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.

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.

That's true. If you *must* brake the vehicle, you may as well do it in a fuel-cut situation.

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.

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.

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.

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.

Under certain conditions, many, if not most, late model cars automagically shut off fuel flow when coasting in gear (and clutch engaged).

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.

CarloSW2

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.

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').

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 :thumbup:

RH77

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.

I wasn't aware that modern cars already used this fuel cut-off feature...its good that they do that.

My car has a manual tranny and manual steering, so no worries there...I just mentioned auto trannies and power steering so that maybe my idea could extend to other cars.

I'll have to do a little research on vacuum canisters. If anybody can offer some decent links that would be awesome. In the meantime I'll check the archives on this site. I'll also have to practice that handbraking technique in some low traffic areas, it sounds worthwhile.

By the way, here are the selenoid valves I was thinking of using:

http://www.bucksales.com/Principals/...v/GPValves.htm

Part # SV430. Its more of a general purpose valve and isn't automotive specific, but I think it should work. If anybody can think of a valve that would be better suited please let me know.

Here ya go! :thumbup:

Summit Racing vacuum canister page

BTW, VW TDIs have speed sensitive fuel cutoff. For instance, in 5th gear if you take your foot off the "throttle" pedal (leaving the car in gear), injection stops completely until you slow to ~25 mph (actually 40 kph), whereupon it picks up smoothly.

Couple of interesting side notes. First, VWs also have a system to permit persons not comfortable with manual transmissions to drive them effectively. Simply start the car in neutral with the clutch depressed, shift into 1st gear and let the clutch out slowly and steadily. The ECU adds fuel and starts the car rolling without the driver having to coordinate power and clutch. You can go right up to 5th gear without touching the "gas" pedal, and the car will roll right along at 40 kph on the flat or even uphill. Press down on the go-pedal at any time to go even faster. :)

Second, there is some controversy in TDi circles about which is better, coasting in neutral or the automatic fuel cutoff feature. I used to use the automatic feature, but lately have been taking the gearbox out of gear for much longer coasting distances on the premise that since I can coast out of gear at least twice as far as with the engine engaged, I burn less fuel in idle than I do under power to cover half the distance. The jury is still out though, as I haven't collected a tank's worth of data for each case. I share this car daily with my wife, too, which complicates good data collection.

Dane-ger,

Cool, Another fellow early Z car person!
Have you tried driving without the booster? My 260Z booster has been dead for about 10k miles and giving no effect. It isn't easy but defiitely live-able. My old calipers wouldn't even lock up while standing on the brake pedal to the floor. I went to bigger (yota 4x4 with solid rotor) brakes and now I have much better stopping power without the booster (Can lock up the tires... and stop MUCH better without locking them up. I.E. not scary)
Just an option...

With the pre TDI I have a momentary switch right on the shift knob. Cuts power to the fuel injection pump. definitely helps brake when I come in too hot. Easy way to kill the engine without reaching, when I want to coast. The pre TDI's go to idle on decel not off. If trying to restart with the clutch always use high gear on a diesel. Or its hard on the clutch. On the little ladies van I want to put a momentary switch on the Idle Air Control (IAC), that thing really tics me off.

olderthanme,
look to see if there is a plastic "thing" inline in the hose that brings the vacuum from the intake manifold to the booster. If so, it is probably a check valve. These can get stuck in the "shut" position, and now you never get vacuum to the booster.
An easy test would be to pull off the hose right at the booster with the engine running, you should have vigorous suckage into the hose. Plus the engine may run poorly while the hose is off.

If you just cut fuel going to the carburetor the carb still has fuel in the float bowl, enough to go maybe a quarter mile or more, then it will slowly lean out, at this point your engine is running really lean and this seems like a bad idea as an extremely lean burning engine tends to over heat the engine, this is why when honda started implementing this very idea in the mid 70's they put 4-5 solenoids on the carb, one on each supply to each jet, it added some complexity to the carb, but made the car use alot less gas.

Down shifting is an easy way to take it easy while braking and also a great way not to waste so much gas. When your engien revs and the rmps increases, especially when you downshift, you're just using more gas than what is needed at the same speed.

