Dueling Fuel Stretchers

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Dueling fuel stretchers

One VW Jetta turbo diesel faces off against a Toyota Prius hybrid to see which technology makes more cents

Sunday, June 20, 2004 Don Sherman Special to The Plain Dealer

In my humble opinion, fuel cells are a long way off. Perfecting hydrogen-fueled, electrically driven automobiles and creating the infrastructure to support them is likely to take 20 more years.

So what do we do in the meantime, now that pump prices have crested $2 per gallon and some worry that they might go much higher?

Aside from bicycling or jogging to work, there are two viable means of trimming your fuel bills already on the market: diesels and hybrids.

One VW Jetta TDI and one Toyota Prius face off here in a head-to-head driving comparison to help decide which technology makes more sense for saving fuel and curbing emissions.

Diesels are fundamentally more efficient than gasoline engines, on several counts. Each gallon of diesel fuel contains 10 percent more energy than a gallon of gasoline.

Diesels don't squander energy by pumping the air they need for combustion past a throttle as gas engines do.

Their expansion ratios (the flip side of an engine's compression ratio), are at least 50 percent higher, to extract the maximum amount of useful work out of every drop of fuel.

The turbocharger fitted to the Jetta's 1.9-liter four-cylinder diesel engine helps recycle energy ordinarily dumped out the exhaust.

The Toyota Prius hybrid approach employs a tag team of gasoline and electric power.

The 1.5-liter gasoline engine contributes 76 horsepower and an electric motor kicks in 67 horsepower more for acceleration and passing. At stoplights and during low-speed operation (below 25 mph), the gas engine plays dead to save fuel and eliminate emissions.

Nickel metal hydride batteries mounted in the trunk are automatically charged on the roll by a second motor-generator driven by the engine and by the main motor, which becomes a generator during coasting and braking so you never have to plug in at night.

On the road Compared with the noisy, smoky diesels of yore, the Jetta is a heartening turnaround. Its clatter is muted and exhaust stream is commendably transparent.

Nudge the accelerator and there's an eager surge forward with little vibration. Your neighbor will never know you're driving a diesel unless you confess.

A heavy flywheel used to smooth out engine operation and a 4,500-rpm redline make the Jetta feel like it's sprinting in concrete shoes. At least the turbo helps minimize sluggishness during passing.

Even though this is the last model year for the current Jetta design, its steering cuts smartly and holds corners confidently. What it lacks in pure speed it makes up in poise and agility.

Compared with the Jetta, the Prius feels like a TomorrowLand attraction. Its envelope is shaped like an airplane wing to slip through the air without a ruffle. The interior is bright and airy. Instead of a normal instrument panel, there's a centrally positioned energy monitor and a digital speedometer.

Starting, shifting and driving procedures are all slightly eccentric. Since the Prius rolls on super- narrow energy-saving radials, it has less of a grip on the road, and it is less of a driver's car.

The powertrain is silent except for the click of relays and the hum of electric motors. The energy monitor reports that amazing things are going on to stretch a mile or two more out of each teaspoon of gas.

Since EPA mileage figures aren't always dependable, a couple of test drivers spent a day negotiating city traffic in Ann Arbor, Mich., and a second shift journeyed at the speed limit across southern Michigan freeways.

A Jetta and a Prius were driven nose-to-tail, and the two drivers swapped seats in a concerted effort to minimize speed, weather, traffic and driving- habit variables.

Results were very consistent with EPA ratings in the case of the Jetta, less so for the Prius.

In city driving, the Prius was the clear winner, logging an amazing 52 mpg versus the Jetta's not bad 34 mpg.

Highway mileage was practi cally a tie -43 mpg in the Jetta, 44 in the Prius. Both of these fuel squeezers are capable of tripling an average SUV's mileage.

Highway mileage that's lower than city mileage is a hybrid quirk.

When you're cruising at 70 mph, key efficiency boosters such as shutting off the engine and recouping braking energy to recharge the batteries simply don't apply.

So the Prius has to rely on its aerodynamic shape and the inherent efficiency of its gas engine to wring maximum miles from each gallon on the highway.

At the test track, each contender earned its fair share of the glory.

The Prius was quicker accelerating both from rest to 60 mph and from

30-70 mph during a simulated pass.

The run to 60 took the Toyota 10.7 seconds and a second longer for the VW.

The passing gap was wider: the Prius went from 30 mph to 70 mph in 10.7 seconds (a coincidence) while the more sluggish Jetta required 13.6 seconds to accomplish the task.

Skinny tires proved to be the Prius' Achilles' heel in braking and cornering tests. The Jetta stopped 20 feet shorter from 70 mph (172 versus 192 feet) and was able to corner better.

The tailpipe competition One category where the hybrid Prius defiantly and victoriously flashes its taillamps at the Jetta is exhaust emissions.

In terms of regulated pollutants, there's a world of difference between these two technological points of view.

Thanks to a broad range of special features, the Toyota Prius is certified as a super-low- emissions vehicle. No cleaner hydrocarbon-fueled vehicle exists and the only emissions category beyond SULEV is a true zero-emissions vehicle such as an electric or fuel-cell car.

The VW Jetta resides at the opposite extreme. The Environmental Protection Agency category it meets allows 30 times the nitrides of oxygen, 15 times as much unburned hydrocarbons, and four times the carbon monoxide allowed by the SULEV standard.

That's OK for most parts of the country, but five states - California, Maine, Massachusetts, New York, and Vermont -don't accept cars of the Jetta's emissions caliber, starting this model year.

The hope is that the advent of low-sulfur fuel (due in 2007) and the arrival of emissions controls already in use in Europe will save the day for diesels and allow them to be certified in more stringent emissions categories.

That makes the Toyota Prius today's true green machine - a car that's just as friendly to the environment as it is to your wallet, although to some driving enthusiasts the Jetta probably will be more appealing.

Sherman is the technical editor of Automobile Magazine and a free- lance writer based in Michigan. He has covered the auto industry for 33 years.

© 2004 The Plain Dealer.
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