The Golden Age of Gears
Transmissions finally earn their day in the sun.
Engine advancements hogged center stage for a decade. We’ve been bombarded by 4WD traction schemes and hybrid propulsion systems galore. Electric cars had their 15 minutes of fame. Meanwhile, transmission advocates waited patiently in the wings, anxious to remind us that the best engines and traction systems are stuck in the garage without a gearbox as the go-between. Welcome to the golden age of transmission innovation. Manufacturers around the globe are currently digging through an avalanche of developments to determine which of the new-wave transmissions offer the best performance and the greatest competitive advantage per investment dollar.
Two powerful forces are at work here. The first is the never-ending quest for fuel efficiency. Now that car makers have revolutionized their engines with variable valve timing, low-restriction intake and exhaust systems, and sophisticated electronic controls, they’re searching elsewhere for cost-effective ways to meet looming CAFE requirements. The second factor is sex appeal; being the first to market with fresh technology is still an excellent means of attracting customers.
To chart a course through the universe of planetary gear sets and ratio variators, we’ve organized the bold aspirations we gathered polling major transmission manufacturers into five emerging categories.Extra-speed manual transmissions
Four-speed stick shifts are going the way of the buggy whip. Car enthusiasts love snicking through the gears and can’t have too many ratios at their disposal. So five-speed manuals are systematically yielding to sixspeed boxes. Though manufacturers refuse confirmation, the rumor mill continues to grind out evidence that seven-speed manuals are in the early development phase.
Aisin, Getrag, Ricardo, Tremec and ZF all offer six-speed manual transmissions for rear-wheel-drive applications. Aisin and Getrag also manufacture them for frontdrive pocket rockets. That’s an especially challenging application because the length of the transaxle is a serious issue with transverse powertrain packages. A popular solution is to spread the gear sets over three, instead of the usual two, countershafts, trading length for width. The downside is additional cost.
One relative newcomer to the six-speed transaxle business is Ricardo, a British enterprise better known for its powertrain R&D expertise. When Audi won three consecutive 24 Hours of LeMans endurance races using a Ricardo transaxle designed and developed in only 18 weeks, Ford engineers, under the gun to hustle their mid-engined GT sports car from the show circuit to running prototype form in 16 months, identified an attractive partner. Ricardo gathered up shelf components to get Ford GT “workhorse” vehicles on the road and is now preparing a purpose-built cable-shift six-speed transaxle for the car’s March 2003 production launch. Ricardo’s U.S. vice president Peter Brown acknowledges, “We’re actively pursuing opportunities to design, develop, and manufacture transmissions in moderate or niche volumes.”
Extra gears improve acceleration and topspeed performance because they keep the engine revving in its most productive range — between torque and horsepower peaks. Tall overdrive ratios also improve mileage ratings in EPA highway-cycle tests. As a hedge against the gas-guzzler tax, Chevrolet’s Tremec-equipped Corvette uses overdrive ratios in both fifth and sixth plus a solenoidactivated blocker to “help” the driver skip second and third during moderate acceleration. “Even though manual transmission sales are decreasing on a global scale, our volumes are increasing as they evolve into a high-end niche business,” says Aisin’s Senior Vice President Don Whitsitt.Six-speed automatics
|Ricardo’s six-speed transaxles helped Audi capture three consecutive 24 Hours of LeMans wins. Their success caught the eyes of Ford engineers who have opted for a Ricardo six-speed for the Ford GT supercar. |
If six speeds are good for the goose, why not the gander? The logic behind automatic transmissions with extra gears is similar — better acceleration, reduced consumption, less engine noise during cruising — but slightly different. The march from four- to five-speed automatics commenced more than a decade ago. When car makers pressed transmission suppliers for efficiency gains, clever engineers sharpened their CAD tools and set to work achieving across-the-board performance improvements. A case in point is ZF’s 6 HP 26 six-speed automatic that, according to Gerhard Wagner group vice president for product development, boasts five to six percent lower fuel consumption, five percent quicker acceleration, and 13 percent less weight. Introduced concurrently with the 2002 BMW 7-series, ZF’s pride is also two-inches shorter, it contains fewer parts, and it’s more reliable than the 5 HP 24 5-speed automatic it replaced. A “mechatronic” internal module marries electronic controls to mechanical actuators, providing the driver with both fullyautomatic and button-shift modes. Packing six forward ratios inside a smaller box was accomplished by mating a traditional Ravigneau planetary gear set to a recently developed Lepelletier gear set.
