ANALYSIS: New ZF 6-speed auto as efficient as manuals (sometimes)
By just-auto.com editorial team | 2 February 2007
Until recently, and like many other people, I had a somewhat out-of-date perception of light-duty, automatic transmissions, writes CSM analyst Chris Guile. I thought they were slow to respond, slow to shift, bad for fuel consumption and expensive to service.
However, at a recent press launch I was able to experience, at first hand, the very latest in automatic transmissions, in the form of the 2nd generation 6HP range from ZF Friedrichshafen.
The first generation 6-speed 6HP transmissions were launched back in 2001, and have been available in many BMW, Ford Group and VW Group applications since then. This 2nd generation version, however, has been re-engineered to give lower fuel consumption and faster shift times, in order to be more competitive with the new Dual Clutch Transmissions (DCTs) which are appearing on the market now.
Whilst the gap in performance (fuel consumption and shift quality) between automatics and DCTs is getting smaller, the DCT has not suffered from any negative associations, which may cloud perceptions - unlike the 'torque-converter' automatics, which have to constantly battle against old prejudices.
ZF claims that, based on BMW vehicle data, the new transmissions achieve a fuel economy benefit of 3% for gasoline engines and 6% for diesel engines, compared to the 1st generation 6HP.
If we look at the published fuel consumption figures for the new BMW 3-Series Coupé, where most of the engine variants are offered with the ZF 6-speed automatic, and a 6-speed manual, we find that the automatics use 14%, 7%, 2.2% and 0% more fuel than the manual versions.
So, in at least one case, this being the turbocharged 335i Coupé, ZF can legitimately claim that its automatic is as efficient as a manual! Although, to be fair, there can be significant differences between the 'real-world' fuel consumption, and the official fuel consumption which is measured over the European Drive Cycle - sometimes better and sometimes worse.
In practice, manual drivers are frequently in the wrong gear, so the automatic version of the 335i Coupé may actually offer better fuel consumption than the manual when driven on 'real roads'.
When we probe a little deeper, we find that most of the fuel economy improvements are actually down to the torque converter, rather than the transmission.
Leading these improvements is the ability to lock-up the torque converter in all gears, and from a lower engine speed (1000/1100 rpm instead of 1500rpm), to reduce the amount of energy that can be lost to the transmission fluid. In other words, the engine spends more time directly connected to the wheels, like a manual, and less time dissipating energy in the torque converter fluid (a typical torque converter has 80-85% efficiency, but these new versions achieve up to 92%, according to ZF).
Although interestingly, BMW decided it did not want to use the option of lock-up in some of the lower gears. Finally, the torque converter is disengaged at 0mph, which means there is no vehicle creep at junctions, and further economy benefits.
One of the main reasons why the torque converter is able to lock-up more quickly is the use of a newly developed damper system (or double damper system in the case of the diesel applications) in the converter. By reducing the torque fluctuations from the engine, the converter can be locked-up earlier, without subjecting the driver to any undesirable noise or vibration.
The modifications to the transmission itself have led to increased torque capacity, with only a slight increase in weight (see table), reduced shift times and improved shift quality/dynamics. Meanwhile, many of the important oil channels in the hydraulic controller are now shorter and wider, to improve efficiency.
1st Generation Torque Weight
6HP-19 400N.m 72-77kg
6HP-26 600N.m 84-89kg
6HP-32 750N.m 99kg
2nd Generation Torque Weight
6HP-21 450N.m 73.5-78kg
6HP-28 700N.m 92.5kg
(6HP-34) 850N.m 99kg
In the 1st generation 6HP it was possible to jump a gear, although the process involved going through that gear. So, for example, a jump from 4th to 2nd involved going from 4th to 3rd and then 3rd to 2nd, although there was an overlap which reduced the overall shift time. The 2nd generation 6HP can jump over a gear completely, or even three gears when going from 6th down to 2nd, or 5th down to 1st. This compares to the DCT technology, which has to change sequentially, through each gear in turn.
Like many modern technologies, the transmission has an electronic controller, or TCU (Transmission Control Unit). In the 2nd generation 6HP transmissions there is more communication between the Engine Control Unit (ECU) and the TCU. In particular, the TCU is able to ask the ECU to 'blip' the engine speed, at certain times during the shifting process, to ensure a smooth gear transition. In addition to its usual task of determining the right gear for the conditions, the TCU also monitors the driving style of the driver, and adjusts the shift strategy accordingly.
Rather than replacing the 1st generation 6HPs, the 2nd generation versions will be built alongside them at the Saarbrucken plant in Germany, effectively as a higher cost option, to those vehicle manufacturers that can afford to pay more for improved performance. Therefore, we are unlikely to see the new versions appearing on many, if any of the Ford Group applications. Many parts are carried over from the 1st to 2nd generations, whilst other parts have been re-designed for use in the 2nd generation versions only. A third group of parts are those which were improved for the 2nd generation, but which will now be used on the 1st generation as well, to benefit from the economies of scale.
What might seem odd is that ZF has not increased the number of gears, as this not only allows the engine to spend more time at, or close to its optimum operating speed, but also reduces the impact of a gear change on the engine speed, thus reducing the lurch or 'head nod' effect. In addition to this, and quite apart from any technical reasons, there is the market effect, which should not be dismissed lightly, whereby one vehicle OEM will want to keep up with, or exceed what its competitors are doing. ZF has stated that it is not going to develop a 7-speed automatic, but it has also said that a replacement to the 6HP is under development. So, given that Mercedes-Benz has had its 7-speed automatic available for several years (the 7G-Tronic or W7A-700), we can safely say that ZF will launch an 8-speed '8HP', for use initially by BMW. CSM Worldwide expects to see the 8HP unveiled in the replacement BMW 7-Series [F01], sometime in 2008/2009.
So, if you thought that the introduction of dual clutch transmissions would result in the demise of automatic in Europe, you may need to think again.
According to CSM estimates for 2006, 52% of all ZF 6HP 6-speed automatic transmissions are fitted in BMW applications, with 29% going to the Ford Group and 19% destined for the VW Group.
Initial applications of the 6HP-21 and 6HP-28 are the BMW X3sd [E83], mated to a 3.0-litre 210kW diesel engine, together with most engine versions of the new BMW 3-Series Coupé [E90]. The larger 6HP-34 may not ever be manufactured, as there is a suggestion that BMW might be content to use the uprated 6HP-28 instead, in place of the current 6HP 32.
Chris Guile is a Powertrain Analyst with CSM Worldwide
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ANALYSIS: New ZF 6-speed auto as efficient as manuals (sometimes)
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