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Optimized Series Hybrid-Electric System Delivers Best Fuel Consumption Results Yet for Double Deck Vehicle

The Gemini 2 HEV. Click to enlarge.

In a recent independent test cycle at the Millbrook Proving Ground in the UK, a Wrightbus Gemini 2 HEV achieved the best results for fuel consumption and CO2 emissions ever recorded from a double deck vehicle: 10 mpg UK (8.3 mpg US, 28 L/100km).

The test was part of the Wrightbus Product Development team’s collaboration with Queens University Belfast and key component suppliers including Siemens, Valence Technology and Ford. The series hybrid-electric drive system also uses an optimized engine management program developed by Revolve Technologies. Revolve Technologies was formerly the UK-based Roush Technologies Limited. An investor group acquired Roush Technologies from its US parent late in 2007, and changed the name to Revolve in December 2008.

The Wrightbus series hybrid system uses a standard 2.4 liter diesel engine—as used in the Ford Transit—as the generator. Revolve engineers have been able to predict load and speed changes in advance, thus allowing greater freedom with injection strategies and EGR (Exhaust Gas Recirculation) rates.

Drawing on its special knowledge of the engine EMS system, Revolve engineers have also been able to utilise some of the existing vehicle based strategies to carry out functions which otherwise would have required significant software changes. This, coupled with a unique CAN interface module, has allowed the full integration of the engine and its controller into the overall hybrid control system, and at a relatively low cost.

Having the engine control as a fully integrated part of the hybrid system is a major advantage – and, coupled with a unique calibration, we have been able to achieve exceptional improvements in fuel consumption. The Wrightbus programme involves optimizing Series Hybrid Drive systems through a detailed analysis of generator load patterns. As a result of this analysis, we have been able to recalibrate the engine to operate at its peak performance throughout the drive cycle, by using smart charging and load control technology. Overall engine performance is significantly improved when compared with normal applications.

—Paul Turner, Revolve’s Technical Director of Product Development

In terms of hybrid vehicles in operation, Wrightbus currently has:

  • The original Gemini HEV hybrid double deck in operation with Arriva London
  • Five Electrocity single deck buses with Travel London
  • Seven Electrocity single deck vehicles in operation with Go Ahead Group’s London Central operation
  • One Gemini HEV hybrid double deck with London Central
  • One Gemini HEV under evaluation with Dublin Bus
  • Five Gemini 2 HEV entering service with Arriva London.



10 mpg (UK) in a double decker bus is astonishing - I thaught they got about 2mpg.
So, I wonder
a: how much extra these buses cost, and
b: What kind of MPG they get when normal busmen use them in normal traffic.

If you get 10 people on the bus, and are getting say 8 mpg, you achieve 640 mpgpp (per person) which is very very good.

If it pans out, it sounds like they are really getting the hang of hybrid drivetrains.

Nick Lyons

@mahonj: Wouldn't that be 80 mpgpp? Each of the 10 passengers is responsible for 1/10 of the fuel use, not 1/64.



10 people => 8.3 x 10 = 83 MPG per person

Since the average speed is so low, aero drag is minimal. If they can reduce weight and rolling resistance, and recover braking energy MPG can be very high.

I calculate with 1.9L VW TDI engine efficiency powering a 32000 lbs (guess) wright bus at a steady 25 MPH:

23 MPG ignoring aero drag
18 MPG including aero drag


WTF!!! Hard to believe it runs only on a 2.4L diesel.

This short US school bus uses a 5.4L gas engine.
The first group is lying or the second group is incompetent.


I meant to say 80 people on the bus. (Rush hour).
Then, I derated the 10 mpg to 8 mpg for more reasistic conditions.
80 x 8 = 640 mpg
Sorry for the confusion.

However, at other times of the day (especially weekends) I see buses flying around with about 5 people on them.
Whay you need is an average occupancy for a bus fleet - then you can get a realistic figure.
[ Or just sum the passengers, miles and fuel used for a year].
And so on.


10 mpg is about the same as this baby


From first hand experience, the first generation of hybrid buses in use in London (single deck) were a bit noisy. The engine was small and running continuously at high revs. It sounded like a portable generator at a concert. The second generation (still single deck) was already better in confort level. I hope they improved even more with these double deckers.

Real word usage shouldn't be that different from tests, since driving conditions are really optimal for series hybrids, with lot's of start&stop (bus stops but also traffic jams and crossings). I think they claimed around 40% fuel reduction but I can't find the link to the source.


Goes to show that buses are an efficient and low carbon form of transport, but only provided that it has reasonable passenger levels.

At least 4 passengers, compared to the average UK spec vehicle consuming 40mpg would even out in efficinecy terms, so if theres 5 passengers or more on this type of bus there is a gain (albeit negligible) but then you'd need 8 passengers to equal two people car sharing, 12 for 3, 16 for 4 and so-on.

And this, of course assumes that the 10 mpg is gained through stop-start city driving conditions (stop-to-stop, junction-to-junction etc but I suspect it will be less - something like 5mpg but then the urban cycle of a 40mpg car is also less - say 25 mpg.

Key message is - buses are a good form of urban transport as they will usually have more than 10 - 20 passengers and tend to be uncomfortably full at rush hour. You'd be saving the planet, but you'd be very hot and sweaty standing on a crammed bus. At night time, the car becomes favourable, as this is the time when couples travel for social visits on quieter roads etc - more sustainable than empty buses running everywhere.

In rural areas, an almost empty bus says to me that using a car is equally as sustainable as a bus, with the car giving the added social benefits of door-door transport with no timetable restrictions and so on. Of course this depends on the size of buses used - smaller is better and they are essential still for those who do not have cars. However, playing the rather fluffy 'social inclusion' card as an excuse to price people of rural roads and force them to use infrequent buses that don't travel at convenient times is not a good one. Both forms of transport have their role. Its just a shame that the more radical of environmental transport activists don't see that different forms of transport including the car have their own purposes.

Mass transit excels in urban areas and is more likely to offer a reasonably viable alternative. In rural areas the car becomes more essential, unless you're prepared to wait for one of the two or so buses a day.


Further proof that series-hybrid is the way forward.

Very easy to add grid electrification afterwards too.

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