Why doesn't it build boost if reved??

[skatershawn]

This Might be concerted a noob question.. but i always wondered why when we rev our cars it doesn't build boost!
I understand you will say... "it has to be under load"
Well why is it only our cars??
What causes this to happen?
The way i think of it is, as the exhaust spins the turbine it compresses the intake... the more you rev the more exhaust, (just like driving, im just in Neutral.) then it sould compress more intake air.
There is no bypass for it to exhaust to pass through besides the wastegate.. but that opens when the intake pressure rises so high to the set psi!
Im pretty knowledged on how turbos work.. but this has always blown my mind!

If theres a thread on it some where please link me... i searched and couldn't find anything but people saying, "it need to be under load"
i just want to understand my motor more!


thanks everyone haha

 

[talondave]

hahaha! boost fluid
clever
i am also making 15psi sitting still, and its a 5spd not an automatic
could it have anything to do with having the wastegate solenoid being bypassed?

I want to see a video, without hitting a revlimit!!!

 

[dsmfa9nutter]

I mean i will drop down and say that heat does play a part with a turbo and how much it spins. If you take a volume of cold air and heat it, it expands. But then again the exhaust gases that are coming out of the combustion chamber are already heated so yes heat plays a small part just due to the expansion and contraction of gasses. It all still comes down to the amount of pressure that is exerted on the exducer.

If that were true, the turbo would spin backwards. Hottest air is coming out of the exhaust ports and cooling as it moves through the turbo and then the exhaust. Exhaust pulses drive the turbo, that's why twin scroll designs work so well.

 

[MrBoxx]

Turbos run off of heat energy, for the most part.

The differential in heat on one side of the vanes of the turbine (manifold side) vs. the temperature of the other side (O2 housing side) create a differential. When you have a temperature differential across a surface, you have a pressure differential, and it has the potential to do Work. The Work being done is the turning of the turbine wheel. It's the transference of heat energy to kinetic energy. This is the 2nd Law of Thermodynamics.

From here] 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.

so if i take an acetylene torch (thats really hot!) to my exducer it will spin really really fast??? or should i take high pressure air and force it through the exducer? I vote air!
Seriously do you have any mechanical knowledge??? Turbos are run off of gas pressure whether it be hot or cold. Why do drag diesels use a turbo on turbo system? Take a compressor and air nozzel and put it in the exhaust side of your turbo and see if it spins, and see if there is air being forced out the compressor side. Heat is the enemy in any motor! It causes materials to degrade, reduces performance, and reduces engine life.

Diesels use a compound turbo system to make ridiculously high boost pressures. Since diesel engines don't care about intake air temps, they simply dump more fuel in to compensate. More fuel= more burning = more heat and thus more temperature to run the turbos.

An acetylene torch is hot, but it's not pressurized heat. Compressed air is pressurized, but it's pressurized due to work that has already been done to it (via compressor). The pressure differential that turbochargers use is directly due to the expansion of a hot gas to a colder gas. The fact that there is a huge temperature differential between the inlet and outlet of a turbo, when factored by the 2nd Law of Thermodynamics tells us that this must be true.

Does a motor run off of heat energy too?? No it runs off of combustion but it produces a lot of heat which is actually energy that is lost during the reaction, that is why combustion motors are considered inefficient.

Yeah, actually motors DO run off of heat energy. To get the energy from gas, we heat it up, which is a word you used in your post: combustion. We use a spark, to burn the gas, which heats up, and we know that when you HEAT A GAS, the PRESSURE INCREASES. The expansion of the gas creates a lot of pressure, pushing the piston down.

Exhaust pulses drive the turbo, that's why twin scroll designs work so well.

Twin-scroll designs work off the idea of timing certain exhaust pulses of hot pressurized air at designated times. It's not simply the fact that there are little puffs of air coming out of the engine; it's that they are HOT puffs of air.

 

[ruben2566]

Really they are run by heat??????? Does a motor run off of heat energy too?? No it runs off of combustion but it produces a lot of heat which is actually energy that is lost during the reaction, that is why combustion motors are considered inefficient. They lose a majority of their potential energy through heat and friction.. so if i take an acetylene torch (thats really hot!) to my exducer it will spin really really fast??? or should i take high pressure air and force it through the exducer? I vote air!
Seriously do you have any mechanical knowledge??? Turbos are run off of gas pressure whether it be hot or cold. Why do drag diesels use a turbo on turbo system? Take a compressor and air nozzel and put it in the exhaust side of your turbo and see if it spins, and see if there is air being forced out the compressor side. Heat is the enemy in any motor! It causes materials to degrade, reduces performance, and reduces engine life.

