Q&A with BorgWarner: Dual clutch transmissions
Alexander Moser, BorgWarner
Europe may have long resisted the automatic gearbox but environmental and fuel efficiency legislation combined with major advances in electronics and manufacturing techniques have paved the way for a flood of new automated transmission technologies, including dual clutch transmissions (DCTs). In this interview, Matthew Beecham talked with Alexander Moser, Supervisor Advanced Product Engineering, Clutch Systems, BorgWarner about just how the DCT market is shaping-up.
As we understand it, vehicle manufacturers are recognising that the diversity of gearbox technologies gives them opportunities to map the feel of transmissions to their brand. What’s your view?
One of the important benefits of DCT technology is the possibility to adjust the software according to the individual requirements of an OEM. Transmission manufacturers can offer the same transmission hardware with different shifting characteristics to diverse customers.
With electric vehicles still at an embryonic stage, I guess fuel-efficient transmissions such as DCTs are likely to enjoy a greater market share. Is that right? How do you see this technology evolving?
The increasing living standard in many parts of the world also leads to a growing demand for a comfortable driving experience, resulting in more customers seeking the driving experience of an automated transmission. Depending on the engine, a DCT has the same or even better fuel efficiency as a manual transmission. The driver can enjoy a comfortable driving and let the transmission do all shifts or switch to manual mode and select the gears via paddles at the steering wheel. Therefore, we see an increasing market share for DCTs within the next [few] years.
Furthermore, because DCTs can be produced in existing plants using the equipment required for production of manual transmissions, more transmission manufacturers, especially in China, will start to produce DCTs near term. DCTs are also an excellent base for the increasing number of stop-start systems because no additional oil accumulator or additional pump is required. As far as electric vehicles are concerned, it is just a matter of time until they also realise a need for an efficient transmission to enhance the driving performance.
DCT is still an optional extra and expensive to fit. I guess until one of the automakers makes it standard, we shall not see a leap in installation rates. What are your market fitment expectations for DCT in Europe by, say, 2017?
We will definitely see a rise in installation rates of DCTs within the next [few] years but due to its costs advantages the manual transmission will be the dominating one also in the near future.
Globally, the number of dual-clutch transmissions in the market is expected to more than double over the next five years to approximately 9 million units by 2017. In Europe, the rise in DCTs is expected to move from 2.5 million units in 2012 to 4.7 million units in 2017.
Nevertheless, stringent future emissions regulations and ever rising fuel costs might help automatic transmissions, including the DCT, to a greater market share in the near future, even in the low cost vehicle segment. This is because of the major benefit of an automatic transmission that the engineer has the possibility to adjust the shift points to achieve the best fuel consumption. Also, current automatic transmissions offer up to 8 speeds while the majority of manuals only have 5 or 6 speeds. It seems likely that automatic transmissions will eventually take market share away from manual transmissions.
As I understand it, a number of automakers have introduced DCTs in their fleet, including VW, BMW, Audi and Ford. When these automakers choose a DCT, what factors do they consider (i.e. in terms of torque, driver comfort and performance)?
In the future, we anticipate increases in performance and comfort expectations in all segments. A DCT offers a good performance, comfort and the flexibility to integrate with a hybrid or electric powertrain. The wet dual clutch technology combined with hydraulic actuation and supplied by an on-demand electric pump is the optimum solution to satisfy customers in all segments.
I’ve heard that DCTs require a blend of additives which must be ‘intelligent’ and predictable. In other words, they have to provide reliable characteristics and adapt quickly to the specific friction demands required for the clutch operation to function efficiently. Is that correct? How important are those additives to make the DCT work?
A wet dual clutch is arguably the most demanding automotive application for a wet oil-bath friction material. The clutch is required to launch the vehicle from rest without shudder and execute shock-free shifts in a few hundred milliseconds. Friction coefficients must remain stable and predictable from cold starts to maximum operating temperature. The oil must endure rapid and extreme temperature excursions and transfer heat away from the clutch elements as quickly as possible.
