The Alexander Dennis Enviro500 (previously known as the TransBus Enviro500) is a three-axle double-decker bus built by Alexander Dennis (formerly by TransBus) in the United Kingdom. It was unveiled in 2002 and is one of the Enviro-series bus models made by TransBus/Alexander Dennis. The bus was sold on diesel or hybrid-electric powertrains and on Volvo chassis as a bodywork. In 2012, the Enviro500 was superseded by the new Alexander Dennis Enviro500 MMC. Design The Dennis chassis for Enviro500 (also known as Trident E500) was modified from the Trident 3. It can be fitted with Cummins ISMe335 Euro III engine (later Cummins ISLe340 Euro IV/Euro V engine which uses AdBlue to reduce emission of nitrogen oxides through selective catalytic reduction process), or ISM-330 engine for the buses delivered to North America, and coupled to Voith DIWA864.3E 4-speed or ZF 5HP602 5-speed gearbox. Later Voith DIWA864.5 4-speed gearbox, ZF 6HP602 6-speed gearbox and Allison B500R 6-speed gearbox became available and gradually became the standard configurations. Initially only the version of the Enviro500 was built, but in 2007 Alexander Dennis announced the production of the version, and an 11.3-metre-long variant followed in 2008. Later Alexander Dennis developed the hybrid electric version of Enviro500, known as the Enviro500H, with Allison's parallel hybrid drive system; it was unveiled in late 2008. The body of the Enviro500 was based on the original style of the single-deck Enviro300, and the upper-deck frontal design was similar to the design used on the Plaxton President body. Orders Asia Hong Kong The vehicle exceeded some design limitations in Hong Kong, the first city where the model was used, prompting the local Transport Department to make some exclusions in order to make the buses available to KMB on time. The width of the bus is exceeding the limit of . The width of was approved by the European Union, becoming the current standard. A fixed glass replaced the emergency door at the rear of the upper deck, but with more hammers available for emergency. The front indicators are the smallest of all current models of Kowloon Motor Bus. The facelifted version of Enviro500 body, first built in late 2003, have revised frontal and rear designs. This version of Enviro500 bodywork was also available with Volvo B9TL chassis. Kowloon Motor Bus The Enviro500 is the first model of second-generation low-floor buses for Kowloon Motor Bus, with straight staircase and plug door (only