Transmission Systems Focussed On Enhanced Efficiency, Comfort

Technology November 2019 Transmission Systems Focussed Enhanced Efficiency Comfort Convenience Hybridisation

There has been a growing trend of transmissions offering higher comfort and convenience, along with added advantages of fuel economy and hybridisation

Over the last two decades, there has been tremendous progress around automotive transmission technologies. This space has been constantly witnessing an increase in the number of gears and variation in ratios aimed at offering more efficient and improved transfer of power to the wheels from the engine. The transmission systems have also improved substantially with regards to enhancing the overall fuel efficiency of a vehicle as well as in enabling multiple driving modes in modern day vehicles.

While the two-wheeler segment may have experienced the least amount of traction, in terms of new transmission technologies, the passenger and commercial vehicle segments have taken giant leaps in innovation and technology development. The four-wheeler segment, powered by internal combustion engines (ICE), have witnessed the most amount of transition, in terms of transmission technologies from rudimentary manual gearboxes to various types of semi-automated and fully-automatic transmissions (AT). However, as the future of mobility is expected to transition towards electrification, transmissions are also being developed specifically for such powertrains.

PASSENGER VEHICLES

Powertrains in the future will become more environmentally-friendly, while maintaining vehicle dynamics and supporting active safety systems. The powertrain includes the engine as well as the transmission, and so the gearbox of the future will have to become smarter to deliver higher efficiency, while retaining the joy of driving. Within the transmission system, there has been an increase in the number of gears and the band of their ratios as well as in the types of clutch systems supporting these advanced gears.

The passenger vehicle space largely comprises either front-wheel or rear-wheel, two-wheel drive models apart from certain SUVs and high-performance vehicles offering 4x4 or all-wheel drive (AWD) systems. On a global scale, we see that two-wheel drive vehicle models are mostly offered with ATs that either feature torque converter or continuously-variable (CVT) technologies. It must be noted that over time the traditional AT has been continuously developed to offer the driver a good mix of fuel economy and driving satisfaction. Meanwhile, less-developed markets like India continue to feature manual transmissions (MT) in the mainstream, with innovations in the form of automated manual transmissions (AMT). AMTs are preferred over ATs since their cost is almost on par with manual gearboxes, offer no loss in fuel economy and deliver the convenience of automatics.

The main factor that has driven the improved performance of ATs is the improvement of electronics in these systems. Numerous electronic control units now monitor the transmission in accordance with the vehicle’s driving pattern and make relevant adjustments to provide enhanced driver feedback. The modularity of new-age transmissions enables them to be used in multiple vehicle models, with the capability of being tuned according to the power delivery expected and efficiency required. This theory holds good for AMTs as well since an AMT in all reality is a manual gearbox that is equipped with an electronic actuator that monitors the driving behaviour and shifts between gears. This feature now provides price-sensitive markets with the option of a convenient transmission technology with features more akin to MTs.

Additionally, dual clutch transmission (DCT) is gaining prominence and is emerging as the go-to technology for vehicles aiming to offer drivers an engaging drive. DCTs are said to combine the most important benefits of MTs and ATs – higher efficiency and power shifts, respectively. The lack of a torque converter requires DCTs to have a higher first gear ratio to launch the vehicle, thereby providing a more thrilling driving experience. The dual-clutch module divides between odd gears and even gears, ensuring friction losses occur from one clutch at a time. This system is mainly applied for front-wheel drive vehicles, but requires more number of components to achieve increased speeds, making it difficult in vehicles with limited space in the engine compartment.

While the above factors hold well for vehicles used for passenger commute, the role of transmission systems for performance and sports vehicles does not follow the same pattern. A number of sports cars are offered with manual gearboxes, so as to give the complete control of the vehicle to the driver and make the experience more mechanical in nature. At the same time, a larger volume of performance vehicles features ATs that can be controlled manually as well, providing the driver with the same sense of control as that of a MT.

Accurate designing and manufacturing processes for new-age transmissions are key towards bringing out their performance, reliability and convenience levels

COMMERCIAL VEHICLES

The commercial vehicle space has been the most prominent segment to feature AMTs in recent times. These classes of vehicles can be bifurcated into buses and trucks, and the former sector has been known to feature technologies similar to passenger vehicles, since buses carry passengers. The bus segment has transmissions of all types across the globe. Developing markets still feature MT for buses, with AMT creeping into such markets at a fast pace. More advanced countries in Europe, North America and the Asia-Pacific region have AT as a standard on buses.

