CAFE review drives new approach to meeting economy targets

18th June 2015
Siobhan O'Gorman

Automotive engineering consultancy, Drive System Design (DSD), believes the 2017 mid-term review of future CAFE (Corporate Average Fuel Economy) targets is already stimulating demand in the US for greater efficiency in transmissions and drivelines. The company’s US office reports growing interest from both OEMs and Tier1 suppliers, as incremental improvements in engine efficiency become harder to achieve and manufacturers look more widely for gains in fuel economy.

“Such is the demand that we are currently recruiting and expect to double our headcount in Detroit by the year end and increase our facility footprint by 50%,” said Shaun Mepham, President, North America, DSD. “As CAFE regulations become increasingly challenging, our customers are seeking better efficiency from the entire driveline. We are helping them to achieve that without compromising durability, refinement or manufacturing costs.”

Transmission efficiency is generally quoted at maximum power which hides the true effect at part throttle and lower speeds, where inefficiencies in the transmission and driveline become much more significant for both ‘cycle’ and ‘real world’ fuel consumption. The predominance of speed limits and urban traffic means losses in the speed range between 10 and 50m/h have the greatest influence on fuel economy for most drivers.

“To make the right design choices requires a complete understanding of the part-throttle efficiencies of the entire system,” commented Mepham. “We do this through building detailed in house simulation models of each sub-system to generate a complete picture, including the effects of manufacturing tolerances. Optimising for high production volumes is our primary goal, in order to generate the maximum impact on CAFE figures.”

DSD’s specialist skills include optimising production macro and micro gear geometry for highest efficiency with best refinement, and the management of internal lubrication to minimise churning losses.

Mepham believes that being focused on these key areas of transmission and driveline expertise, instead of diversifying into wider powertrain consultancy, has been a key reason for the company’s growth.

“The challenge faced by the industry is to improve transmission and driveline efficiency without introducing new issues, such as unacceptable noise or vibration,” said Mepham. “When higher efficiency is achieved through the use of lighter structures; new materials with less inherent damping; or hybrid electric powertrains with lower background noise levels, it becomes more difficult to ensure good refinement.”

DSD has specialised in the areas of NVH and efficiency optimisation since its founding in the UK in 2007 and has designed ultra-quiet gears for three of the world’s premium EVs. The company presented two technical papers at the CTI Symposium at Novi, MI. The first showed how casing NVH can be optimised even at first phase prototype level, by using proven analysis techniques. The second looked at compact architectures using multiple 48V e-drives to create a low cost 30-45kW full hybrid system.

“Our ability to include magnetic pole passing as a housing excitation within the analysis enables us to simulate the refinement of the complete system with confidence,” commented Mepham. “In the case of an EV driveline, the motor introduces different magnitudes of excitation to those encountered normally and it is important to fully understand their effects. The NVH simulation methods for gears and electrical machines developed in house at DSD are probably the most effective in commercial use today, anywhere in our industry.”

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