In the April 2008 issue we published a story called "Fuel for Thought" that addressed the European diesel revolution coming to the United States. We gave you a peek at and offered speculation on some of the platforms likely to make the Transatlantic voyage for use on public roads here in America. Though still in its early stages, the revolution is now in full swing, spearheaded by four major German manufacturers. Here's the current state of affairs as we see it.

R8 Le Mans
Don't get too excited-Audi has no concrete plans to build a production version of the R8 Le Mans, equipped with a V12 TDI producing a maximum 1,000 Nm of torque (about 740 lb-ft). Initially, designers had problems fitting a gearbox that could stand up to such a massive amount of twist, and the entire powertrain package proved a problematic fit in the existing R8 rear end. Reportedly, the concept vehicle is a completely new animal from the doors back, and building a road-going car to this spec could drive production costs into impossibility. But that doesn't mean Audi can't or won't build a high-performance diesel version of the car. A diesel-powered super sports car, especially one drawn so directly from Audi's legendary motorsports heritage, could go a long way in selling diesel technology to the unconverted masses.

In contrast to Volkswagen of America, Audi USA may at first seem somewhat indifferent to the diesel revolution even though Audi AG is regarded by many as the world's leading diesel innovator. The company has promoted diesel technology with international motorsports programs and cross-country road events promoting the efficiency and reliability of the technology, but currently only has plans to sell the Q7 SUV with a 3.0-liter TDI engine. Sure, a Q7 with a TDI is what's needed to keep SUV sales moving, but you'd think that with the current demand for smaller, high-mileage vehicles something like a 46-mpg A3 would drive sales sky high. In Europe you're able to buy the A5 with a 3.0-liter TDI. Compare that to a U.S.-spec 3.2-liter gas engine (both are equipped with a six-speed manual transmission):

*A5 3.0 TDI Quattro
(European-spec): 240 hp @ 4000 rpm, 368 lb-ft @ 1500 rpm0-62 mph in 5.9 sec., 35 mpg (combined cycle)

*A5 3.2 Quattro
(U.S.-spec, gasoline): 265 hp @ 6500 rpm, 243 lb-ft @ 3000 rpm0-60 mph in 5.8 sec., 22 mpg (combined cycle)Based solely on the numbers, the choice seems obvious. The A5 3.0 TDI is as fast to 60 mph but gives a combined mpg increase of approximately 60 percent. Clearly, Audi has the tools and know-how to create some amazing diesel products, and it could be that we'll see further products come to the States in the future. But for now, Audi brass just isn't saying.

Current Model:
Q7 3.0 TDI
Engine: 3.0-liter V6, turbocharged, direct injection
Output: 221 hp @ 4000 rpm, 405 lb-ft @ 2000 rpm
Efficiency: 19/25 mpg

Until recently, BMW never surprised anyone with its U.S.-bound vehicle/engine combos. Always predictable, it never really strayed, always building the same naturally aspirated engines, each generation more refined and with a bit more power. That was until, out of left field, the company announced it would be getting back into the turbo business, shortly after releasing the 3.0-liter N54 twin-turbo gasoline engine. BMW kept the surprises coming, shocking me with an even-more-ultimate driving machine equipped with a 50-state-legal, 3.0-liter twin-turbo engine with 265 hp and 425 lb-ft of torque that could propel a 3 Series sedan from zero to 60 mph in just 6 seconds. If you take a quick look at BMW's European heritage you can see that the company has actually been developing and refining diesel technology for more than 25 years; today, more than 67 percent of all BMW sold elsewhere in the world are diesels.

Current Models:
Engine: 3.0-liter I6, variable twin turbos, direct injection
Output: 265 hp @ 4200 rpm, 425 lb-ft @ 1750 rpm
0-60 mph: 6.0 sec.
Efficiency: 23/33 mpg
X6 xDrive35d
Engine: 3.0-liter I6, variable twin turbos, direct injection
Output: 265 hp @ 4200 rpm, 425 lb-ft @ 1750 rpm
0-60 mph: 6.8 sec.
Efficiency: 19/25 mpg

When you think of diesel-powered passenger cars, Mercedes is likely the first to come to mind. You could say that Mercedes first introduced the American public to diesel, and some old Mercedes diesels have even reached a sort of iconic status. In the early 1980s more than half of company sales were diesels, and Mercedes would likely still be the reigning king had not U.S. emission standards constricted as tightly as they did. But today, with fuel prices as they are, it only makes sense that the company would have strategically placed its Bluetec V6 diesel engines in some of its heavier platforms, vehicles that could benefit from an increase in miles per gallon. But being an equal opportunity-diesel maker, all Mercedes that could benefit from a diesel engine could be getting one without a sacrifice in performance. Its modern performance and fuel economy stats clearly show that the new Bluetec is not your daddy's diesel engine.

