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TAKE IT TO THE LIMIT
Page 35
Page 36
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Some manufacturers seem intent on vying to produce more and more power — but acceleration limiters can ensure that such power is not abused.
• Power tends to corrupt, and absolute power corrupts absolutely. Given the growing interest in high-powered tractive units, or so the truck manufacturers would have us believe, one could easily take Lord Acton's words as a warning to operators beguilled by all the advertising hyperbole extolling the virtues of maximum horsepower and torque.
Not that the trend towards more and more power is restricted to glamorous premium-weight tractors. Not so long ago, 112kW (150hp) was considered ample for a 16-tonner — now some buyers wonder if it's enough at 7.5 tonnes. Similarly, 187kW (250hp) was once thought of as plenty for an eight-legger, but nowadays enough six-wheelers can match that kind of output.
Nonetheless, the trend is most prominent in the top-weight tractor market. Demand for tractors with more than 298kW (400hp) might be limited, but try telling that to the marketing men — a manufacturer without an engine over 298kW is seen to be suffering from a distinct marketing disadvantage. It is surely only a matter of time before we get the first 373kW (500hp) artic: and where do we go from there? Why not 388kW (520hp) or even 410kW (550hp)?
Where the current power race will lead to is anybody's guess. But one thing is certain. Give a driver a high-powered vehicle and human nature dictates that he will want to use all the available power — and not always at the right time.
HIGH-POWERED
With the possible exception of longdistance TIR operators who need to maintain high average speeds, or hauliers regularly running over tough hilly terrain, few can justify such high-powered machines. The problem is that many operators are reluctant to accept this. So how do you ensure that the extra power does not go to the driver's foot, let alone his head?
The advent of legislation requiring topspeed limiters to be fitted to new coaches, restricting them to 113km/h (70mph) is a clear indicator of what could be coming for HGVs.
Many truck builders already offer such devices as options on tractive units, mainly as an aid to top-speed fuel economy, although in some cases a limiter is necessary as the maximum geared speed of the truck could exceed the speed rating of its tyres. MAN even fits Lucas Kienzle speed limiters on its F90s as standard.
Unfortunately, while a basic speed limiter can keep a driver from breaking a pre-set limit (such as the 113km/la motorway limit) on the level, it can't stop a truck going faster downhill. More importantly, it certainly won't prevent any abuse of engine power below that pre-set limit. It's also interesting to note that while many operators are prepared to spend extra money on wind deflector packages which allow their trucks to go faster by overcoming air resistance, they are reluctant to invest in a speed limiter, which would arguably repay its investment costs much sooner.
Faced with the relatively narrow performance envelope of top-speed limiters, truck builders have been trying to solve a knotty problem: how do you prevent a driver of a high-powered truck or bus from accelerating too rapidly, with the resultant harm to fuel economy and wear and tear on the driveline and braking system, but without compromising the overall performance of the vehicle?
Scania's engineers have addressed the problem with the Scania Acceleration Limiter (SAL) system. This was originally used on the Swedish company's "pusher" articulated bus to prevent jacknifing under excess power, but is now fitted as standard on the current Series 3 single and double-deck N-model bus chassis, launched in the UK last October at the Motor Show.
The demand for more power in trucks has been matched in the PSV market. The average power rating of 138kW (185hp) in the traditional British city bus has been rising to the point where Scania (Great Britain) is now more than happy to offer its rear-engined N113 CRB single and N113DRB double-deck chassis with an 11-litre turbocharged engine pushing out 189kW (257hp).
LESS DEMANDING
The extra power and acceleration provided by the transversely-mounted DS1171 engine is certainly useful when running fully laden or in hilly areas, but it is clearly not so critical when operating under less demanding conditions.
It was to bridge this kind of performance gap between the two operating extremes that prompted Scania to fit its SAL to the N113 chassis. The acceleration limiter allows a driver to exploit the extra power of the 11-litre engine where necessary, while limiting its use when not needed, In practice, that means preventing the N113 from accelerating at more than 1.3m/sec, regardless of terrain, traffic or load.
