Optare became the fourth manufacturer to enter the Euro 6 double-decker fray when it launched its MetroDecker in May. Following tweaking, the prototype was made available at a ride-and-drive event at Millbrook.
Optare’s keenly-anticipated MetroDecker â€“ its first venture into the integral double-decker market â€“ broke cover at a high-profile launch in London’s Covent Garden earlier this year. It promises much, and headlines with a sub-10 tonne unladen weight, believed to be a first for a low-floor double-decker.
But what’s it like on the road? Thanks to Optare arranging a ride-and-drive last week at Millbrook, a simple answer to that can be delivered. It’s excellent: responsive and smooth, with Optare’s typically good passenger environment.
The MetroDecker also provides further evidence that the small engines offered at Euro 6 are more than man enough for the job. It’s powered by a twin-turbo, 5.1-litre Mercedes-Benz OM934, coincidentally the same cubic capacity as Volvo’s D5K fitted to its B5TL chassis.
The compact, four-cylinder OM934 is transversely mounted to the nearside, and drives through ZF’s EcoLife gearbox. It’s inclined rearwards around the crankshaft to give good access and reduce the engine bay’s intrusion into passenger space. Standardising components has been the key to the MetroDecker, developed in less than a year. No other drivetrain options are available, and nor will they be in the future. That’s for a reason: â€œWe have done this to get driveability and fuel efficiency exactly right,â€ says Optare’s Engineering Product Manager Tim Hampshire.
â€œMercedes-Benz engines have proven to be the most reliable in our other products, and to get the most fuel efficiency it’s important to make sure that the engine is working in exactly the right speed range. The six-speed EcoLife allows us to do that; it is very quick to shift at lower speeds, but its TopoDyn software – fitted as standard – also gives the best performance and economy on hills.â€
Much of the rest of the bus is also standardised with a focus on weight saving. That includes the integral construction method, use of GRP composites for most body panelling, and framework which has been carefully thought through to allow the use of as little as possible without compromising strength and rigidity.
The MetroDecker’s heating and ventilation system, including upper deck air chill as mandated by TfL, is also very simple and consists of one small rear-mounted vent downstairs and two teardrop-shaped openings above the rear window on the upper deck. No ducting is necessary and Optare says that careful design means that air will circulate well in both saloons.
One aspect of ventilation Tim acknowledges will be improved in production buses is the opening window arrangement. On the prototype they are integral parts of the larger pane, but strength limitations mean that upper deck hoppers are low and intrude into passengers’ space when open.
That will be rectified by use of conventional-style hoppers on both decks, resulting in the opening section being mounted at the top of the window. Other modifications will include additional engine compartment heat and noise insulation, while the fuel filter will be moved away from heat sources. The exhaust will also receive lagging.
On the road
Millbrook is used by First for fuel comparison trials and vehicle manufacturers for durability testing, and gives a good variety of circumstances typical of those a bus is likely to encounter during its working life.
That includes high-speed running, steep climbs and very poor surfaces. Many of its roads and tracks have bus stop laybys installed, giving a reasonable representation of real-life road conditions and permitting the MetroDecker to be given a stern workout.
Stern is the operative word; although the OM934’s 5.1-litre displacement and 900Nm torque output are both moderate, in reality the twin-turbo set-up means the unit packs a punch which, until not long ago, would have only been available from something twice its size.
The dual turbos give a smooth power delivery across the rev range, and take-off from stationary is effortless. The EcoLife up-shifts rapidly to keep engine speeds low, but progress is good and gearchanges are imperceptible.
Tim Hampshire speaks of extra sound insulation in production buses, but engine noise intrusion is negligible.
Most impressive of all in terms of the OM934’s power delivery is its rabid keenness to climb, which combined with ZF’s TopoDyn gradient sensing software is superb. From a standing start at the foot of a winding 7.6% gradient the MetroDecker accelerated to, and held, 30mph before the summit was reached.
Equally importantly from an operator’s perspective, even after a very spirited drive around Millbrook’s alpine circuit, involving hard climbs coupled with prolonged retarder use during descents, the indicated coolant temperature remained unchanged.
That should help dispel worries of excessive heat generation at Euro 6, while also notable was temperature in the engine bay. Optare has installed a rudimentary indicator, and it showed around 85C immediately after this period of hard work.
Good for passengers
One thing noticeable is how smooth the MetroDecker is. Unlike some other smaller engines of earlier generations, the OM934 avoids ‘peaky’ power delivery and behaves in the manner of a larger unit.
Benefiting the passenger’s experience is excellent road holding, helped by independent front suspension. The MetroDecker was pushed very hard through the alpine course’s hairpin bends with almost non-existent body roll, although the acid test would be with a laden bus.
Similarly, the damping has been a focus for Optare’s engineers. On an undulating section of course designed to upset suspension composure, the bus remained planted firmly to the tarmac, with nothing in the way of uncomfortable oscillations.
The only area worthy of improvement is, in common with almost all other buses, the mirrors. Trucks have been blessed with two-mirror set-ups which give good views without creating excessive blindspots for almost two decades, but bus manufacturers stick to much more basic installations.
The MetroDecker’s are on a par with any other bus’, but that is more a criticism of the market in general than this particular vehicle. It is also undoubtedly down to operator demands for the lowest possible replacement costs, although Tim Hampshire explains that electric adjustment is an option.
The bottom line
Both saloons are clutter-free and have a well-thought-out lighting arrangement, which appears to benefit operator and passenger alike. The bright LED strips cast few shadows, are durable and mean the ceilings are easy to clean. Careful consideration has gone into minimising reflections in the cab.
In many ways Optare has taken bus design back to basics with the MetroDecker. It’s as simple as could be hoped of a vehicle at Euro 6, and integral construction means that unnecessary weight and body structure has been removed. That leaves a bus which is straightforward and standardised inside and out.
Add to that a keen driveline and it’s well suited to the task at hand, whatever that might be. As set up in the demonstration vehicle, the gearbox is configured for interurban-type running, and a limited speed of almost 60mph was reached with no fuss.
For city operations, its shifting strategy can be altered to take account of the different requirements and ensure optimum fuel economy. Optare says the 11.1m, single-door version of the MetroDecker will break cover soon, and the model’s order book will formally open at Euro Bus Expo next month.