Current Models:
Engine: 3.0-liter V6, turbocharged, common-rail injection
Output: 210 hp @ 4000 rpm, 398 lb-ft @ 1600 rpm
Transmission:Seven-speed automatic0-60 mph: 8.0 sec. (ML320); 8.6 sec.
(R320)Efficiency: 18/24 mpg

Engine: 3.0-liter V6, turbocharged, common-rail injection
Output: 210 hp @ 3800 rpm, 398 lb-ft @ 1600 rpm
Transmission:Seven-speed automatic0-60 mph: 9.1 sec.
Efficiency: 17/23 mpg

Engine: 3.0-liter V6, turbocharged, common-rail injection
Power: 210 hp @ 3800 rpm, 400 lb-ft @ 1600 rpm
Transmission: Seven-speed automatic0-60 mph: 6.6 sec.
Efficiency: 23/32 mpg

Volkswagen has spent the latter part of a year prepping the American consumer for the coming diesel invasion. This campaign has included various industry and consumer shows and drive programs designed to dispel common myths and prove that diesels no longer make strange noises or spew clouds of smelly exhaust. In addition to this educational blitz, the company created a Jetta TDI Cup racing series to highlight the viability of modern diesel-powered race cars. The first batch of TDIs sold out before they hit the showroom floor, so it looks like the public got the message. To round out the product line, Volkswagen will be releasing the Touareg with a V6 TDI in early 2009, and by the end of 2009 a Rabbit TDI.

Current Models:
Jetta TDI (Sedan, Sportwagen)
Engine: 2.0-liter I4, turbocharged, direct injection
Output: 140 hp @ 4000 rpm,236 lb-ft @ 1750 rpm
Transmission: Six-speed DSG with Tiptronic0-60 mph: 8.2 sec. (Sedan); 8.4 sec. (Sportwagen)Top Speed: 128 mph
Efficiency: 30/41 mpg

Future Models:
Touareg V6 TDI, Rabbit TDI

The future-and what we'd like to see come down the road next
A major hurdle when trying to maximize engine efficiency is planning how to reduce the parasitic drag created by a belt-driven engine's accessories like the alternator, air conditioning and power steering. Audi engineers set out to tackle the problem on the A4 TDI Concept e, developing a secondary voltage source independent from the engine to power the electronic rear brakes, electromechanical steering, and a host of other electronic goodies. They accomplished this by using an energy-recuperation system that converts kinetic energy during deceleration into usable electrical energy which is stored for use during acceleration, effectively reducing drag, increasing performance, reducing emissions and increasing fuel economy. There's also a start/stop function that shuts the engine off at a standstill. Put it in neutral and the system disconnects the air conditioning's compressor from its drive whenever possible. All this results in a vehicle that produces 120 hp and 213 lb-ft of torque, zero-to-62 times of 11.4 seconds, and 58.95 mpg.

With the early success of the 1 Series, we'd hope that BMW would add the 123d to the list of vehicles it'll be bringing to the U.S. market. Powered by 2.0-liter, four-cylinder engine, the 123d produces 204 hp at just 4000 rpm and 295 lb-ft of torque at 2000 rpm. It's enough to propel the vehicle from zero to 62 mph in seven seconds and maintain a 45 mpg efficiency rating. Behind the 123d's impressive numbers is the world's first four-cylinder diesel engine to use Variable Twin Turbo technology, where sequential turbos are employed to optimize power and torque across the rpm range. Each turbo has been sized to do a specific job. The smaller, low-inertia unit spools instantly to effectively create immediate throttle response. As demand increases, the larger unit takes over to ensure there's continuous boost as engine revolutions increase.

The line dividing diesel and petrol performance has become so blurred that it's impossible to simply look at a vehicles' performance numbers and determine if it runs on petrol or diesel. A perfect example is Mercedes' C-Class. Based on the vehicle's zero-to-62 time of seven seconds, you might assume it had a V6. Then you notice the vehicle is able to net a combined efficiency rating of 45 mpg. It has the power of a six-cylinder with the fuel economy of a four-cylinder. That's better than a Prius... plus it's prettier and handles better, too. The C250 CDI is also somewhat unique when it comes to its high-performance diesel powerplant. Instead of a turbocharger, its 2.2-liter inline four utilizes a dual-stage supercharger to produce 204 hp and 368 lb-ft of torque.

While a highly efficient engine design is important, the Volvo DRIVe package for the C30, S40 and V50 demonstrates that if you start with the engine and tailor the vehicle itself and drivetrain around it, you end up with increased fuel economy and reduced emissions. To reduce drag, the DRIVe package consists of a series of aerodynamic enhancements, specially designed wheels, reduced vehicle ride height, and a front end designed to optimize airflow into the engine bay. It is further enhanced with a next-generation Michelin tire with decreased rolling resistance, tweaked engine management, and optimized cooling and power steering systems. To round out the package, the transmission has been re-geared and filled with a new low-friction gear oil. All this adds up to a 4 to 7 mpg increase on a C30/S40/V50 equipped with a 1.6-liter diesel engine. In addition to the mileage increase, zero-to-62 times were reduced from 11.4 to 10.8 seconds and emissions reduced by 8.5 percent. If you're not into economy-sized engines, Volvo also offers a 2.4-liter, five-cylinder turbo diesel engine that can propel a S40 to 62 mph in just 7.5 seconds.

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