TfL has issued performance figures for its NBfL. For some time Boris Johnson has been describing it as ‘four times more efficient’ than other hybrids although no official data has been available to the media. TfL released this information to the BBC last week.
TfL’s figures are based on the eight prototypes which have been operating on route 38 and show the buses achieved a fleet average of 6.74mpg in recent weeks. TfL states fuel efficiency is significantly better than diesel buses but admits these figures are not definitive. The buses only run during the day and the comparison is being made with hybrids operating on route 73 where the hybrid double deckers are achieving 6.1mpg against standard diesel buses delivering 5.3mpg.
Improvements are anticipated when the next 27 NBfLs are placed in service towards the end of June on route 24. These are full production specifications, rather than prototypes, which are said to be lighter and should use slightly less fuel.
The BBC was told: “We estimate the production vehicles will deliver approximately 1mpg improvement on the prototype vehicles and also expect improvements to result from the buses operating over longer distances and over longer periods of time. We intend to make data available next year when the production vehicles have run for at least six months on route 24.” Data from a prototype which had been in service for eight months were measured at Millbrook:
- 2.048g/km of nitrogen oxide (NOx) – this is a quarter of the emissions from the fleet average for standard hybrid buses (7.7g/km) and diesel buses (9.3g/km).
- 690.23g/km of carbon dioxide (CO2) – which is better than the fleet average for standard hybrids (864g/km) and is almost half the fleet average for diesel buses (1295g/km).
- 0.012g/km of particulate matter (PM) – this is a quarter of the emissions from the fleet average for standard hybrids and diesel buses – which emit 0.048g/km of PM.
Bear in mind, there can be large differences between one type of hybrid and another. TfL’s ‘fleet average for standard hybrids’ does not specify a particular bus. We already know the parallel hybrids have proved fairly even in delivering regular, reliable improvements of 25% or more in service whereas the serial hybrids are much more variable and average 14-17%. The next generation series hybrids with engine stop-start will even the playing field slightly.