Features - Technical
MARCH 8, 1999
Supply and Demand
BY PETER WRIGHT
How can the best driver in the world stall his $2,000,000 car trying to get it into first gear for the start of two consecutive races?
The hydraulic system supplies high-pressure oil to the gear-change, clutch, differential, drive-by-wire throttle, variable length intakes and the power steering systems. An engine driven pump pressurises oil, which is then stored in a small accumulator and supplied to actuators. Moog electro-hydraulic servo-valves, controlled by the appropriate computer, meter the oil flow according to the demands of the control algorithm. The size of the pump and accumulator are kept as small as possible as they are heavy items and the pump consumes power. The pump is driven at some ratio of engine speed and, on the track, the flow is greater than the demand. The surplus oil is either spilled through a pressure-relief valve or, if a variable-displacement pump is used, the pump changes its stroke to match the demand, while the accumulator deals with peak flows to the various servos. Unfortunately this is not the critical design case; the start of the race is.
Coming up to the line, engine revs are low as the driver tries to prevent overheating, maybe 20-30% of peak. Pump flow rate is therefore only 20-30% of its peak too. Just as the hydraulic system is struggling to keep the accumulator full, the driver is making maximum demands on his control servos with frequent use of the gear-change and clutch, weaving about with the power steering, and blipping the throttle, which also causes the intakes to try and keep up with the engine rpm - all of which use hydraulic oil. At the same time the servo-valves bleed oil away continuously, needing the power to drive their second stage hydraulic amplifiers. Demand is at its highest just as supply is at its lowest. If the accumulator empties, pressure will instantly fall, causing the computers to lose control of the actuators. Engaging the clutch and selecting first gear is just the sort of action to do this, while the driver, controlling the clutch via a steering wheel paddle, cannot feel that it is not fully disengaging. Either first gear fails to select, or in it goes and the dragging clutch stalls the low-inertia engine, as the anti-stall software cannot control the throttles to catch it.
There are dozens of other possible reasons why Schumacher was unable to leave the line in those two races, both technical failures and driver errors. However, the hydraulic systems are designed so close to the limit, and the critical start-line case is so different from the requirement when the car is running at speed, that it is easy to see how even the world's best driver could be caught out by it.