VW’s 2.0L Turbodiesel Remarkably Likeable and Fuel Efficient

The Ward’s 10 Best Engines competition has recognized outstanding powertrain development for 17 years. In this installment of the 2011 Behind the 10 Best Engines series, Ward’s looks at VW’s 2.0L turbodiesel.

TDI boasts two combustion-enhancing technologies that reduce emissions.

It’s no secret Ward’s editors are fans of Volkswagen’s quick, quiet, fun-to-drive and super-fuel-efficient 2.0L turbodiesel I-4.

Testing the engine in the all-new ’11 Jetta TDI sedan, coupled to VW’s excellent 6-speed dual-clutch automatic transmission, they achieved average fuel economy of 35.3 mpg (6.7 L/100 km).

That was better than some of the hybrid-electric vehicles tested, even though editors flogged the engine hard on their daily commutes rather than loping along freeways with the cruise control engaged.

So it’s no surprise this small but delightfully torquey turbocharged direct injection oil-burner has earned its third straight Ward’s 10 Best Engines award “for being so efficient, quiet, and powerful and an utter joy to drive,” as one editor put it.

On their score sheets, editors called the engine “the car’s best attribute” and “the best diesel out there” for the money.

VW’s 2.0L TDI generates 140 willing horses and a burly 236 lb.-ft. (320 Nm) of torque, along with impressive 30/42 mpg (7.8-5.6 L/100 km) city/highway with the 6-speed automatic.

The I-4 features a twin-cam, 16-valve aluminum head and an iron block fed by a high-pressure common-rail direct-injection system and a variable-geometry turbocharger.

The engine meets U.S. 50-state emissions standards with oxidation catalysts and regenerating particulate filters.

Unlike other U.S. passenger-car diesels, it does not use a selective catalytic reduction urea-injection system to comply with tough Tier 2 Bin 5 emissions.

How does it manage that? Partly because it’s a smaller engine powering a car that is lighter than most other U.S. vehicles equipped with diesels, but mostly due to leading-edge technology.

Emissions cleansing starts with an oxides-of-nitrogen storage catalyst, which is purged periodically by letting the engine run momentarily ultra-rich to create additional heat.

But the 2.0L TDI also boasts two combustion-enhancing technologies that help reduce emissions while at the same time improving its smoothness, drivability and fuel economy.

The first is low-pressure exhaust gas recirculation, which combines with a conventional high-pressure system to provide EGR under virtually all conditions.

The high-pressure system routes exhaust gas from the manifold back into the intake charge, while the low-pressure system takes much cooler gas from downstream of the particulate filter and dumps it into the turbocharger intake air.

The second technology is cylinder-pressure balance control. This system ensures a very controlled combustion process for similar combustion in all four cylinders, which improves noise, vibration and harshness characteristics and emissions.

The secret is a sensor in each glow plug that continuously measures cylinder pressures and communicates them to the engine control unit, which adjusts air intake, fuel injection, EGR and other variables to keep them as equal as possible.

“With these signals, I can work on the injection side, on the air-intake side, on the turbocharger side and on the EGR side to do the same in every cylinder,” says Jens Hadler, powertrain engineering director for Volkswagen.

The system ensures each cylinder gets exactly the same mass of fuel, for example, and it compensates for differing diesel fuel qualities in different areas of the country.

Piezo injectors also contribute 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, Hadler says.

“We also have multiple injections, instead of a single injection, over a period of milliseconds, which reduces noise during the combustion process,” Hadler says.

He says excellent NVH is necessary for North American customers to appreciate the engine. “If you want to sell a diesel in the U.S. today, you must achieve even better NVH than in Europe. That has been a problem with diesel engines.”

Another major diesel challenge is to ensure it will function equally well in all regions of the world – in cold and hot climates, low and high humidity and high altitudes – and accommodate different fuel qualities.

Beyond specific summer and winter fuels regionally distributed by oil companies, this is mostly a calibration issue, and VW’s pre-glowing system provides very high temperatures inside the combustion chambers to ensure immediate light-off in very cold weather.

According to VW Group spokesman Christian Buhlmann, VW’s TDI engines date back to a 2.5L 5-cyl. engine introduced in 1989, followed by a transverse-mounted 1.9L I-4 first available in the ’93 Golf.

A second-generation 4-cyl. TDI with unit injectors debuted in 1998, and the larger 16-valve 2.0L arrived in 2003. The current third-generation TDI with common-rail direct injection was introduced in Europe in 2007 and the California Air Resources Board Bin 5-compliant version came to North America in 2008 in the ’09 Jetta.

VW Group is the world market leader in diesel passenger cars, Buhlmann says. “Today, we offer 3-, 4-, 6- and 8-cyl. TDI engines in Volkswagen cars, plus a V-12 TDI in the Audi Q7 and R8,” he says.

“Our latest prototype XL1 Hybrid is equipped with a 2-cyl. TDI of only 0.8L displacement. No other car maker offers such a large variety of diesel engines, paired with manual, DSG or torque-converter automatic transmissions. They are fun to drive, offer extraordinary fuel economy and outperform many hybrids in fuel efficiency,” Buhlmann says.

Available today in the U.S. market are the Golf TDI, Jetta TDI, Jetta Sportwagen TDI and all-new Passat TDI, which debuted at the 2011 North American International Auto Show and will arrive at VW dealerships this summer.

All will be equipped with the 140-hp 2.0L TDI. In addition, VW offers milder and higher-performance versions, ranging from 110 hp to 180 hp, in a variety of models in other global markets.

For future improvement, as both customer and government demands for ever-higher fuel economy accelerate, “the main development targets are to lower fuel consumption by lowering internal friction and reducing emissions such as particulates and nitrogen oxides,” Buhlmann says.

“Today, the maximum torque of 236 lb.-ft. (320 Nm) is reached at 1,750 rpm and kept until 2,500 rpm. By using longer gear ratios in the transmissions (preferably DSG), the engine can stay within the lower rev range at higher vehicle speeds, as well as in uphill driving, and achieve 40-plus mpg (5.9 L/100 km) highway fuel economy. Our target is to improve these numbers even further.”

Current hybrid-electric vehicles offer outstanding city fuel economy but far less efficiency advantage at highway speeds. For those whose travels include substantial highway miles, a somewhat less-expensive 40-plus-mpg diesel can offer real savings over many HEVs on the market, even when diesel fuel costs more than regular gasoline.