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Don't be fuel-hardy

Diesels are cheaper on the highway, hybrids save cash in the city

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To the surprise of no one, fuel economy has become the rallying cry of the automobile industry. Environmentalism, radicalized by a few and largely ignored by many, remains a hard sell. But its byproduct, the wholesale reduction of automotive fuel consumption, has surged to the forefront of purchase criteria for a majority of consumers. And, whether it be the U.S. government's adoption of stringent 54.5 miles per (U.S.) gallon corporate fuel economy standards or Ford and Hyundai/Kia's recent fallout with the U.S. Environmental Protection Agency's fuel-economy testing standards, fuel economy is headline news.

The problem with any comparison of automobile fuel economy is that there are too many variables. Sure, you can amass a coterie of hybrids, diesels and conventional gasoline-powered cars in one place, measure their fuel economy under specific conditions and choose a winner -- a specific car that, in this one circumstance, sipped the least amount of fuel. But of the larger question -- which type of powertrain (hybrid, diesel or gasoline-powered) best cuts fuel consumption, having cars of different sizes and weights and made by different manufacturers introduces too many wild cards to be authoritative. Assumptions can be made, but definitive answers are fleeting.

VW to the rescue

Enter Volkswagen. In a move surely planned to give us driving wonks the experimental control required for accurate analysis, the company recently added a hybrid to its Jetta lineup, meaning it now offers hybrid, diesel and gas-powered versions of what are otherwise standard Jettas. Better yet, all appear to offer roughly the same power/outright performance and, while that may make VW's marketing job a little more difficult, it's a godsend to curious automotive engineers-cum-Motor-Mouths. To the laboratory, boys!

Our three testers

The three VWs in question are the base 2013 Jetta GL powered by a conventional, naturally aspirated (as in no turbocharger) 2.5-litre in-line five-cylinder, the Jetta Turbo Hybrid, powered by a 1.4L turbocharged four-cylinder mated to a 27-hp electric motor, and the much ballyhooed Jetta TDI, with the company's evergreen 2.0L turbodiesel four-cylinder under its hood.

The base model's in-line five boasts 170 horsepower, the Hybrid's gas and electric combination works out to an identical 170 hp and the TDI boasts 140 ponies (but with 236 pound-feet of torque compared with the 2.5L's 177 lb-ft and the Hybrid's 184 lb-ft). The main differences in performance is that the 2.5L's maximum torque occurs at 4,000 rpm, while both Turbo Hybrid and TDI produce their peak torque below 2,000 rpm, so these two can easily maintain speed up hills without downshifting, while the GL needs to kick down a gear or two. As well, the Hybrid's little 1.4L engine is the smoothest of the bunch, while the 2.5L is a little quieter at low speed than the TDI's rattling idle (although the order is reversed at highway speeds). Otherwise, the cars, as expected from their similarity in construction and specification, feel remarkably homogeneous.

Fuel ratings

What is very different is their official government fuel economy rating. In this category, the base 2.5L trails markedly. Its official Natural Resources Canada rating is 9.1 litres per 100 kilometres in the city and a more miserly 6.2 L/100 km on the highway. The TDI's corresponding numbers ring in at 6.7 city and 4.6 highway, VW's 2.0L turbodiesel showing decent around-town economy along with the open-road parsimony for which it is famous. Still, the Hybrid trumps all, its 4.5/4.2 L/100 km ratings showing the efficiency expected in city driving for a gas/electric combo, but also better highway economy than the TDI, which is a little surprising. A hybrid's electric motor's added boost is usually most effective at low speeds and generally less efficient at highway speeds. Could Volkswagen have finally unlocked the secret to diesel-beating highway fuel economy?

Unfortunately for the government ratings and Volkswagen's engineers (who continue to profess that the hybrid is more thrifty than their lauded oil burner), the answer is, quite simply, no.

Test controls

In all our tests at steady-state highway cruising, the diesel trumped the Hybrid (and, naturally, the 2.5L gas engine). Please note that all three cars were driven at the same time, tail-to-front, their cruise controls set to identical speeds. Even at the lowest speeds, when we thought the Hybrid's turbocharged engine might be at its most frugal, the TDI always triumphed, though by a smaller margin.

Test results

For the record, with a speedometer reading indicating 100 kilometres an hour the TDI sipped 4.2 L/100 km, while the Hybrid consumed a still-economical 4.8 L/100 km. The 2.5L GL, unsurprisingly, trailed at 5.9 L/100 km. Jacking things up to 115 km/h saw the gap widen, if only slightly. The three Jettas averaged 4.7, 5.4 and 6.4 L/100 km respectively. At our final speed of 125 km/h, the TDI's diesel extended its lead with a 5.4 L/100 km/h reading, while the Hybrid and GL recorded 6.1 and 7.0 L/100 km.

And the faster we went, the greater the diesel's advantage became (though the Hybrid hung in there better than other gas/electrics we've tested). While we never managed to average more than 120 km/h, our return ride was directly into a significant headwind that emulated greater speed sufficiently that fuel economy dipped significantly for all three cars. And, while we can't determine the exact speed these conditions replicated, the fact that the diesel sipped but 6.7 L/100 km while the other two required 7.6 and 9.0 respectively cements the fact that the turbodiesel was the king of the highway.

