Friday, October 11, 2013

Fuel economy comparo: 2013 Audi A6 2.0T vs. 3.0 TDI

If Scrooge wanted an A6, would he pick the gas or diesel?
A test of the two engines in the Audi A6 gives a clear advantage to the diesel.
With ever-tightening fuel economy standards and the high price of gasoline these days, there has been much talk of the best way to squeeze more miles out of a tank of gas. Besides the obvious — electric cars and the ever-popular hybrid — there's been a rather confusing array of upgrades, twists and improvements to the conventional internal combustion engine, all promising to stretch those hydrocarbons a few extra kilometres.
Having blown through all manner of internal combustion alternatives — cylinder deactivation systems, supercharging, etc. — the North American automotive market has settled on two basic technologies — diesel and turbocharged engines — as the future of fuel economy reduction without resorting to electrification.
Initially, most companies chose one or the other as the best route to non-electrified fuel economy. Ford, like most of the domestics (as well many of the Japanese manufacturers), replaced many of its larger engines with smaller, turbocharged versions, typically a 3.5-litre V6 being replaced by a blown 2.0L four-cylinder. The Europeans, meanwhile, stuck to what they knew best — diesels — almost 50% of the continental market already converted to the oil-burning engine.
More recently, those distinctions have become blurred. BMW has unveiled an entire array of turbocharged engines to supplement its diesel technology while normally diesel-phobic General Motors has added a 2.0L turbodiesel to its Cruze lineup, which, by the way, already features a downsized and turbocharged 1.4L engine.
The company with the longest experience in both alternatives, however, is Audi and, conveniently, it just happens to produce its A6 powered by the most popular forms of both technologies — a turbocharged four makes up company's ubiquitous 2.0T while six, oil-burning pistons in a vee create the increasingly popular 3.0L TDI.
The great thing, for our quasi-scientific purposes, is that the cars, other than the TDI's 100-kilogram weight penalty, are essentially identical, using the same transmission, the same brakes, the same steering mechanism and tires, a perfect crucible to establish which technology is the superior high miler. Of course, both have their adherents, the small turbocharged engine an obvious choice because it only displaces two litres while the diesel engine's reputation for frugality is well established.
So, which A6 would Scrooge drive?
Well, unfortunately for those thinking the small turbocharged engine is the salvation to our fuel economy woes, this wasn't even close. At a steady 100 kilometres an hour, for instance, the A6 TDI did a fair impression of a Toyota Prius averaging just 5.1 litres per 100 kilometres. Yes, a car weighing more than two tons and boasting six pistons managed a 5.1 L/100 km rating. The 2.0T's 6.4 L/100 km, while still impressive, was some 25% more profligate.
Boosting the speed up to 120 km/h just highlighted the diesel's advantage a little more. The TDI still sipped along at 6.0 L/100 km, an incredible feat for a large luxury sedan cruising at a supra-legal speed. The 2.0T sucked back a more typical 7.5 L/100 km, an identical 25% in arrears of the TDI.
We thought that the 2.0 might get back a little of its own during our short city drive, but here again, the turbodiesel proved surprisingly frugal. At the end of our cruise through the scenic little burg of Port Hope, Ont., the TDI's digital fuel economy readout displayed 8.3 L/100 km while the 2.0T was indicating 9.5, the margin in town a slightly smaller 15%. It is worth noting that the 2.0T actually bettered its Natural Resources Canada rated fuel economy (10.5 L/100 km in the city and 6.8 on the highway), the first time a gasoline-fueled car has accomplished that in our testing. The diesel, by contrast, was almost spot on its rated consumption: 8.5 L/100 km in the city and 5.3 on the highway.
On top of its obvious fuel advantage, the 3.0 TDI was also more enjoyable to drive than the 2.0T. There is much more low-end torque (though the little four-banger is no slouch in this arena), passing power is more convincing and it feels just as smooth at speed. At idle, the diesel is a little noisier but that's more apparent to passersby than occupants.
The biggest detriment to the diesel, at least in this case, is the premium charged for the compression-ignition engine. In this case, Audi asks for a whopping $9,800 more for the base TDI than the 2.0T. To be sure, some of that margin includes some non-powertrain upgrades (as well, I'm guessing, for some healthy profits for Audi). As well, that price differential also accounts that the A6 TDI is a V6 being compared with a lowly four-cylinder gas engine (Mercedes-Benz E and GLK Class diesels are cheaper than their V6 gasoline counterparts because they're four-bangers). Nonetheless, it doesn't change the fact than one couldn't expect to recoup the difference between these two models in less than 25 years. It should be noted, however, the A6 TDI is but $2,500 more expensive than Audi's 3.0 gasoline-fuelled V6, a differential that should easily be recouped in less than three years considering its even more significant fuel economy advantage.
What the cost issue doesn't change is that in our quest for ever-increasing fuel economy, Rudolf Diesel's old oil-burner is still putting it to even the most frugal of gasoline engines.
We Canadians have a lot to learn from our continental cousins in this regard.

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