2010s, bus operators retrofitted older buses to improve fuel economy, reduce air pollution and meet emission standards. A three-year £86m project to improve 5,000 buses to Euro VI emission standards was completed in 2021. A Kinetic energy recovery system using a carbon fibre flywheel, originally developed for the Williams Formula One racing team, has been modified for retrofitting to existing double-decker buses. 500 buses from the Go-Ahead Group will be fitted with this technology from 2014 to 2016, anticipated to improve fuel efficiency by approximately 20%. The team who developed the technology were awarded the Dewar Trophy of the Royal Automobile Club in 2015. New Routemaster Programme The New Routemaster double-decker was specified and constructed to a hybrid design. The bus was designed to be 40% more fuel-efficient than conventional diesel buses, and 15% more than London hybrid buses already in operation, reducing nitrogen oxide emissions by 40% and particulate matter by 33% compared with diesel buses. The first eight vehicles entered service with Arriva London on route 38 in February 2012. By 2018, a total of 1,000 New Routemasters were in service. However, the buses have suffered from problems with their battery systems with some operating solely as diesel vehicles, and in total 200 buses will have power units replaced under warranty. Introduction of battery electric buses In December 2013, the first battery electric buses entered service in London as a trial on routes 521 and 507, using BYD Auto buses built in China. BYD estimated that the cost savings could be up to 75%, owing to the cost of electricity compared to diesel fuel. The trial was successful, and further orders for battery electric buses followed. In 2015, the world's first battery electric double-decker bus entered service on route 98. The first routes in London solely served by battery electric single decker buses were routes 521 and 507 in 2016. The first route in London solely served by electric double-decker buses was route 43 in 2019. In 2021, it was announced that all buses in the fleet meet or exceed Euro VI emission standards, following the phasing out of older buses, the retrofitting of diesel vehicles and the introduction of new hybrid & electric buses. By March 2022, over 700 electric buses were in service. In October 2022, rapid charging using a pantograph was introduced for battery electric buses on route 132, the first use of this technology