This is something thats been making me feel curious for a while now.

You´re driving your car downhill, in gear (lets assume a manual transmission), without pressing the gas pedal. Only the force of gravity and momentum is making your car move. In these situations, that happen practicaly every time you drive, what exactly is happening in the engine? Is there any fuel consumption, or is it merely a fuel cutoff? If it is a cutoff, then isnt the engine running only on air, instead of an air/fuel mixture?

Im kind of confused..

Same happens as when the car is ticking over I believe, it has to be using SOME fuel, otherwise it wouldnt run........unless you turn the ignition off, then the engine isnt running at all.

Dan,

Quote from vic_gt :

This is something thats been making me feel curious for a while now.

You´re driving your car downhill, in gear (lets assume a manual transmission), without pressing the gas pedal. Only the force of gravity and momentum is making your car move. In these situations, that happen practicaly every time you drive, what exactly is happening in the engine? Is there any fuel consumption, or is it merely a fuel cutoff? If it is a cutoff, then isnt the engine running only on air, instead of an air/fuel mixture?

Im kind of confused..

It's running on the cars idle throtle, a very low fuel to air ratio. It's using the same amount of fuel as it would if you leave it running in neutral without pressing the throtle.

When driving like that the engine produces very little power, actualy it's generating more resistance then power, so it's slowing the car down, rather then accelerating.

Thanks dev.

I tought that while coasting fuel would be completely cutoff, because wheel movement is already keeping it running.
You know, something like: fuel is only injected when idling(to keep it from shuting down) and when pressing the throttle pedal.

Anyway, in downhills its more economical to just let it in neutral, right?

Quote from vic_gt :

Anyway, in downhills its more economical to just let it in neutral, right?

I wouldn't really know, depends on the car I guess, but with engine braking you reduce the usage of brake disc and break pads, so they last longer and don't overheat as fast as they would.

As dev said, you use virtually an identical amount of fuel going downhill in gear (without accelerating) as to being in neutral. However, you may as well knock it into neutral, you're putting less strain on the engine. On relatively low angle downhill stretches, I tend to shove it into neutral and just let it roll - however, on higher angel downhill stretches I tend to keep it in a low gear, for the purpose of ensuring the car doesn't run away from you so easily

But remember folks, always shift back from neutral into a gear suitable for your speed... or significnatly shorten the life of your synchromesh

Oh, and for crying out loud... don't ride on the brake pedal... polished brake pads are no fun. Use it properly when you need it

i dont have the habit of running the engine in neutral. im also very aware, and used to brake fading, because i have the habit of driving hard, which leads to braking real hard while heel and toeing, etc. im no rally driver, i just like to have fun.

anyhow thanks for the help

Quote from JamesF1 :

But remember folks, always shift back from neutral into a gear suitable for your speed... or significnatly shorten the life of your synchromesh

You can always double clutch

Hold yer horses, I could swear I've seen evidence of modern cars using even less fuel during easy engine braking than idleing. Gonna check if I can find some driving school material of the subject, as we all know, Finnish driving schools are hands down the best in the world... just as our race drivers.

Quote :

Anyway, in downhills its more economical to just let it in neutral, right?

Please bear in mind that in some countries, the UK for instance, this is actually illegal as you are "not in full control" of the vehicle when coasting in neutral, ie: You have no available power to avoid an incident if necessary. It's a bit of a silly law, but it's obscure laws like that which they use to catch otherwise honest people.

A friend of mine was recently ticketted for parking his car facing the wrong way, we now laughinglly reffer to our local police as the "Feng Shui" police. Apparently there's some regulation that says you have to be parking in the same direction as the traffic when parking on a bus route.

Quote from vic_gt :

Thanks dev.

I tought that while coasting fuel would be completely cutoff, because wheel movement is already keeping it running.
You know, something like: fuel is only injected when idling(to keep it from shuting down) and when pressing the throttle pedal.

Anyway, in downhills its more economical to just let it in neutral, right?

As for VW Polo built between '90 and '94 with the "Digijet" or "Digifant" motor managment i can tell you, that if don't completely off the throttle and the engine revs over 2,500 rpm, the fuel is cut off. But I doubt the saving of fuel would be great, because - as mentioned before - very little fuel gets injected if you're off the accelerator (unless you travel steep long distances downhill, that is ).

I'm fairly sure modern engines cut the fuel when they're being motored. I run tests on diesel engines all day every day and when they're being motored by the dyno during simulated driving the AFR goes sky-high, i.e. there's no (or very, very little) fuel being added.
Having said that, I've never actually checked the fuelling during motoring, so I could be wrong.

most efi cars cut fuel at 0% throttle above a certain rpm.

note that the throttle isn't really a "gas" pedal. it's an "air" pedal. the fuel injection system adds the right amount of gas for the air that's going into the motor.

also note that the throttle is never really fully closed, even when your foot is all the way off the pedal. if it did fully close while the engine was running, a vaccuum would be created that would suck oil into the cylinders and potentially cause other damage.

so put two and two together, and you'll see that when coasting downhill, the motor is sucking at least some air into itself, and that the injection system will add some fuel to the mix, even though it usually wont be very much.

Quote from evilgeek :

note that the throttle isn't really a "gas" pedal.

It is on a carburated car though. And, isnt it also called the "accelerator"?

Quote from evilgeek :

so put two and two together, and you'll see that when coasting downhill, the motor is sucking at least some air into itself, and that the injection system will add some fuel to the mix, even though it usually wont be very much.

You don't think it's a simple matter to combine a tps signal of 0% and rpm signal above some arbitrary value, and cut power to the injectors? Or does everything still run on carburettors in Canada?

The amount of people keen to explain stuff without basic knowledge of how and why things works is always interesting...

