Some vehicles have what is known as deceleration fuel cut-off. So, if you're engine braking, and the RPM is above a certain threshold, the fuel is completely cut off. Under that threshold, there is a little bit of fuel being added to keep the engine rotating. On my car I believe this threshold is 2500RPM. But it may be lower or even higher than that I suppose depending on the vehicle. Some vehicles don't even have this feature.

Speaking of fuel economy, I just remembered this one time, I actually glided a good 7 or 8 miles or so with my engine off. The speed limit was 25, and it happened to be mostly downhill. I was at 25 +/-5mph the whole time. Unfortunately, this was at night, and I had my headlights on. My battery actually got noticeably depleted, it took a very tad bit longer to start than usual.

EDIT: Also, I just read back a couple posts and it seems that what I said has already been said a few times. Oh well.

The threshold on most cars is closer to 1500 than 2500.

Also, you need a new battery and/or alternator. The alternator was driven the whole time so the fuel cut is definitely not to blame

If the engine was off, and the ignition was off, then the alternator wouldn't have been doing anything. Headlights on many cars work independently from ignition.

Quote from tristancliffe :

If the engine was off, and the ignition was off, then the alternator wouldn't have been doing anything. Headlights on many cars work independently from ignition.

I just assumed by "off" he meant no fuel injected, because who in their right mind turns the ignition off while the car is moving? Especially downhill? You lose power steering and the brake booster. I would never even have such a stupid idea, but just in case someone would, we're taught never to do that in driving school, also my car's manual says never to turn the ignition off when the car is moving.

€: Besides his battery should still be replaced because ~20 minutes lights, even high beams, shouldn't make a noticeable difference.

Quote from morpha :

I just assumed by "off" he meant no fuel injected, because who in their right mind turns the ignition off while the car is moving? Especially downhill? You lose power steering and the brake booster. I would never even have such a stupid idea, but just in case someone would, we're taught never to do that in driving school, also my car's manual says never to turn the ignition off when the car is moving.

€: Besides his battery should still be replaced because ~20 minutes lights, even high beams, shouldn't make a noticeable difference.

I meant off as ignition off. True, I do lose power steering, but with my car, if you're going more than 20mph or so, it's not very noticeable. Anything under that it becomes a little more difficult to steer.

You you shouldn't lose power brakes as soon as you turn the ignition off if the system is working properly. Because the brake booster has a vacuum reservoir that stores vacuum. Obviously not all air can be sealed off from the system, and it will lose vacuum over time. But over the course of 20 minutes or so, the vacuum lost is negligible. It's enough to assist to jam the brakes once if you need to, just as you would with the engine running.

Also, I know that my battery needs to be be replaced. It's been depleted to the point that my car wouldn't restart once, when my alternator goes bad. But I'm waiting till it dies completely, as I don't travel very far from home and I can easily get a friend to pick up a battery for me from a parts store, and then I can just replace the battery on the side of the road!

Poor batteries are, according to the bit of paper that arrived with an alternator for a customer today, responsible for over 80% of alternator failures.

So by waiting for your battery to die you risk killing your alternator. Plus, the alternator will have to work a lot harder, which increases your fuel consumption. Lose-lose.

How can a dying battery damage an alternator? I imagine it would have to work longer to recharge the battery, but would just that be enough to significantly reduce alternator's lifespan?

Who knows. Something to do with the clever electronics that are inside it...???

Well, my alternator is cheaper to replace than my battery so I'm not too worried. Plus, the battery has already lasted 8 years, and the warranty only covered 2yrs.

Quote from MadCatX :

How can a dying battery damage an alternator? I imagine it would have to work longer to recharge the battery, but would just that be enough to significantly reduce alternator's lifespan?

Ohms law. When a battery is old and/or damaged, the internal resistance is higher. The alternator tries to keep a consistent voltage output, usually between 13.8 and 14.4V. When the internal resistance is increased, more current is required to keep the same voltage. This causes the alternator to be working harder, and as a result making more heat. More heat over a long period of time tends to affect electronics negatively.

Quote from wheel4hummer :

You you shouldn't lose power brakes as soon as you turn the ignition off if the system is working properly. Because the brake booster has a vacuum reservoir that stores vacuum. Obviously not all air can be sealed off from the system, and it will lose vacuum over time. But over the course of 20 minutes or so, the vacuum lost is negligible. It's enough to assist to jam the brakes once if you need to, just as you would with the engine running.

It won't last 7 or 8 miles downhill when you occasionally need to apply the brakes to maintain a reasonably low speed, which I assumed was the case since you didn't engine brake. Even just little corrections will deplete the reserve and you'll be left with nothing, should the need for an emergency braking manoeuvre arise. As you said, it's enough for one or two fairly aggressive stomps, but that's it.

But you were all alone in the middle of the night, so the risk was negligible, I'll give you that. I still wouldn't recommend doing it, especially if your car cuts fuel when coasting.

