Saturday, February 2, 2002
Audi's CVT is unconvincing
By Alan Vonderhaar firstname.lastname@example.org
The Cincinnati Enquirer
I never met an Audi I didn't like till now.
Just kidding this week's examinee is one of the nicer cars on the road, but still, by Audi standards, it didn't measure up.
Those are very high standards, indeed it wouldn't be fair to hold anybody but premium makes to them.
The car in question is the new A4, which was sampled here a few weeks back in a somewhat more ordinary form. That one was the sporty quattro edition with 3-liter V-6 engine and the iron-pumper's special, 6-speed manual transmission. Very hot and very poised, also rather expensive about 38 grand.
This time our guest is closer to the low end the A4 with the 1.8-liter turbo engine, front-wheel drive and Audi's new continuously variable transmission (CVT). Since the lattermost was the hook for a second look, let's examine it first.
The $1,150 option is the only alternative to the base 5-speed manual transmission on front-drive A4s; you can get a conventional 5-speed automatic with all-wheel-drive (quattro) machines.
The purpose of any gearbox is to multiply the torque an engine supplies, by trading off revolutions against twisting force. Until the vehicle reaches cruising speed, the driven wheels are rotating fewer times per second than the engine's crankshaft is, but more torque twisting force is applied at the output shaft than is being sent to the input of the gearbox.
At cruising speeds, the driven wheels are rotating at the same number of rpms as the engine, or even faster, which is called overdrive.
The basic point to gearing is overcoming inertia and static friction in bringing the vehicle from a dead stop to a suitable cruising speed. A great deal of torque is needed to overcome starting resistance, much less to sustain the vehicle's momentum once it's under way.
(Horsepower is somewhat different from torque in that it is time-sensitive, measuring the work done in an interval, whereas torque is purely a measure of force. Throughout automotive history, horsepower has been cited more because a given engine generally makes more horses than foot-pounds, thus giving the ad department something to toot about.)
Anyway: Early cars had one or two forward gear ratios, which was sufficient with relatively torquey engines and very limited speeds. In time, three, four, five and now even six forward ratios have become prevalent.
The point is to match the engine's torque to the needs of the moment as efficiently as possible.
Generally, a torque curve is shaped somewhat like an inverted U, rising rather quickly to a peak and then falling off that peak equally rapidly as engine rpms rise. Modern methods of fuel delivery, and various variable-valve timing schemes have allowed the torque curves of sophisticated engines to look more like mesas, with an early sharp rise to a wide plateau, followed by a sharp dropoff.
Such treatments actually argue against forward-ratio proliferation in most cases. A five-speed transmission (or even four-speed) is generally quite adequate except where huge amounts of torque allow (or mandate, for economy reasons) two overdrive ratios and thus a six-speed gearbox the Corvette and Viper come to mind.
The more gears, the more complexity, to say nothing of parasitic losses and compromised operation.
A CVT uses two pulleys which share a belt. One pulley is conical, the other flat (see illustration on Page G2). As the belt moves up and down on the slope of the conical pulley as that pulley advances and retreats, the speed ratio between their two shafts varies. It's rather like a bicycle's derailleur, but with an infinite number of gears.
By allowing electronic controllers to manage things, engineers can design in maximal efficiency, which is supposed to translate to seamless shifting, high gas mileage and low emissions.
CVTs have existed for decades, used mainly in industrial applications. Early automotive designs used a rubber belt and were limited to relatively low-power situations, such as the 1989 1.2-liter Subaru Justy. With rubber, reliability was an issue, so now steel belts are favored. They allow CVTs to be used with much more energetic powerplants, such as the A4's 1.8-liter turbo, which makes 166 foot-pounds of torque.
So equipped, the A4 delivers mileage very close to what the five-speed manual transmission affords, and slightly better dynamic performance. EPA ratings are 20 mpg city, 29 highway. I experienced 25.1 in an equal mix of highway cruising and back-road charging.
Audi's implementation of the CVT (which they call multitronic, with a small initial letter) is pretty good, although it makes some strange noises at times and its launch from a standing start, as the pulleys engage, feels rather like a stumbling hesitation. This car, I would submit, is not a good showcase for the CVT, because this turbo engine has a broad, flat torque curve, and Audi's conventional five-speed automatic transmissions are superb. A CVT would show up better if used as an alternative to a peaky engine and so-so four-speed automatic.
The multitronic allows some degree of manual control; by sliding the console-mounted shifter to the right, one engages one of six preprogrammed forward ratios (which one depends on road speed), and the driver can force downshifts or upshifts by sliding the shift lever fore or aft, a la Porsche's ubiquitous Tiptronic mechanism. The transmission computer won't allow selection of an inappropriate gear, to prevent lugging or overrevving.
I could adjust to multitronic, but FrontTrak, Audi's front-wheel-drive setup, is another matter. I've always been a fan of Audi's magnificent quattro (they are really weird about capitals and lowercase), but approached this alternative with an open mind.
I was really amazed at how much less satisfying the front-drive version was. The sureness and agility of the quattro was gone, replaced by a fractious setup that didn't measure up to the best of the FWD crowd, even with electronic stabilization.
Trust me on this one: the extra $1,750 you spend for quattro will be a solid investment in safety, performance and comfort.
The ride quality of this tester was markedly inferior to that of the one driven earlier. Undue harshness was felt, beyond what should be necessary for a sporty feel. The tester had the optional sports suspension, which was probably to blame, along with the 235/45/17 high-performance tires that come in that package. If I were speccing this car, I'd have gone for the upsized 215/55s on 16-inch rims, and let it go at that. (The standard rubber consists of 205/65/15 all-seasons).
The cabin of the A4 is a very pleasant place to work, with good ergonomics and senses-pleasing materials. Slightly larger than last year, the new A4 can accommodate two average-sized adults in the back for trips of modest duration.
Neither the government nor the insurance folks have crash-tested a 2002 A4 yet. The newfound chassis solidity, along with front, side and head air bags, argues for a high degree of collision-worthiness.
Base price on the A4 with the 1.8 turbo and CVT is $26,050. The sacrificial victim also had special paint, canvas beige metallic (why?), for $450; power moonroof, $1,000; sport package (17-inch cast alloy wheels, sport suspension and ultra-high-performance summer tires, $1,000; heated seats, front and rear, $525, and a "premium package," $350, which includes a Homelink transmitter and auto-dimming rearview mirrors, inside and outside.
With freight, total was $29,950.
Edmunds, the auto information resource, says the new A4s are going for a slight discount from sticker.
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