[MisterMister]

Sorry if this is a dumb question, but Ive only had one other turbo car (aftermarket) and that was stolen 2 months after I had it put in.

Anyway, is the turbo not supposed to spool when the engine is reved while not in gear (or with the clutch pushed in)? this seems to be the case for me. is it just that there isnt enough load or is there some sort of bypass mechanism? I have been wondering this for a while and decided to finally ask. I guess this is as good a place as any to do so.

 

[[email protected]]

no boost when free revving. (maybe like .2 psi). no load, no boost.

 

[MisterMister]

can someone explain this in more detail? the way i understand it, the exhaust spins the turbine which creates boost. since exhaust is still exiting even when not in gear, shouldnt there still be boost?

 

[billwot]

can someone explain this in more detail? the way i understand it, the exhaust spins the turbine which creates boost. since exhaust is still exiting even when not in gear, shouldnt there still be boost?

1) When the turbo spins, it creates flow. But in order for that flwo to be pressurized, there must be some restriction. When there is no load, the engine doesn't create much restriction.

2) When the engine is under load, more exhaust energy is available to drive the turbine, and it spins faster and flow more air. More flow and more restriction means more boost.

When you free rev the engine, it uses more air than the turbo can provide. You will also learn that for the same reason, you will make less boost in low gears, and more boost in higher gears.

This might also be of interest:

http://www.mr2oc.com/showthread.php?s=&threadid=49

bill

 

[MR2000GTE]

really it's because the turbo has intertia (it takes time to accelerate it to speed). Yes, full throttle in neutral is the same as full throttle in gear; it's just that in gear you can stay at full throttle for a longer period of time.

You'll notice that if you rev it hard at high rpm over and over you'll develop boost. I could do this even on the ct26.

Since installing a CT20b (ceramic turbine and thinner shaft) I can make 15 psi by revving in neutral.

 

[torn81]

Since installing a CT20b (ceramic turbine and thinner shaft) I can make 15 psi by revving in neutral.

I really doubt this ! I want to see a short video

If it would be true ... why use an anti-lag system if you are already boosting ? This dosn't make sense. Also, it wouldnt make a difference of 0 psi to 15 psi in neutral just by switching to a "slightly" better turbo.

Well, it's just that I never saw a turbo car made boost in neutral without antilag ... I have a 3sgte ct26 right now, and never had a ct20b. But I highly doubt you on that 15 psi on neutral !

really it's because the turbo has intertia (it takes time to accelerate it to speed).

Also I think this sentence is not true. The turbine has inertia, like any moving part, but it's not because of inertia that you can keep more boost.

It's more about when the engine has load, you need more power. When you need more power, you have to have more air and fuel. If you have more air and fuel, it means the flow will be greater on the turbine. More flow at the turbine will get you more air compressed by the compressor wheel.

And it goes like that in circle and building more boost until the compressor can't compress any more air, or the turbine side is too restrictive to flow the exhaust flow, or that you have exceeded the fuel capacity of the injectors.

Also, if you're in first and the wheels spins, you will need less power from the engine to move the spinning wheels and you will get less boost. And as a vehicle gain speed, it takes more and more energy ( power ) to make it even faster to face the aerodynamic friction and contact surface (wheels) friction. Thats why you boost less in low gears, and more in high gears.

So in neutral, you have no load. No load means no need for power. That means not a lot of air to spool the turbine and turn the compressor wheels.

The only way to build boost in neutral is by applying a load on the engine (slipping the clutch for exemple), or using an alternative way to turn the turbine wheel like an anti-lag system (anti-lag system is to make detonation in the exhaust manifold instead of the combustion chamber to turn the turbine wheel while in neutral or shifting).

Feel free to say where I'm wrong and all .. but I'm pretty sure this is a good description of how a turbo would gain boost.

-Phil

 

[silent765]

ok then how come in some cars they can just sit in one spot and just go VROOOOM PSSHHH VROOOM PSHhh there has to be some presser

 

[Alex W]

Part of it also comes down to what is actually driving the turbo. Contrary to popular beliefe, the major source of energy used to drive a turbo is not the exhaust FLOW but the THERMAL ENERGY in the exhaust. No load=much less thermal energy=no boost.

