Alt on (DFCO) alt off (drive)

[California98Civic]

For the last 2 or 3 tanks I have been experimenting with a technique and I am getting interesting results, though I don't know for sure if there is any fuel economy benefit. I would welcome your thoughts. First some preliminaries:

1) Alternator cut-off switch: I installed one a couple years ago. I can turn the alt on when I want it and shut it off fairly easily from the dash.

2) DFCO or Deceleration Fuel Cut Off: when the engine control computer shuts off the fuel injectors to if the car is in gear, foot is off the throttle, and the RPMs are at maybe 1800 or more (I usually use 2000 as the llwer limit to be safe).

3) Hilly route: My commute features several very long and steep hills that end in stoplights that are usually red, or at which I must turn. I always have to break at these locations.

THE TECHNIQUE I AM TESTING: going down these hills I bump start the car, turn on the alt, and use the engine for breaking instead of the friction breaks. The result is a sort of DFCO-exclusive use of my alternator, exclusively under conditions where I would normally coast with the engine off and use the friction breaks. If you look at my fuel log, you'll see that the power I use from the grid to charge the battery at night has gone way down, but my fuel economy has been flat.

QUESTIONS: Do you think DFCO is really working? My FE numbers could be flat because I have been driving a lot on the freeway, more than in past spring seasons, because of my super tall transmission. Do you think the couple seconds it takes to get into DFCO after bump starting and the couple seconds after DFCO to cut the injectors could mean significantly more fuel use when repeated scores of times during a tank?

Thanks for any and all replies.

james

[redpoint5]

I'm not clear on what you have changed from your previous method of driving, so I can't speculate as to why your mpgs are flat.

From this,

If you look at my fuel log, you'll see that the power I use from the grid to charge the battery at night has gone way down, but my fuel economy has been flat.

it sounds like you have been driving without the alt and just grid charging at home. If that's the case, then you wouldn't see any benefit to your mpg by charging it during DFCO. As you said, the power used to charge from the grid would be reduced, and that's it.

[cbaber]

Sounds like a kind of regenerative charging but not in a hybrid. You aren't seeing fuel economy improvement because the modification (alternator delete) hasn't changed, but your method of charging the battery has. Rather than use electricity from home you are using the alternator but using no fuel to generate the charge.

Therefore you will only see a benefit in terms of lower electricity usage charging the battery at night, rather than an outright fuel economy improvement. It's still an improvement since you said you are using less grid charge, so that is where your savings come in.

[California98Civic]

I guess i was not clear enough. I should have highlighted how normally at this time of the year my average fuel economy is climbing, not flat, because of warmer weather. So my numbers should be climbing.

[j12piprius]

Do you usually turn the engine off down the hill, instead of dfco?

I've personally found that engine off coasting uses more fuel, as measured by fillups, compared to dfco or eonc. My interpretation is that the fuel injectors are not off when my car is moving in neutral, even though the ignition is off.

However, this might not relate to what you are doing.

[California98Civic]

Yes, I usually coast to a stop with the engine off. Now in those situations where i will have to brake, I use DFCO with the alternator on. But before and after each DFCO braking event, the injectors will run normally for a second, so I wonder how much fuel that is.