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In contrast to these benefits, the hybridisation of the drive system has a number of drawbacks. These predominately amount to the additional capital cost, where a diesel hybrid typically costs £300,000, this is £110,000 more than a conventional diesel bus and constitutes an increase of about 50% [23]. The additional complexity of both the drive system and its control results in additional maintenance time and cost, where a diesel hybrid typically requires 50% more maintenance time than a conventional diesel bus [22]. 2.2. Case study 1: TFL Initially a trial consisting of eight diesel hybrid buses was carried out in 2007 and was found to have very high (96%) customer support [24]. After analysing the trial, the official deployment of diesel hybrid buses began in central London. The number of diesel hybrid buses has steadily increased, where in 2015, more than 1200 diesel hybrid buses were in operation in London, as can be seen in Figure 4, and exceed the target of 1700 in 2016 [12]. This consists of old buses redesigned for hybrid operation and new designs such as the new Routemaster. The impact of the deployment of the low emission bus fleets has already begun to have an impact on emissions in London, as shown in Figure 5. In the last few years, emissions of NOx and CO2 have begun to drop due to the introduction of diesel hybrid buses into the TfL fleet and the retrofitting of selective catalytic reduction measures into the existing fleet. The level of PM100 emissions dropped considerably due to the introduction of PM filters in the early 2000s. It is expected that these will continue to drop as further deployment of diesel hybrid and zero emissions vehicles continues. The performance of the diesel hybrid bus fleet in London is very variable, as might be expected due to differing models and routes. It was claimed that the average Euro V bus 0 500 1000 1500 2000 2500 0 4 8 12 16 20 2010201120122013201420152016 Percentage of hybrid bus Diesel hybrid bus total number Figure 4. Total number and percentage of the TfL bus fleet of diesel-hybrid buses in operation. Data from Ref. [12]. Hybrid Electric Vehicles38