I was on a mountain Saturday night and I definitely wouldn't want to roll down without power steering and brakes, but then it's quite steep and my car is pretty heavy (some 1650kg). Was much more fun going up than down I must say.

Sidenote: In German, the fuel cut when coasting is called "Schubbetrieb", my favourite dictionary translates it as "coasting mode".

I put 50some thousand miles on cars without using power steering or brakes. You aren't going to die because you don't have power steering or brakes. Cars use to not have that.

Both my first 2 cars I owned did not have power steering or brakes.

I know, my first car also didn't have power steering or brakes, I even drove a modern car (Tesla) without power steering and brakes and I'm still alive. But deliberately disabling them on a car where they're fitted as standard is a different matter.

I know some automatics avoid DFCO when in "overdrive" or whatever the highest drive position is labeled, and only use in when in plain "drive" or with an actual gear selected.

Running the engine at the optimum brake specific fuel consumption point and then coasting in neutral is the essence one of the more effective hypermiling techniques, pulse and glide.

Quote from wheel4hummer :

Ohms law. When a battery is old and/or damaged, the internal resistance is higher. The alternator tries to keep a consistent voltage output, usually between 13.8 and 14.4V. When the internal resistance is increased, more current is required to keep the same voltage. This causes the alternator to be working harder, and as a result making more heat. More heat over a long period of time tends to affect electronics negatively.

Isn't it actually the other way around? AFAIK U = R . I, right? So higher resistance leads to higher voltage at a certain current. Alternators regulate voltage by altering the DC voltage on the rotor, the higher this voltage is, the higher is the current induced by the alternator (at least alternators in bigger cars use this instead of permanent magnets).
The only thing I can think of is that a bad battery needs more current to recharge. Because of the higher resistance the alternator can push less amps at a certain voltage to the battery so it needs to work longer to recharge it. I imagine that if a battery is really on the verge of dying, alternator can have a hard time if the engine speed is constantly low.
However, that's just my excess of physics knowledge and lack of practical experience talking...

Quote from MadCatX :

Isn't it actually the other way around?

V(voltage) = I * R
I(impedance, current) = V / R
R(resistance) = V / I

Quote :

So higher resistance leads to higher voltage at a certain current.

that's what keeps your car from dying on the road when you've got a bad battery (the exception being when you end up using all your available electrical power)... if you can't charge the battery, you can't crank the engine, regardless of the voltage.

i could be wrong, but i think the current is more important than the voltage for starting.

Cranking power is important and I'm not a battery expert but it's what amps the battery can provide without choking on itself that's important I think..? As in when I left my parking lights on my bike on once the voltage was like 11.72 but it wouldn't start, where as at 11.85 it started right up...

If your battery dies completely your car / bike will cut out, it's not a closed charging circuit (as in power from the alternator is not directed directly back into the electrical system) - it passes through the battery first and if, in the process of passing through it, the battery can't hold it, it looses charge till there's not enough power to provide juice to the fuel injection system. A fully carb'd vehicle, however - in theory, should work. Alot of mid 90's motorcyles (and early 2000s actually) were carbed but they had electronics which controlled some of the butterflys (think they're called something different but I'm too tired to care/remember) so again wouldn't work without a steady charge.

Quote from S14 DRIFT :

If your battery dies completely your car / bike will cut out, it's not a closed charging circuit (as in power from the alternator is not directed directly back into the electrical system) - it passes through the battery first and if, in the process of passing through it, the battery can't hold it, it looses charge till there's not enough power to provide juice to the fuel injection system. A fully carb'd vehicle, however - in theory, should work. Alot of mid 90's motorcyles (and early 2000s actually) were carbed but they had electronics which controlled some of the butterflys (think they're called something different but I'm too tired to care/remember) so again wouldn't work without a steady charge.

Typical S14 nonsense... with conventional engine wiring the all the earths (including battery and alternator) are connected together usually via the bodyshell. The battery and alternator live are conventionally connected directly together, you can take full battery/alternator power off either the positive terminal of the battery or alternator. Any engine with conventional wiring will run just on the alternator, although the rectifier/regulators on some modern alternators will get damaged if they have no battery to charge.

In some cases batteries that have gone properly tits up end up drawing so much charge out the circuit that the alternator is unable to cope, in these cases simply disconnecting the battery will allow the engine to run on the alternator.

Your idea a 'fully carb'd' (whats a part carb'd vehicle? Carburetors on 1 & 2 and fuel injection on 3 & 4?) can run without any electrical power shows you have no clue what you're on about. Spark ignition engines (and compression ignition engines don't use carburetors!) use electrical magic to make the sparks and that electricity has to come from a power source ie. an alternator/battery/some other form of generator.

I would love to see you come up with an alternative to electronic ignition, maybe mechanically driven flint spark plugs could be the future?

