Bob's Blog

While diesel engines have long been popular in heavy-duty full-size pickups, they’ve recently become more common in cars and SUVs as well. And according to Lars Ullrich, the number of vehicles offering diesel engines is expected to triple in the next couple of years.

How does he know? Because Ullrich is one of the diesel-engine gurus at Bosch Corporation, which is a major supplier of fuel-injection components for both gas and diesel engines. Manufacturers developing diesel vehicles often consult with Bosch during the early design phase, so Ullrich essentially has access to an automotive crystal ball.

Of course, that doesn’t mean he’s willing to share what it says. Manufacturers don’t always want to tip their future-product hand this far ahead of official introduction, so Ullrich wouldn’t be specific about all the cars and trucks that are due to get diesel engines. But a chart he showed at a recent presentation contained a timeline listing the current 15 diesel models along with 29 little blue boxes positioned over the span of the next two years. And it didn’t seem as though those boxes were haphazardly placed; it was pretty clear they represented new vehicles due to offer a diesel engine.

Some we know about. Mercedes-Benz is bringing in an S-Class diesel during the 2012 model year, when Volkswagen will add diesel versions of its redesigned Beetle and Passat. Chevrolet has already announced a diesel engine will be offered in the compact Cruze sedan in calendar-year 2013. Jeep is planning a diesel for the Grand Cherokee, and Mazda has hinted a diesel will be coming–possibly in a redesigned Mazda 6 sedan–in the same timeframe. That’s also roughly when diesel versions of the Audi A6, A7, A8, and Q5 are expected. Granted, many of those blue boxes probably represent different diesel models from manufacturers that already offer a few (Audi, BMW, Mercedes-Benz, and Volkswagen all have multiple diesels for sale now), but some are likely new to the fold. When asked what size range those blue boxes covered, Ullrich replied, “Think of where diesels do the most good in terms of fuel savings.” That implies midsize and larger vehicles. A few years ago, GM and Ford hinted that smaller V8 diesels would soon be offered in upcoming half-ton pickups, but those programs were put on the back burning during the recent financial unpleasantness. Ullrich’s statements, however, rather implied they might be back on the table.

When they think of diesels, many people picture big trucks and buses belching smoke. If that’s happening, the engine is worn out or something’s amiss, because diesel exhaust is inherently quite clean–with one exception.

There are several exhaust emissions scrutinized by the Environmental Protection Agency (EPA), but it’s nitrogen oxides (often written as “NOx” and pronounced “knocks”) that has been the diesel’s Achilles heel. NOx is a by-product of combustion heat, and due to their high compression ratios, diesels make a fair amount of it. But that can now be greatly reduced, thanks in large part to a change in diesel fuel itself. 

Just as unleaded fuel allowed the use of catalytic converters on gas engines back in the mid-’70s (lead clogged the converters), the advent of low-sulfur diesel fuel that’s been phased in over the past few years has allowed the use of similar after-treatment systems on diesels. In some cases, a simple catalytic device will do the trick, while other engines require the use of urea-based Diesel Exhaust Fluid–better known by the trade name “AdBlue” – which is a liquid that’s sprayed into the exhaust and periodically needs to be replenished (see next page).

Most 2.0-liter 4-cylinder diesels offered by Audi and Volkswagen don’t use AdBlue, but all current six-cylinder automotive diesels do. So do the newest crop of heavy-duty V8 diesels in ¾- and one-ton pickups. An odd exception – and why we had to say “most” 4-cylinder diesels don’t need AdBlue–is the new 2012 Volkswagen Passat. It carries the same 2.0-liter 4-cylinder diesel as its slightly smaller Jetta stablemate, but uses an AdBlue system. Yet despite being heavier, the Passat with manual transmission gets 1-mpg better EPA fuel-economy numbers than a manual-transmission Jetta. And that corresponds to something else Ullrich said, which is that AdBlue systems result in a slight fuel-economy increase over using the diesel version of a catalytic converter.

Stiff Competition

It used to be that Volkswagen’s diesel models were the undisputed Kings of Economy. Then a slew of gas-engine hybrids came along that easily topped them, though at a price. But recently, even conventional gas-engine cars have threatened the diesel’s dominance, as some of these “40-mpg” entries boast diesel-like fuel-economy figures.

So why are more diesels coming if conventional gas engines are nearly as thrifty? Two reasons.

First, while some of the 40-mpg cars are about equal in fuel economy to current diesels, their engines produce nowhere near the torque. For instance, the Honda Civic HF carries an EPA rating of 29/41, while the Volkswagen Jetta TDI diesel gets a 30/42–just 1 mpg better. But the Civic’s 1.8-liter engine makes only 128 lb-ft of torque–at a lofty 4300 rpm–while the Jetta’s 2.0-liter turbodiesel churns out a stump-pulling 236 pound-feet at just 1,750 rpm. The ability to make lots of low-end torque is why diesels work particularly well in heavier vehicles.

Another reason is that diesels are likely to get even thriftier in the near future. According to Ullrich, some will be “downsized” in displacement, which would increase economy, with higher turbocharger pressures making up for some of the power loss. They also might get stop-start systems–such as used in most hybrids–that shut the engine off at stoplights. The next step would likely be a reduction in the number of cylinders, which would further reduce fuel consumption.

But the ultimate goal might be the diesel-engine hybrid. Since hybrids tend to do particularly well in city driving and diesels do better on the highway, it seems like a natural marriage. However–like Kim Kardashian’s–it wouldn’t be a cheap one. The problem is that it would be a combination of two expensive technologies, so the fuel savings would have to be significant to pay off over the long run.

Just how much might each of these diesel advancements add in terms of fuel economy? According to a chart published by Bosch reflecting a Volkswagen Golf-sized vehicle with a 134-horsepower engine that gets 44 mpg, downsizing and a start-stop system could bring a 36-percent gain to 60 mpg. Cylinder reduction would add another eight percent for 65 mpg. Adding a hybrid system might result in another 12 percent gain, not a particularly impressive increase for the cost involved. But the bottom line is that the four advances together could result in a small family car that gets a stunning 73 mpg.

Let’s see a conventional hybrid match that.

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