In town, the results, as expected, reversed. The 2.5L was left even farther behind during the stoplight circuit, averaging 10.8 L/100 km during our Port Hope, Ont., test run. The TDI, meanwhile, posted a still-credible 7.9 L/100 km, while the Hybrid's electrical assistance proved its worth by sipping just 5.9 L/100 km. Indeed, the relative hierarchy -- as well as these absolute numbers -- was the least surprising of the entire test.

What does it mean?

So, what does all this number crunching mean to the everyday consumer? Well, in our not-official-at-all "overall" economy circuit (said Port Hope run and an equal measure of highway cruising immediately thereafter), the TDI and Hybrid posted almost identical numbers (7.3 and 7.2 L/100 km), with the 2.5L sucked back 9.7 L/100 km over the same distance. What those numbers would seem to indicate is that, if you spend equal amounts of time driving in the city and on the highway (indeed, up to a 60/40 split either way), it doesn't really matter which car you buy -- Jetta TDI or Jetta Hybrid -- your fuel savings will be the same. Only as you gravitate toward either extreme will the Hybrid (in the city) or the TDI (on the open road) differentiate itself. Of course, both beat the 2.5L in all circumstances.

Buyer beware

But, just because they consume less fuel doesn't mean either will save you money. There is the premium to be paid for the alternative power plants' greater efficiency. Comparing base prices and factoring a rough cost of $1.30 for both gasoline and diesel fuel (while historically there have been differences, currently the pricing is roughly the same), our calculator says the diesel will start paying back its premium at about 71,000 kilometres and the Hybrid after 147,000 klicks. To be fair to the Turbo Hybrid, it includes a host of equipment not available on the 2.5L and TDI, such as the multi-link rear suspension and a seven-speed transmission, as well as other luxury upgrades. If an apples-to-apples price comparison were possible, the hybrid technology would obviously pay off its premium sooner.

One, of course, could factor resale value and lifetime maintenance costs (i.e., how much will it cost to replace the Hybrid's lithium-ion battery when it wears out?), but this is strictly a fuel economy evaluation, so we'll stick to simple math. It is also worth noting that, if reducing the total cost of ownership is really your goal, the Jetta to buy is the bargain-basement $14,990 2.0L. Its 115 horsepower is nothing to write home about but with fuel consumption approximately the same as the 2.5L, it would take either the Hybrid or the TDI almost until the next millennium to pay back the price differential.

Conclusions

Besides the obvious conclusion that diesel engines are better on the highway and hybrids are better in town, a few other conjectures can be made. One thing that's obvious from the results (and reflects our anecdotal evidence as well) is that while the TDI's real-world highway fuel economy is quite close to Natural Resources Canada's ratings, the 2.5L's are inflated considerably and the Hybrid's even more. These conclusions are no reflection on the Jetta Turbo Hybrid's performance -- its fuel economy is among the best hybrids we've tested -- but the methodology for testing fuel economy seems to greatly favour hybrids over diesels.

Also, considering how thoroughly the Jetta Hybrid thumps the recently tested Ford C-Max Hybrid in highway consumption, it might be safe to posit that a small turbocharged engine (as in the Jetta's 1.4L turbo) is a better choice for hybridization than a naturally aspirated motor (the C-Max's Atkinson-cycle 2.0L). And perhaps the best combination would be a diesel/electric hybrid or, better yet, a plug-in diesel hybrid.

Whatever the case, this test should cement the thought that diesels offer superior fuel economy on the highway and hybrids rule in the city.

-- Postmedia News

 

How does a diesel achieve better fuel economy than a conventional engine?

There are many reasons for diesel's superiority over the conventional gasoline-fuelled engine. The first might be the greater volumetric efficiency resulting from a diesel engine not requiring a throttle valve (diesels alter engine power output by varying how much fuel is delivered into the cylinder, while gas engines generally require a throttle valve in the intake manifold that impedes air flow and, thus, efficiency). Diesel fuel is also slightly more energy-dense than gasoline. But one of the major reasons is the greater thermal efficiency that comes from a diesel's typically higher compression ratio. The more fuel is compressed, the bigger bang there is per explosion (while the 2013 Jetta GL's conventional 2.5-litre in-line five-cylinder engine squeezes its air-gasoline mixture 10.3 times, the Jetta TDI's evergreen 2.0L turbodiesel four-cylinder compresses its raw fuel 16.5:1). And bigger bangs equal more power per litre of fuel.

How does a hybrid achieve better fuel economy than a conventional engine?

Contrary to popular myth, the hybrid's batteries do not magically produce energy not otherwise supplied by the gas engine. Indeed, other than the energy regenerated by the brakes (significant, but not nearly enough to warrant the costs of hybridization alone), all of the energy to recharge the Jetta Hybrid's lithium-ion battery pack comes from gasoline burned in the conventional engine. But, in the automotive equivalent of a Rob-Peter-To-Pay-Paul Ponzi scheme, what the hybrid does is kick in a bit of "free" energy when the gas engine is at its most inefficient -- under acceleration -- and then recharges its battery when the gasoline motor is at its most efficient -- cruising at a steady speed. Borrow low and lend high -- the bond seller's mantra -- applies equally well to hybrid powertrains.

Republished from the Winnipeg Free Press print edition April 19, 2013 F1

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