Quote from wheel4hummer :

It is on a carburated car though. And, isnt it also called the "accelerator"?

It's not! The carby adds fuel because of the air flowing through it, it doesn't add fuel because you pressed the throttle.

On a gasoline engine, if the throttle is close, the rpms are high and no fuel is added, then it would all go very very lean - this is bad for catalysts, and could potentially cause detonation in the worse cases. Thus a certain amount of fuel is added to combust, so that the catalyst and engine remain safe, but it's a sufficiently small amount so as not to accelerate the engine and nullify the closing of the throttle.

But I might be wrong...

I'm certain that in some case Tristan is correct, as I know for a fact my Mk4 Vauxhall Astra cuts off fuel during prolonged engine breaking, and I wanted to turn it off for the reason he stated above, for the same reason my motorcycle carburation is set up how it is, detailed below.

You shouldn't coast down hill as you have comparatively little control over your vehicle.

Break pads are cheaper to replace than engine parts so I tend to brake using the breaks more than using my engine to slow the car.

I have a high idle on my motorcycle because it is highly tuned and has been known to have pre-ignition issues (usually from lean mixture and overly high running temps), the extra fuel cools the cylinder (there isn't enough fuel to ignite with any real expansive force, most of it just gets thrown unburnt into the exhaust system) and also leads to backfires/burbling exhaust on lift off which I quite like. Being a two stroke, more fuel means more oil getting to the engine and this helps reliability, I'm bored of replacing scorched pistons.

Usually in a car I drive between 3-5 thou rpm, it ensures good power delivery, and you don't need to boot the loud pedal every time you want a few extra mph. Driving under 2500rpm (unless you're driving a diesel) strains the engine (evident in the running in period where 30mph in 5th, like every sodding uk driver seems to do, is forbidden), similarly ragging it everywhere does the same thing (yes, I am talking to you, mr 1.1 106 with your foot flat on the floor in second gear EVERY SECOND OF EVERY DAY, enjoy those stretched con-rods!).

My bike on the other hand, which requires 8-14 thou rpm for average driving, is almost devoid of torque. If I really want to get up to speed in a dangerous situation i have been known to scream it to 15,500rpm.... it doesn't sound all that pretty, and I wouldn't imagine it does my fuel consumption much good.....

Quote from tristancliffe :

On a gasoline engine, if the throttle is close, the rpms are high and no fuel is added, then it would all go very very lean - this is bad for catalysts, and could potentially cause detonation in the worse cases. Thus a certain amount of fuel is added to combust, so that the catalyst and engine remain safe, but it's a sufficiently small amount so as not to accelerate the engine and nullify the closing of the throttle.

But I might be wrong...

wrong
it's really not an uncommon feature at all.
I don't know heaps about stupidly lean combustion mechanics, but I doubt it's particularly similar to 18:1 style, sucking a gutful of air and running out of fuel, melt your pistons combustion - we'd be talking much less air.

http://fordfuelinjection.com/?p=64 see under deceleration
http://www.stealth316.com/2-fuelinjection.htm see deceleration fuel cut

2 links, no doubt plenty more if you're keen.

I tweaked the tps on my rx7 a while back to prevent this injector shutoff (15 years of age seems to have made it a little clunky) and the car drove really nicely but gave a lot of backfiring on overrun.

Quote from Becky Rose :

Please bear in mind that in some countries, the UK for instance, this is actually illegal as you are "not in full control" of the vehicle when coasting in neutral, ie: You have no available power to avoid an incident if necessary. It's a bit of a silly law, but it's obscure laws like that which they use to catch otherwise honest people.

A friend of mine was recently ticketted for parking his car facing the wrong way, we now laughinglly reffer to our local police as the "Feng Shui" police. Apparently there's some regulation that says you have to be parking in the same direction as the traffic when parking on a bus route.

That law makes a lot of sense, when you park on the wrong side of the street car head lights light up under light and it can get rather confusing. Plus it means your passanger has to get in on the road side, and based on the fact the women sat on the opposite side to men, this would put a woman in the street, and that's just not gentalmanly.

Also it means you had to cut traffic to park. The law isn't enforced much (more paper work then it's worth), however it is still a law.

Well it's news to me, and I like learning new things, so I'll give them a read later. Thanks.

Quote from Blowtus :

You don't think it's a simple matter to combine a tps signal of 0% and rpm signal above some arbitrary value, and cut power to the injectors? Or does everything still run on carburettors in Canada?

The amount of people keen to explain stuff without basic knowledge of how and why things works is always interesting...


.

of course it would be simple to program the ecu to tell the injectors to do whatever you want, but unless you have an actual example of a motor that cuts them off, then you are speculating just as much as anyone else. i don;t doubt that some motors reduce the amount of fuel injected at high revs and low throttle, but i am still highly doubtful that any engine would cut the injection completely off, because of the potential damage to the motor. that was my only point, and i don't see why you gotta be so sarcastic.

I did some google-ing and I didn't find any engine that completly couts of fuel when engine brakeing. There are some engines that cut of fuel for some cylinders when there is no need for a lot of power. Like Chryslers 5.7 V8 HEMI, it has a system called MDS (Multiple-Displacement System) which cuts fuel to 4 out of 8 cylinders.
AMG used a similar system with the V12 engines, but I couldn't find any info about it.

Don´t forget that older fuel injection cars (until l-jettronic comes up around 1980) don´t use any kind of fuel cutoff. Same to engines using carburators. With a carburator engine you better never lift complete in downhill because this will close the throttle valve (damn language barriere) completly, no air is taken and your engine uses a huge amount of fuel.

Catalyst? Waste of power. Throw it out. =)

Actually, modern catalysts aren't that restrictive, and de-catting your car will not give you the 10hp most people seem to think it will.