During hill climbing, or when starting off fully laden, the engine's full power is still available to the driver. Under more lightly-loaded conditions, however, when the bus would naturally accelerate more quickly, the limiter prevents a sudden surge of speed which not only saves fuel, but avoids sending standing passengers hurtling towards the back of the bus as the driver puts his foot down.
Even with the acceleration limiter working there is still plenty of performance to allow the bus to pull out safely into traffic and maintain good journey times. Furthermore, it enables the fitting of a lowerspeed (higher numeric) rear axle ratio, which certainly benefits those operators in hilly areas, while ruling out the fierce acceleration which could otherwise result from such ratios.
The choice of a lower-speed axle, together with the acceleration limiter, has the added benefit of lowering the gearchange point in the N113's two-speed automatic gearbox when the torque convertor locks up and direct mechanical drive comes in, thereby reducing fuelwasting power losses in the transmission.
On a more practical note, the N113's electronic throttle system, like that in the Scania 143-470 tractor (CM 16-22 March) is basically a 'drive-by-wire' system, which does away with the need for a complicated mechanical linkage between the throttle pedal and engine.
The acceleration limiter system consists of a microprocessor control unit, an accelerator pedal sensor and a control motor. The accelerator pedal sensor is fitted next to the accelerator pedal and measures its position. The control motor is fitted next to the fuel injection pump and operates its accelerator arm. According to the input signals that the control unit receives from the pedal sensor, it actuates the control motor to deliver the corresponding acceleration.
The accelerator pedal sensor is little more than a potentiometer, connected to the pedal by a link rod. When the pedal is depressed its movement is transmitted to the potentiometer, so that the potentiometer's output voltage is directly controlled by the pedal.
The control motor on the injector pump acts via a gear train to rotate a lever which actuates the accelerator arm of the injection pump through a link rod. This also activates a potentiometer in the control motor: the output voltage from this second potentiometer is also controlled by the throttle arm position, providing the control unit with information as to the degree of acceleration.
The accelerator control unit senses the position of the accelerator pedal — the "set point value" — and compares it with the position of the control motor — "the actual value". If they do not agree the control unit will then run the control motor forwards or backwards until the correct acceleration value is obtained. In effect, moving the throttle pedal varies the voltage from the potentiometer into the microprocessor, which feeds voltage to the motor controlling the throttle linkage to the pump.
PEDAL TO THE FLOOR
Pushing the pedal to the floor makes the control motor respond quickly for the first part of the operation, then its movement is slowed down as the linkage approaches the open position.
A sensor in the gearbox feeds a speed signal to the microprocessor which compares the speed against a time base, which tells the computer what the acceleration is. The rate of acceleration (determined by the motor on the pump linkage) is consequently adjusted.
So far the SAL system is only fitted on buses, but there is obvious potential for it to be used on trucks.
With the advent of electronicallycontrolled fuel injection systems, like the Bosch EDC unit fitted on Scania's 143-470 tractor, truck builders have been striving towards far greater control of the fuelling process, as well as engine speed. However, while such systems offer greater control of fuel metering and the injection timing, depending on load, they cannot prevent a driver from wasting fuel through thoughtless excess acceleration, so the SAL system could well be beneficial on a truck, particularly when running empty.
There is, of course, the argument that the quicker a laden truck gets up to cruising speed, and uses the rolling momentum of its mass to full advantage, the better. But by slowing down the actual rate of acceleration, and the rate of engine speed, an accleration limiter could help keep the engine speed within its optimum fuel and torque range for a greater period. There is no point blasting off the line if you have to stop 400 metres up the road for traffic lights, but many drivers still seem to engage in the stop light grand prix.
An acceleration limiter, combined with a top speed limiter and an electronicallycontrolled fuel injection system, might well ensure that the drivers of the latest generation of high-powered artics are less likely to be corrupted by the power beneath their right foot.
El by Brian Weatherley