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      06-09-2010, 05:04 AM   #1
fatjoez
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Angry Why does the 135i burn 39.5L/100KM petrol at idle, 0.0L/100KM while coasting?

Can someone please, do me a favour, and explain the science/logic behind the 135i burning

39.5L/100KM petrol @ a traffic light/idling
0.0L/100KM petrol @ rolling down a hill with foot off accelerator

much appreciated..
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      06-09-2010, 05:19 AM   #2
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Quote:
Originally Posted by fatjoez View Post
Can someone please, do me a favour, and explain the science/logic behind the 135i burning

39.5L/100KM petrol @ a traffic light/idling
0.0L/100KM petrol @ rolling down a hill with foot off accelerator

much appreciated..
When you're idling, you're moving zero distance while using fuel. That should theoretically give you a consumption of infinite litres per 100km. You are burning fuel but not moving at all. Our consumption metre can only show a maximum of 39.5L.

When you're coasting down a hill in gear, no fuel is injected into the engine. The spinning of the wheels keep the engine running, so you're effectively using no fuel to travel the given distance. Hence, 0L/100km.

Make sense?
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      06-09-2010, 06:02 AM   #3
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I've brought this point up before. Also, Jeef Beef is correct in the above points. Our cars do burn quite a bit of fuel at idle though.
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      06-09-2010, 06:05 AM   #4
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Quote:
Originally Posted by Jeef Beef View Post
When you're idling, you're moving zero distance while using fuel. That should theoretically give you a consumption of infinite litres per 100km. You are burning fuel but not moving at all. Our consumption metre can only show a maximum of 39.5L.

When you're coasting down a hill in gear, no fuel is injected into the engine. The spinning of the wheels keep the engine running, so you're effectively using no fuel to travel the given distance. Hence, 0L/100km.

Make sense?
Except that you must be using SOME fuel or the engine would stop running. It's not like the engine is OFF while you are coasting down hill.

I've wondered which uses less fuel....coasting down hill in gear or coasting down hill out of gear. Logically if you are out of gear in a down hill coast you are using the same amount of fuel as you would be while idling at a light. Are you using the same, more or less if you are coasting down hill while still in gear?
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      06-09-2010, 06:10 AM   #5
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of course coast downhill out of gear would consume the least. When you in gear, the engine revs at a certain RPM according to the gear you in, the lower the gear, the higher the RPM. When you clutch in, or is in Neutral, you are idling and remain at somewhere around 800 rpm. duh. :P
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      06-09-2010, 07:26 AM   #6
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Quote:
Originally Posted by ichiban View Post
of course coast downhill out of gear would consume the least. When you in gear, the engine revs at a certain RPM according to the gear you in, the lower the gear, the higher the RPM. When you clutch in, or is in Neutral, you are idling and remain at somewhere around 800 rpm. duh. :P
That would only be true if gravity wasn't pulling the car downhill.... You seem to be assuming that the amount of fuel that is being consumed is simply a function of engine speed, ignoring load. Rolling downhill in-gear there is a negative load on the engine. To quote yourself "duh :P"
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      06-09-2010, 08:17 AM   #7
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Originally Posted by TP 135i View Post
Except that you must be using SOME fuel or the engine would stop running. It's not like the engine is OFF while you are coasting down hill.

I've wondered which uses less fuel....coasting down hill in gear or coasting down hill out of gear. Logically if you are out of gear in a down hill coast you are using the same amount of fuel as you would be while idling at a light. Are you using the same, more or less if you are coasting down hill while still in gear?
You're using more fuel when coasting out of gear, as your engine now has to idle. If you're in gear, the engine is still turning, it's not off, just the fuel injectors aren't injecting any fuel. Everything is still turning in there without using any fuel at all, which is what provides engine braking.
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      06-09-2010, 01:45 PM   #8
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You're using more fuel when coasting out of gear, as your engine now has to idle. If you're in gear, the engine is still turning, it's not off, just the fuel injectors aren't injecting any fuel. Everything is still turning in there without using any fuel at all, which is what provides engine braking.
So if the injectors aren't injecting any fuel, there is NO combustion going on...just a spinning engine. That would mean that the engine would have to restart it's combustion cycle when you, once again, activate the accelerator pedal. Is that right?
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      06-09-2010, 01:47 PM   #9
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Like said above, coasting with gear engaged = engine spinning with no fuel being injected. I wouldn't be surprised if the plugs werenít firing either. But your engine is still going through the four strokes. Air is being pulled in, compressed, valves opening/closing, etc...