Extra gears facilitate a broader ratio spread between launch and cruising ratios. The new ZF automatic has a 6.05 ratio spread between first and sixth compared to 4.0 for the typical four-speed, 5.0 for fivespeed automatics, and 5.5 or so for wideratio five-speed manual transmissions. According to ZF’s Executive Vice President for R&D Michael Paul, the 6.05 ratio spread is optimum for today’s engines but directinjection gas and diesel powerplants of the future may appreciate even broader spreads. When asked if that means ZF is investigating seven-speed automatics, Paul acknowledged that single-digit fuel efficiency gains are possible with them but refused to confirm that his engineers are actively pursuing that approach.
ZF is by no means the only 6-speed automatic transmission booster. Aisin offers them for front- and rear-drive applications. Jatco has them under development. Last year GM and Ford agreed to cooperatively engineer such a unit for their front-wheel drive automobiles. GM has also announced intentions to introduce a six-speed Hydra- Matic for upscale rear-drive cars and light trucks. Spread-ratio six-speed automatics make perfect sense for a broad range of applications. BorgWarner expects them to be the most popular type of transmission in the U.S. within ten years.Servo-shifted manuals
As prosperity and traffic builds around the globe, drivers are anxious to escape clutchpedal and gear-shift drudgery. At the highest levels of motorsports, Formula One drivers change gears consistently and expeditiously by pressing buttons on their steering wheels. Deep within their racers’ innards, there are still clutches and gears but they’re activated by computer-controlled servos and solenoids instead of the driver’s impetus.
Servo-shifted manuals (SSMs) are both the poor man’s automatic and the latest example of racing technology trickling down to civilian use. Transmission experts insist that the first role won’t play in America because the majority of us made the leap from sticks to automatics decades ago and few will tolerate the jerks and jolts inherent with interrupting power flow to shift gears. But those shopping for an Aston Martin Vanquish, BMW M3, Ferrari, Maserati, or Toyota MR2 love jerks and jolts. Nine out of ten Ferrari buyers spend the extra $10,000 for an F1 paddle-shift system in models that offer it as an option. Under the Maserati banner, the take for a similar six-speed Cambiocorsa transmission is eight out of ten buyers. Drop down to a BMW M3 and the price for a Getrag-supplied sequential manual gearbox (SMG) is a mere $2,400. Toyota’s automatic alternative in the MR2 Spyder is $1,000 extra.
Performance tests by Car and Driver magazine revealed that, even though SSMs stir the gears in less than a tenth of a second, expert hands and feet operating a conventional clutch and shifter still win the acceleration race. In spite of that, high-end buyers relish the paddle-shift approach’s entertainment value.
Current SSMs consist of a conventional clutch and gearbox equipped with add-on actuators. While weight, cost and bulk savings are possible using more integrated designs, the necessary investments are difficult to justify given the niche nature of this transmission alternative. The prospect of better ideas just around the corner also counts against SSMs gaining popularity in the U.S. Dual clutch automated manuals
|BorgWarner’s DualTronic dual clutch automated manual uses two wet plate clutches to route engine power through two input and two counter shafts. Pictured is the system’s clutch module (left) and control module (right). |
In the golden age of gears, better mousetraps keep popping up. Another concept with motorsports roots that’s now arriving in the U.S. market is dual clutch automated manual (DCAM) transmissions. Six years ago, Volkswagen tapped BorgWarner Transmission Systems to develop what the former markets as a Direct-Shift Gearbox and the latter calls DualTronic. BorgWarner group president Robert Welding acknowledges that the dualclutch concept has been around for decades but making it work hinged on three key breakthroughs: powerful electronic controls, new friction materials with higher temperature capability, and a special fluid formulation with the flexibility to lubricate synchronizers, meshed helical gears, bearings and wet clutches.
DCAM is a hybrid design which borrows freely from conventional automatics and servo-shifted manuals. Two wet multi-plate clutches route engine power to the wheels through two input and two countershafts. Computerized servos shift the helical gears and operate the wet clutches. At launch, one clutch slips as the other progressively engages. Successive gears are preselected so the 1-2 upshift is a seamless passing of the baton — from the clutch driving first gear to the one routing torque through second gear. The process repeats for each ratio change. The driver pushes no clutch, pulls no lever, and experiences no power-flow interruption unless a manual mode is selected.
