The reality of clutchless auto-blip downshifts

We often receive enquiries asking if it's possible to use our 'Easyshift' ECU to make clutchless downshifts with a manual gear lever using a throttle blipper. Well, our simple advice is this: don't bother, for the reasons explained below....

First of all, the following information relates only to manual stick shifting and not the Geartronics paddleshift system!

To understand why manual shift 'auto-blipping' is usually a pointless exercise, you must first understand a bit about transmission dynamics and the loads that the transmission is experiencing under various conditions.

We all understand that when we make an upshift, the engine torque puts a 'positive' load on the gearbox. This load 'locks' the drive dogs together, and it's almost impossible, without using extreme force, to disengage the current gear until this load has been removed. This is easily achieved by reducing the torque output of the engine, either by ignition/fuel cut or by ignition retard. Under most circumstances, we simply pull the gear lever, cut the engine and wait for the load to be released from the dogs, at which point the selector barrel will rotate to disengage the current gear and select the next. As the next gear engages, we re-apply engine torque and the vehicle accelerates away. Simple, relatively at least.

Downshifting is obviously different, and actually far more complex to do properly. Downshifts are almost, but not quite, the opposite of upshifts. When you come to make a downshift, the throttle will be closed (or at least it should be), and the forward momentum of the vehicle maintains a high engine speed. In effect, the vehicle is driving the engine, otherwise the RPM would naturally fall to idle. So, under these conditions it can be seen that the gearbox is transmitting a 'negative' load. Just like the upshifts, this load locks the dogs together, preventing disengagement of the current gear. As we've learned from upshifts, we must reverse this load to un-lock the dogs and allow the shift to be made. The obvious solution is to 'blip' the throttle to produce some torque to increase the RPM. It's not quite that simple though, as we will learn shortly. Now, here comes the problem...

When the car has a manual shift, the only way we can trigger a throttle blip is to detect when the driver starts to apply force to the gear lever. We can use strain sensors or load cells to detect the pushing of the lever, and then the electronics can switch a solenoid coil or pneumatic valve to operate a small actuator to open the throttle, or in the case of electronic (DBW) throttle, send a command to the engine ECU to open the motorised throttle electronically. Unfortunately, this takes time, especially in the case of DBW throttles which can typically take 50mS or more to move from the closed position to the 30-50% required for the blip. Not only is the physical speed of the throttle movement an important consideration, but the actual response of the engine is far more significant. Most engines, even when accurately calibrated, don't respond instantly when the throttle is opened, in fact we have worked with many engines that are so badly calibrated that the RPM initially continues to fall when the throttle is suddenly opened. Turbocharged and supercharged engines pose an even bigger challenge because of the generally poor off-boost performance and often large inlet plenums and large distances between engine & throttle, resulting in delayed air delivery to the engine.

This delayed throttle response wouldn't be too much of a problen until you consider the load on the transmission during hard braking. As we mentioned previously, the forward momentum of the car drives the engine via the gearbox to maintain the high RPM. This is responsible for the load on the dogs that prevents gear disengagement. However, imagine the situation where the car is braking very hard, so hard in fact that the rate of retardation of the car matches the natural decay rate of the engine RPM. In this situation the gearbox is completely unloaded and the dogs become unlocked. Now, when the driver pushes the gear lever to make a downshift, the gear will disengage instantly because of the unloaded dogs. So, what happens is that the shift completes very quickly, so quickly that the throttle blip has had no opportunity to produce the necessary RPM increase, even if the physical throttle movement is very fast. At the point of next gear engagement, the RPM will be too low and the shift will be very aggressive as a result of the sudden engine braking brought about by the RPM being mechanically forced to match the road speed. On a slippery surface this can, and often does, cause the driven wheels to momentarily lock-up, potentially leading to a loss of control.

As stated at the top of this page, the above information relates only to manual shifting and not semi-automatic 'paddle' shifting. When we use our paddleshift system, the situation is very different because we are in complete control of the downshift sequence, including throttle blip and gear lever movement. When a driver commands a downshift using paddles, an electrical signal is sent to the gearbox control unit (GCU), which then takes over to co-ordinate the sequence of events. To compensate for the delayed throttle response, we actually open the throttles well before pushing the gear lever. How and when the engine responds is then precicely controlled by the application of carefully timed ignition or fuel cuts. This ensures that as the selector barrel rotates from one gear to the next, the engine RPM increases to first unload the dogs and then to match the next gear. The result is super-smooth downshifts that don't damage the dogs or cause a loss of vehicle control.

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