Hi, I'm sure everyone has noticed in LFS when the engine is revving down, the throttle (green bar) disappears completely. But is that really accurate? I like to watch cars on the roads, and when I look at their exhausts, there is always smoke coming out, whether they are accelerating or decelerating. Always. Doesn't this mean that there is always a minimum amount of throttle applied? I was wondering if anyone can shed some light on this.

One more thing...when you run out of gas in LFS, and your engine is still revving, and you press the gas, the sound gets louder as if you are really giving it gas, even though you're not. Just thought I'd mention it.

The fuel pump would still be on and the engine is still turning fast so would be sucking in some fuel and air mixture from the vacuum the pistons would be creating, and the spark plugs would still be sparking. That would create exhaust gasses.

At the moment LFS automatically applies a small amount of throttle to stop the engine rpm falling below the idle point. I'm not sure if that throttle is always present through the whole rev range on a real engine, perhaps Trist can fill in the (large) gaps in my mechanical knowledge.

The green bar is not the throttle applied to the engine. It is the amount of pressure on your pedal.

There is always gas burnt in the engine even if you don't touch the throttle while it is running.So it can keep the valves and things running.And that makes the smoke coming out of the exhaust.

Edit: i mean in neutral.

when a modern car is coasing to a stop in gear, with no throttle being applied 1) there is no throttle, so a "bar" wouldnt be there anyway
2)no fuel is being burnt on the overrun in most cars nowadays
:?

Quote from ZORER :

There is always gas burnt in the engine even if you don't touch the throttle while it is running.

That's not true, what about diesels with "jake brakes?"

Quote from ZORER :

The green bar is not the throttle applied to the engine. It is the amount of pressure on your pedal.

I'm not sure I agree. If you take your foot off the pedal in neutral a small amount of throttle is added in automatically to keep the engine idling.

Quote :

There is always gas burnt in the engine even if you don't touch the throttle while it is running.So it can keep the valves and things running.And that makes the smoke coming out of the exhaust.

This might be true in older, carburetted, cars but not any more. An engine with fuel injection (diesel or petrol) will cut the fuel completely on a motored deceleration (foot off the pedal but coasting with the car in gear). I could imagine that racing engines might keep some injection going without spark to cool the pistons but I have no evidence for this, it's just a (slightly) educated guess.

Your car's just gonna run richer while coasting. How do you suggest F.I. cars afterfire?

Quote from shiny_red_cobra :

Hi, I'm sure everyone has noticed in LFS when the engine is revving down, the throttle (green bar) disappears completely. But is that really accurate? I like to watch cars on the roads, and when I look at their exhausts, there is always smoke coming out, whether they are accelerating or decelerating. Always. Doesn't this mean that there is always a minimum amount of throttle applied? I was wondering if anyone can shed some light on this.

I know on real cars there the throttle bypass to keep the car ticking over so there will always be air / fuel being sucked into the engine. As for LFS as bob said there is a little bar alwyas present which seems to effectivly be to keep the engine at tick over.

Quote from wark :

Your car's just gonna run richer while coasting. How do you suggest F.I. cars afterfire?

AFAIK with FI the mass air flow sensor - amazingly for the name senses the mass of air flowing into your engine - then the ecu will calculate how much fuel needs to be injected for the engine load conditions. so if you lift off then very little fuel will be injected causing no backfire. Also with todays exhaust systems and legislation about cats and silencers etc cars dont backfire.
Only time i know of 'road cars' backfiring is when the engine has been modified and a turbo added which lead to over fuelling on lift off.

This is only from my pretty basic understanding so if im wrong correct me.

Quote from shiny_red_cobra :

Hi, I'm sure everyone has noticed in LFS when the engine is revving down, the throttle (green bar) disappears completely. But is that really accurate? I like to watch cars on the roads, and when I look at their exhausts, there is always smoke coming out, whether they are accelerating or decelerating. Always.

Get in your car, start it up and then take your foot off the throttle. It should still be running?

Quote from Greboth :

AFAIK with FI the mass air flow sensor - amazingly for the name senses the mass of air flowing into your engine - then the ecu will calculate how much fuel needs to be injected for the engine load conditions. so if you lift off then very little fuel will be injected causing no backfire. Also with todays exhaust systems and legislation about cats and silencers etc cars dont backfire.

Fuel injected petrol-engined cars use tables to predict the correct volume of fuel to inject for a given mass air flow (MAF). There is also a lambda sensor (also known as a heated exhaust gas oxygen sensor, or HEGO) in the exhaust manifold which is used for trimming the fuel. These sensors exhibit a very sharp change in voltage output as the exhaust mixture passes through stoichiometric (lambda = 1). The fuel injection system uses a process called limit cycling whereby the fuel will be increased until the HEGO voltage switches (rich mixture) and then decreased until the HEGO voltage switches again (lean mixture), the whole process being repeated a few times per second. Using the tables (feed-forward) and HEGO sensor (feed-back) the engine operates very close to a stoichiometric mixture at all times, which is essential for the proper performance of a catalytic converter.

