Mellor Bus Stresses Lightweight Construction for New Midsized Battery-Electric Buses
U.K.-based bus manufacturer Mellor Bus has recently launched a completely new range of battery-electric midsized buses under the Sigma brand name. First to market later in 2022 will be the 7-meter (23-ft) long Sigma 7 and 10-meter (32-ft 10-in) long Sigma 10. These two models will be followed by four others: Sigma 8, Sigma 9, Sigma 11 and Sigma 12.
The focus for the company has been to produce the Sigma range using light- weight construction methods, designed to allow for smaller battery capacities to consequently reduce energy consumption and shorten recharging times. Partner component manufacturers include CATL, Wabco, Webasto, ZF, Dana and Ventura. CATL supplies all the batteries for the range and the drive motors all are supplied by Dana. All Sigma models will carry full EU Type Approval.
Mellor is part of the U.K.-based Woodall Nicolson Group of specialist vehicle manufacturing companies, which includes Binz International, Coleman Milne, J M Engineering, Promech Technologies, Treka Bus and VCS Ltd. Mellor Bus chief technology officer for the Woodall Nicolson Group John Randerson told Truck & Off-Highway Engineering that the company had applied its strategy of making the vehicles as size-, cost- and weight-appropriate as possible.
“Our job is to provide as much flexibility in terms of the solution to our customer base, be that in the blue light sector, the bus sector, even in the funeral car business, where we have just launched a Tesla-based hearse vehicle, so if you look across the wider Woodall Nicolson business, there are touch points of zero-emission vehicles in all our businesses,” Randerson explained. “Bus primarily has had by far the most significant uptick in terms of business investment, people and facilities because bus in this country and elsewhere globally has been way ahead of all the commercial-vehicle sectors in terms of uptake.”
Mellor Bus’s background has been in converting vehicles into small buses based on vehicles built by mainstream vehicle manufacturers. It has no history of building completely new combustion-engine buses of its own design in the midsize bus category. “We have no preconceived ideas, no back catalogue of diesel-engine vehicles to have to work around,” Randerson said.
The company is not working with a vehicle originally designed to be powered by a diesel engine that now needs to accommodate electric power, including the power electronics and batteries. “If you have no preceding design and you have a history and expertise in lightweight technology, it allows you to almost roll up into a ball what went before, chuck it and then come up with something that is entirely different,” Randerson said. Entirely different in this case includes the design, appearance, size and battery capacity of the vehicle, while maintaining a competitive range.
Lightweight build methodology
“If we can make this vehicle 3-tonnes lighter than competitor vehicles of the same size, then what can we do with that 3 tonnes?” he said. “Well, it’s what you don’t do with it; you don’t carry it around day after day, night after night, charging it up.” Randerson noted that while the new Sigma 10 range is powered with a 210-kWh CATL battery pack, some competitor vehicles would need a 400-kWh battery pack.
“We have spent a whole host of time looking at the best possible build methodology for these vehicles,” he said. “Already within the business, we use quite exotic stainless steel throughout the construction of our existing vehicles, and the same is used in the Sigma range. I would say for the past 20 to 30 years, larger buses have used aluminium, quite a lot of it and structural stress panels. This build is entirely different in that we use an integral stainless-steel welded fabricated structure, which although it is stainless steel, there is less of it and it saves weight.
“The chassis technology uses exactly the same methodology,” Randerson continued. “You’ll see on these vehicles, there is not just a decorative finish on the outside — that is indeed carbon fiber. The floor structure is not a traditional 18- or 15-mm birch-faced ply- wood; it’s a composite and it’s one that we have been using in our vehicles for probably the last 10 years. We have actually got such a degree of experience in how to do this from the smaller buses that we’ve been able to adapt and tweak it into these kinds of platforms to make them as light as possible.
“You take those three items and we save weight significantly,” he said. “We go out of our way to fit lightweight seats, we’ve got 17.5-inch alloy wheels. Once you start on that journey towards light weight, a 17.5-inch tire and alloy is a lot lighter than a 22.5-inch or a 19-inch, so there is an increasing payback. If you don’t need an eight-tonne rear axle, you need a six-tonne rear axle, it’s a lighter axle, it has a smaller motor, so the payback from just some, on the face of it, minor tweaks to a build methodology and a body design continue right the way through.”
The front axles on Sigma 8, 9 and 10 models all are rated at 5.5-tonnes (12,125 lb). “It’s far lighter than an 8-tonne [17,637 lb] axle. So, once you play around with the methodology to get the lowest possible weight, you can then manipulate where you apply that weight to take best advantage,” Randerson said.
Mellor Bus has adopted a similar approach to the drive axle. Sigma 7, 8 and 9 models will be driven by a Dana eaxle. “The 10 and 11 will use a ZF portal rear axle with a traditional short propshaft and motor setup, and the Sigma 12 is different again in that it will have a ZF hub drive,” Randerson said. “So, we have selected the best possible solution for each of the applications, instead of saying, ‘We have an axle, we can buy it in volume, it goes in everything.’”
The vehicles will be equipped with a CCS-type charging socket enabling DC charging. This will enable a Sigma 7 to be re-charged in about 1.5 hours and a Sigma 10 in around 2.25 hours. Mellor Bus quotes a range of about 160 miles (257 km) on a full charge.