Honda Forms Solar Cell Subsidiary
British Government Proposing Transportation Tax Measures to Tackle Climate Change

ZF Introduces Second-Generation 6-Speed Automatic; Up to 6% Fuel Savings

Fuel savings with ZF automatic transmissions compared to earlier models. The new 6-speed delivers up to 19% in fuel savings compared to an old 3-speed. Click to enlarge.

ZF has introduced a new second-generation 6-speed automatic transmission that delivers fuel savings of up to 3% in a gasoline-engine vehicle and up to 6% in a diesel.

The use of a new torque converter in combination with optimized transmission control and hydraulics make the multi-ratio automatic 6-speed transmission of the second generation even more efficient than its predecessor, thereby contributing to the increased fuel savings. BMW began offering the new rapid shift transmission in the 3 Series Coupé and the X3 in September.

Response times have been reduced by up to 50% and, for the first time, direct double shifts without interruption are possible. The rapid shift transmission is designed for an input torque of up to a maximum 850 Nm—i.e. for light-duty vehicles with engines of up to twelve cylinders.

A new torque converter enables reduced shift times at low fuel consumption while maintaining comfort.

In order to optimize fuel consumption, ZF has developed a consistent shift strategy for the torque converter lock-up clutch with the objective of closing the clutch completely as soon as possible.

Further development of the hydraulic system enables downshifts of up to four stages to occur without any time losses or noticeable deceleration. A separate pressure regulator directly controls the respectively need clutch so that no additional time is required for jumping a drive stage. Called the target gear finding mechanism, this makes the shift process precise.

On the basis of the data received on load conditions, the driving situation, and the speed of acceleration, the improved control software determines whether one-stage or multiple stage downshifts are required.

ZF reduced the shift time by half compared to its previous transmissions, and response time is rapid whether one gear step or several are shifted. If—at an approximate driving speed of 70 kph in sixth gear—the transmission receives a kickdown signal, it needs just 100 milliseconds to trigger the change from sixth to second gear.

As with earlier ZF transmissions, the planetary transmission system—based on a Lepelletier gear set—is at the center of the new automatic ZF 6-speed transmission. Thus, six gears can be realized with significantly less components. In the past, only three gear sets and six shift elements were needed for the automatic 5-speed transmission; today, a simple planetary gear set suffices as well as a Ravigneaux gear set in combination with five shift elements.

This results in a weight reduction of approximately eleven kilograms (24 pounds) compared to the 5-speed automatic. Increased use of plastics contributes to the reduction of weight and consequently fuel consumption.


Rafael Seidl

Having a lock-up clutch that is active in all gears (incl. first and reverse) is a key factor in improving fuel economy in vehicles with AT. In effect, the lossy torque converter is then used only during the initial acceleration from standstill and gear shifts, not in regular driving.

The other key feature is that high-powered modern engines, specially those equipped with boost systems, increasingly exhibit a torque-speed diagram with a flat top. This is due to fuel flow rate limiting by the engine controller, to avoid excessive mechanical stresses in the engine. In many cases, 90% of maximum rated torque is already available somewhere in the 1500-2200 RPM range. This high load capacity permits longer gearing ratios and earlier upshifts, such that the engine is operated closer to its point of optimum fuel efficiency.

It's not immediately clear if ZF's number refer to one and the same engine fitted with their first and second-generation ATs or, if the old AT was paired with the old engine and the new AT with the new one. The transmission control logic needs to be tuned for each engine model.


I presume a DSG transmission can attain even better fuel efficiency than this automatic with lock-up converter.


Very impressive results, but Rafael has hit on an important point. In what context should we view the claimed fuel consumption savings? Are these back to back tests with the same engine and vehicle - almost definitely not. Are they predictions based on some extrapolated data made to look nice by the marketing department - probably. Unfortunately it is rare to ever see genuine comparison data. In this case the "fuel savings" are not qualified - which drive cycles (hopefully that's what the data represent), what type of vehicle and engine etc.

The old weight saving chestnut gets a lot of play but has anyone stopped to see if vehicle curb wieghts are decreasing for each new model year? They are getting heavier and the engines are getting more powerful to ensure the same vehicle performance. The so-called weight savings get soaked up in additional goodies like passenger safety devices, numerous electrical/electronic gadgets and anything else that can be seen as attractive to the consumer. That should highlight the remarkable achievements of Powertrain engineers to just maintain the fuel consumption numbers, let alone reduce them.

Rafael Seidl

Jorge -

DSGs with wet clutches actually suffer a small penalty compared to those with dry clutches, which are indeed better even than straight manual transmissions. Unfortunately, dry-clutch DSGs are limited to ~250Nm rated torque and do not offer quite the comfort level of wet clutch designs, traditional ATs or CVTs.

Modern ATs with torque converters and lock-up clutches are actually not a whole lot worse than manuals and offer similar performance. Most European consumers still prefer manuals, either for perceived performance/connectedness to the vehicle or for lower cost (A-C/D segments, especially those with diesel engines). AT market share is expected to grow over the next decade, though not explosively so.

The advances in AT transmission technology, including the new types, are really more significant for the US and Japanese markets, where manuals are rare because few drivers ever learn how to operate them. Unfortunately, the trannies in many SUVs and pick-up trucks are actually older 3- or 4-speed designs without any lock-up clutch at all. Those do raise fuel consumption by 8-10% over a manual.


Oh, no, Rafael, automatic transmissions universally employ lock-up torque converter for at least 30 years, and 3 speed automatics are going to extinction on the roads. The quest is to maximize use of lock-up to reduce fuel consumption further.

Good calibrated automatics already for a long time (by switch of the button) could facilitate brisk engine braking, or forbid downshift during WOT, or lock engine to transmission, quite successfully mimicking prized precise control inherent to manuals. Thought, very few of American drivers use these capabilities.

Slow shift response always was very frustrating feature of automatics, so ZF 0.1 second shift delay is very welcome.

One feature where automatics shine. Even economical 1.8l Japanese engine with low level low-RPM torque, if mated with automatics, could launch a car from stand-still with wheel-spin. Hydraulic torque converter effectively doubles torque going to the wheels versus same engine mated to manual transmission (from stand-still to about 40km/h on WOT). That’s why American drivers, 90% driving automatics, are not so sensitive to high low-end torque, so prized by Europeans preferring manual transmissions.


Lock up torque converters became common on US 3 speed automatics in the late 70's, although only on 3rd gear. Four-speed automatics have been almost universal since the mid-late '80's. The switch to 4 speeds came quicker with SUV's/pickups/RWD cars, because they were more common then (as a group). The first generation FWD transaxles were 3 speed, but most were switched/converted to 4speed/OD by late '80's/early '90's. The last 3speed to my knowledge was Chrysler's FWD transaxle, which was still put it in Neon and their mini-vans as a base automatic to keep sticker prices low, as recently as 2 to 4 years ago, but no more.

Verify your Comment

Previewing your Comment

This is only a preview. Your comment has not yet been posted.

Your comment could not be posted. Error type:
Your comment has been posted. Post another comment

The letters and numbers you entered did not match the image. Please try again.

As a final step before posting your comment, enter the letters and numbers you see in the image below. This prevents automated programs from posting comments.

Having trouble reading this image? View an alternate.


Post a comment

Your Information

(Name is required. Email address will not be displayed with the comment.)