Drive-by-wire for dual carbs/throttle bodies???, would you run them?? Options

[Mueller]

A lot of newer cars do not have the traditional cable that goes from the gas pedal to the throttle body...instead the pedal has a potentiometer*, this in wired to the ECU which tells the throttle body how much to open, just because you have your foot buried to the floor, the throttle body might only be open 50% since the ECU is dictating what is going on.

Now a drive-by-wire throttle body could be wired directly to the pot. on the gas pedal to be 1:1 at all times......with a setup like this on dual carbs or individual throttle bodies, no need to worry about thermal expansion of motor and how that effects the linkage.


* (similar to a variable resitor which varies the voltage output depending on foot placement)

[davesprinkle]

Drive-by-wire pros:

Pro #1. Emissions. Good fuel control boils down to a problem of estimating airflow into the cylinder. If the airflow estimate is wrong, then the engine will have an undesirable A/F ratio, and emissions will suffer.

In old-fashioned systems, where the driver controls the throttle, the ECU must operate in a "reactive" mode, responding after-the-fact to throttle movements. In transient conditions, ie when the driver makes rapid throttle movements (either opening or closing), the complex dynamics of manifold filling and wall-wetting make accurate fuel-control very difficult.

In the more modern drive-by-wire systems, the ECU meters both air AND fuel into the engine. In this way, the ECU is able to avoid those unpredictable and large-magnitude throttle transients that wreak havoc with A/F ratio and emissions. In these systems, the throttle pedal is no longer a throttle-actuator, but rather a throttle-request. The ECU says: "I see you just planted the gas pedal on the carpet. I will respond by slowly opening the throttle in a manner that allows me to avoid a troublesome A/F ratio transient that might cause me to fail an EPA emissions test."

Pro #2. Torque smoothing. All drivers (racers and street drivers) like to have a linear pedal vs. torque response. A computer can easily deliver this in a drive-by-wire system. I might add that clever mechanical design can do the same with a conventional throttle system, although with less flexibility -- pulleys, levers, and/or cams are more difficult to iterate upon than is a software table.

Pro #3. Torque limiting (rev-limit, pit-lane speed limit, traction control, launch control, etc.). This is the big advantage of drive-by-wire for motorsports. In a conventional system with a driver-coupled throttle, the ECU can limit torque by ignition retard, or by spark & fuel cuts. This is effective, but in many cases is very brutal, with big torque transients that can cause potentially damaging driveline oscillation. The F1 guys, running drive-by-wire, have figured out that the smoothest way to reduce torque is by limiting the air entering the cylinder -- just close the throttle.

OK, for the Cons:
Con #1. Lousy throttle response. My own personal experience is that the Suby and in particular the Lexus DBW systems are far too aggressive at attenuating throttle transients. They feel lazy and sluggish, even dangerously slow. The engine doesn't respond on a downshift throttle blip. If you plant the throttle, the ECU will "think" for a noticeable fraction of a second before it slowly feeds in more torque. Probably good for emissions, but not so good for avoiding that SUV bearing down on you. I should point out that DBW systems don't HAVE to be so bad. The Boxster that I drove felt exactly like a cable-driven system. I've heard the same about the Corvette.

Con #2. Safety. This is fairly self-explanatory. Do you really want a complex and potentially failure-prone computer system intercepting your throttle requests? There's just something reassuring about a direct mechanical connection between the driver's foot and the throttle blade.

Con #3. Complexity. The DBW systems out there have tens if not hundreds of man-years invested in their design and validation. And for good reason -- refer to Con #2.

Con #4. $$$ All those computers, actuators, sensors, and man-years add up to a substantial cost. You can rest assured that not a single auto manufacturer would have designed DBW if not for first item in the list -- emissions.