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Predictive Powertrain Control for buses – clever cruise control helps touring coaches to save fuel
- Up to four percent lower fuel consumption
- Digital route information makes it possible to "see ahead"
- Numerous setting options for individual adjustment
With Predictive Powertrain Control being introduced in 2014, Daimler Buses is taking a further step towards improving economy while at the same time reducing the burden on the driver. The "predictive" cruise control system is familiar with the topography of the road ahead and thereby avoids the systemic weaknesses associated with an automatic speed control regulator. A conventional speed control function is unable to distinguish between uphill gradients which by their nature slow the journey and affect fuel economy and flat terrain. Predictive Powertrain Control, by contrast, can do precisely that.
World's first cruise control system for coaches that can both "see ahead" and regulate the transmission
The Predictive Powertrain Control driver assistance system launched by Daimler Buses is the world's first automatic speed control function for touring coaches on the market with integrated transmission intervention.
Up to four percent lower fuel consumption
Predictive Powertrain Control primarily intervenes when negotiating uphill and downhill stretches. In doing so, the system turns the classic and inherent disadvantage of a conventional cruise control system into clearly measurable advantages: fully laden touring coaches such as the new TopClass 500 can achieve fuel savings of up to four percent in long-distance operations compared with vehicles fitted with a classic cruise control system.
The driver must simply be well-briefed on the "predictive" capabilities of intelligent cruise control system. Whether test vehicles are driven by a professional coach driver or not, buses featuring Predictive Powertrain Control always consume less diesel on laps than comparable vehicles driven in the conventional way.
On comparison drives on typical long-distance routes, such as Stuttgart to Hamburg and back, the system also performed well in terms of driver friendliness. Operating the cruise control is only minimally different to when driving a TopClass without Predictive Powertrain Control.
Predictive Powertrain Control relies on data fusion
Predictive Powertrain Control is representative of a new type of technology that brings together the technology that is already in the vehicle with external data from its surroundings. Specifically, it goes further by linking intelligent vehicle computers (already fed with comprehensive data from engine and transmission control units) with parameters that previously only well-trained drivers were able to use to inform the process of predictive driving.
Professionals are fully aware that they should ease their foot off the accelerator pedal before reaching the top of a hill. This saves a lot of fuel and adds very little time to the journey. A conventional cruise control system does not consider this aspect. Good drivers also know that it helps to gain speed in good time before an uphill stretch. This avoids one or more gear changes during the ascent, thereby reducing diesel consumption and the journey time and benefiting the bottom line. A standard cruise control fails to take such factors into account.
At the end of a steep incline, an environment-minded bus driver knows to maintain full throttle only until kinetic energy takes over and helps to propel the vehicle mass down any downhill slope that may follow.
Once again, a normal-style cruise control would be found wanting here, as will, in many cases, a bus driver operating the vehicle manually if they are not familiar with the route, for example, or if darkness restricts their perceptive abilities. Situations such as a telephone call may also divert attention from considering aspects of the route ahead affecting economy. Towards the end of driving time, of course, it may simply not be possible to maintain the same high level of performance in this regard as at the start of the journey.
Predictive Powertrain Control is ever-ready
None of these factors affect Predictive Powertrain Control. The system is always ready and never distracted from its key task of ensuring that driving is as economical as possible. This requires the right on-board expertise: with the help of topographical data, Predictive Powertrain Control is familiar with virtually every gradient en route.
For countries in central Europe, route information is stored for almost 100 percent of all motorways and federal highways (A-roads). The vehicle's position within the road network is pinpointed using GPS data.
Apart from exercising control via the conventional speed control function, Predictive Powertrain Control also takes on gearshift management from case to case. This means that Predictive Powertrain Control is able to intervene in gear selection on the basis of exact route knowledge. In addition, the system may conveniently deploy the fuel-saving EcoRoll technology on-board, which is already in use on trucks. The predictive function reduces the number of gear changes, especially in hilly areas.
Data from inside and outside
As a whole, the Predictive Powertrain Control package incorporates the input data profiling the route ahead and the vehicle's current GPS position. The latest data provided by the vehicle itself includes the vehicle mass, the road speed, the engine torque and output, and the PowerShift transmission's currently selected gear.
The key target value added to this mix is the desired speed, which is input by the driver using controls on the steering wheel.
Safety always comes first
In individual cases, and armed with this data, the predictive cruise control takes charge of the accelerator, auxiliary brake and transmission. Naturally, ultimate control still lies with the driver. And of course the optional package consisting of adaptive cruise control and automatic emergency braking function Active Brake Assist remains active and takes priority over Predictive Powertrain Control, in the interests of vehicle and traffic safety.
Supplemented by advance knowledge
When adaptive cruise control with integrated Active Brake Assist gives the go-ahead, Predictive Powertrain Control is able to concentrate fully on the route. The predictive function is actually "prescient", has advance knowledge and focuses firmly on maintaining the desired set speed within hysteresis curves set by the driver (permitted value above and/or below the target speed) while achieving the best possible diesel consumption.
Pre-programmed with expert knowledge on how to appropriately handle the vehicle mass, the system negotiates uphill and downhill stretches. Away from totally flat routes, Predictive Powertrain Control can really play to its strengths. On flat sections, the intelligent speed control system works precisely with the control strategy and accuracy of a standard cruise control.
