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have provided a local driver to change due to their detrimental impacts on human health [3–5]. In 2008, it was estimated over 4000 deaths were brought forward as a result of long-term exposure to particulates in London [6]. In order to combat these concerns, many cities have introduced measures such as the ‘low emission zone’ in London and emission control targets [7]. London is to introduce the first ultra-low emissions zone (ULEZ) in 2020, which, amongst other targets will aim to replace conventional diesel powered buses with low emissions alter- natives [8, 9]. Despite this drive for change, it is evident that finding a replacement for diesel buses is not simple. In addition to the low cost, simplicity, reliability and maturity of the tech- nology, diesels also offer excellent characteristics to meet the required power demands and operational needs of city buses. It can be seen from Figure 1, the diesel engine that is a type of internal combustion engine (ICE) provides high output powers and uses energy dense fuel making them ideal for both the range and operating times expected of city buses and also for meeting the high transient power requirements during acceleration. In order to address the environmental concerns posed by diesel buses, a number of technolo- gies are being investigated and implemented. The most widespread of these are diesel-hybrid buses, which make use of an on-board energy storage system to effectively recycle captured kinetic energy obtained through regenerative braking. Although hybrid buses are capable of significantly reducing fuel consumption, they are still reliant on diesel as the primary fuel source and hence do not address the fundamental problems associated with emission that come from using diesel as a fuel. As such, there has in recent years been an increased focus on the development of zero emissions buses, with two main competing technologies. These are battery electric buses and hydrogen fuel cell (FC), both of which exhibit zero operat- ing emissions, hence eliminates the environmental and health issues associated with diesel buses [11]. Such technology solutions are less mature and result in significant changes to the Figure 1. Comparison of various technologies for the power and energy densities (based on Ref. [10]). Hybrid Electric Vehicles34