Drive-By-Wire

Posted by murmini Sat, 23 Sep 2006 13:29:39 GMT

The throttle pedal in the MINI is just yet another reminder of the BMW connection that MINIs have. That reassuring underpinning of BMW tried and tested performance technology from which we benefit.

The gas pedal on many cars connects with the throttle body via either a metal linkage, or a flexible steel wire (just like a brake cable on a bicycle). When you step on the gas it opens the throttle and allows more air to flow into the engine, (probably should be called the air pedal) in turn, the engine demands more gas and the engine goes faster. With the advent of fuel injection the engine management system sees the throttle open via a throttle position sensor and instructs the fuel injectors to provide more gas with the same overall effect.

The standard throttle linkage is prone to issues effecting it's general overall reliability that can result in a lack of responsiveness. If it snaps your engine is going to be stuck at idle, if it sticks open, then its white knuckle driving until you can turn off the engine. (see remark below!) It is also prone to the linkage wearing out, becoming sluggish due to lack of lubrication and so forth. It is a mechanical device prone to the ailments of all mechanized things.

For some time now, BMW has been employing a 'drive-by-wire' technology that converts the motion of the throttle pedal to a signal that the Engine Control Module (ECM) computer can use to determine how much throttle is being depressed and how fast you depressed it.

The MINI gas pedal, shown above (with BMW Group identifier) has no physical connection to the throttle body. It is a stand alone device with a 6 pin electronic connection. Simply, it is a device that converts the spring-loaded up/down action of the throttle pedal into a rotary action. This rotary action is measured or gauged by a Hall Effect sensor that converts the motion (and subsequent position) into a voltage signal almost like a volume control determines how loud a radio is. Below is a view of the box containing the two Hall Effect devices and the 6 pin connector.

Now, just in case your overtaking a car one day and your worrying if this thing might fail, they have built in a level of redundancy and subsequently made this gadget extra safe. I mean, after all, it's your 'go fast' pedal! The extra level of redundancy is incorporated by using two hall effect devices on the rotational shaft evident by the six pin connector on the throttle pedal assembly. A five volt input/output and ground drives one hall effect device or throttle pedal sensor and a second, completely independent 2 volt input/output and ground provided by a completely separate circuit, drives the other one. This acts as a redundant system seeing a drivers request for 'more gas' and a 'plausibility check from the second or backup sensor. The following diagram is a very simplified version of what is going on.

Its a clever system that constantly monitors both throttle position, 'how fast you want to go' and rate of throttle opening, 'how fast you want to get to that speed' . The signals received from this are processed by the ECM and subsequent required quantities of air and fuel are calculated and delivered by opening the throttle body (air) and by opening the fuel injectors (gas) accordingly.

Many other devices are also providing simultaneous data to the ECM such as the TMAP (Temperature and Manifold Absolute Pressure) sensor and the Manifold Air Pressure MAP sensor informing engine management of the ambient conditions. Additional devices also deliver data input, like the camshaft positioning sensor which assists in providing the synchronization necessary to fire the ignition spark and burst of injected fuel at the critical moment.

This is certainly more reliable than the traditional throttle cable; a piece of flexible, braided steel wire, within a plastic sleeve, that ran from the throttle peddle, through the firewall into the engine compartment, to the throttle body. I recall in an early 1970's mini I used to own, where the engine grounding was so bad that every time I started the engine, a good load of amps found their way down the throttle cable, sufficient anyway to heat it up to a point where the plastic housing would melt and cause a recurring sticky throttle cable. A problem that took a while to solve. Believe me, sticky throttle cables can be quite alarming!