The case is not the same with trucks, whatever be their size and capacity. Trucks across various categories such as road haulage, off-road, mining, construction, etc. have specific driving patterns, for which each requires a specific type of transmission. Trucks plying on roads generally follow trends of buses as far as transmission systems are concerned. However, more specialised models like tippers in mines are put through continuous running for around 20 hr in a day. These trucks generally run at low speeds over extremely tough terrain, while almost always featuring full payloads. AMTs are the most-commonly found transmission systems on such off-road commercial vehicles, due to their mentioned benefits.

Trucks also feature ATs, with the most common being six and eight-speed transmissions. Functions like creeping prove very useful for such vehicles, since they generally carry heavy loads and need to be driven along different types of environments to transport the goods being carried. ATs also find place in commercial vehicles, since they are mostly driven in environments that lead them to carry a specific speed without making too many stops. This ensures a fairly good level of fuel efficiency, which is usually not the case with passenger vehicles with AT.

ELECTRIFICATION & AUTONOMY

The eventual future of mobility is set to move towards alternative fuels, with electrification emerging as the top contender on a mass scale. The transition from ICE vehicles to pure EVs may be achieved along different timelines across various geographies, but the transitioning period will witness a large number of hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs) in the mix. In the case of HEVs and PHEVs, the electric motor can be integrated into conventional ATs, DCTs and CVTs by adding a hybrid module. These add-on solutions make it possible to share most parts between conventional and hybrid transmissions, thereby facilitating modular design and manufacturing. On the other hand, the development of dedicated hybrid transmissions is also taking place. These transmissions generally feature lower mechanical complexities than that of conventional multi-speed transmissions.

Pure EVs at present mostly feature single-speed gearboxes due to the torque and efficiency characteristics of the motor. Work is being carried out for the development of multi-speed transmission systems for EVs, which could lead to reduced motor peak torque and lower energy consumption through optimised motor operating points. While most companies have developed two or three-speed EV transmissions, there has also been speculation over the development of four-speed electric transmissions.

EV transmissions generally tend to be fully-integrated with the motor and power electronics, in order to improve efficiency and reduce weight. It leads to increased electronics content and lesser mechanical systems, which further enhance the efficiency of these transmissions, while resulting in low friction and enhanced refinement. Another type of technology nearing mainstream production in the area of EVs is that of in-wheel motors, which have the components transferring the power inside the wheel itself. This technology is in a nascent stage, and is expected to create increased space for cabin and interiors, since there would be minimal intrusion from axle and transmission components.

The ultimate transition of mobility will be towards attaining autonomy, and numerous experts believe that electrification and autonomous driving synchronise together effectively. Further, dedicated transmissions for electric motors and new-age technologies like in-wheel motors provide additional space for vehicle interior design, which can be utilised efficiently to cater to the needs of occupants in ensuring the cabin is used effectively. Fully-autonomous vehicles will not require conventional forms of seating, since there is no requirement for a driver, and this can also aid in the adoption of shared mobility. Therefore, seating can be made to offer vehicle occupants a place to meet and move together without conforming to traditional seating applications.

CONVENIENCE, REFINEMENT

Transmission technologies are not limited to addressing features of efficiency, driving dynamics and safety systems, but are also integral in achieving levels of refinement related to noise, vibration and harshness (NVH). The level of NVH in a vehicle is mainly a result of the friction caused by a number of factors. Friction caused by gears travel through various airborne and structure-borne paths to eventually reach passengers, causing discomfort. Some of the noises arising out of vehicle transmissions are whining, rattling, clunking, shifting and bearing noises.

A method to reduce the transmission rattle in ICE engines is by equipping a wide-angle dual-mass flywheel. This splits the mass of the flywheel into two using springs that also helps in reducing the amount of noise arising from the gears in the crank. In the case of EVs, the transmission is usually single-speed, which has less inherent friction and lower NVH levels than traditional MTs or multi-speed ATs. This feature of refinement of EV transmissions is required especially since EVs are far quieter than ICE vehicles since their motors are also highly-efficient, while remaining very quiet.

Off late, there has also been an increase in the joint development of transmission systems by OEMs and suppliers, with many top names collaborating. Such partnerships enable the development of transmission systems that effectively address customer requirements in a timely manner. They also help considerably cut overall vehicle development times, leading to newer products reaching markets more quickly than before. All these developments will enable companies to offer transmission technologies, and vehicles that bring improved efficiency and driveability, while also improving the level of convenience offered to drivers. This especially holds good since fully-autonomous vehicles are still some time away, and until then vehicle will continue to be driven by humans.

(Inputs from SpringerLink, Engine Technology International, M&M)

TEXT: Naveen Arul