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Location:
W.A
Registered:
May 2004
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 Another turbo question
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Thu, 09 September 2004 10:05
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I've been thinking about this for quite some time...
Why is it that a turbocharged engine, when cruising (not accelerating), is not under boost?
Eg. When accelerating to 100km/h boost is on, but when that speed is reached, and held at the same rpm, there is vacuum and not boost.
I would think that an engine reving in neutral at say 4000rpm would produce the same amount of exhaust gases as an engine reving at 4000rpm under load..
Meaning the turbine should be spinning at the same speed for both cases.
If this is true then the turbo should be providing boost all the time...
except it doesn't when not under load!
Can any enlightened person explain this? (so i can sleep at night)
Thanks!
Another example, in case I didn't explain it very well:
When driving on a straight stretch at a constant speed, there is a vacuum in the inlet manifold, so no boost. However when the car starts to go up a hill, the turbo spools up, and u have boost!
But this is all at the same rpm! So can anyone explain why this is so?
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Location:
Brisbane, QLD
Registered:
February 2003
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Re: Another turbo question
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Thu, 09 September 2004 10:30
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from what i can understand
when the throttle is close you'll theoretically produce -1bar(gauge) vacuum
this is due to the fact that the engine is still sucking in "air but there is no replenishment of it from where the throttle closes
think about a paper tube, suck through it and "nothing" happens to the tube
close the opposite end from where you are sucking and watch the tube collapse(due to the "vacuum" produced)
ok
sooo pulling figures out of my arse
vacuum will be produced between 0-50% open throttle(so your gague will read between -1 bar and 0 bar)
at 50% open you'll read 0 bar
when cruising along you'll be say, pulling another figure out of my arse, at 10% open throttle and read -0.2 bar(a vacuum)
so even though you are producing exhaust gases, which make the turbo spool up its probably not enough to compress the intake more than atmospheric pressure
what boost is, from my understanding is the compressor's(apart of the turbo) ability to compress air to MORE than what the atmospheric pressure is, this is a by-product of the turbo's RPM(i think?)
i could be wrong...its just me trying to put what i think i understand into words
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Location:
Brisbane
Registered:
May 2002
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Re: Another turbo question
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Thu, 09 September 2004 10:43
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| d-shiznit wrote on Thu, 09 September 2004 20:05 |
I would think that an engine reving in neutral at say 4000rpm would produce the same amount of exhaust gases as an engine reving at 4000rpm under load..
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Definitely not. The volume of exhaust gases produced is proportional to the amount of power generated by the engine. An engine revving at 4000rpm is developing bugger-all power (just enough to overcome internal friction and pumping losses), but an engine accelerating a 1000+ kg lump of steel at 4000rpm is obviously developing a great deal more power!
So there's your answer; the lightly-loaded engine is developing very little exhaust gas, hence the turbine is developing very little power, hence there is no boost.
On top of this, the throttle butterfly regulates the manifold pressure - so even if the turbo is developing 3 bar of boost, there will still be a vacuum in the manifold if the throttle is mostly closed.
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Location:
Brisbane, QLD
Registered:
February 2003
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Re: Another turbo question
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Thu, 09 September 2004 10:47
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ok
well looks like my description was only partly correct
there you go...
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Location:
W.A
Registered:
May 2004
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Re: Another turbo question
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Thu, 09 September 2004 11:09
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Norbie,
So from what you are saying an engine reving in neutral, and an engine under load produce different amounts of exhaust gas..
But how can this be the case?
1 revolution and 1.6L gas should come out (of a 1.6L engine)
When boosting, more exhaust gas will come out, but what if it's going from vacuum to boosting (without opening the throttle any more)
Is this true for NA cars? so reving in neutral and under load at the same rpm will produce different amounts of exhaust gas also?
Take the example of a car from level ground going up a slope, I don't accelerate any harder, so the throttle body is open the same as when the car was on level ground...
Ok so how is it that i start getting boost? With the throttle body at the same position, engine at the same rpm.
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Location:
Brisbane, QLD
Registered:
February 2003
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Re: Another turbo question
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Thu, 09 September 2004 11:12
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try thinking of it like this
what uses more energy
walking up a hill at 5km\h
or walking along a flat peice of ground at 5km\h?
you're still moving at the same speed, but burning up more energy going up the hill(if you were a car, this would mean you'd produce more exhaust gas)
i hope thats what norbie was trying to get across
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Location:
W.A
Registered:
May 2004
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Re: Another turbo question
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Thu, 09 September 2004 11:51
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Ok I understand that using up more energy part.
But how does using up more energy = more exhaust gas
Keeping in mind that I haven't opened up the throttle any more, and the rpm is still the same
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Location:
W.A
Registered:
May 2004
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Re: Another turbo question
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Thu, 09 September 2004 11:53
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If the rpm is still the same, and the throttle at the same position, then the amount of air going into each cylinder is the same.
Therefore how can more exhaust come out?
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Location:
Melb
Registered:
June 2004
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Re: Another turbo question
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Thu, 09 September 2004 12:39
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Think about it. Is your throttle still in the same position when you are cruising as when you are accelerating? Not likely. Have you changed gears etc?
The walking up a hill analogy is quite good. Even though you are still going the same speed and maybe your legs are taking as many steps per minute- you wont get up that bloody hill unless you apply more force through your legs.
