Torque control

Wednesday, February 19, 2014

Tags :  electronics

To many, the thrill of balancing steering against throttle opening while cornering hard is what motorsport is all about. That feeling of keeping a racecar at the edge of its performance envelope and only inches away from potential disaster may satisfy the most basic primeval instinct, but between driver and oblivion there is a web of maps and algorithms that is becoming more complex as time goes on.

At its simplest, the throttle pedal exists as an air valve; opening and closing according to the demand by the driver, the air is mixed with fuel and burned to deliver torque to the vehicle wheels. At its most basic level therefore, an electronic fuelling system is a device for delivering torque over a range of engine speeds and loads using a series of ‘maps’ specifying lambda (the air-to-fuel ratio) and ignition timing to give optimum performance. Carefully calibrated and stored electronically in what’s called a look-up table, this torque can be controlled away from its maximum by the simple expedient of retarding the ignition timing, which can be done quickly and accurately. This in principle therefore was the basic building block behind the original engine management systems.

However, as engines become more complex and the complexity of control therefore increases, the process of calibration becomes more onerous, and the more complex things become, the more onerous it gets. In the early days the cylinder charge was affected by the throttle angle, the fuel by the calculated injection time and the engine efficiency by the control of the ignition. If we then add a desire to include boost pressure, camshaft phasing or variable intake systems – not forgetting powertrain control, of course, since many of these interactions will occur simultaneously – it is difficult to predict the overall effect and consequently the output of the engine. And an engine whose torque output is inconsistent does not inspire confidence in the driver.

What is needed is a clear control structure and a system of prioritisation to ensure that the torque demanded by the throttle pedal is the same as that produced at the vehicle wheels. Fortunately, the invention of the electronic throttle – sometimes referred to as a fly- or drive-by-wire throttle – has made all this possible, and we now use for engine control what is commonly known as the torque-based approach.

The first stage in any system is to convert the driver demand into a torque requirement gleaned from the pedal map. A function of engine speed and pedal position, the resulting output is the precise torque to be delivered, and how that is to be delivered will be down to what is known as the system architecture.

Modern engine control can use many different devices to alter the output torque. This architecture must therefore provide the basis by which all engine actuators (throttle position, spark advance, variable intake devices and so on) are coordinated to produce the required output demanded by the pedal map.

Furthermore, control has to be prioritised between fast and slow response in order to achieve the correct transient demand, and all of this must be achieved using the minimum of fuel.

The overall system also includes feed-forward and feedback subsystems. The feed-forward system provides the calculation to produce the estimated desired actuator position to give the torque requested using compressible flow models. Inputs of desired manifold pressure and desired air per cylinder are fed into the computer model to produce the required throttle position commensurate with the required torque. The feedback system then corrects this feed-forward subsystem based on this estimated torque. In the end, the driver should get precisely the torque requested.

While perhaps not as important in most motorsport applications in the past, the new breed of Formula One powertrains will never be able to function without some form of torque-based control.

Fig. 1 - It’s all in this little box

Written by John Coxon

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