Brake expectations11 November 2010
Future brakes and braking systems are promising even better levels of safety and performance, particularly for truck combinations. John Challen and Brian Tinham investigate
What good is an environmentally friendly truck, if it is out of service, having crashed under braking? This – and concerns about injuries, insurance, litigation and the rest – is among the driving forces behind developments, particularly with emergency braking systems that are looking to dramatically reduce stopping distances.
Developments come in various guises, attacking the problem from different angles. The latest innovation from systems supplier Wabco, for example, debuted at the IAA Commercial Vehicle show, builds on the company's OnGuard technology, which is a collision mitigation system for truck drivers.
According to the company, its aptly named OnGuardPlus is the first in the commercial vehicle industry to comply with the European Union's expected regulation to make AEBS (advanced emergency braking systems) mandatory on new HGVs from November 2013. The technology applies full brakes in imminent collision situations, reacting to moving obstacles, decelerating vehicles and stationary vehicles, for example, when approaching traffic congestion. Wabco explains that it initiates emergency braking, bringing the vehicle to a complete stop, if necessary.
Jacques Esculier, Wabco chairman and chief executive, says that the new system will be available worldwide from 2012 for trucks and buses. "OnGuardPlus demonstrates Wabco's technology leadership as we progressively increase levels of vehicle safety and performance, while striving to reduce the number of commercial vehicle accidents by 2020," he comments.
Meanwhile, having made great strides in the bus and coach industry, Allison Transmissions is now looking to to improve braking on RCVs (refuse collection vehicles), using its hydraulic output retarders. These can be fitted on vehicles above 15-tonnes, and create a braking force transmitted via the transmission to the differential.
Allison's retarder uses a vaned flywheel in the transmission housing. The transmission directs oil into the retarder housing to counter the vehicle's momentum via the drive shaft. Energy is converted to heat and dissipated through the vehicle's cooling system. Resistance from the flywheel, augmented by stators on the inside of the housing, effectively delivers braking force to the drive wheels. More oil in the housing translates into more aggressive braking, and since there is no mechanical friction or wear to shock the drivetrain, maintenance costs can be reduced too.
Tests at Ohio's Transportation Research Centre in the US prove the retarder's effectiveness during stop-and-go duty cycles. Without the retarder, front brake temperatures reached 266°F, while the rear linings topped out at 390.2°F. However, with the retarder engaged, brake lining temperatures on the front axle reached only 120.2°F, while the rear brake pads were measured at 172.4°F – less than half the temperatures measured without the retarder.
Retarders may have been around a while, but Allison Transmission area manager Riccardo Sardelli, suggests that transport engineers shouldn't imagine that development has stopped. "We see the future in more control, and greater integration," he hints. "The only way forward is to change the metrics. We are changing designs to give higher retardation levels for heavier load applications, such as articulated dump trucks. It'ss about changing the internal geometry."
Waste not, want not
This is important, because when retarders are used on RCVs, brake wear can be improved by as much as 30%. That's why, in the future, Sardelli believes they could become standard on up to 60% of RCVs. "Vehicle sizes, weights and horsepower are all increasing, so you need a more effective [and less costly and maintenance-intensive] method of stopping the vehicle. This is where the retarder comes into its own – by significantly reducing the effort, and hence the damage, on the foundation brakes."
Sardelli says that Allison is also looking at trucks for pickup and delivery in the urban environment, where CNG-powered vehicles are becoming popular in some countries. "Gas engines give less engine braking than conventional diesels, and you don't want to use the engine to brake too much anyway, because you might break it," he comments. Hence the retarder.
What of magnetic retarders – a relatively new concept? "Magnetic retarders are being offered by Voith, and are something Allison doesn't have today, even though various OEMs are trialling them," concedes the Allison man. "It looks like a very good concept, because it is light, maintenance-free and doesn't generate a great amount of heat."
As for the future, one very interesting project that looks set to come to fruition for the next generation of trucks is emerging out of the Cambridge Vehicle Dynamics Consortium (CVDC) – a network of transport industry companies and the University of Cambridge Engineering Department, focused on truck safety, productivity and environmental improvements.
Cambridge post-graduate Jonathan Miller is part of a team that includes high performance valve company CamCon and Haldex Brake Products, working to update two of the mainstays of truck braking systems – the hardware and the software. He points out that hardware on trailer brakes, for example, hasn't been modified to any great extent since the 1990s, while ABS algorithms also "haven't changed much". He contends that it is shortcomings in both the hardware and the software that cause the familiar 1-2Hz pulsations of ABS-assisted braking on trucks, during emergency stops, which, on analysis, results in the braking wheels spending too much time either locked or free-rolling. That, in turn, leads not only to suboptimal deceleration, but also the characteristic loss of steering control.
"So we have modified ABS algorithms to implement slip control that massively reduces trucks' stopping distances. Effectively, our software continuously optimises the braking forces so that the braking wheels ride the perfect point on the curve between locked-up and free-rolling, for maximum braking," he explains. And that includes dealing with varying road surfaces, having different friction coefficients – automatically varying the braking forces to maintain that optimum.
But that's far from the end of it. "We have also developed and built a new brake valve that is 10 times faster at switching than conventional units," reveals Miller. And it's not just the valve, important though that is: unlike existing trailer brakes rely on a central solenoid valve to modify air pressure to the brakes, in CVDC's design its new fast valves sit on each wheel's brake chamber, acting directly. That direct action both cuts out air pressure lag and enables independent braking force per wheel. Miller reckons that, based on simulations with test hardware on Cambridge University's laboratory rigs, the new valve design will reduce stopping distances by up to 30% (within a whisker of passenger cars' capabilities).
Incidentally, it will also cut compressed air usage by up to 50%, leading to an additioal potential payload and energy usage advantage for hauliers. "The valves use less air, which is typically an energy source and needs to be stored in reservoirs on the vehicle," states Miller. "If we can build the brake systems to operate using less air, there will be smaller reservoirs, and more space to haul cargo." Who knows, it might even lead to lower insurance premiums, given the shorter stopping distances achieved.
Unfortunately, engineers and fleet managers will have to wait a little longer before they see the technology in action – but maybe not too long. "At the moment we are still prototyping, but we are about to put our system on the CVDC demonstrator vehicle – a two-axle tractor, three-axle semi-trailer combination." Once that is proven on the MIRA track, which is likely to be sometime in the next 12 months, the university man expects to have more news. And he maintains that it will be retrofittable for any truck. "Because the system is fitted to the individual brakes of each wheel, it shouldn't matter if it is a rigid truck or a dual trailer combination," he asserts.
Such an elegant solution will probably be more attractive – at least initially – than another approach to improving brake efficiency, which uses electro-mechanical braking (EMB) systems that are not so easily retrofittable. However, Bob Prescott, chief engineer for truck projects at Haldex – who is responsible for the company's development work on its EMB – believes these may well have a future, not least because they can compensate for changing disc friction.
Haldex's system uses a brushless dc electric motor to apply the brakes, rather than air – which sounds simple enough, until you consider the forces and computing required to make it work. "Normally the electrical energy required, if you were simply replacing the air actuator with an electric motor, would be huge," concedes Prescott. "However, rather like a drum brake, our system uses the vehicle's own momentum to pull the pad onto the disc. So the motor just moves the pad up a ramp at an angle such that friction from the disc pulls it on, meaning we only need tiny currents to control and then release the pad," he explains. "If the pad friction is low, then the system has to apply more power to maintain braking – but that's better than losing braking efficiency, as you would in an air system."
As for the hardware, the system uses two microcomputers sited at each wheel, one controlling the foundation brakes and the other the parking brake, which is held open against a spring by an electromagnet. Electrical power comes form two batteries, sited in an enclosure on the chassis – one for each braking function.
Although, according to Prescott, as many as three-to-five years from production, the EMB has already featured at previous IAA shows in Hannover, and has even been fitted to a Mercedes-Benz truck. Since then, improvements have been made, specifically in low friction road conditions, where Prescott claims stopping distances are reduced by as much as 25%.
Sounds great, but Prescott warns there are still hurdles to overcome – one being cost. "On a 4x2 tractive units you get 4S4M ABS controls – meaning individual wheel speed sensors and modulators per wheel. But on a 6x2 or 6x4 the rear axles share the modulator function. So, to get the individual wheel control we need, we have to add additional modulators, and that means more cost." That sounds worse than it is: most of the money is in the electronics, and prices there will only go down.
Meanwhile, there remains the dual electrical system required to ensure fail safety on the brakes. "In the test vehicle we have two dc to dc converters connected to the vehicle single circuit, and they charge our two batteries while providing galvanic isolation from the vehicle systems and each other. That also adds more cost, but in the future, designers would probably share the batteries with, for example, electrically powered steering – which means we can amortise the additional cost.
With the promise of such advanced braking systems, it is likely that operators will gain in terms of efficiency and reduced downtime. The only question is when those benefits may be realised.
Allison Transmission Europe BV
Denby Transport Ltd
University of Cambridge
WABCO Automotive UK Ltd
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