In summary, the additive package for a dual clutch transmission fluid plays a critical role in achieving maximum performance from a DCT.
What are the main improvements in the technology used to develop clutches and transmission fluids?
Optimised friction material and controlling clutch pressure accurately allows better micro slip of the clutches. Operating the system at the micro slip level allows the overall system pressure to be lowered. This lowering of the system pressure then improves fuel economy. With regard to the friction material, for optimum results, wet friction elements need to withstand the higher temperatures, higher torque and limited oil flow inside modern clutch designs. We are applying our expertise in solenoids and hydraulics to the next generation of electric, on-demand control systems, suitable for both wet and dry dual clutch applications. We optimise our solenoids to reduce leakage and parasitic losses. As a specialist with many years of experience in the field of transmission friction technology, we are continuously optimising our friction material and groove design.
From your perspective, how is DCT being promoted to the consumer (i.e. is it promoted as a comfort feature or is there more emphasis on its sporty and responsive performance)?
Both. One advantage with a DCT is to be able to tailor the drive feel. The transmission can be setup depending on the customer and market to be a comfort feature as well as provide sporty and responsive driving. A DCT features cost improvement in combination with high-speed resistance, high thermal robustness, high comfort and no torque limitation. This makes the wet dual clutch transmission attractive for all vehicle segments and provides benefits for all driver types.
In future, I guess we could see a variety of DCT systems. What will tomorrow’s DCTs offer and on which segment (i.e. in terms of torque, driver comfort and performance)?
With a DCT, drivers benefit from no perceptible power loss during gear changes and the advantage of lower emissions. In addition, the dual clutch transmission offers high potential for application with electric vehicles and hybrid electric vehicles. The strength of a DCT to be able to shift without any torque interrupt would make it interesting for commercial applications as well. Furthermore, as stop-start systems are becoming more and more mainstream and are available in combination with almost all DCTs on the market and further improve fuel efficiency.
Why is a DCT well suited for application with EVs and HEVs and how does it differ from a DCT for an internal combustion engine?
In hybrid applications, the benefit of the DCT is that it does not have a torque converter. So, during the re-start you can contain the pump oil flow allowing for a quick pump prime. In EV applications, the DCT provides high efficiency and low parasitic losses allowing for smooth shifts without torque interruption. What is new or different with a DCT for EVs and HEVs is the emphasis placed on the electronic communications needed to match the electric machine to torque and speed requirements during shift events. The transmission enables more robust shifts and control strategies while the clutch sizing and cooling requirements are lower.
With eGearDrive, BorgWarner already offers a multispeed electric drive transmission. What are the benefits of a multispeed transmission in an EV?
In general, the multispeed transmission enables improved launch performance, gradeability and increases maximum vehicle speed. The battery savings can really only be quantified at the vehicle level by the OEM. However, transmission efficiency is a key enabler which can help optimise battery range and cost. The multispeed transmission also allows the OEM the opportunity to operate the motor and the generator in a more efficient range.
What are the engineering and technical challenges that are involved in developing a multispeed transmission for EVs?
Collaboration is a key competence in being able to successfully develop a multispeed transmission. The challenge is to perfectly match a transmission shift event with the electric motor controlling torque management and speed. The shift transient needs to be seamless between the transmission clutches and the electric motor torque control. However, shift feel can be replicated if desired, but the usual expectation for shift changes with EVs is to be seamless, quiet, and undetectable by the driver. As compared to an IC Engine, an electric motor has much higher rotational speeds. Thus, the transmission should be robust and efficient to handle high rotational speeds and high ratio steps between two gears. Wet dual clutch transmissions are excellent to answer this challenge.
What have you finally learnt from your EV applications and demonstrator vehicles so far? What still needs to be done to refine and optimise these systems?
The remainder of this interview is available on just-auto's QUBE research service