Over-run fuel cut with IAC valve wide open

[16vboost]

I'm wondering if others would be interested in this feature. The idea is to open the idle valve during over-run to reduce the engine pumping loss. This would allow you to coast farther / longer and stay in fuel cut longer (saving fuel).

So the idle valve would still close during normal driving and remain closed between shifts but when entering over-run fuel cut, the injectors would cut AND the idle valve would ramp to open, letting you coast without using fuel. Then when the conditions for exiting over-run are met, the valve will immediately close THEN the injectors will resume spraying so you don't feel a jump. Maybe the ignition timing can start way retarded when the injectors turn on and advance to normal over a half second or so to further reduce the jump back to fueling.

Thoughts?

-Alex

[piledriver]

Actually that will INCREASE engine braking.
The only way to really eliminate it is push the clutch in.

It would reduce sucking oil past your rings due to overrun vacuum. Has been suggested for that reason previously.

[Laminar]

It would reduce sucking oil past your rings due to overrun vacuum. Has been suggested for that reason previously.

This is one reason that OEMs let the throttles "hang" after throttle lift via either the IAC or TBW control. The first time I let off the gas in my MS-controlled car I jerked forward because I wasn't used to the car reacting so suddenly, I was used to the dampened response of the OEM setup.

[piledriver]

I've seen it as well, they also keep fuel and spark on in some regimes for this and emissions reasons.

Kill fuel and spark sometime and see if it slows faster at WOT or full vacuum. I have.

[dontz125]

Kill fuel and spark ... it slows faster at WOT

The heavy diesel guys call that a Jacobs brake ...

[16vboost]

Actually that will INCREASE engine braking.

Really? A thermodynamics professor told me otherwise. He suggested turning the engine off during over-run and applying throttle. Of course I didn't believe him so I tried it and he was right. The engine brake is almost eliminated. Just make sure you don't go locking the steering column while doing this.

Anyway I like the over run oil consumption benefit too. My valve seals are a little "relaxed"

-Alex

[piledriver]

When I tried it it was just about a wash, seemed to have a little faster decel at WOT.
(have a kill switch that keeps the inj and coil power off for firmware burns, makes for a great immobilizer if I forget about it)

Kill fuel and spark ... it slows faster at WOT

The heavy diesel guys call that a Jacobs brake ...

That's a great example...but not the same thing.
A Jake Brake works a little differently by opening the ex valves on what would be the power stroke, eliminating the energy recovery from the compression stroke... the energy put into compression is normally largely recovered on power stroke.(minus heat/frictional losses)

With the ex valves open ~tdc of the power stroke, you release the high pressure gas (the "Jake Brake" sound), the energy of compression is then a ~dead loss so braking is increased further than even WOT.(which is all the time on a diesel of course)

For more fun and Carnot cycle games, consider the (modern implementation of) Atkinson/Miller cycles, where the intake valve duration is much longer than exhaust, making (effectively) for a longer power stroke than compression stroke, resulting in higher thermodynamic efficiency.

I stopped closing my IAC, or at least limited the zero to ~25%, reduced my oil consumption to almost nothing, before doing that I was sucking a quart in ~1K, closer to 4K now. (also run limited crankcase vacuum, but that setup has been like that for years)


The OEMs tend to open the IAC to a fast idle and keep the fuel @~ stoich and spark on, reducing the braking effect and greatly reducing HC emission (sucking oil past the rings) makes for a smoother drive in lower gears/light throttle esp.

[NorwegianScum]

This would probably allow me to have over run fuel cut engage alot more in rush hour traffic, without sacrificing a smooth driving experience. I will literally donate money if this gets made, cause I know this will save me money.

It even has a safety aspect to it when letting go of the throttle (engaging fuel-cut) mid corner on slippery conditions as it removes the abrupt engine braking, which is some cases can make you loose grip.

Slightly related: Perhaps idle valve opening could be controlled on a table in relation to coolant temp or intake temp x RPM as well, in order to influence how fast engine speed falls when letting off the throttle, and how much engine braking you want for your particular motorsport and driving style.

I am not ashamed of bumping this perfectly good suggestion FYI