DFCO for non-DFCO cars.
 

I know many people don't have Deceleration Fuel Cut Off (DFCO) on their cars. DFCO is useful to have when coasting and light braking is desired, like when coasting up to a light and you think you'll need to stop.

DFCO detects a geared deceleration, sometimes by torque and sometimes using other methods...

This thread is to develop an OPEN SOURCE upgrade circuit to add DFCO to non-DFCO cars at low cost.

I have a simple notion that first we'll talk about how that might behave, and what the systems and logic are. Then, we'll combine skills to work out the logic and connections, design a PCB and software or whatever we decide is required, then we'll test it, and finally make a few and see how they work.

At every stage the knowledge, designs and so forth will be freely available for the benefit of the community.

To begin, does anyone have a good knowledge of the issues involved with adding DFCO to a non-DFCO car?

I'm not ready to do it yet, but I was planning to add extra DFCO to my auto Datsun.

The Datsun only shuts off the fuel if you're above 3000rpm with the throttle closed, holding it down to 2700rpm, where the fuel turns on again. Almost useless in every day driving.

I was going to use a digital vacuum switch (because I already have one) set to switch at a bit higher than normal (idle) manifold vacuum (maybe with a 1 sec timer?). That would then control an injector kill switch circuit.
The Bosch L-jetronic EFI the Datsun uses isn't very sophisticated.:o

So, we should add rules for operation of the DFCO, like min revs, etc....

On one of my older carburated cars I fitted an aftermarket fuel cut-off device.
It used the car's anti-run-on fuel solenoid and used a vacuum sensor to switch the fuel off when it detected high vacuum ie overrun conditions. Simple but effective.

you need a timer also. Otherwise you go to switch gears and DFCO would engage.

Where was the vacuum sensor located? Lower in the throttle body?

So far, we have the following rules:

Vacuum > triggering level (does this need to be adjustable?)
RPM > (idle + anti-stall) (should cut fuel in a safe amount above idle/stalling speed)
Car must be in gear

This is a good idea.

More aggressive DFCO where light braking might be used also means you keep turning the alternator (vs. hitting your kill switch and feathering the brakes up to your a stop).

Splitting hairs, yes. But it may make sense for some drivers to add this to DFCO-equipped cars just to expand the "DFCO envelope".

(Note: the obvious option would be to just hold my ignition kill switch while leaving the car in gear and the engine turning. However, mine will soon throw a CEL, so that's not really ideal.)

A more conventional method (vs. vacuum levels) of controlling additional DFCO would be to base it on engine revs via an adjustable frequency switch like this.
Frequency Switch - Jaycar Electronics

This method has DFCO being selected if the throttle position sensor shows the throttle as being closed, with the engine above your set rpm. Still using a time delay of a few seconds to avoid unwanted power cuts while changing gears.

you know what the fastest way to make the ECM angry is? having the O2 sensor signal drop to full lean when it expects it near stoich.

when you cut fuel, you should probably simulate a normal O2 voltage, otherwise, the ECM will add a bunch of fuel into the short term fuel trim until one of two things happen:

it hits its limit and sets an O2 error code

or

the engine comes back on, runs rich temporarily until the ECM takes back out all of the extra fuel it commanded during the DFCO event



of course, if going into DFCO while in open loop, neither of these things will happen. in any situation, do yourself a favor and have the DFCO event also start generating a false O2 sensor signal, otherwise you'll probably end up using more fuel than without it. if you do see gains, they won't be as high as they could be.

I was thinking it wouldn't be necessary to simulate the O2 sensor outputs if you could just force an open loop condition. Or would that also cause a code?

I have tried to simulate O2 sensors before and I can't speak for others with the right skills, but it's hard! I had to monitor where the ECM was in its rich/lean cycle and create the expected O2 responses. I'm not at all a good enough programmer ;)