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Old 01-02-2008, 05:31 AM   #1 (permalink)
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Engine braking without using fuel idea

I have read that some hypermilers will shut off their engines and coast in neutral to save gas. While this is definitely fuel efficient as long as you don't lose too much speed and end up having to accelerate again...it can be dangerous because you lose brake vacuum, power steering assist, electrical charging, etc.

In order to maintain these other functions though...you don't need to burn fuel...you just need to keep your engine spinning. That way the cylinders are still producing vacuum for your brakes and the alternator and power steering pump are still working.

So my idea is to add a fuel cut-off selenoid in my fuel system both upstream of the carburetors (dual carbs) and between the float bowls and needle valves (SU carburetors, the float bowls and venturi bodies are connected by an external fuel line). The selenoids will be electrical, so I can just flip a switch to cut off fuel to the engine...and as long as the clutch is engaged and the tranny is in gear with the car moving, the engine will still spin and everything will still work (I don't have power steering though, so no worries about that). If I need engine power again, I just flip the switch, fuel will flow to the carbs, and as long as I'm still moving the engine should make power again.

Obviously since this is engine braking instead of coasting, I'll slow down more quickly. This would probably be most usefull going downhill or slowing down to a stoplight.

I haven't actually built this yet since I just had the idea and money's a bit tight right now, but I plan to sometime in the future.

Actually this would be much easier and cheaper to do with a fuel injected car. Since the entire fuel system is controlled electrically, you could probably just put a switch inline with the ECU or something like that and not have to screw around with selenoid valves like I would with my carburetors (the fuel injectors are the selenoid valves).

Also, I'm not sure if this would work with an automatic transmission since it uses a torque converter instead of a clutch. Anyways, just wanted to see what you guys would think of my idea.

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Old 01-02-2008, 10:54 AM   #2 (permalink)
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Quote:
Originally Posted by Dane-ger View Post
I have read that some hypermilers will shut off their engines and coast in neutral to save gas. While this is definitely fuel efficient as long as you don't lose too much speed and end up having to accelerate again...it can be dangerous because you lose brake vacuum, power steering assist, electrical charging, etc.
On the engine-off coasting danger issue, a blanket statement doesn't really work. EG: my car has manual steering, so no loss there. The vacuum reserve is good for two additional stops with full assist (I suspect most vacuum assisted cars are similar).

Also, if you haven't read up on "pulse and glide" yet, have a look for some info about it. It counters your point that coasting followed by more acceleration isn't efficient.

Quote:
Obviously since this is engine braking instead of coasting, I'll slow down more quickly. This would probably be most usefull going downhill or slowing down to a stoplight.
That's true. If you *must* brake the vehicle, you may as well do it in a fuel-cut situation.

Quote:
Actually this would be much easier and cheaper to do with a fuel injected car.
So easy and cheap that in fact it's already programmed into most modern vehicles. My 10 year old Metro has a fuel cut mode (which rarely gets used, since normally the engine is off when I'm decelerating). It will cut injection when the throttle is released and engine RPM is above approximately 1500.

Quote:
Also, I'm not sure if this would work with an automatic transmission since it uses a torque converter instead of a clutch.
That's one area I'm less familiar with (I rarely drive an automatic). I suspect they can only fuel cut when the torque converter is locked, otherwise the engine would stall out.
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Old 01-02-2008, 11:24 AM   #3 (permalink)
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Just an additional note, you can always add a vacuum canister if you believe you will need more than one or two brake applications before restart.
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Old 01-02-2008, 11:40 AM   #4 (permalink)
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And a related driving tip: if you need a light brake application and want to save the vacuum reserve, I know some drivers use the hand brake.
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Old 01-02-2008, 07:23 PM   #5 (permalink)
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Under certain conditions, many, if not most, late model cars automagically shut off fuel flow when coasting in gear (and clutch engaged).
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Old 01-03-2008, 02:43 AM   #6 (permalink)
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Dane-ger -

I want to do (emphasis on my TO-DO list) this too, but in a simpler way like you said. I want to copy other ideas where they have a momentary off switch for the fuel injectors. On my stick-shift Saturn, others have found that pressing a momentary switch to disable the fuel injector fuse is all it takes. You are right that it is easier for me because I have fuel injectors and a manual tranny.

My main issue with EOC is the density of traffic in Los Angeles, so I am sympathetic with your desire to maintain full braking/steering power in your car. The second vacuum canister is definitely a good way to supplement your braking power (yet another to-do, *sigh*). Also, if your car is light enough and was offered with manual steering (plausible if you have carbs), then you could convert to manual (that's also a to-do).

So much to-do, so little to-time.

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Old 01-03-2008, 02:52 AM   #7 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
That's one area I'm less familiar with (I rarely drive an automatic). I suspect they can only fuel cut when the torque converter is locked, otherwise the engine would stall out.
I don't think it cares about the TC at all. It cuts until the RPMs drop to a certain RPM and I think that's as complex as the programming gets. All the TC does is make it feel jerkier when DFCO kicks in then kicks out once down to where the FI resumes to keep slushboxes from stalling.

I wish it was selectable... shifters always seem to be lacking for buttons.
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Old 01-05-2008, 03:25 AM   #8 (permalink)
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In terms of autos... my 1989 auto actually does care about the TC. When slowing down, it does fuel cut above 80km/h, but once the torque convertor lockup deactivates, it instantly starts using the same amount of fuel per engine cycle as it would at idle - which means it is better to put the transmission into neutral - so at least it 'idles' more slowly.

I've tried doing manual engine-cut using the keys (in gear) once or twice; after a few seconds, when you switch back to 'engine-on' mode, the engine seems to be turning at about 500rpm. I think it is quite bad for the transmission to be honest (which is why, if you have to tow an auto, it must not be in 'drive').
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Old 01-05-2008, 11:07 AM   #9 (permalink)
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2 things

2 Things:
  • Perhaps look into wiring-up an indicator for fuel-injector firing -- there's a how-to floating around on wiring an LED to show injector pulse activity.
  • With my automatic, I've performed a full, emergency ABS stop with the engine off, key on, second pump. Use at your own risk

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Old 01-05-2008, 01:10 PM   #10 (permalink)
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Quote:
Originally Posted by landspeed View Post
In terms of autos... my 1989 auto actually does care about the TC. When slowing down, it does fuel cut above 80km/h, but once the torque convertor lockup deactivates, it instantly starts using the same amount of fuel per engine cycle as it would at idle - which means it is better to put the transmission into neutral - so at least it 'idles' more slowly.
Mine's the same... without TC lockup it can quickly drop to where fuel delivery is restored. That's why I think it is just a simple FI RPM control that doesn't look at other inputs like tranny state.

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