Coasting with gear disengaged = nothing is able to keep your engine running, so it runs as if itís idling, using quite a bit of fuel.

You can see this in practice with your MPG indicator on your console:

Coast down a hill both in gear and out of gear. When the injectors are off, MPG will be 99.9 MPG, basically meaning no fuel is being used. While out of gear, it will be much, much lower. I think around 20 - 30 MPG, maybe less. I don't know. I dont coast out of gear.

Also, you can see the point where you arenít coasting fast enough for the computer to be happy with the engine RPM, and it will start injecting fuel and running as if itís idling. You will clearly feel the engine do this switchover, and your MPG reading will change from 99.9 MPG to a lower value.

Bottom line, cost as long as you can, and raise your MPG's. Maybe even cool off your engine from that 0-100mph pull you just did.

Google "pumping losses" and "engine braking" for some more info.

Also, I wonder if the Valvetronic system alters valve lift during coasting to minimize pumping losses and maximize the distance you can coast? Where are all the master BMW techs when you need one.
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      06-09-2010, 10:39 PM   #10
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Quote:
Originally Posted by TP 135i View Post
Except that you must be using SOME fuel or the engine would stop running. It's not like the engine is OFF while you are coasting down hill.

I've wondered which uses less fuel....coasting down hill in gear or coasting down hill out of gear. Logically if you are out of gear in a down hill coast you are using the same amount of fuel as you would be while idling at a light. Are you using the same, more or less if you are coasting down hill while still in gear?
Actually no, as others have stated, the injectors do not fire at all. There is NO fuel being used. Modern engines like ours simply let the pistons go through the 4 strokes - using the spinning of the wheels to do so. When you put your foot down again, the injectors will fire.

If you're out of gear, then the engine must use fuel to maintain idle speed. So in essence, it should be more efficient to coast down in a high gear.

Which means the most fuel efficient way to coast down a hill is to select a gear which keeps engine speed just above idle (so the injectors won't fire) as this minimises speed loss through engine braking.
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      06-10-2010, 01:57 AM   #11
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can someone show me a diagram or explain a bit further how the "wheels rolling" turns the engine?

i sorta get the rest, I just dont get why BMW can't tell the car to stop injecting when the car is idling?

like this whole start-stop why didnt they put it in the 135i whats so hard?
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      06-10-2010, 02:27 AM   #12
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Quote:
Originally Posted by fatjoez View Post
can someone show me a diagram or explain a bit further how the "wheels rolling" turns the engine?

i sorta get the rest, I just dont get why BMW can't tell the car to stop injecting when the car is idling?

like this whole start-stop why didnt they put it in the 135i whats so hard?
I'm pretty sure a diagram isn't necessary. The engine turns the wheels when you're accelerating - so the wheels must also turn the engine when you're not. It's a pretty simple concept - there's a physical connection between the engine, transmission and wheels when you're in gear. Rolling down a hill means your wheels are spinning and that turns the engine too. I don't see how I can make this simpler

If the car stopped injecting fuel then you wouldn't be idling. With no force to move the pistons, the engine would shut off. Sure there are many cars with start/stop technology but this idea hasn't caught on to most cars yet.

Look in the end idling doesn't use THAT much fuel. The consumption metre shows a high figure because mathematically, the figure should be infinity. Consumption = fuel use/distance traveled. Idle = 0 distance traveled.

Therefore, consumption = x/0 = infinity.
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      06-10-2010, 02:54 AM   #13
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There are small elves pedaling away inside your engine bay, hidden under some scrap metal and plastic bits. To make the car move, they have to pedal pretty hard, and they get very thirsty, so they drink petrol (yes, these are petrol-drinking elves... they're also german and eat grass... but I digress).
So when you are coasting down a hill 'in gear', it makes life nice and easy for them, so they just sit there and pedal every now and then, and sip little bits of petrol - less petrol consumption.
But when you take your car out of gear, or are sitting at the lights idling, they have nothing to do, so they get bored and throw a party and drinks lots (tsk tsk.. bludgy elves), so more fuel is used - high petrol consumption
Remember this when you think about putting alcohol (E10) into your car. They will just get drunk and fall off their bikes, and your car won't move properly
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      06-10-2010, 05:55 AM   #14
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In the BMW manual for the 135i it confirms that no fuel is used when rolling downhill in gear. Well in the UK manual anyway.

