Allison EP System

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The Allison Hybrid EP System is a two-mode compound split parallel hybrid electric system for diesel vehicles. The system is comprised of a dual pulse interval modulator (DPIM2), Panasonic nickel metal hydride (NiMH) energy storage batteries, a control module, and an electric Drive Unit.

Pre-production began in 2001, with a number of transit agencies trialing buses equipped with the Hybrid EP System, then known as the EP System, and giving their feedback. Allison began commercial production in October 2003. King County Metro in Seattle, Washington was one of the first to order buses with the EP System, taking 213 buses from 2003 into 2004.[1]

Allison announced in December 2009, that for 2010, the EP System for buses and coaches would feature new and improved components for greater reliability, durability and performance.[2]The 2010 Hybrid EP System uses Allison's Fourth Generation Electronic Controls which has greater processing power and memory capability. It also features a the new DPIM2, improved energy storage system (called ESS2), and updated Drive Unit for EPA 2010-compliant engines.

The DPIM2 unit is usually located behind the NiMH batteries on the roof. It converts energy that is transferred between the drive/generator, which uses and produces alternating current (AC), and the batteries, which store direct current (DC). The energy that is stored in the batteries is produced by the generator, which is connected to the diesel engine, and though regenerative braking. The current DPIM2, which has more robust internal hardware and enhanced redundancy for greater reliability, replaced Allison's DPIM in 2010.

The Hybrid EP Drive Unit (originally known as the EV Drive) is an infinitely variable transmission in common with the Allison Bus Series. It features two generator/motor that acts as variable clutches and control the the planetary components.


Type Max. input tower of Drive Unit Max. input torque of Drive Unit Available engines Mass of Drive Unit (dry) Notes
H 40 EP 280 hp (209 kW) 910 lb-ft (1235 Nm) Cummins ISB 280 hp (offered by ElDorado, Gillig, Nova, NFI, and Optare)
Cummins ISL 280 hp (offered by NFI and NABI)
919 lbs (417 kg) Originally known as EP40 System
H 50 EP 330 hp (246 kW) 1050 lb-ft (1420 Nm) Cummins ISL 330 hp (offered by ADL, MCI, NFI, NABI, and Nova)
Caterpillar C9 330 hp (formerly offered by NFI and NABI, no longer offered as of 2010)
919 lbs (417 kg) Originally known as EP50 System


  1. Allison H 40 EP and H 50 EP Specification, 04/2012. Allison Transmission.
  2. 2010 Allison Hybrid EP System,, retrieved on 2010-01-29