A computer and an hydraulic pump are necessary to make this electro-mechanical circus perform but eliminating the torque converter is a boon to efficiency. According to BorgWarner, the DCAM’s fuel efficiency beats a conventional five-speed automatic by 13- 15 percent and six-speed automatics by seven to nine percent.
Shortcomings associated with this best-oftwo- worlds approach are minor. Skipping gears is not possible. If the driver mentally shifts gears from leisurely acceleration to flatout passing, extra reaction time is necessary to drop the taller gear and pick up a lower one. Unlike SSMs, DCAM’s are not a hang-on scheme so dedicated tooling is necessary, but at least existing gear-cutting and shaftgrinding machinery is applicable.
Audi TTs equipped with DCAMs arrive this fall and VW is thinking about importing a few Golf R32 hot rods fitted with them. Ricardo is tooling up a seven-speed DCAM for the 16-cylinder Bugatti Veyron due next year. Porsche will join the party using BorgWarner DualTronic components in the 2005 911. A competing ZF system is allegedly under development for the next generation Boxster due as a 2006 model. Getrag is working on a design for release by 2007 and Aisin is at the investigation stage.
BorgWarner’s Welding, who regards DCAMs as the greatest boon to transmissions since syncromesh, believes they have tremendous growth potential, especially in Europe where they could soon match conventional automatics in popularity.Continuously variable transmissions
|Nissan’s belt-type CVT has more torque capacity and a slightly narrower ratio spread. |
Why not eliminate gears altogether? That’s the proposition offered by the continuously variable (CVT) crowd which consists of three distinct camps. The belt contingent squeezes a multi-link drive element between twin cone-shaped devices called ratio variators. The chain gang uses the same approach except the drive device operates in tension instead of compression. A third alternative in limited production but five years behind belt and chain designs is a system that places rollers between twin toroidalshaped surfaces. In all three cases, an infinite number of ratios is possible and steps between shifts are eliminated.
The CVT Audi introduced for sale in the U.S. in 2002 is a chain-type design with sufficient torque capacity to live behind a 3.0L V-6 and a ratio spread of 6.05:1. This year Nissan introduced a belt-type CVT for the 3.5L Murano with more torque capacity and a slightly narrower (5.40:1) ratio spread. Honda has quietly offered CVT-equipped Civics since 1996. Subaru imported a few Justy models fitted with them beginning in 1989.
Historians credit Leonardo da Vinci with conceiving the CVT in the late 15th century. Today’s applications stem from the efforts of engineer Hubertus van Doorne whose twinrubber- belt Variomatic transmission propelled Dutch-built DAF microcars to a 52-mph top speed in 1958.
Fuel efficiency gains of seven to 10 percent over a five-speed automatic are typical with CVTs. They lose a few points to six-speed automatics because of the parasitic losses associated with the hydraulic pump that’s needed to squeeze variator elements tightly in contact with the drive element. CVTs are inherently simple and contain far fewer parts than other automatics. The short, fat proportions of this box make it ideally suited to both transverse and longitudinal front-drive applications. An even grander claim to fame is passing poise: step into the throttle and a CVT quickly alters its ratio to build rpm and forward thrust in one smooth, seamless rush.
CVTs are perfect for Japan where speeds are low, traffic is congested and cars are compact. Thirty-five models already offer them. Europe is a tough sell because of higher average speeds. BorgWarner predicts slow growth in the U.S. with front-drive entry-level models as the best fit. Nevertheless, Aisin, Getrag, GM, Jatco and ZF are all pursuing CVTs even though they require dedicated tooling because their mechanical makeup is unique.
Prospects for toroidal-type CVTs are less auspicious. One expert who prefers to remain anonymous exclaimed, “The toroidal era is over before it started. They are history and their makers should close shop and go home.” Problem areas are weight, cost, and a very expensive operating fluid that doesn’t yet meet U.S. manufacturers’ standards for temperature range.
Right now transmission engineers have never had it so good. World car makers are beating a path to their doors and years of toil and hefty investments are about to pay off. The luckiest of all benefactors are customers who are about to enjoy higher fuel efficiency, better performance, and a heightened driving experience as the golden age of gears unfolds.