I cant believe i actually had to make this post.

Not to be rude, but an internal combustion engine also converts heat energy from combustion into mechanical energy. Your reply tells me you don't have any formal training in the operation of an internal combustion engine.

Your also confusing heat with temperature, you mention an acetylene torch, take a match, it has a very hot flame, but doesn't produce much heat. Heat from the combustion process is what provides the gas pressure to run the engine and turbo.

A compound turbo setup on a diesel is used to produce higher air pressure, but without adding more fuel to increase the heat, it won't make more power.

 

[falcon8r]

Turbos run off of heat energy, for the most part./QUOTE]

Thank you for that. I just shook my head , even the wiki does not explain this. I looked, to illistrate this point also. Your explanation is perfect. Heat is what works a turbo. Tres...

 

[quiksilver20004]

I never said heat doesnt play a part in a turbo i simply stated that pressure is what causes the most work. The two are tied together. You can take cold air compress it then introduce it to an area of low pressure and it will expand, just not with as much force if that same volume of air had been first heated due to the molecules not being as active. I will admit that i was a bit wrong in what i said and the way i explained it. I know that heat does work, take a look at the stirling motor. It is a prime example of the work that heat does when combined with a gas. Yes heat plays a factor due to differential pressure. You are correct in what you said about more heat=more pressure and it going from a small space into a larger one and expanding, thus causing work to be done on the turbine. But if you look at what you said you will see that this concept works off of pressure and pressure differences, the heat is just a catalyst for the process. A turbo will work off of pressurized air that is not heated, though it may not work as well. YES heat does work in a motor there i said it! You just dont want too much heat.

The #principle behind any reciprocating internal combustion engine: If you put a tiny amount of high-energy fuel (like gasoline) in a small, enclosed space and ignite it, an incredible amount of energy is released in the form of expanding gas. (expanding gas is high pressure)

As far as the statement that diesels dont care about intake temps this isnt completely correct. More fuel doesnt just automatically cause more burning. Fuel will not burn or burn well without first being mixed with o2 to oxidize it. The cooler that the intake temperature is, then the more dense the air which gives you a greater potential energy. You cant just add more fuel without adding more air or you would reduce the amount of energy that the fuel can produce. Now given a diesel helps itself in this by having much higher boost pressures. A diesels combustion reaction works off of straight heat. You add high pressure air then compress it causing a tremendous amount of heat, then inject a high pressure well atomized pulse of fuel and you get detonation. If they wernt concerned with intake temps then why do they put an intercooler on it? Now tell me im wrong on that

 

[DSMopar]

Well you're not totally clueless, just completely wrong. I'm a mechanical engineer, there's a whole section in thermodynamics called 'heat engines' which includes I.C engines. So no the entire process is heat driven, heat is what drives the pressure differences.

 

[quiksilver20004]

Didnt i just say that heat is what causes the pressure differences??????????

I know heat is what drives the pressure differences, that and compression due to the piston. But what im trying to say is the PRESSURE caused by the heat is what pushes the pistons back down and helps drive the turbine. Im well aware of thermodynamics, we just arn't looking at things from the same point of view. Heat causes pressure which causes work to be done. If i have a vacume in a cylinder then added as much heat as i wanted, would it cause the cylinder to move? No, due to the fact that there is nothing in the cylinder to expand, thus not causing pressure, and thus not driving the piston down.

 

[tkelly27]

http://www.engineersedge.com/thermodynamics/power_plant_components.htm

http://www.engineersedge.com/thermodynamics/images/second24.jpg

As you can see, the only thing that determines the amount of work going into a turbine is the enthalpy.

You're welcome to read more about enthalpy here.

Enthalpy - Wikipedia, the free encyclopedia

Theoretically, given the proper ambient conditions, a turbine will be driven with the same amount of work with a high pressure low temp fluid as it would be with a high temp low pressure fluid, as long as they had the same amount of energy in it.

The turbine uses temperature and pressure as the combined heat energy, and kinetic energy to drive it.

The pressure from your air compressor created kinetic energy to drive the wheel. There was likely a small amount of heat energy transferred. The pressure in was exactly the same as the pressure out, save the pressure created by the velocity of the air slowing down on the blades.

This topic came up before, and I was on the pressure side of the argument. He knows a hell of a lot more about turbines than I ever will and explained how there is not only a significant pressure drop, but a significant temp drop as well. Maybe he'll chime in, but I'm too lazy to look him up to ask him.