 

[MisterMister]

ok then how come in some cars they can just sit in one spot and just go VROOOOM PSSHHH VROOOM PSHhh there has to be some presser

IIRC this only happens in the F&F movies.

thanks for the answers guys.

 

[torn81]

Part of it also comes down to what is actually driving the turbo. Contrary to popular beliefe, the major source of energy used to drive a turbo is not the exhaust FLOW but the THERMAL ENERGY in the exhaust. No load=much less thermal energy=no boost.

I'm pretty sure thermal energy, and exhaust flow can be related in some type of equation.

This is from a web site:

Thermal energy - The total internal kinetic and potential energy of an object due to the random motion of its atoms and molecules. An object that feels hot has more thermal energy inside it than it does after it has cooled down. Although technically incorrect, the word "heat" is often used to mean thermal energy. See Heat.

So the thermal energy of an exhaust flow is not how hot it is, but whats its potential energy. In a way, I think it's right to say that what states the exhaust flow CFM would be the thermal energy generated by the combustion chamber. More load on mechanical parts = particles have more potential energy due to more stress in the explosion, thus making more exhaust flow.

So you're right to say that the thermal energy from the exhaust flow is what is driving the turbine wheel. And that if the exhaust flow is cooling down, less energy will be available, and the exhaust flow will be flowing at lesser speed.

And from another angle: in my book, cold air going at 500cfm (random value) and hot air going at 500cfm will spin a wheel at the same speed. BUT, for the hot air, there will be less air molecule for the same volume as the cold air.

So hotter exhaust could lead to increased CFM because the exhaust molecules expands and thus makes a larger volume to spin the turbine wheel.

I think that understanding the physic behind a turbo-compressor is more difficult than I though

-Phil

 

[Skywalker]

hehehe sounds a bit Fast and furious to me :smile:

anywho, my hks dv will go off with a bit of a rev.

 

[billwot]

Part of it also comes down to what is actually driving the turbo. Contrary to popular beliefe, the major source of energy used to drive a turbo is not the exhaust FLOW but the THERMAL ENERGY in the exhaust. No load=much less thermal energy=no boost.

That is correct.

It is a common misconception that the exhaust turbine half of a turbo is driven purely by the kinetic energy of the exhaust smacking into it (like holding a kid's tow pinwheel behind your tailpipe) While the kinetic energy of the exhaust flow does contribute to the work performed by the turbo, the vast majority of the energy transferred comes from a different source.

Keep in mind the relationship between heat, volume, and pressure when we talk about gasses. High heat, high pressure, and low volume are all high energy states, low heat, low pressure, and large volumes are low energy states.

So our exhaust pulse exits the cylinder at high temperature and high pressure. It gets merged with other exhaust pulses, and enters the turbine inlet - a very small space. At this point, we have very high pressure and very high heat, so our gas has a very high energy level.

As it passes through the diffuser and into the turbine housing, it moves from a small space into a large one. Accordingly it expands, COOLS, slows down, and dumps all that energy - into the turbine that we've so cleverly positioned in tho housing so that as the gas expands, it pushes against the turbine blades, causing it to rotate. Presto! We've just recovered some energy from the HEAT of the exhaust, that otherwise would have been lost.

This is a measurable effect: Stick an EGT upstream and downstream of the turbo, and you see a tremendous difference in temperature.
So, in real world terms, what does this tell us?
All else being equal, _The amount of work that can be done across an exhaust turbine is determined by the pressure differential at the inlet and outlet_ (in English, raise the turbo inlet pressure, lower the outlet pressure, or both, and you make more power) Pressure is heat, heat is pressure.

This is probably the best site i have found for real engineering info about turbos. The author is a Chrysler engineer, and a well-known performance car guy.

http://www.turboclub.com/turbotech/index.htm

 

[torn81]

That makes sense.

 

[MISTER2]

Since installing a CT20b (ceramic turbine and thinner shaft) I can make 15 psi by revving in neutral.

Not possible unless your car is operating in a manner against the laws of thermodynamics.

There just isn't enough energy or restriction in the system under zero load to allow for that much boost to be generated. Under a zero load / free rev the turbine is being spun primarily by the kenetic energy of the exhaust gases hitting the turbine blades...at best you might generate 1-2psi from the kinetic energy alone.

You can't build significant boost unless you add massive ammounts of heat energy as previously discussed.