Predictive Powertrain Control performs well on uphill and downhill stretches
In hilly areas, which account for a considerable proportion of the European road network, Predictive Powertrain Control is in its element. The driver is able to set the target speed for the cruise control target and the upper and lower hysteresis curves if required. This defines the custom top speed for the descent (= target speed + upper hysteresis curve) and is no different from a standard cruise control system.
There is a third variable, though, which is a new feature of Predictive Powertrain Control the lower speed limit below the cruising speed. The driver uses this value to set the lower speed range which the system uses to reduce speed and fuel consumption when operating circumstances are suitable, for example, when driving up a gently climbing hill. As such, Predictive Powertrain Control detects an incline and whether the coach in its current configuration of rated output and gross vehicle weight has the dynamic driving power to climb the incline in the current gear without falling below the desired, specified speed.
Predictive Powertrain Control takes advantage of the situation
If applied engine torque is sufficient to maintain speed, Predictive Powertrain Control takes advantage of the situation before the uphill gradient ends. Based on the route information known to the system, a subsequent downhill gradient is taken into consideration in a timely way that, above all, allows fuel to be saved. Engine torque is cut back at an early stage and then, depending on the subsequent downhill gradient, the bus coasts under power without consuming fuel.
The vehicle speed then falls to the lower of the limits set by the driver, with Predictive Powertrain Control calculating this ahead of time from the dynamics of mass and the known route profile. If driven by a non-regulated cruise control system, the bus would spend much too long in the phase of fuelled operation while moving, which burns fuel. When travelling downhill, therefore, the vehicle applies the permanent brake sooner than necessary.
Supportive intervention in the powertrain
Predictive intervention in the powertrain may occur on a case-by-case basis, thereby complementing this first relevant operating situation the gentle incline and predictive intervention by the cruise control system. If it makes sense from the parameters, the EcoRoll function is activated at a much earlier stage than with a conventional control system.
This reduces the time and mileage spent at a constant, fuelled speed and instead extends the distance travelled with the engine idle while consuming minimal fuel.
Handling a steep uphill gradient is another situation in which Predictive Powertrain Control excels. Here the manual of economical bus driving teaches that the driver should raise their speed above cruising level in good time before the start of an incline. The aim of this exercise is to leverage the coach's kinetic energy while consuming comparatively little additional fuel by subsequently avoiding a gear change when travelling uphill thanks to momentum.
Predictive Powertrain Control overcomes the limits of cruise control
For a normal cruise control function, this kind of driving style is off-limits. A driver operating the vehicle manually intuitively takes into consideration factors such as the vehicle mass, percentage gradient and length of gradient, but may nevertheless make mistakes. Predictive Powertrain Control has no problems with any of that. The data available is backed by expert knowledge. As a result, a touring coach with an activated system will independently gain speed in a timely manner above the desired speed that has been set, so as to utilise additional momentum and successfully contend with an incline in the highest possible gear.
If PPC detects the need for multiple downshifts on the section of route ahead (e.g. in the hilly area around Kassel), these are consolidated as required.
Depending on the set driving program, the vehicle may shift down two gears on the flat to avoid two gear changes on the ascent and thereby minimise any loss of speed on the uphill gradient.
The perfect driver provides the benchmark
Exactly like a precisely executed double downshift and in line with the general aim of the technology, changing gear early saves fuel over the entire uphill stretch. At the same time, driving in a textbook way in mountainous terrain also saves time. Coach drivers who strive for perfection may well find they are more than happy to defer to Predictive Powertrain Control simply because they often do not have the requisite knowledge of route topography and how the current vehicle mass affects the best way to drive the vehicle.
Predictive Powertrain Control is not just a profitable instrument in hilly and mountainous areas, it can also be of assistance on downhill stretches. The system is programmed according to what every professional knows: to ease their foot off the accelerator pedal early, prior to reaching the top of an uphill gradient. In conjunction with downhill force, the vehicle mass in motion does the rest.
Predictive Powertrain Control also shines when coasting
How does Predictive Powertrain Control work? If there is no major downhill slope at the end of an uphill stretch, early upshifting (where necessary incorporating a double upshift) can bring the revs back to a low level at a very early stage for increased fuel economy. With or without EcoRoll on a case-by-case basis, the speed on the downhill stretch is subsequently managed too. Timely downshifting – or preventing upshifting – helps the auxiliary brake(s) kick in with the best efficiency.
As a whole, the Predictive Powertrain Control system allows the driver to fine-tune a whole host of parameters, including cruising speed and the upper and lower speed limits to set a range.
The general shut-off capability after the ignition starts with Predictive Powertrain Control and/or EcoRoll is only mentioned here for the sake of technical completeness. In practical driving situations, this option is rarely utilised.
Individual setting options
Depending on whether the Standard or Power gearshift program is in use, Predictive Powertrain Control will work with different parameters again. For example, the driver may set the lower speed limit between 0 and 10 km/h. The predicted build-up in speed prior to a steep gradient is limited to 4 km/h in standard mode, whereas if the power program is active Predictive Powertrain Control is able to automatically increase the speed by 6 km/h above the set cruising speed. In both cases, the top downhill speed set by the driver (target speed + upper hysteresis curve) is the absolute upper limit.