EGR, Idle Air, and downhill pumping losses

[Interlocutionist]

I was driving today and as I was going downhill with my foot off the gas I was thinking about the pumping losses and EGR and the IAC that was probably open because the throttle was shut and I thought, "what about using the EGR instead of the IAC?"
Both the EGR and IAC bypass the throttle plate, so when coasting downhill or just decelerating, one or both of those valves have to be opened. So I was thinking, if the EGR was routed to the IAC input, or the IAC was disabled somehow and the closed-throttle airflow was all from EGR, what effect would this have and would there be any benefits or problems?

[Olympiadis]

Originally Posted by Interlocutionist

I was driving today and as I was going downhill with my foot off the gas I was thinking about the pumping losses and EGR and the IAC that was probably open because the throttle was shut and I thought, "what about using the EGR instead of the IAC?"
Both the EGR and IAC bypass the throttle plate, so when coasting downhill or just decelerating, one or both of those valves have to be opened. So I was thinking, if the EGR was routed to the IAC input, or the IAC was disabled somehow and the closed-throttle airflow was all from EGR, what effect would this have and would there be any benefits or problems?

Most newer cars will engage DFCO during heavy DECEL conditions. During DFCO the fuel injectors do not fire, so there would be no advantage to program out the throttle-follower calibrations for the IAC valve. Normally the IAC is commanded to go to full-open.
The EGR normally doesn't function during heavy DECEL. One reason is because of the lack of pressure in the exhaust during these conditions.
Each case would be different, - pressure varying with different cars and different driving conditions.
You could induce some EGR flow during DECEL by over-riding the valve, but the mass of the flow would be less than if the engine were loaded.

The catch for increasing air-flow during heavy DECEL is that most calibrations watch the MAP output to check for DFCO qualifications. If you increase your manifold pressure, then DFCO will disengage and you're back to injecting fuel.

You can get around all of this with an injector kill-switch, and simply open the throttle during coast-down.

[Old Mechanic]

With DFCO working the exhaust would basically have no combustion byproducts to recirculate. No fuel to burn so no exhaust to recirculate. I guess you could recirculate the exhaust but for all practical purposes the bypass circuit does the job of preventing super high manifold vacuum which would tend to pull the oil up past the piston rings.

regards
Mech

[California98Civic]

Originally Posted by Olympiadis

You can get around all of this with an injector kill-switch, and simply open the throttle during coast-down.

Are you suggesting kill the injectors and then open the throttle in gear? Not trying to be negative. I just don't understand.
Thanks,
james

[Interlocutionist]

Some of the more primitive cars probably do not have DFCO built in, such as the one I'm currently working with. Which is why I'm trying to figure out ways to attempt to retrofit more advanced technology to relpace the old, not so effective controls.

An injector shutoff does sound like an interesting idea. Maybe even tie it in with an alternator kill switch; injectors turn off and alternators turn on while coasting downhill, injectors back on and alternator off when the engine needs to go again.

[Olympiadis]

Originally Posted by California98Civic

Are you suggesting kill the injectors and then open the throttle in gear? Not trying to be negative. I just don't understand.
Thanks,
james

That is what some people do.
I have an automatic and only do engine-on coasting.

[Old Mechanic]

Originally Posted by Interlocutionist

Some of the more primitive cars probably do not have DFCO built in, such as the one I'm currently working with. Which is why I'm trying to figure out ways to attempt to retrofit more advanced technology to relpace the old, not so effective controls.

An injector shutoff does sound like an interesting idea. Maybe even tie it in with an alternator kill switch; injectors turn off and alternators turn on while coasting downhill, injectors back on and alternator off when the engine needs to go again.

I know DFCO was used on the 1981 Nissan 280Z, possibly even the 1975 model, the first year they were injected. If your car is fuel injected just place a screwdriver tip on one of the injectors with the plastic handle in your ear, making it a crude stethoscope. Rev the engine and listen to the injector. If it stops clicking completely when you let off the throttle, it has DFCO.

regards
Mech

[California98Civic]

Could more people chime in on this point raised in this thread about engine off coasting in gear on manual transmissions? Can a manual transmission car with an injector cut-off switch can safely coast-down in gear with the injectors off? With the car on, would such a scenario mean alt charging but no fuel? Wouldn't it be damaging to the engine and/or transmission? Would lubricants be circulating?

[Daox]

Its perfectly fine to do in a manual trans. I've done it a few times with my Paseo down larger hills to help charge the deep cycle battery I have (because I'm running an alternator delete). There is still very noticable drag due to the engine still pumping and compressing air. But, there is a noticable difference between WOT and no throttle. I don't do it often because IMO its really not worth it. Its also a pain to flick on the alternator switch, hold down the injector kill switch, hold down the gas pedal, select a gear to get rpms up a little... then at the bottom of the hill reverse it all to start the engine back up.

[California98Civic]

Originally Posted by Daox

Its perfectly fine to do in a manual trans. I've done it a few times with my Paseo down larger hills to help charge the deep cycle battery I have (because I'm running an alternator delete). There is still very noticable drag due to the engine still pumping and compressing air. But, there is a noticable difference between WOT and no throttle. I don't do it often because IMO its really not worth it. Its also a pain to flick on the alternator switch, hold down the injector kill switch, hold down the gas pedal, select a gear to get rpms up a little... then at the bottom of the hill reverse it all to start the engine back up.

Forgive me for being this dense, Daox, but here is how I hear this: I can coast down a hill near work that is a mile and a half and very steep in 4th or 5th gear, injectors OFF, pedal to the metal, and key in the run position and thereby charge my battery via the alternator without risking damage to the engine? That's what you have done?