I suppose a "part carb'd" would be one of those nonsensical electronically controlled carbs, i.e. electronically controlled mixture.

When I say "fully carb'd" I mean where there are no electronic fuel pump (done by a vacuum), no electronic butterflies or any major components such as those). alot of motorcyles which are caburetted (like cadburys only better) have 2 butterflies, one which you control and one which the ECU controls, as well as the ECU telling you how much fuel to push through.

Typical Alex as well. (as in trying to belittle people as usual). I have no issue with being wrong (we all are from time to time - or all the time in my case) but when you come across as such a cock I can't help but want to throw a pie in your face. You have such an air of arrogance that, unlike Tristan, you don't deserve. Maybe that's what you get when you grow up in Oxford

I tried this on my Triumph when I had 2 bikes, the tracker in it decided to eat all the battery, drained it to the point of the alarm thinking it had been disconnected from the battery (so it went off for 3 hours while I stood there looking like a right penis)

I linked them across via jump leads and I guess the battery on my Triumph was completely drained as I mentioned and when started it would die unless the jump leads were on even after it was jump started. It was showing 14.7v on the old voltmeter which is fine... Then again it had a CANBUS system and many lights and moving nozzles which did many things. It took a night of being on a trickle charger to be able to start off a jump and run under it's own generator.

When I left my parking lights on my Fazer on by accident, I was able to get everything up and running without needing to leave the cables on...considering the Triumph was brand new (had it for about 2 weeks at the time) and the Yamaha was on a 99/v and I'd had it for a year previous without changing the battery (so can't assume it was any more healthy), it ran fine with what I am presuming to be no or very little battery. Unfortunately then a worm decided he'd eat my loom and my bike said I was doing 145 while I was actually doing about 40...I was like "wow either I'm going very fast and moving through the space time continuum or something wrong here" and then it cut out when I turned my headlights off (I was testing)

However my fazer had an electronic fuel system (as in electric pump etc, no vacuum pumps and "prime/run/reserve switches" for me. But hey idc.

So hence why I presumed, rightly from my experience that a carbed vehicle which doesn't have fancy electronic components making it run either requires far less electrical power or can function without.

Tits? no? gtfo then.

Quote from S14 DRIFT :

When I say "fully carb'd" I mean where there are no electronic fuel pump (done by a vacuum)

Are you sure there are vacuum powered fuel pumps? Usually mechanical fuel pumps are either powered by a camshaft or the crankshaft.

Quote from S14 DRIFT :

alot of motorcyles which are caburetted (like cadburys only better) have 2 butterflies, one which you control and one which the ECU controls, as well as the ECU telling you how much fuel to push through.

I think you're just talking about electric choke.

If the engine uses a magneto for ignition, has a mechanical fuel pump, and the carburetor is completely mechanical (no fuel cut-off solenoid or other electronics,) it could run fine without a battery, just like a lawn mower.

Also, modern alternators don't start generating voltage without a external power source because the rotor uses a coil instead of permanent magnets to generate a rotating magnetic field. This means you can't push start a car with a totally dead or missing battery. You can always put a booster on and then get the engine turning, at which point the alternator will keep itself operating even without a battery.

I've driven an older car that had an electronic carburetor (ECU, O2 sensor, and a solenoid to control mixture) and used a mechanical fuel pump (driven by a extra cam on the cam shaft.) The alternator went bad once and I drove over a hundred miles, mostly at night, on the battery without realizing it (the charging system warning light never lit up when running.)

Quote from wheel4hummer :

Are you sure there are vacuum powered fuel pumps? Usually mechanical fuel pumps are either powered by a camshaft or the crankshaft.

No to be honest with you chum. I just know that the engine sucked through what it needed rather than being injected electronically what it was asked for.

Quote :

I think you're just talking about electric choke.

Choke on alot of bikes is done with a cable which moves bits of the carb around to make it work...on carburetted ones anyway.

My 2004 Z750 had fuel injection (no choke) but instead had a fast idle level which was again cable operated. The triumph had a fast idle built in with the whole electronic fuel injection/ECU so asked for more fuel and higher idle speed.

Quote from S14 DRIFT :

Maybe that's what you get when you grow up in Oxford

You presume wrongly...

Quote :

So hence why I presumed, rightly from my experience that a carbed vehicle which doesn't have fancy electronic components making it run either requires far less electrical power or can function without.

That's great... but you're still talking nonsense, how then in your experience can a spark plug function without electricity or are you just typing drivel without thinking as usual?

Quote from ajp71 :

That's great... but you're still talking nonsense, how then in your experience can a spark plug function without electricity or are you just typing drivel without thinking as usual?

A magneto? It's not functioning without electricity, but it is functioning without a battery. Or does your lawn mower have a battery :P

Quote from RasmusL :

A magneto? It's not functioning without electricity, but it is functioning without a battery. Or does your lawn mower have a battery :P

...and what do the magnets provide power to the ignition system how exactly?