In my VAG cars the instantaneous mpg display would read --- when this was happening, signifying infinite mpg, in the 135i it says 99.9mpg as described above.
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      06-10-2010, 10:30 AM   #15
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as for why the start stop isn't implemented in the 135i, from my understanding, cars that shut off when at a stop are diesel, no spark plug. The spark plugs would die so fast if it was implemented on petrol cars. This is just my guesstimation.

oh and clived, i'm not understanding where this negative load is coming from, the engine cannot transmute a negative load to better fuel efficiency, iono i'm just not getting it, if i'm just stupid right now, please enlighten me XD.
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      06-10-2010, 03:07 PM   #16
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oh and clived, i'm not understanding where this negative load is coming from, the engine cannot transmute a negative load to better fuel efficiency, iono i'm just not getting it, if i'm just stupid right now, please enlighten me XD.
I believe that by "negative load", he is referring to pumping losses. Basically, when an engine is idle, the butterfly valve is basically closed, which limits the amount of air that can enter the intake manifold. This limited air, plus the intake stroke of the engine creates a negative pressure (vacuum) in the manifold. So, on the intake stroke, the engine has to work harder to fight this vacuum to pull the air into the combustion chamber. Again, you can see this in effect while coasting in gear vs out of gear.

While in gear, the pumping losses will limit how fast you can coast, because the pumping losses are affected by the current gear you are in.

While out of gear, your costing speed wonít be affected by any pumping losses.

If you test this, you should be able to clearly notice the speed at which you can cost while in gear vs while out of gear. Of course, like mentioned by Jeef Beef, coasting in a higher gear minimizes the effect of pumping losses because the gear ratio is very low, like 1:1 in 5th and like .8ish:1 in 6th. Compare that to 1st which is like 4.something:1, meaning that the engines pumping losses are being multiplied by > 4.

It's worth noting that the 128i (N52B30), or new 2011 135i (N55B30) probably doesn't even have a butterfly valve like most cars do, because I believe the Valvetronic system on these engines uses the intake valves like a butterfly valve: i.e. changing the lift and duration based on the specific situation. I believe the ECU with these engines minimizes pumping losses while at idle/low RPM. It would be interesting to compare the coasting speed of an N54B30 vs an N55B30 to see if Valvetronic would affect this.
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      06-10-2010, 03:37 PM   #17
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Pretty much every modern car turns off the injectors when coasting in gear. It literally uses no fuel. That's why the mileage indicator shows "--", because no fuel is being used.

Just like the engine drives the wheels through the transmission, the process works in reverse when you are coasting, the wheels keep the engine spinning. The momentum of the car keeps the wheels turning, which are still connected to the engine through the transmission (auto or manual).
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      06-11-2010, 03:43 AM   #18
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thanks for the explanation caleb.

^ then why does the rpms stay at a constant 800 ish when the car is coasting?
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      06-11-2010, 05:26 AM   #19
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thanks for the explanation caleb.

^ then why does the rpms stay at a constant 800 ish when the car is coasting?
If you are in gear the rpm are relative to your speed and the gear that you are in.
If you are out of gear the rpm would stay at idle 800 ish rpm.

And when I am in gear and coasting OR out of gear and coasting my mileage-O-meter is reading 99.9. I think that it only reads "---" when I am sitting and idling at a light. (I'll check that one today).
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      06-11-2010, 07:41 AM   #20
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You're still better off coasting out of hear in most cases because you can coast so much FURTHER before slowing down than coasting in gear. Often times if I see a distant red light I will coast in neutral and then if I need to come to a stop or slow down significantly I will throw it in to gear. I have found that if I coast in gear I will unnecessarily slow down more than I need to, and then have to speed up again to get to the speed limit. Save a little fuel when coasting but use a lot more to speed back up. Bad!
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      06-11-2010, 07:59 AM   #21
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I tried this yesterday: get up to about 40mph on a slight downhill: coast out of gear and it reads about 70-80mpg (i.e if you coast in idle at 40 mph you'll still use a gallon of gas to keep the engine idling because it has to keep itself running on gas for a couple of hours while you do the 80 miles)

pop it into 4th or 5th without touching the gas and let gravity turn the engine and it will immediately go up to 99.9
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      06-11-2010, 07:17 PM   #22
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My 135 just shows --- when I'm stopped and idling.
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