Features - Technical
SEPTEMBER 24, 1997
BY PETER WRIGHT
One technology that survived that ban was drive-by-wire, (see: V6N6), but the control laws and permitted parameters used for feedback, were severely limited in order to ensure that it was not used for traction control. As the procedures for checking software matured and confidence was gained that it was possible to police these regulations, the FIA began to consider easing the restrictions to permit developments that were in line with road car development, but did not take away driver involvement in car control. The development of brake-balance systems and active differentials falls into this category, but are still closely scrutinised to ensure compliance with the letter and spirit of the Technical Regulations.
One of the prime purposes of road car drive-by-wire systems is to refine the process by which the driver demands torque from the engine, and the engine - along with its EMS - delivers it in the most efficient and environmentally clean manner. Mechanically-linked throttles can only control the air flow to the engine at any given moment, while the EMS strives to provide optimum fuelling. Even so, the driver needs skill to modulate the throttle pedal to control the engine output to his needs. Drive-by-wire turns the throttle pedal into a true torque demand - a fixed throttle pedal gives a constant torque output, irrespective of engine RPM, temperature etc, within the overall capabilities of the engine. To do so, the control system must include engine RPM as one of the feedback's.
Until the middle of this year engine RPM was one of the banned parameters in Formula1 drive-by-wire algorithms, because of the fear that it could be used as a surrogate for rear wheel speed in a traction control algorithm. Now it can be used as a feedback, provided it is only used to adjust throttle opening according to an engine map, and in no way can it be interpreted that it is being used to modulate the engine in response to wheel speed or wheel acceleration.
What advantages does this concession give? The answer is twofold: there are significant driveability advantages; and there is potential for tuning the engine for higher peak power.
First it is necessary to understand the driveability problem. A racing engine is tuned, using intake and exhaust lengths and valve timing, to give maximum torque over a working RPM range. With the current Formula1 engines/transmissions, which have a maximum RPM of around 18,000 and 6 or 7 gears, this range is around 3,000 RPM. The torque curve approximates to FIGURE 1. Below the working RPM range, the engine is "out of tune" and torque is significantly lower, transitioning into tune over some quite small, RPM range. Logically, the driver would never use a gear that resulted in an RPM below the lower limit of the working range, in order to maximise acceleration at all times, modulated only by the throttle pedal to control wheelspin and handling. However the range of throttle pedal movement to control torque from zero to maximum (actually less than zero, as closed throttle gives negative torque due to engine drag) is so small that the sensitivity, or gain of the throttle as a control input is too great for most drivers to be able to control handling. To overcome this problem they select a lower gear than optimum for a corner, and force the engine down into the "out of tune" region. Because the torque is so much lower, the throttle pedal gain is also much reduced. Now the driver can control the car. Once full throttle can be applied coming out of the corner, the engine accelerates so quickly in the lower gears, that it is soon up into the optimum RPM range and little time is lost at less than maximum acceleration.
However, if while on part-throttle, trying to control rear wheel grip, the RPM rises into the steep transition region, torque can increase so quickly that it catches the driver out and spins him, particularly on a wet track. This is the region engine tuners try to shape in such a way that the transition is as smooth and gradual as possible. Non-linear, drive-by-wire or mechanical linkages that reduce throttle pedal gain for the initial movement of the pedal, help the driveability.
Feed back RPM into the drive-by-wire control algorithm and suddenly the problem goes away. Now the throttles can be progressively closed as RPM rises to give a constant torque, irrespective of RPM, for any given pedal position. The engine can be run in the transition RPM range, and precise throttle control used to give any torque up to the maximum. No longer will there be a sudden torque increase as the engine starts to come into tune, and by the time full torque is demanded, the engine can be in the working RPM range and ready to deliver it.
Not only is the driver happy, but the engine tuner can discard all the compromises he has had to make to smooth and fill the transition region, at the expense of tuning the engine for maximum torque. He can now fully optimise the working range - the width of which is only governed by the number of gears available.
It will be interesting to see whether, in the longer term, this tips the favour back to a V12, a configuration that is known to produce more power than a V10, but to be "peakier" and to be less driveable.