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Monday, February 8, 2010

Why Throttle-by-Wire not Cable?

I've seen many comments in discussion forums bemoaning the move from cable to electronic throttles. Here are a number of reasons why that's necessary. Part of this stems from a visit in January to a supplier that developed the vehicle stability system for the BRP Spyder three-wheel motorcycle. Though I'm not an engineer, I've sat through enough engineering presentations to give a sense of the "why."
First, e-throttles improve fuel efficiency; even a skilled driver will give too much or too little gas as they change speed, climb a hill, etc -- an electronic throttle acting through fuel injectors can adjust 50 times a second, probably faster... That's even more important if you have a turbocharger as it can take a while for them to adjust to a change in speed. And of course the output signal can also be used by the transmission to adjust the shift point if, for example, it senses a hard acceleration.
Second, e-throttles also allow emissions to be cut: add in not just throttle and wheel speed but also the oxygen sensor and an electronic throttle lets the engine adjust to reflect a host of other parameters, something impossible with a cable.
Third, you need it for traction control and stability control, as the car must be able to automatically ease off the throttle if it senses the wheels slipping.
Fourth, in a hybrid you have to coordinate the electric power with the engine power, there is no way for a person to process the data needed to do that, much less process the data and drive at the same time.
Fifth, you need it for fast stop/start where easing off on the gas or braking turns off the engine, and then turns the engine on again, all so fast that you don't notice it. That boosts fuel efficiency immensely, esp in city driving.
Sixth, cables are probably worse than an electronic throttle in terms of intrinsic quality, you have to keep dirt and moisture from getting in yet allow the cable to move in and out, you have to get the physical connection right in assembly, kinks or excessive vibration and you can have friction in one spot until it seizes. And you still need springs, and you can still get the pedal stuck on carpet or get your shoe jammed under it or. I've had to replace the gear shift cable, on a car but not a throttle cable that I can recall. Experienced boaters know to check their cables periodically -- I had a cousin killed when a steering cable failed and threw him from a boat.
Seventh, a cable is physically bigger than wires, weighs more, and costs more than a pedal sensor plus wires.
In short, the combination of consumer and regulatory constraints on a modern vehicle mandate an electronic throttle.

Mike Smitka