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achieved a fuel economy of 32.9 l/100 km in London [9]. The reported fuel economy of diesel hybrid buses operating in London is presented in Table 1. As may be expected, the type of bus and bus route significantly affects the fuel economy, where a single decker bus generally exhibits a higher fuel economy than a double-decker bus. It was found that the introduction of diesel hybrid technology improved the fuel economy on nearly all routes; however, there were a couple of discrepancies to this, such as on the E8 bus route where the fuel economy actually decreased. The introduction of the new Routemaster bus appears to provide a slight improvement over previous diesel hybrid buses; however, there appears to be significant discrepancies between the recorded and expected performance. Results released by TfL in 2014 suggest a fuel economy in the range of 38.2–45.6 l/100 km, whereas it is claimed by the manufacturer that a fuel economy of 24.4 l/100 km was recorded on the 159 bus route. Unfortunately, the details for these results are not available and so it is difficult to determine the validity of the results. This discrepancy could be the result of a number of factors such as the route topology, traffic conditions, driving style and passenger conditions. In summary, TfL has successfully introduced a large number of diesel hybrid buses into their bus fleet. This has resulted in a decrease in the emissions associated with the bus fleet, with considerable further reductions expected. It provides an example of the successful deploy- ment of diesel hybrid buses into a large operational bus fleet to achieve reductions in emis- sions and fuel consumption. However, the increased cost and system complexity remain problematic. Figure 5. Expected reduction in CO2 and NOx emissions from the TFL bus fleet with the deployment of diesel/electric hybrid buses [1, 15, 25]. Development of Bus Drive Technology towards Zero Emissions: A Review http://dx.doi.org/10.5772/68139 39