___________________
Autopartswarehouse.com Car Mods.

Same goes for my 1989 auto. If Im going down a long steep hill in gear with foot off the gas, itll cut fuel after a few seconds, but then bring back the fuel for a few seconds, keep doing that till the car quits gaining speed. If I pop it into N then the idle speed will drop and itll pick up a lot of speed. But I dont like how it reengages when I shift back into D, I feel like Im hurting the trans, so I avoid doing that. But one thing I just figured out last night, is if I hold the throttle in just the right spot on a long downhill, itll keep the TCC locked but cut the fuel. The fuel will stay cut all the way to the bottom of the hill and Ill maintain speed. And no funny actions at the end since I dont have to shift back into D. Something I have thought about is a momentary switch to cut the fuel. Then with the fuel cut, the throttle can be really opened up, keeping the TCC locked and reducing restriction at the intake, maybe getting you farther down the road.

When you have an SUV.... Power steering in traffic is a must...

In a geo... power steering is pretty much not required at all.

Thats what I've figured out heheh. :D

DifferentPointofView -

110% agreement. I am on the cusp, but I know it will work for me, so it's on the todo list (edit: convert to manual steering).

CarloSW2

How are you all determining when your injectors have stopped firing? Are you going by the cutoff feature in the Scangauge? Or have you wired up LED indicators? I'm still very curious about this. I'm not convinced that my car does this, but I can set the "fuel cutoff" setting in the SG according to the manufacturer's instructions and see cutoff indicated by "9999MPG" under certain coasting conditions. But as best as I understand it this is just a guess. On the other hand, one of my manufacturer-specific X gauges is fuel pulse width, and I never see this register zero under any coasting conditions. Thoughts?

The mpguino reads all 9s when fuel is cut. But long before having that, I could tell when mine cuts the fuel. It feels different. The exhaust note disappears. And when I tried an O2 sensor monitor once, it shows nothing when the fuel is cut.

You could wire an LED with a 470 Ohm series resistor directly across a fuel injector. The LED would light up to its full brightness whenever the injector is NOT firing. It will be progressively dimmer when the injector is injecting larger amounts of fuel.

Better yet, connect a voltmeter directly across the injector's two wires. Turn on your key but do not start the motor. Read the voltage. This is the full source voltage when the injector is NOT firing. Now start the motor and notice that this voltage will drop and drop further when you are accelerating (using more fuel).

Anyway, when driving, coasting, etc. just look at that voltage. If it reads the same as the key-ON, engine-OFF voltage then you know that your engine is not using fuel.

EDIT: I have that a bit backwards... If the key is ON but the motor is not running, the voltage measured across the injector will be zero volts, not full source voltage. My above numbers would be correct if you wired the voltmeter the way I did to my vehicle, which was to connect the meter between circuit ground and the switched side of the injector (not the powered side of the injector). Same applies to the LED circuit.

I know my tbird cuts fuel until it drops to 2000 rpms coasting, not sure about the festiva though, Im gonna guess lower rpm, as its a total economy car and has much more usable low end power.

Since Dane-ger said he had carbs, not fuel injection, it reminded me of what another member of my car pool once did (this will give away how old I am!).
He had a 1956 Ford with the 312 T-bird engine. On a couple of downhill stretches on our commute he could flip a switch to a valve that let outside air into the intake manifold, and that killed the flow of air through the carburetor. Supposedly this saved gas, but I wonder what kind of damage this might have done to the engine, plus the engine cooled too much on long downgrades.
He had an automatic transmission--no lockup in those days-- but the engine kept turning. Maybe the early Ford automatics had a rear pump like the early Mopars did that allowed this to happen.
Ray Mac.

if you have to have power full time power steering - go electric!
one of my diesel rabbits has 4 additional vacuum tanks i can do reeeeeeeally longs hills
that way...

I changed over steering to manual and brake booster to electric for coasting (and elimination of engine belt drains).
(If I don't need power steering in my giant truck, nobody does)

But on long steep descents I really want to be able to engine brake to preserve the brakes.

