September 10, 2006
Technology
Is 8 Enough? Transmissions Meet the Need for Speeds
By KEVIN CAMERON

AMONG the technology features that will distinguish the 2007 Lexus LS 460 when it goes on sale next month is an eight-speed automatic transmission. An industry first, the Lexus gearbox will top the mere seven speeds offered on certain Mercedes-Benz and BMW models — even the $1.2 million Bugatti Veyron — and handily outclass the four-, five- and six-speed transmissions in widespread use today.

Is there a need for so many speeds? If so, is it a need based on engineering or on marketing?

A little of both, actually: Officially, Lexus says that eight ratios were necessary to meet its goals for acceleration and fuel economy. Sure enough, specifications released by the company confirm that the new car is quicker than its predecessor and uses less fuel, even though it weighs more.

As for marketing, the chief engineer of the LS 460, Satoru Maruyamano, told Automotive News, a trade publication, that Lexus wanted to be the first with eight speeds.

To understand why the number of gears has proliferated, consider how powertrains have evolved since World War II.

In the immediate postwar years, big cast-iron engines of American cars were paired with either a three-speed manual-shift transmission or with an automatic that had three forward gears, or even just two. (A well-known two-speed unit was Chevrolet’s ubiquitous, slow-off-the-line Powerglide.)

Because the engines of that era were so big and slow-turning, with a lot of low-end torque, three gear ratios were enough to start a heavy car from rest and reach a modest top speed.

Drivers who have inadvertently tried to pull away from a stop in the highest gear of a manual transmission car quickly understand why a car needs more than one gear ratio between the engine and drive wheels. With little help from the high ratio of a transmission’s top gear, an engine’s low-speed pulling power is not enough to heave the vehicle smoothly into motion and then accelerate it strongly.

A low ratio, like 4 to 1, makes it easier to move away from a stop because of the mechanical advantage provided by gear reduction. The principle is similar to using a longer lever to move a heavy object. (To confuse matters, lower ratios are numerically higher; 3 to 1 is a lower ratio than 2 to 1).

If the start-off gear were the only ratio in the transmission, it would limit the car’s top speed to perhaps 50 miles an hour — with the engine screaming. At least two gears are needed, one for starting and one for highway cruising.

Why add an intermediate gear? Try starting in first gear, then shifting to the top gear. You will start out smoothly, with initially strong acceleration, but when you shift, the car’s acceleration drops to almost nothing. The wide gap between the ratios has pulled the engine’s r.p.m. below its range of useful torque output. This, then, is why the transmissions of so many American V-8 automobiles of the past had three forward speeds.

But if you downsize that big V-8 engine — a necessity given 21st-century fuel prices — everything changes. To obtain acceptable power from a smaller engine, it must be tuned to a finer edge, sharply narrowing the range of engine speeds over which it delivers strong torque. With only three speeds, the ratio gap at each upshift would pull the engine r.p.m. out of this range, making acceleration anemic.

To keep the engine revs always within the torque range, four speeds, or even five, are needed. That is why modern smaller-displacement engines are usually paired with four- or five-speed transmissions.

The coming of federal fuel economy standards made the quest for higher mileage permanent. This had the effect of requiring a high top-gear ratio so a car could maintain highway speeds at the lowest possible engine r.p.m. This defines approximately what a transmission’s top-gear ratio has to be.

The heavier the vehicle, the lower the first gear must be to give the engine the leverage it needs to accelerate briskly. Racing cars, being light, have tall first gears, but heavy luxury sedans need a low first.

How do engineers fill in the space between the first and top gears? They start by looking at the best pulling range of the engine. What is the r.p.m. difference between peak power and peak torque? If the shift from first to second pulls engine r.p.m. down too far below peak torque, acceleration in second gear will suffer. That pretty much defines the ratio splits between first and second, and between the other lower gears.

As the car gets moving at higher speeds, horsepower becomes more important than torque because it is power that must overcome aerodynamic drag and rolling friction. The engineers have to make sure that at the upshift, engine revs do not drop so far that the power to continue accelerating becomes unavailable. This forces ratios to be more closely spaced toward top gear.

A final consideration is that the wider the ratio separation, the more perceptible is the “thump” of each upshift. The shock is created by the energy of the spinning internal parts as the rotating speed is suddenly pulled down in the next-higher gear. Programming of the timing and force of shifts in modern electronically controlled automatic transmissions is very sophisticated, but there are limits.

Now that the car is at highway speed, why not just stay in top gear? Thirty years ago, the torque converter on every automatic transmission allowed the engine to freely speed up and make more power as the driver opened the throttle slightly to meet an upgrade.

Today, fuel economy requirements have made it necessary to add a small clutch inside the torque converter to eliminate slippage at highway speeds.

As we approach a hill today, the car is in a very tall economy gear, with the engine revs low. Just opening the throttle won’t get the car over the hill, because the engine needs to spin faster to make the necessary power. That requires a downshift. Hills come in a variety of grades, so the car may need more than two closely spaced upper ratios to keep the engine working efficiently, on hill and dale. In fact, all four top ratios in the Lexus eight-speed are separated by less than 20 percent drops in engine revs. This allows efficient cruising with almost imperceptible shifts.

Therefore, it was the constraints of a low first gear, a very high top gear and manageable ratio splits in between that resulted in Lexus’s choice of eight forward speeds.

This surprises people accustomed to the gear ratios used in racing, but the world of heavy luxury sedans is more akin to heavy trucks than to high-performance sports cars. Lots of big trucks have eight-speed gearboxes for the same reasons — they need a low first to accelerate their weight, then progressively closer ratios in the higher gears.

You can get a rough idea of the design intentions behind a gearbox by dividing its top ratio into its first-gear ratio. This number — the overall ratio spread — generally lies between 1.9 and 2.5 for racing cars. To keep a racing engine within its typically narrow useful rev range, transmission ratios must be closely spaced, and first gear can be quite tall because the vehicle’s weight is low.

For the Lexus LS 460, the ratio spread is much larger, from 4.60 in first to 0.68 to 1 in eighth, which works out to 6.7. This suggests that first gear is very low for brisk starting of the car’s considerable mass, while the top gear is very high for fuel-efficient cruising. At the engine’s peak revs, 6,400 r.p.m., this car could easily reach its governed top speed of 130 miles an hour in fifth gear.

Then why eight speeds? Maximum r.p.m. at freeway cruising speed would be inefficient — and uncivilized to boot. The three higher gears are there to enable smooth gear changing while holding engine r.p.m. at unobtrusive and fuel-saving low levels.

Automakers have begun using transmissions with an unlimited number of ratios — the continuously variable transmission — to do even better. While the concept is valid, limitations of the mechanical parts, especially when paired with large engines, have so far kept this design from universal use.

imagine an 8-speed MT?

going through gates 1-6 just to get to 60mph, then, 7-8 for cruising...