Allison Transmission Holdings Inc. is now offering an option to fully electrify components used with its H 40/50 EP hybrid propulsion system for transit buses and coaches. Co-developed with Vanner Inc., Increased Accessory Power II (IAP II) is a customizable electric distribution platform that provides up to 30 kW of continuous power from the hybrid system to accessories such as electric air conditioning, electric air compressors, and power steering systems.
IAP II includes either a single or dual Hybrid Beltless Alternator (HBA) for up to 600 A-at-idle 24-V dc charging.
“[The platform] is made up of major building blocks that have been brought together as IAP II. The Hybrid Beltless Alternators are solid-state devices that replace the traditional engine-mounted belt-driven alternators and operate at efficiencies of 92-96%,” Chris Collet, Vanner Vice President, Bus and Hybrid Markets, shared with Automotive Engineering.
HBAs have been in commercial production since 2010 and more than 1400 are operating with Allison hybrids, according to Collet.
"Allison's standard H 40/50 EP system typically improves fuel economy up to 25% over standard diesel buses," Michael G. Headly, Senior Vice President of Global Marketing, Sales and Service for Allison Transmission, said in a statement announcing the new electrification option. "When using a Vanner HBA with our system, a number of transit agencies in the U.S. have reported an additional average fuel-economy improvement of 16%, for a total improvement up to 41% or more."
Other demonstrated benefits of IAP II, according to Allison Transmission, are that it allows accessories to operate at the most efficient speeds, improving energy management by reducing parasitic loads; increases brake responsiveness by building air brake pressure faster than a conventional engine-driven pump; improves low-speed vehicle maneuverability due to smoother power steering; and enables faster cabin cooling, without straining the engine or burning fuel by running at high idle.
IAP II builds on Allison's parallel hybrid propulsion system and its release with the Vanner HBA and IAP I. As with IAP I, the second-generation platform uses a high-voltage distribution module (HVDM) for intelligent control of high-voltage electrical power and an HBA (or dual HBAs) for dc to dc conversion of high voltage to 24 V dc.
“The HVDM acts as a smart high-voltage grid for the Allison H 40/50 EP that controls and channels the export power...from the hybrid to electrically powered accessories,” Collet explained. Development of the HVDM began in December 2011 with testing beginning in March 2012 and Death Valley hot-weather testing in summer of 2012.
“The HVDM was the enabler to power fully electric air conditioning from the hybrid system and future full electrification of accessories,” he said.
IAP II adds a water-cooled Vanner exportable power inverter (VEPI)—“the final building block,” said Collet—that converts high-voltage dc to high-voltage ac that can power accessory systems, while eliminating the cost and need for the A/C system's integral inverter. The VEPI produces 230 V ac 3-phase for full bus electrification.
The VEPI is in commercial production and was part of the overall validation of IAP II. Transit agencies are now transitioning to IAP II in their specifications, Collet said.
"Full electrification of a hybrid bus is the single most advantageous option a transit authority can realize that will improve fuel economy, reduce emissions, and lower maintenance costs," claims Steve Funk, President of Vanner Inc.
The IAP II system is available in standard and articulated bus applications. All components are delivered to OEMs on a pre-assembled and pre-wired rack. The racked solution allows customers to specify only those electrified components that they demand, and offers better troubleshooting and easier maintenance than a boxed or individually sourced solution, according to Allison Transmission.
Vanner will coordinate warranties for the IAP II racked components.
“An advantage over a fully electrified hybrid vs. other electric vehicles is that the hybrid is its own electrical generation power plant. It is not dependent on a daily charging cycle to keep it on the road,” explained Collet. “An IAP II-equipped hybrid bus continues to use the best of both worlds of mechanical and electric efficiencies.”