be much above the gas engine. The prices we have now are competitive, and we are looking into the cost very deeply.”

The Jetta TDI sedan starts at $21,990, and the TDI SportWagen at $23,590, about $2,000 higher than similar gas-engine models, but the car also qualifies for a $1,300 income-tax credit.

Beyond the obvious priorities of ultra-low emissions, high fuel economy and low noise, Krause says another major diesel-engine challenge is to ensure it will function equally well in all areas of the world — in cold and hot climates, low and high humidity and high altitudes and accommodate different fuel qualities around the world.

“It's mainly a calibration issue,” he says. “For warm and cold climates, there is summer and winter fuel. The oil companies take care of that. And in very cold weather, we get very high temperatures inside the combustion chambers with our pre-glowing system, so the winter fuel will ignite immediately.”

Excellent as this engine is, there always is room for improvement. “The major issue for the future is still-lower California emissions standards, the LEV 3 program which starts in the '13 model year,” Krause says. “We are working at getting even better fuel economy because of the upcoming corporate average fuel economy and greenhouse gas standards.”

As both efficiency and emissions rules get tougher, what will have to be traded off to meet them? “It cannot be performance,” Krause says, “because people want the performance. And it cannot be fuel economy. So it is a technology/cost issue.”

Despite extremely weak overall new-vehicle sales, VW's Jetta TDI has been doing fairly well. Of the 5,199 Jetta sedans sold in the U.S. in February, 1,308 (25%) were diesels, as were 440 (56%) of the 784 Jetta SportWagen sales.

If increasing numbers of Americans begin to embrace diesel-powered cars, this surprisingly fun-to-drive, civilized and affordable VW diesel may play an important role.

“We expect diesel-powered cars to gain market share this model year,” says Kelley Blue Book Executive Editorial Director and Market Analyst Jack Nerad, “largely because of appealing new offerings like the Volkswagen Jetta TDI. But consumer acceptance of diesels in the United States is still a strong uphill battle.”

Volkswagen AG: 2.0L SOHC I-4 Turbodiesel
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The Ward's 10 Best Engines competition celebrates 15 years of recognizing outstanding powertrain development. In this latest installment of our 2009 series, Ward's highlights the engineering behind VW's affordable and highly advanced new 2.0L I-4 turbodiesel.

ENGINE SPECS

VOLKSWAGEN AG
2.0L SOHC I-4 TURBODIESEL

Displacement (cc): 1,968

Block/head material: cast iron/aluminum

Bore × stroke (mm): 81 × 95.5

Horsepower (SAE net): 140 @ 4,000 rpm

Torque: 236 lb.-ft. (320 Nm) @ 1,700 - 2,500 rpm

Specific output: 70 hp/L

Compression ratio: 16.5:1

Assembly site: Salzgitter, Germany

Application tested: Jetta TDI sedan

EPA city/highway (mpg): 29/40