The 8HP70 replaces the six-speed 6HP26 in ZF’s product catalogue – the latter a highly successful transmission which is currently fitted to a raft of the world’s leading prestige cars, including Maserati’s Quattroporte.  However, the 8HP takes those strengths, and builds on them, to deliver a truly world-class product.  Already fitted to premium cars such as the Bentley Mulsanne, Audi A8, BMW 5- and 7-Series, and the Rolls-Royce Ghost, it makes significant strides in all areas; in particular, its technical innovations will allow Chrysler to meet U.S. fuel economy regulations when it is fitted to the new 300 sedan later this year. Initially the 8HP70 unit will be sourced from ZF (where it is built in Saarbrücken, Germany), but within two years Chrysler will licence the unit to produce it at its new transmission factory in Indiana, a plant it is setting up in conjunction with ZF.

Due to changes in its internal design, shift times compared to its six-speed predecessor are reduced, now down to 200 milliseconds.  Also new is the ability to shift in a non-sequential manner – even offering the ability to go from eighth to second gear in a single shift.  Chrysler’s chosen version, the 8HP70, weighs 90kg, and features a torque handling limit of 700 Nm.

The Thema will debut on the European market in the fourth quarter of this year with two engine options – the first, expected to account for some 20 per cent of sales, is Chrysler’s new 3.6-litre ‘Pentastar’ petrol V6, developing 292bhp and 353Nm.  However, the lion’s share (80 per cent) of sales are expected to be claimed by the turbodiesel engine option, VM Motori’s 3.0 MultiJet V6.

However, a potential spanner in the works is the potentially high price of the ZF transmission. Sourcing the 8HP70 units directly from ZF for the next two years is likely to result in a significant cost premium, as Chrysler/Lancia have very few other options – a problem recently seen with the U.S. market 500, where Fiat was forced to pay a high price to secure an automatic transmission supply from Aisin, necessary in order to suit that market’s preferences.

Meanwhile, Automotive News Europe has this week reported that Lancia will also offer a hybrid version of the Thema in 2013 – a system which is expected to share components and control software with a similar project currently being developed for the next-generation Maserati Quattroporte.

ZF 8HP70 8-speed automatic transmission

ZF engineers had set the bar high to produce a new benchmark for automatic car transmissions. The second generation of the ZF 6HP 6-speed transmission, which entered production only in 2006, included reaction times faster than human perception, direct engine linkage by early-stage torque converter lock-up, and intelligent, adaptive control software, that almost reads the driver’s intentions from his foot. But the new transmission is capable of even more – it saves (even more) fuel. It also guarantees ultimate driving enjoyment, as well as the variability needed to be able to use future technologies.

Fuel saving

Several innovations have been introduced in the new 8HP to reduce fuel consumption: the completely new transmission concept with four gear sets and only two shift elements open, a higher overall gear ratio, a variable oil pump, new torque converter, and optimised hydraulic and transmission control. A stop-start function is offered as an optional function. Idling at a standstill, the engine automatically switches off. When engaged again, the engine automatically starts up. Even at standstill times of just 10 seconds, switching off the engine cuts average consumption noticeably – and also CO2 emissions.

The start/stop function is enabled by the development of the hydraulic impulse oil storage (HIS). It supplies the hydraulic oil that the transmission’s shift elements need for starting. When the engine is switched off, it allows for a quick start – as it is required with the start/stop function. Already 350 milliseconds after starting the engine, the vehicle is ready for setting off. With the start/stop function of the hydraulic impulse oil storage, it is possible to reduce fuel consumption by another 5 per cent. Compared with the worldwide most efficient 6-speed automatic transmission by ZF, the newly-developed 8-speed automatic transmission saves another 11 per cent.

Benefits of HIS

The hydraulic impulse oil storage makes more complex solutions obsolete, such as a more powerful oil pump in the transmission or an electric hydraulic pump. As a larger-dimensioned transmission oil pump would considerably neutralise the fuel savings in continuous operation, an electric pump is an additional burden on the main power supply, has disadvantages in terms of noise, and leads to a considerably higher integration effort. In contrast, with the hydraulic impulse oil storage, ZF engineers have already considered the lowest possible system costs and easy installation of the unit. Other expensive adaptations of the transmission are not required; after all, with the new development of the transmission, ZF engineers have already considered the integration of the hydraulic impulse oil storage by ensuring a favorable design of the components and their respective routing.

Farewell to conventional gear set concepts

The ZF engineers went on the lookout for a completely new transmission system and discarded all previous established gear set concepts. Armed with a comprehensive list of criteria, the development engineers systematically appraised thousands of possible epicyclic gearing systems and their arrangement variants. The outcome was an 8-speed automatic transmission system, leaving just four simple gear sets and five shift elements.

Lower drag losses and higher gear meshing efficiency

The clever thing, however, is the shift plan. There are only 5 shift elements – multi-disc clutches and brakes in the heart of the transmission – and only 2 are open in each gear. The fewer open shift elements there are, the fewer transmission parts there are rotating relative to one another. On the bottom line, this results in a significant reduction of drag losses in the transmission. The development engineers were also able to increase the gear meshing efficiency with the new transmission concept. Energy is lost in some gears when power is transmitted through gear wheels. In the new automatic transmission, these losses are below 2 per cent throughout – a further factor helping to reduce consumption.

Significantly lower CO2 emissions

The logical consequence of lower fuel consumption is a significant reduction in CO2 emissions – in real terms, this produces a further reduction of 200g/km to 178g/km CO2 emissions in a 3.0-litre 6-cylinder engine – with improved performance.

Faster acceleration

In addition to its positive effect on fuel consumption, the higher overall gear ratio improves the acceleration values of the ZF product. Shorter gear steps, particularly from first to second gear, give vehicles fitted with the 8HP faster acceleration and enable improved shift quality in the lower gears. Two additional gears also helped the ZF engineers achieve an increase in the overall gear ratio, without loading individual gears with excessive torque or speed. This harmonious torque and speed distribution is less harsh on parts and allows reliable operation with fewer ‘hefty’ gear sets.

Precise gear shifting

200 milliseconds is how long the 8HP 8-speed automatic transmission takes to carry out a gear shift – fast and smooth. Even a professional driver can’t shift as fast as this. This gives gear shifting that impressively combines driving pleasure, efficiency, and maximum comfort. Direct gear jumps over two or more gears – including the extreme of shifting from eighth to second – are also possible. In conjunction with the intelligent adaptive shift strategies, this guarantees maximum agility and driving fun.