Originally developed for motorsport applications, dual clutch transmissions have been utilised in road vehicles as diverse as the VW Golf and the Bugatti Veyron. One of the first commercial vehicles to adopt the dual clutch technology has been Fuso which developed the Duonic transmission for selected Fuso Canter light truck models. Transmission manufacturer ZF has a dual clutch version of its TraXon truck transmission but Volvo has been the first manufacturer to offer the dual clutch setup in its trucks in Australia and New Zealand.
It’s important to acknowledge the distinction between a twin clutch and a dual clutch driveline. Twin plate clutches involve two friction discs sandwiched between the clutch pressure plate surface and the engine’s flywheel, with a floater plate in the middle. The intention is to spread the torque load over a larger surface area and such arrangements have been employed for decades in vehicles ranging from heavy trucks to performance cars. A dual clutch setup, however, is much more than what it says – not only does it involve two independent clutch assemblies but a significant re-engineering of the gearbox as well.
In the Volvo dual clutch I-Shift, the two dry clutches operate independently of each other and each drives one of the concentric input shafts of the 12-speed transmission. Unlike the conventional I-Shift, the dual clutch version is essentially two parallel gearboxes in the one transmission case. One clutch connects to the odd numbered gears while the other is used when engaging the even numbered gears. The basic difference in function of the dual-clutch transmission is that it pre-selects the next required gear while still driving in the current one. The fundamental benefit of this arrangement is that while one clutch and gear is engaged the next gear in the sequence is already selected and ready to accept the driving forces. As one clutch disengages, the other engages the next gear and the process is repeated sequentially either up or down the ratios.
Gear shifting occurs in a fraction of a second without interrupting the power delivery and the result is an almost seamless delivery of torque as the driveline between the engine and the tail shaft is constantly connected or ‘locked up’. The only exception is on the shift between sixth and seventh gears as this is the ‘range change’ and a conventional shift is required.
The ‘standard’ Volvo 12-speed I-Shift is noted as already one of the smoothest automated manual transmissions on the market and in recent years has become available with crawler and ultra-low crawler ratios and this latest dual clutch innovation brings a new dimension to the transmission’s ability to extract the maximum efficiency from the engine and deliver tangible benefits to the operator.
We had the privilege of experiencing a Volvo FM equipped with the dual clutch I-Shift transmission in Sweden just over 12 months ago and again in a Volvo FH at Queensland’s Mt Cotton truck testing facility several months later. After being impressed with the performance of the transmission on the closed test tracks we were presented with the opportunity to take a relatively extended drive on Australian roads with a realistic gross vehicle weight.
The inherent design of the dual clutch I-Shift with its concentric twin input shafts dictates that its maximum torque capability is 2,800Nm which results in it being limited to 13-litre applications and the dual clutch I-Shift is not currently available with Volvo’s larger 16-litre engine. The Volvo FH prime mover for this test is powered by a 13 litre engine Euro 5 engine rated at 540HP. The dual clutch transmission is also available with the same engine rated at 500HP. The dual clutch version of the Volvo I-Shift adds 100kgs to the truck’s weight and the overall transmission is 120mm longer than a standard version.
For this test drive the trailer set is ballasted to give us a GVM of 55 tonnes and our total trip length of 363 kilometres on various Queensland roads includes travelling up the Toowoomba Range and descending Cunningham’s Gap. This particular unit has a 0.78:1 overdrive ratio for its top gear combined with a 3.4: 1 final drive ratio. At the conclusion of our trip the electronics show an average consumption of 61.7 litres per 100 kilometres which translates to 1.62 kilometres per litre. As our standard assessment drive practice is to not consciously aim for the most efficient consumption, preferring to put each truck ‘through its paces’, this is an impressive result considering the load, terrain and traffic conditions.
This transmission always starts in first gear and on the flat the difference in shift quality is immediately noticeable even when automated skip shifting is possible under reduced throttle application when rolling with the traffic. It is when we start pulling up the slope of the Gateway Bridge and the transmission begins upshifting under more load the effect of the power shifts is astounding. As we travel further into the countryside and become more familiar with the box’s characteristics that we can take advantage of the constant delivery of torque to the drive wheels and can confidently change gears in the middle of a bend without upsetting the stability of the vehicle. Because the power is transferred continually to the wheels the risk of getting stuck or losing traction in slippery conditions is also greatly reduced. The almost constant torque combines with high turbo pressures so engine revs can be kept at lower levels which contributes to reducing fuel consumption and noise.
As with a regular I-Shift this AMT generally performs best in its own ‘auto’ mode and for circumstances where driver intervention is required there are up and down override buttons on side of the shifter. There is also a ‘limp’ button on back of shifter which can be used to drive the truck to a safe parking situation should the electronic transmission management have an issue. Typical of Volvo to include this fail-safe feature even though it is unlikely ever to be required. Due to external factors beyond our control and with little warning we are reluctantly forced to stop on the steep grade just above the ‘saddle’ on the uphill section of the highway on the Toowoomba Range. Ordinarily this would have proved to have been somewhat of a minor challenge in a traditional three-pedal truck equipped with a crash box but by sticking to the correct protocols we hardly raise a sweat. And neither does the Volvo.
It’s just a matter of selecting the hill hold function to hold the brakes, manually selecting first gear using the buttons on the shifter and then simply pushing down on the accelerator. The driveline smoothly delivers full torque to the drive wheels and once we start moving we can quickly manually click into second gear and then progressively go up through the ratios to sixth gear with no interruption to the truck’s momentum thanks to the dual clutches. We ease off the throttle slightly and continue to manually hold sixth gear because, although the Volvo seems more than willing to keep upshifting, the momentarily longer ‘range change’ between sixth and seventh gears may affect the momentum and lug the engine below 1,000 rpm. We probably could have just as easily started off in ‘drive’ and let the electronic transmission management take over the up-changes, but this is a good opportunity to experience the smoothness of the transmission when using the manual control.
Returning to Brisbane via the steep descent through Cunningham’s Gap we are able to take full advantage of the three stage engine brake and as the transmission automatically downshifts to boost the effect of the retardation provided by the engine those shifts are as instantaneous as the upshifts.
The dual clutch is particularly suitable for applications that involve sensitive freight including livestock and will be a definite advantage in carrying liquids by reducing the ‘slosh’ effect due to the almost imperceptible gear changes. The additional stability will also be favoured by carriers moving loads with high centres of gravity.
The advantages of the dual clutch I-Shift are many. Driver comfort and load stability are obvious, as is improved fuel efficiency in most applications. Long term maintenance costs are kept in check as well with transmission oil changes stipulated at 450,000km and as each clutch is only performing half of the shifts the service life of those components are extended significantly as well.
The ‘I’ in I-Shift probably stands for ‘Intelligent’ but in the case of the dual clutch derivative of Volvo’s automated manual transmission it can just as easily stand for ‘Instant’.
Fast Fact
The dual clutch I-Shift is less expensive than a full automatic with similar torque capabilities and is without the disadvantages of excess weight and heat generation.
Fast Fact
Fuel consumption on flat terrain will most likely be similar to that obtained in the same circumstances as the traditional I-Shift. Volvo say that fuel consumption benefits of between three and five per cent should be easily realised when travelling on undulating roads.
Fast Fact
Globally the dual clutch I-Shift transmission has been quickly accepted by the markets with Volvo reporting around 4,000 of its 13-litre trucks being ordered during 2018 with the transmission.