As for the original question, I'm not sure what you mean by 'there is always smoke coming out'. I've not noticed this unless the car is knackered or it's a cold morning. A well-tuned petrol engine should not produce any visible smoke. Diesels, on the other hand...

Quote from Woz :

Get in your car, start it up and then take your foot off the throttle. It should still be running?

Thats because the throttle body is always open a little bit, so the car adds fuel. The car adds fuel when the A/F ratio gets above a certain amount, and takes fuel away when the A/F ratio is below a certain point. I think....

When you shut off your car, the fuel pump and ignition stop.

Quote from shiny_red_cobra :

One more thing...when you run out of gas in LFS, and your engine is still revving, and you press the gas, the sound gets louder as if you are really giving it gas, even though you're not. Just thought I'd mention it.

That's because the engine is still running and pressing the gas opens up the butterfly which then allows the sound of the engine to be heard through the intake. So this would be accurate.

Close the throttle suddenly, and the engine will run lean (not rich) which causes the popping.

All intake systems have an idle air bypass, so that at idle, with the throttle fully closed, a certain amount of air and fuel is correctly metered to the engine, thus allowing it to run at idle. When you close the throttle at higher revs the engine is kept running (or rather, kept turning) by the inertia of the engine.

As Davo says, opening the throttle will change the note of a pumping engine (i.e. turning but not firing), so this is accurate behaviour.

Your engine shouldn't smoke!

You can also create backfiring and a lick of flame on the overrun by sparking very late at load site zero. So late in fact that the exhaust valve is fully open.

Quote from ZORER :

There is always gas burnt in the engine even if you don't touch the throttle while it is running.So it can keep the valves and things running.

I mean, in neutral. not in gear.

Quote from ZORER :

I mean, in neutral. not in gear.

ECUs don't know what gear you are in. The map is the same in gear or not.

Quote from Gentlefoot :

ECUs don't know what gear you are in. The map is the same in gear or not.

yea that's right.silly me.

But if you're in a gear and the car is moving that means all the valves and pumps are running depending on the crank. But in neutral, the engine should produce its own power and rotation to keep itself alive.


i don't know .

Oddly enough, the valves and pumps work exactly the same way in neutral as in gear. The gearbox is a different part of the car.

If you don't know the absolute basics of engine workings then maybe, just maybe, this thread isn't the one for you

This thread isn't the one for me. Although I do know 'what' a gearbox is. 'Where' it is, there you have me.

I'm leaving you to it now...

Quote from ZORER :

But if you're in a gear and the car is moving that means all the valves and pumps are running depending on the crank. But in neutral, the engine should produce its own power and rotation to keep itself alive.

Below is approximately what's going on inside, regardless of what the gearbox is doing:



A typical ECU will just adjust the fuel feed depending on the current RPMs and how much you press the throttle. Which, roughly speaking, is how it seems like it "knows" to cut the feed if you're coasting in gear.

Quote from thisnameistaken :

It's in the big greasy box at the front with all the other confusing big dirty metal things.

Unless you own something that's got the engine in the rear.
In that case if you find a big greasy box with all the confusing big dirty metal things in it at the front then it will very likely not be the big greasy box with all the confusing big dirty metal things in it that you should be looking for, because if it is then it will possibly have your brains plastered all over it.

EDIT:
Because nothing is complete without a fancy marketing pitch.

Quote from Gentlefoot :

ECUs don't know what gear you are in.

Not true.

Quote :

The map is the same in gear or not.

This is probably true, though not necessarily. I am aware of ECU software which is capable of changing the throttle mapping depending on the current gear but I don't know if the functions are actually used.

i know the basics,i know a bit more than basic actually. i think i should improve my english... let's start all over again,

1-if the car is in a gear and it is moving and you don't touch the throttle, the kinetic energy coming from the rotation of the wheels will go through the gearbox to the engine and keep the engine turning.
2-But if in neutral, no kinetic energy will come from the wheels because the gearbox won't transfer it.So the engine should be burning gas to keep turning.
nothing wrong with this.

Here comes the question, will the engine burn gas in the first condition?? i think yes.

Yes, but with exceptions. Modern ECU's sometimes completely stop the injectors on overrun (when the engine is being turned primarily by the gearbox) with the throttle fully closed, but this is in the interests of economy and emissions (though I'm not sure of the benefits to emissions, as it seems like a boat of false promises to me).

But on 'older' cars, especially on carburettors, the fuelling was always on (and with carbs proportional to the pressure drop across the carbs caused by the throttle position and engine speed).

Also, in your example above, in case #1 the engine will restart delivering fuel around the idle rpm to try and stop it from stalling. The ECU will detect falling rpms (and probably won't know the car is in gear) and just try to maintain a smooth idle.

Quote from StewartFisher :

Not true.

This is probably true, though not necessarily. I am aware of ECU software which is capable of changing the throttle mapping depending on the current gear but I don't know if the functions are actually used.

Sure, you can add an input for gear if you want. But this is not needed and I do not know of any ECUs where this is standard.

Ofcourse you can add input into your ECU for wheel speed, EGT, Air Temp, Fuel Temp whatever you want if you really want to.