Does DFCO truely cut off the fuel?

[pete c]

I thought it did, but, got into a debate with some guy who appeared to be quite knowledgable about the subject, that this is not the case.

This took place on a hyundai forum where I went asking about wiring a FI cutoff switch. After the obligatory scolding from the anti-hypermiling crowd, this guy said that the injectors do still fire, but the pulse is much shorter. I argued that a hyper lean condition would result and that ain't good for the engine. He said that hyper lean is not an issue at such a small load.

I can see how this might be done just to keep the cat hot.

What say you ECU gurus out there? Is DFCO truely DFCO?

[California98Civic]

Great setup to a significant question. I'm experimenting heavily with DFCO right now too. And i am skeptical that it is a total fuel cut-off. Not certain though. Wouldn't an MPGuino be the best way to find out, for a given car?

[user removed]

We listened to the injectors with a tech's stethoscope. All injector activity ceased when you revved the engine and closed the throttle completely.

1981 Nissan 280ZX.

regards
Mech

[markweatherill]

I've fitted oxygen sensor gauges to some cars, and always noticed that in conditions where you'd expect DFCO, the gauge stops reading until DFCO stops, then it springs back into life.

[cbaber]

If you really wanted to verify that the injector is shut off, you can look at the signal through an oscilloscope. This is what an injector pulse looks like:



When I was a senior in college, I developed a microprocessor application that functioned like the MPGuino. This software read the length of an injector pulse, and was very precise. From this signal length, and the injector rating, you can calculate how much fuel is used. If an MPGuino is reading 9999, it's because it's not picking up a length for the injector pulse, and therefore assumes no fuel is being used. It's quite possible that the amount of fuel is so tiny that the MPGuino reads 9999 but you might actually be using some fuel, but it would be so little that you could consider it to be no fuel.

Also, my Honda manual confirms that DFCO exists under certain conditions. I don't think the manufacturer would call it fuel cut off if it wasn't actually cutting off fuel.

[user removed]

DFCO is the cheapest way to inject air into the exhaust and remove any fuel delivered previously. In the old carb days closing the throttle quickly lead to a huge increase in unburned hydrocarbons since the vacuum skyrocketed and compression was too low to light off the fuel anyway.
That's why they used air injection, pumps lines and other stuff to break. With EHI, when you slam the trottle closed ony air is passing through the engine. To reduce the peak vacuum which could actually suck oil past good rings they use various types of devices to delay throttle closing to keep peak vacuum from spiking.

regards
Mech

[RedDevil]

Inhowfar does it cut off air intake flow on DFCO?
I imagine that the cat will cool down quickly if you keep pumping air through without burning fuel with it, but it won't if the air is stagnant. And it will not offset the oxygen balance then.

I ask because the Insight lifts the valves in DFCO/EV mode to reduce pumping losses.
I expect it does not pump air towards the exhaust.

I set my UG instant economy readout to l/100 km, so the lower it reads the less fuel is used.
During DFCO it reads 0.00.

[user removed]

I'm sure they have many improvements in the application and control strategies developed over the last few decades. Bottom line is listen to the injectors and blip the throttle. If they stop ticking when the throttle is released then you have DFCO. You could still have it and it might not work without a certain load signal when the throttle is released but ultimately when the injectors stop, there is no fuel delivery.

regards
Mech

[cbaber]

Here is a patent for deceleration fuel cutoff submitted by Honda in the early 1980's:
ÈÑÇÁÉ ÇáÇÎÊÑÇÚ US4434769 - Deceleration fuel cut device for internal combustion engines - ÈÑÇÁÇÊ ÇÎÊÑÇÚ Google

I was also able to find a deceleration fuel cutoff patent from the early 1960's. In both documents the terminology indicates total fuel supply cutoff, not partial.

The main goal of deceleration fuel cutoff is to reduce emissions and damage to the engine. When you don't cut off fuel during deceleration hydrocarbon emissions increase due to more fuel escaping the cylinder. This fuel can heat up the catalytic converter, and cause backfiring.

DFCO is a condition that has to have specific conditions met. If you read the Honda patent, the MAP and RPM are taken into consideration. If fuel is cut off at to low of an RPM the engine can stall.

If you read about the Honda IMA system it mentions how the valves actually rest and seal off the cylinder during DFCO, allowing for less pumping losses (less engine breaking) so that the regenerative braking is soaking up more energy, rather than being wasted by the resistance from the engine.

Honda Worldwide | CIVIC Hybrid

It's pretty clear from the sources I posted that fuel is cut off 100% during DFCO.

[Mista Bone]

On the Honda's I deal with DFCO = injectors are NOT cycled until lower rpm limit where they are brought back online for normal idle issues. It is covered in the Honda factory service manual.