Drawing up a specification for a transmission or driveline may be relatively routine for a conventional vehicle, but for an autonomous vehicle (AV), it's anything but routine, claims automotive consultancy executive Lee Sykes.
“The way we design AVs may require an entirely new approach. We have to address the different requirements of progression through the various SAE Levels of autonomy to understand fully each transition from 1 to 5 and their impact on the functional priorities in order to apply appropriate solutions,” explained Sykes, Commercial Director of driveline engineering consultancy, Drive System Design (DSD), which recently opened a new test facility in Michigan to help vehicle manufacturers and their suppliers increase the efficiency of conventional, electric and hybrid drivelines. DSD specializes in the engineering, development, test and control of future transmission and future driveline systems.
Deciding whether to concentrate on Level 5 or the interim autonomy steps will affect how much vehicle architecture will be carried over and how much purpose designed, he said. But he added that a clearer picture emerges when designing for fully autonomous vehicles where the systems can be fully optimized, without the constraint of accommodating legacy systems.
Transmission and driveline design is steered by a number of key attribute targets defined in the specification, including package, durability, weight, efficiency, NVH, performance, safety comfort and cost. Traditionally, some of these are driver demanded but, if there is no driver, the attribute may have no relevance or, at the very least, the relative weightings of the design priorities will change substantially.
The package demands for an AV transmission will be heavily influenced by cabin space requirements; in the extreme case of a “skateboard” platform, the most suitable solution may involve wheel motors, each with integral reduction gearing. Package size is inevitably related to durability and the required duty cycle.
A summoned-on-demand taxi/rental AV could operate 24/7, clocking up a very high mileage and greatly increasing the duty cycle; on the other hand, eliminating the human driver will avoid mechanical (and passenger verbal!) abuse, or frequent demands for peak performance, so reducing its duty cycle, stated Sykes.
Controlling NVH, refinement and noise have long been a concern for transmission and driveline engineers but a connected AV with an electric powertrain brings additional challenges in an otherwise quiet cabin: “And with no manual driver to blame for any sudden irregularities in acceleration deceleration, smooth progress will be expected at all times. Refinement expectations will include vertical as well as horizontal accelerations, so ride comfort will need to be tailored accordingly. Although the majority of aircraft and train passengers accept relatively poor levels of comfort and NVH, we believe AV users will not.”
Minimizing energy drain from the battery pack will always be an AV priority, which in turn drives a need for vigilant weight reduction of all the vehicle’s systems. Architecture decisions will need careful optimization of the transmission in terms of basic mechanical efficiency and actuation losses, together with a systems approach to balance electric motor performance with the need for multi-speed ratios. DSD is working with its customers to map out future requirements and advise on architecture direction.
Sykes added: “Another conventional aspect of performance, the handling or responsiveness of the vehicle, is largely irrelevant if no driver is in control; provided the vehicle has safe and adequate road holding, the suspension can be optimized for ride comfort. Driveline technologies, such as torque vectoring, used on manually driven vehicles to combine safety with responsive handling, may well be replaced by AI algorithms which prevent the vehicle from ‘exploring’ the full dynamics’ envelope in the first place.”
Easy going takes effort, too
At lower speeds, satisfactory performance includes the ability to carry out maneuvers in a controlled yet timely manner, without discomfort to the occupants. Shuttling into a parking space or inching forward in stop-start traffic typically require precise clutch modulation, which will be more challenging for a robot driver with no human intuition: “This has implications for powertrain design; it may be more effective to use motor current control than conventional clutch control. If so, it will be important to evolve the motor design concurrently with the transmission – again a systems approach is necessary.”
While a human driver might become perhaps dangerously distracted by too much data, an AV has much greater bandwidth available for data processing, so can monitor far greater input from sensors while still driving safely.
Sykes stressed: “This enables an AV to investigate and process more detailed information continually, such as powertrain and driveline condition. It also permits more intelligent and lighter design, based on pre-emptive replacement of failing parts, rather than over-engineering for the 99th percentile user. This approach to monitoring in-field life will impact durability specification and have a knock-on effect on package, weight and serviceability. It should also be noted that what constitutes a ‘99th percentile user’ will need a re-think in the world of AVs.”
Sykes believes that in addition to classic engineering questions, the industry also needs to consider the price/ownership models most relevant to an AV; should it be designed as a low cost consumer product for individual ownership or a more robust commercial product, made to different standards as a revenue generator on a rental basis? An appropriate strategy must be adopted for powertrain and driveline longevity compared to the other major vehicle systems, such as battery packs or fuel cells.
Understand duty cycles
“The interactions between the different attributes of an AV mean that the driveline and chassis engineers will still have important roles, but with different priorities to today,” he said. “Instead of maximizing straight line performance and juggling comfort against handling, there will be new challenges and opportunities. The fundamental duty cycles for an AV will require much future thought too.
In addition to these optimization challenges, understanding how the propulsion systems are developed during the transition phases to full autonomy also brings opportunities. What is appropriate for modest levels of autonomy will not be the best solution for SAE Level 5. Clearly there will be plenty for engineers to get their teeth into.”
His words are echoed in the U.S. by Jon Brentnall, President of DSD Inc. speaking at its new test center in Farmington Hills, Michigan: “The current focus on real-world emissions means the efficiency challenge has suddenly become substantially more critical, yet parasitic and other losses are still draining energy unnecessarily.”
DSD’s UK test center has developed highly advanced processes and systems to ensure that transmission and driveline areas that have not previously received sufficient attention can now be investigated for application in the burgeoning EV world: “It is our intention to build similar test capability tailored to the North American market.”
The U.S. facility will initially house a loaded transmission efficiency test rig and will be developed to include three driveline test cells. The current rig, which is fully operational, is suitable for all transmission types, including engine accessory drives, such as supercharger gearboxes. It will largely be used for transmission efficiency testing and the data produced will also ensure that in-house transmission efficiency math models are well correlated.
Further expansion throughout the year will include a hydraulic test stand for hydraulic valve body development and a tilt rig, which provides enhanced lubrication flow analysis capability. “We are delighted to be offering this opportunity for aspiring engineers looking for their next challenge,” underlined Brentnall.
The facility’s first project is the test and development of a full parallel-hybrid transmission for a front-wheel drive application for a North American vehicle manufacturer.
Author: Stuart Birch
Source: SAE Automotive Engineering Magazine