Optare’s single-deck buses already have a fine reputation for fuel economy. That has been furthered by adoption of Allison’s xFE range of transmissions across the board with the FuelSense 2.0 Max package
Optare believes that it has stolen a march on the rest of the industry by fitting Allison’s xFE range of transmissions with FuelSense 2.0 Max software as standard in its single-deck range.
It may well be right; the Metrocity, Solo and Versa already have an enviable reputation for fuel economy, and the xFE with FuelSense 2.0 Max reduces consumption over a comparable non-xFE unit by up to a claimed 7%.
With the xFE range, Allison has utilised several principles that major on efficiency. It has been careful to maximise the time that the torque converter lockup clutch is engaged, and also to reduce the steps between the first, second and third ratios. Both keep engine speeds as low as possible during the fuel-hungry launch from stationary phase.
Previously, Allison gearboxes would engage the lockup clutch no sooner than second gear. In the xFE range, direct drive is achieved in the first ratio, and at little more than walking pace.
That is an important contributor to economy. When the lockup clutch is open, the engine speed is higher than it would otherwise be, increasing fuel use.
In earlier Allison transmissions, engaging the lockup clutch in first gear was not possible. The torsional damper in xFE units’ torque converters have been modified to allow it to occur in the lowest ratio, and that change also delivers a secondary benefit.
High-torque, low-speed four-cylinder engines as used by Optare are torsionally active, says Darron Griffiths, Application and Technical Training Manager at Allison agent Mitchell Powersystems. The revised damper prevents that from being transferred to the transmission.
Five-year warranty
“Engagement of the lockup clutch in first gear in xFE transmissions has no effect on the unit’s life,” Darron continues. Demonstrating the confidence that Allison has in the T2100 xFE, used in some Solos and Metrocitys, and the T3270R xFE, used in other Solos and Metrocitys and the Versa, both have a five-year unlimited-mileage warranty.
Each unit has six gears, but in Optare buses only the lower five are used. That may sound at odds with the focus on fuel economy, but it is for a reason, says Darron.
“The Metrocity, Solo and Versa are limited to 59mph, and at that speed there would be a risk of ‘hunting’ between fifth and sixth gear,” he explains. “That would have an effect on fuel consumption.”
xFE units weigh the same as earlier models and the oil cooler has the same capacity a comparable non-xFE transmission, despite much less time being spent with the lockup clutch open. The operating temperature of an xFE unit is also the same.
Software-based
Heat generation, albeit not within the transmission, is part of the thinking around the most significant fuel-saving aspect of the xFE application with FuelSense 2.0 Max: Acceleration Rate Management (ARM).
ARM is one of several software principles that feature in the xFE package. “It is possible to tailor the maximum rate of acceleration that the bus can achieve. It works regardless of passenger loading or topography,” says Darron.
“It is important to strike the right balance for the application the bus is used for: On interurban work in tougher terrain, more performance will be required than in a city, but the ease of entering a roundabout, for example, should also be considered.”
ARM prevents drivers from using all of the available power where doing so would be inappropriate, but it only functions in first and second gears. That is because of the need for enough heat within the exhaust unit for the aftertreatment process to work correctly.
Other fuel savers
DynActive is a further part of the FuelSense 2.0 software package. DynActive follows the engine’s torque band, and it prompts gear changes at the optimum point when accelerator position and topography are considered. The shift pattern is also influenced by detection of the passenger loading among other factors, and change points are varied as required to deliver the best economy, rather than observing a fixed ‘table’.
Neutral at stop is also part of FuelSense 2.0 Max. It eliminates the load imposed on the engine by the transmission when a gear is engaged and the bus is stationary, and Allison’s application of the concept is done smartly.
The output shaft is locked by two clutches, preventing the bus from rolling backwards on an incline if the brake is released without pressing the accelerator. Then, the torque converter is fully disengaged.
Specific to FuelSense 2.0 Max, neutral is selected before the bus reaches a complete stop. Instead, it occurs at around walking pace.
Standard Optare fit
Engine stop-start does not currently form part of the xFE package, but regardless of that, fuel performance of xFE-equipped Optares in service has been good, the bus manufacturer says.
xFE with FuelSense 2.0 Max has been fitted in the Metrocity, Solo and Versa as standard since July 2018, and operators that have taken delivery of such vehicles include First UK Bus, Go North East and Transdev Blazefield.
In the Solo, the T2100 xFE transmission is available in conjunction with the Cummins ISB4.5 engine rated at 150bhp, and in the Metrocity and the Solo with the Daimler OM 934 at its 154bhp output. In the Metrocity, the Solo and the Versa, the T3270R xFE is used with the OM 934 when it is rated at 174bhp.
Optare does not currently offer the Metrocity or the Versa with a Cummins engine, but it does not rule out adding the option later.
Optares with xFE Fuel Sense 2.0 Max driven
routeone was able to put two of Optare’s demonstrators with Allison xFE gearboxes and FuelSense 2.0 Max through their paces First up was an 11.5m Metrocity with the Daimler OM 934 engine rated at 174bhp and the T3270R xFE transmission. It performs well, and there is no lack of power; there is also no obvious difference from an earlier Metrocity with a non-xFE unit.
The T3270R xFE has an integral retarder. It applies with varying levels of force, and is very capable, if slightly abrupt in its application at times. Otherwise, the Metrocity is an easy bus to drive and the xFE’s focus on economy does not come at the expense of driveability.
Second was an 8.5m Solo SlimLine. This bus has the OM 934 rated at 154bhp and the T2100 xFE transmission; while no gearbox retarder is fitted, Optare adds an electric secondary brake.
The Solo’s gear selector is a traditional T-bar example, while the Metrocity has buttons. The Solo is just as driveable as its big brother, and both buses are easily capable of higher speeds despite only five out of the six gears in each transmission being used.
In particular, the Metrocity is well able to cruise at its 59mph limited speed without undue engine noise intrusion, and both buses demonstrate Allison’s ‘clustered’ approach to ratios at the low end of the road speed range.
The lockup clutch engages very early and the first two gearchanges are made in quick succession, but once again that does not come at the expense of driveability.
Climbing hills from a standing start, such as one to a bridge over a railway line near Optare’s Sherburn-in-Elmet factory, is also accomplished easily, reflecting the adaptive nature of the FuelSense 2.0 software. If the promise of up to a 7% reduction in consumption is true, the xFE is a tempting proposition.
