Jason C
05-04-2005, 08:59 AM
So I got back from a very long impromptu conversation with someone in the know - and damn! It was one of those "always thought it might have been that but needed to confirm it" type of deals.
I've seen people here complain about throttle-by-wire, mostly talking about perceived "lag" but also mentioning that in some instances revs were slow to come down during a throttle lift - making the engine seem lethargic. This is in (extreme) contrast to, say, a racing engine, supersport motorcycle engine, or that lovely V10 in the Carrera GT.
Now I have always had a suspicion that it was emission related, and after learning about what happens to the fuel/air ratio upon a sudden throttle lift, I was pretty sure. Now I'm definitely sure:
First of all, when you use an electronic throttle, your right foot is merely sending a *request*. That's it! That's all - a "request" to the computer that you want more air or less air to flow to the combustion chambers. What the computer does with that request is entirely up to the programming. Comforting, eh? :)
Most (if not all) cars have some sort of IAC along with the throttle. IAC stands for Idle Air Control, and the main reason it's there is to provide air for the engine when the throttle plate is completely closed. So when you're idling, the air that the engine receives is going through an IAC plumbing and bypassing the closed throttle plate. Some cars have tiny IAC's (certain GMs) while other cars have a relatively large IAC (BMWs, Hondas).
What does that have to do with that despised DBW? Well, consider...
When you abruptly lift the throttle, especially if you were at WOT just a split second before, you cut off the main flow of air to the combustion chambers - and create a momentary rich condition. This is because the fuel injectors are still a half-beat behind, they have to first get a signal from the PCM to alter pulse width. The PCM in most instances is waiting to get a signal from the pre-O2 sensor before doing anything. The O2 sensor has to wait a little further for some combustion gasses to analyze. All this takes a little time.
Meanwhile, as you drastically reduced airflow via a throttle lift, the injectors haven't responded instantaneously. They're still assuming big throttle opening = big pulse width, so they continue to dump a lot of fuel into the combustion chamber for a moment before the PCM signal gets to them. This is not good (emission wise): Rich condition = high HC/CO levels and accelerated wear of the catalytic converter. As automakers are under pressure to reduce emissions and increase vehicle longevity, they'll do all sorts of tricks to reach that end, damn the means.
Getting back to the DBW BMW: Accelerating, big lift, what happens now? When the computer sees above-idle RPMs and a closed throttle, it assumes you are trying to decelerate. It will do what it can to make the process gradual. By doing this, all the components named above have more time to react and sort things out and you don't dump as much fuel into the exhaust. The BMW throttle-by-wire uses the IAC in order to accomplish it, like this:
• Big throttle lift after acceleration.
• Throttle plates fully closed, computer recognizes this via the TPS
• Computer is pre-programmed to prevent a sudden rich condition under a throttle lift
• Computer initiates preprogrammed setting(s) while you are decelerating
• IAC is fully opened by the computer, allowing a large amount of air to flow to the combustion chamber
• Computer juggles IAC opening (like a throttle body!) and pulse width to gradually bring RPMs down, minimizing any momentary rich condition
• Carmudgeon curses as engine sluggishly responds.
• Carmudgeon vows to swear off DBW. Goes on to ditch DBW and buys E36 M3/E30 M3/Red 325is with tan interior/etc.
• Carmudgeon spends the rest of his days bad-mouthing DBW on the interweb. :P
(PS: I know I haven't said a word regarding the Michigan trip, I'll post it eventually :flipoff: )
I've seen people here complain about throttle-by-wire, mostly talking about perceived "lag" but also mentioning that in some instances revs were slow to come down during a throttle lift - making the engine seem lethargic. This is in (extreme) contrast to, say, a racing engine, supersport motorcycle engine, or that lovely V10 in the Carrera GT.
Now I have always had a suspicion that it was emission related, and after learning about what happens to the fuel/air ratio upon a sudden throttle lift, I was pretty sure. Now I'm definitely sure:
First of all, when you use an electronic throttle, your right foot is merely sending a *request*. That's it! That's all - a "request" to the computer that you want more air or less air to flow to the combustion chambers. What the computer does with that request is entirely up to the programming. Comforting, eh? :)
Most (if not all) cars have some sort of IAC along with the throttle. IAC stands for Idle Air Control, and the main reason it's there is to provide air for the engine when the throttle plate is completely closed. So when you're idling, the air that the engine receives is going through an IAC plumbing and bypassing the closed throttle plate. Some cars have tiny IAC's (certain GMs) while other cars have a relatively large IAC (BMWs, Hondas).
What does that have to do with that despised DBW? Well, consider...
When you abruptly lift the throttle, especially if you were at WOT just a split second before, you cut off the main flow of air to the combustion chambers - and create a momentary rich condition. This is because the fuel injectors are still a half-beat behind, they have to first get a signal from the PCM to alter pulse width. The PCM in most instances is waiting to get a signal from the pre-O2 sensor before doing anything. The O2 sensor has to wait a little further for some combustion gasses to analyze. All this takes a little time.
Meanwhile, as you drastically reduced airflow via a throttle lift, the injectors haven't responded instantaneously. They're still assuming big throttle opening = big pulse width, so they continue to dump a lot of fuel into the combustion chamber for a moment before the PCM signal gets to them. This is not good (emission wise): Rich condition = high HC/CO levels and accelerated wear of the catalytic converter. As automakers are under pressure to reduce emissions and increase vehicle longevity, they'll do all sorts of tricks to reach that end, damn the means.
Getting back to the DBW BMW: Accelerating, big lift, what happens now? When the computer sees above-idle RPMs and a closed throttle, it assumes you are trying to decelerate. It will do what it can to make the process gradual. By doing this, all the components named above have more time to react and sort things out and you don't dump as much fuel into the exhaust. The BMW throttle-by-wire uses the IAC in order to accomplish it, like this:
• Big throttle lift after acceleration.
• Throttle plates fully closed, computer recognizes this via the TPS
• Computer is pre-programmed to prevent a sudden rich condition under a throttle lift
• Computer initiates preprogrammed setting(s) while you are decelerating
• IAC is fully opened by the computer, allowing a large amount of air to flow to the combustion chamber
• Computer juggles IAC opening (like a throttle body!) and pulse width to gradually bring RPMs down, minimizing any momentary rich condition
• Carmudgeon curses as engine sluggishly responds.
• Carmudgeon vows to swear off DBW. Goes on to ditch DBW and buys E36 M3/E30 M3/Red 325is with tan interior/etc.
• Carmudgeon spends the rest of his days bad-mouthing DBW on the interweb. :P
(PS: I know I haven't said a word regarding the Michigan trip, I'll post it eventually :flipoff: )