This was easy for me, because I have a diesel.
The engine kill switch I already installed on the shifter cuts off the electric fuel pump. All I have to do is hold it down for as long as I want to engine brake. A couple jobs in the hills past couple days, used this feature a whole lot.

Not necessarily, my father's Bravada will cut if I shift down to 2, and EB down a hill at 25-30 mph. I do not know if it's locking the converter or not, though.

I can't tell if my father's truck does it or not, the 2.2L doesn't have much back torque to begin with. But on his Bravada, it's very noticeable when the engine stops receiving fuel. It also has a factory gauge, which hits 99mpg when the injectors stop.

Pretty much anything prior to the 60's had rear pumps, and most from the 60's did, too. But even without a rear pump, there's enough pressure built up in an automatic going at speed that it won't drop the motor until you get to a very low speed.

Many modern cars including my Yaris already come with DFCO. Does yours?

caution!
 

Bosch VE pumps (and japanese versions), except for those fitted in cummins engines are lubricated by fuel, yeah as if ULSD will lubricate ;~> same with the stanadyne pumps if you run them in hydraulic braking mode with the fuel shut off you can harm or destoy these already fragile IPs.
the inline Bosch pumps especially the 5 cylinder version automatically shuts of fuel on downhills...

Make sure yours does not already use a form of DFCO and save yourself grief. A Scanguage will read 9999 mpg in DFCO mode.

See disclaimer about my general lack of familiarity with automatics. :)

Maybe there's enough pressure in the unlocked torque converter to keep the engine turning in some cars. I wouldn't be surprised.

See disclaimer about my general lack of familiarity with automatics. :)

Maybe there's enough pressure in an unlocked torque converter to keep the engine turning. I wouldn't be surprised.

I've found that this isn't always the case.

From the Integra shop manual:

"During deceleration with the throttle valve closed, current to the fuel injectors is cut off to improve fuel economy at speeds over following rpm..."

Basically standard engines are 950 rpm U.S. and 1050 rpm Canada.

...but I don't get the 9999 mpg indicator (and never have in tons of rental cars). So anyway, I often downshift on exit ramps or up to lights if I have to stop unexpectedly.

You can generally see and feel the transmission/TC give a bump in revs right below the cutoff.

Modern fuel injection probably has it -- at least OBD-II, maybe earlier...

RH77

You don't need a solenoid for the fuel pump, it turns itself off when it senses the pressure increase from the closed carb float valve. Fuel flows into the engine because air pressure across the carb venturi is lower than that in the float chamber, to stop the fuel you just need to equalise the air pressure. SU did this themselves by connecting the float chamber vents to manifold vacuum and atmosphere. By closing off the atmospheric vent with a solenoid valve full manifold vacuum was communicated to the float chamber. Pulsing the valve stopped fuel being drawn up the vent pipes

I watch the Loop gauge on the Scangauge. It goes to Open Loop when it's in DFCO mode. There's no need to monitor the exhaust O2 levels if you're not injecting any fuel, right?

On both my Hondas, it holds DFCO down to about 1200 rpm, then fires the injectors back up. My civic is a manual, but the 04 Odyssey is an automatic. As long as I downshift to keep the rpm above 1200, I get DFCO.

That said, it's almost always more efficient to idle the engine and coast in neutral - momentum is more valuable than the light fuel usage.

cutting fuel to a carb(s) may change bowl level. It may not throttle right away back into power...
just a thought.
I run carb too. if to add air bypass for a carbed system, engaging when throttle is off, that works good. I have yet to get a way for it to work on the carb system,
An idea is an egr not used that is a valved source into intake. maybe convert it to electric like fuel injected, and be it from a filtered line using the air intake filtered. Long engine braking on a carbed engine and no air bypass, can pump vapors into intake from the base (hence carbed engines stay clean for 50 years versus an air bypassed one that pigs itself all the time).

so there is drawbacks in any direction.A truck's jake needs no vacuum, they smack the valves around..so there goes ideas of "vtec" as a jake..vtec sucks at anything really, but that is not the subject here.
one could add a bigger vacuum canister, but on little engines, it will be a rough idle until satisfied.
manual brakes and manual sterring retirning to the primitives of man and machine would save the world. But we cannot have that either can we? :confused: