Buses continue to get powerful. While most buses have their engine located at the rear, even the most basic of designs today employs an engine that can generate power upwards of 250bhp! A power output of 250bhp or more when combined with mammoth torque turns a bus into powerful machine, which if handled carelessly can be dangerous and a source of accidents with fatal results. Accidents common to buses, especially powerful buses, is roll over. This translates into cause of injury to the occupants. Thus, when a bus makes the headlines, its usually because it is involved in a serious accident. What is often forgotten though is that 6.2 billion people use buses every year and often as a source of reliable travel.
If one looks at the accident figures per one billion passenger-kilometers traveled in Europe, buses are by far the safest of all means of transportation. This means that although public attention is drawn to accidents involving buses, theyre still rare occurrences in Europe. On Germanys roads, for example, buses are involved in less than one per cent of all accidents leading to injury. But although such incidents are rare, the number of people affected when they do occur is generally greater than in many other types of traffic accidents and thats what makes them news- worthy.
India is no different and accidents involving buses is on the rise. According to a study conducted by accident researchers in Europe, 67 per cent of all bus accidents resulting in fatalities involve a rollover of the vehicle. The other types of fatal bus accidents are collisions with an obstacle (28 per cent) and frontal crashes (five per cent). For Schreiber and his team, it is the rollovers that have come to haunt the most.
The high centre of gravity of buses makes them prone to roll over under unfavourable conditions. Whats more, because coach passengers want as smooth a journey as possible, the springs and shock absorbers in the chassis are given a "soft" tuning to enhance travelling comfort. When the two factors are combined, they reduce driving stability if the bus driver has to suddenly swerve into another lane to avoid an obstacle ahead or approaches a long curve too quickly. In such situations, a coach (bus) can run into a situation thats difficult to handle. It either breaks away from its lane or starts to roll because the driver has steered too sharply or attempted to overcompensate.
Laws of physics cannot be ignored, help of electronics is taken to keep the dynamic behaviour of buses as stable as these laws allow. Electronic Stability Program (ESP) is one of them. Driver of passenger cars and trucks in Europe have already been relying on ESP and in case of Mercedes Trucks, electronic safety systems like Telligent Stability Control, for several years now. But ESP in buses is a new proposition altogether. They need to be tailored for use in buses, as dynamics of buses are different from cars and trucks. To start with, a bus engine is generally located in the rear of the vehicle, which means the mass is distributed differently than in a truck Overhangs - front and rear - are particularly large and axles are fitted with air suspension, with "soft" setting.
Daimler Chrysler subjected it: Setra buses to a good amount of testing to arrive to a final ESP solution. ESP is designed to detect a difference between the drivers control inputs and the actual response of the vehicle. When differences are detected, the system intervenes by providing braking forces to the appropriate wheels to correct the path of the vehicle. This automatic reaction is engineered for improved vehicle stability, particularly during severe cornering and on low friction road surfaces, by helping to reduce over-steering and under-steering. To implement ESP, additional sensors must be added to the ABS system. A steering wheel angle sensor is used to detect driver input with a yaw rate sensor and a low-G sensor that measures the vehicle response. Some ESP systems include a connection to the powertrain controller of the vehicle to enable reductions in engine torque when required. Electronic stability programs are based on existing ABS systems with the addition of several enhancements. The inclusion of analogue electro-hydraulic valves requires closed- loop, current-controlled pulse width modulation (PWM) output: from the electronic control unit (ECU). The yaw and low-G sensors are typically located in a separate cluster near the vehicles centre of gravity. Additional communication interfaces are required to enable data transfer to the powertrain ECU and sensor cluster. An ESP system is required to operate as fail-safe during the event of a fault. The system relies on the electronic components to provide a high level of operational fault coverage to correctly initiate the fail-safe state.
In addition to Mercedes Benz, Scania too is developing ESP system for its buses to enhance safety. Like the one in Setra, the ESP in Scania Irizar bus has sensors that measure lateral acceleration at the vehicles centre of gravity. If lateral acceleration exceeds a predetermined limit, the engine torque is cut and wheel brakes are applied. The system is able to detect driver reaction by comparing the yaw angle with the steering angle. If these do not correspond, engine torque is cut and wheel brakes applied to stabilize the vehicle. Precisely which brakes are activated (on which wheels and which axles) depends upon the nature of instability. The Scania system is said to have a switch-off mode though no manufacturer will recommend the ESP system to be switched off, not Scania either. When ESP is triggered, data is stored within the systems memory and frequent triggering indicates a need for driver training.
To ensure driver fitness and awareness, both Daimler Chrysler and Scania conduct driver-training courses specifically for bus drivers. Daimler Chrysler is also developing a proximity control system. This electronic unit automatically activates the brakes of a bus on highways and other major roads if the vehicle gets too close to the vehicle up ahead. Not only does the unit help to relieve the pressure on bus drivers, it also boosts the safety of the overall system. The proximity control system is already available for Mercedes-Benz passenger cars under the name of Distronic.
Also at an advanced stage of development is a lane-departure warning system for travel coaches. In this system, an electronic analysis unit uses a camera to detect if drivers have unintentionally swerved from the right lane and warns them accordingly. This warning system is already offered in heavy-duty Mercedes-Benz and Freightliner trucks. For buses however Evobus has had to do a good amount of modifications and the process is on, yet. If the vehicle leaves its lane, the system warns the driver via a "rumble-strip" sound emitted by the onboard loudspeaker. Researchers and engineers at Evobus claim to have found that drivers instinctively interpret the "rumble strip" noise correctly and steer their vehicles back into the right lane. There is an awkward side of the story, according to Schreiber. It may not be desirable for the occupants of the bus to hear the audio warning and Evobus researchers are looking at fitting the drivers seat with a vibration alarm instead, similar to those used in cell phones.
Whats more, the lane-departure warning system will be activated only at speeds above 80 kpl. The warning system will automatically stop as soon as the driver uses a turn signal or applies the brakes. "This way, we can reduce the number of unnecessary warnings that could disturb the driver," says Schreiber. With the aim of realising the vision of accident-free driving and not just in passenger cars but also in travel coaches the Evobus engineers are already working on additional systems to enhance active and passive vehicle safety. "We have a clearly defined road map that will take us through to 2014," says Schreiber.