Dont let questions like this make you lose sleep at night, hope you get some sleep!
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Location:
W.A
Registered:
May 2004
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Re: Another turbo question
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Thu, 09 September 2004 12:53
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yes the throttle is in the same position, havent changed gears.
I'm talking about putting your foot down, in any gear. Eventually you will stop accelerating and reach a constant speed, throttle will still be in the same position. (foot still in same position on pedal)
At this point, boost will change to vacuum correct?
Then say you go up a hill, still with your foot in the same position, still in the same gear, you will start to get boost again.
 I understand the energy analogy, just looking for a scientific explanation as to why there is more exhaust gas!
I don't really lose sleep over it btw... hehe
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Location:
Brisbane
Registered:
May 2002
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Re: Another turbo question
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Thu, 09 September 2004 13:45
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It's really very simple guys!
When the engine is under load, that's another way of saying it's developing power. Where does the power come from? Burning fuel (in the presence of oxygen). What is the byproduct of burning fuel? Exhaust gas!
More engine load = more power = more fuel = more exhaust.
Also be aware that engine load is not directly proportional to throttle opening - there are several other factors which affect this. Engine speed and volumetric efficiency are two important factors, and of course boost in the case of a turbocharged engine.
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Location:
Western Australia
Registered:
April 2004
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Re: Another turbo question
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Thu, 09 September 2004 14:09
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if the same throttle posistion is used in each circumstance then to my understanding the exhaust output SHOULD be the same.. if someone can prove me wrong i love learning 
if you say it uses more energy to go up hills in cars have you heard of torque multplication ? give the correct gear ratio's and my understanding *could be a bad thing* you should be able to use the same throttle posistion thru out all gears and make it up the hill..
that aside.. drive your car @ 90km/h underload.. with lets say 25% throttle.. depress the clutch or put it in neutral keep your foot on the pedal on the throttle and look at the rpm.. underload it produces X amount of gaas from exhaust @ 2,000rpm and not underload it produces X amount of gas from exhaust but at 5,000rpm ?
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Location:
MACKAY
Registered:
May 2002
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Re: Another turbo question
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Thu, 09 September 2004 14:45
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OK I will try to explain ,
If when on cruise at 3000 rpm the throttle is say 10 percent open and the manifold vacuum is at 10 inches mercury the load point in the ecu map will be on a lean cruise mixture and producing say 15 kw's power , there is not enought exhaust gases to spool up the turbo , but when you come to a hill the motor is then loaded up a bit and rpm drops a little and the manifold pressure rises to say 5 inches mercury , now the load point in the ecu map at this pressure will increase the fuel and inturn will produce more power which = more exhaust gases which will start to spool up the turbo and lift the pressure in the manifold .
Dose that make any sense !!
Trevor
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Location:
Campbelltown, NSW
Registered:
November 2002
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Re: Another turbo question
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Thu, 09 September 2004 15:30
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Boostis depicted by two things RPM and Throtle position (air in)
When cruising at 4000rpm your throtle position is 10% (little air in, little air out) not enough air out to spin the turbo enough to create positive pressure.
When accelerating at 4000rpm your throttle position is at 95% (lots of air in, lots of air out) now you have a lot of gas flow out through the turbo which spins the turbine at a greater rate than before so it creates more positive.
so more air in= more air out= more boost
Hope this helps
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Location:
Perth
Registered:
August 2004
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Re: Another turbo question
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Thu, 09 September 2004 15:52
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When an engine is under load more fuel is used. when the fuel mixes with the air and gets combusted the resultant gas takes up more room or volume than just the origional air and fuel. Kinda like if you shake a coke bottle... So when the engine is under more load all theings being equal throttle etc... More fuel gets combusted and it produces more gas... Load can be seen as how much you shake the coke bottle. When a constant speed is reached the amount of fuel required to maintan that speed is less therefore load decreases and you have less exahust and hence the drop in boost... Hope this helps
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Location:
W.A
Registered:
May 2004
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Re: Another turbo question
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Fri, 10 September 2004 04:28
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| TA22-3SGTE wrote on Fri, 10 September 2004 00:45 |
when you come to a hill the motor is then loaded up a bit and rpm drops a little and the manifold pressure rises
Trevor
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How does the manifold pressure rise?
it can only rise if the turbo provides boost right?
Thanks to everyone that replied by the way, I agree with more fuel producing more exhaust gases..
Mx62, thats the way I see it!
Norbie, stark, ta22 3sgte I understand what you're all saying
But now another question:
Which sensor detects the engine coming under load (to then provide more fuel)?
(Same throttle position, same rpm, same gear)
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Location:
Brisbane
Registered:
May 2002
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Re: Another turbo question
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Fri, 10 September 2004 04:54
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Engine load is determined by the MAP sensor (manifold pressure) or the AFM/MAF sensor (airflow).
And to answer your other question: the manifold pressure rises because the RPM dropped. You have to remember this is a dynamic system, and it's not a linear relationship between throttle position and airflow/manifold pressure/power output!
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Location:
W.A
Registered:
May 2004
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Re: Another turbo question
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Sat, 11 September 2004 04:18
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 Ok big THANKS to everyone that replied!
I C THE LIGHT!
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