Tuning Wiki

Pedal response and surge damping

How the accelerator becomes a torque or fuel request, and why surge damping, cruise and speed limits change what the driver feels.

Updated by llandowtuninguk@gmail.com   Revision 2

The pedal is a request, not the output

On an EDC diesel the driver no longer pulls a cable to the injection pump. Bosch describes the accelerator-pedal sensor as a voltage that the ECU turns into pedal travel through a programmed characteristic, with a second sensor or an idle switch so the signal can be checked. In normal driving the injected quantity is then a function of that pedal setting and engine speed, using maps that also take in influences such as fuel and intake-air temperature. Bosch, accelerator-pedal sensor; drive mode.

From EDC16 onwards the same pedal input is interpreted as a request for propulsive torque. Cruise control and the vehicle-speed limiter are processed in the same selection. Traction control can raise the request when a wheel is about to lock and lower it when a wheel is about to spin. The quantity that follows is whatever remains after those demands and the limits. Bosch, torque-controlled EDC, printed pages 75–77.

A petrol torque-structure ECU does the same kind of translation into an air charge rather than straight into a diesel fuel quantity. ME7 documents a pedal and engine-speed contribution to requested relative torque. Simos 18 and 19 examples document pedal interpretation and further caps that can depend on gear, speed, air density and drive mode. Those are family examples, not a common map.

Why a sharper pedal can feel worse

A sudden torque change excites the drivetrain. Bosch says the occupants feel that as unpleasant periodic changes in acceleration, and that active surge damping exists to reduce it. Two actions are described. A filter matched to the drivetrain slows the driver's torque step so the drivetrain is not kicked as hard. A second, active correction uses engine-speed signals: it reduces injected fuel when speed is rising through the oscillation and increases fuel when speed is falling. Bosch, active surge damping.

That is the calibration version of a car that shunts, rocks on the engine mounts, or feels sharp and then loose. Making the pedal map steeper asks the filter and the damper to do more work. Removing the filtering without looking at the mounts, the clutch or the flywheel moves the oscillation to the driver. On a diesel, the same tip-in is also where the smoke limit and the turbo decide whether the torque the pedal asked for can exist yet.

An accelerator-pedal box creates that steeper request outside the calibration. It sits on the pedal-sensor wiring and rescales the signal, so less travel is read as a larger demand. Full travel is already the largest request the maps produce, so the box does not raise the full-load ceiling. The risks, and the separate diesel and petrol tuning boxes that falsify rail or manifold pressure, are in tuning boxes and pedal boxes.

Pull-away, cruise and a speed limit

Idle speed is kept low for emissions and consumption, and Bosch notes the disadvantage for smooth running and for pulling away. The first part of pedal travel therefore has to leave idle, take up the clutch or the converter, and meet a drivetrain that may already be close to surge. A map that is very flat at the top and very steep at the bottom concentrates that problem in the first few millimetres of travel.

Cruise control is a separate request for a road speed. On some applications the pedal can override it, and the brake or clutch releases it. A vehicle-speed limiter holds fuel down to the selected road speed even if the pedal goes further; Bosch describes a kickdown switch as one way that limit is released on the variable limiter. A fixed limiter is a different function and is not described as driver-switchable. None of these is the engine rev limit, which cuts fuel back as engine speed approaches the maximum the manufacturer set, using a ramp so the cut is not a step. Bosch, idle, maximum-rpm control, cruise and the speed limiter.

If the car stops accelerating while the pedal is still down, log whether engine speed or road speed was the thing that stopped rising. The two limits feel similar from the driver's seat and they are not the same table.

A practical reading

Compare pedal position with the requested torque or quantity, then with the value after limitation, then with measured air or cylinder charge and with engine speed. A pedal change that never appears in the request is a sensor, a switch or a drive-mode selection. A request that never appears in delivered torque is a later limit: smoke, temperature, a gearbox shift, traction control, or the turbo still catching up. Those later limits are collected in torque requests and the limit that wins.

Sources