2JZ head porting.

[Billzilla]

Can you explain how heat can spin a turbine other than the fact that heat is related to volume (ie expansion). How many RPM will my turbine spin if I put it in the oven at 220 degrees? If it doesn't spin, why is this?

Well obviously it won't.
But heat is how we percieve the energy of the atoms of an object, so the hotter it gets the more energy it contains.
It's not a very efficient process expanding the air to get some energy our of the heat it contains, but it's still there.

You know the old story of a bicycle pump getting hot when you pump a tyre up? It's kinda the same thing but in reverse .... you're turning that heat into a bit of work.
The second law of thermodynamics or the like.

[2JZR31]

*sigh*

You need a little more physics knowledge, energy is a constant, it cannot be created or destroyed (discounting nuclear energy/mass reactions), to do any work requires energy.
Energy can be changed in form easily tho, heat energy can be change to kinetic energy - that is the process that causes the turbine to spin.

Ah, fuck it, I haven't studied physics for over 14 years, it hurts my head - can't you just accept that it's true?, I really don't want to think about it enough to explain it, plus it's past my bedtime, gotta get up at 5.45.

edit - thanks bill, now I can go to bed

TK

I know about the conservation of energy.

Heat expands the air which means it has a higher velocity to spin the turbine. Which means it is flow related. This follows the law. However heat driving a turbine by its action of being hot alone does not seem to make any sense.

[2JZR31]

Well obviously it won't.
But heat is how we percieve the energy of the atoms of an object, so the hotter it gets the more energy it contains.
It's not a very efficient process expanding the air to get some energy our of the heat it contains, but it's still there.

You know the old story of a bicycle pump getting hot when you pump a tyre up? It's kinda the same thing but in reverse .... you're turning that heat into a bit of work.

So does this mean that heat can't do anything on its own to drive a pump, and only by products of the heat can. IE expansion?

[Billzilla]

So does this mean that heat can't do anything on its own to drive a pump, and only by products of the heat can. IE expansion?

Not quite - there's Crookes Radiometer but that's not the best example.




And yes, really hot air will want to expand more, that's a good analogy.

[Rz]

More heat in ex = faster spool, regardless of turbo size

TK

so more heat in the exhaust is what improves spool ?
how is a port job generating more heat after the waste has left the cylinder ?
the only thing i really know about in this arguement is the turbo , and what leaves the engine has to pass through the exhaust housing , and the nozzle size between turbos and their respective a/r's can vary greatly.
ie : a turbo like a gt35 (600 hp rated) cant possibly pass more exhaust than what can get past the 12 ports / valves in the 2j , but something like a gt4788 (1500hp rated) i could easily see outflowing the stock ports.
im not arguing for arguements sake here , just interested.

[2JZR31]

Not quite - there's Crookes Radiometer but that's not the best example.




And yes, really hot air will want to expand more, that's a good analogy.

Guess what? The link says that it works via the expansion of air caused by heat.

a turbo like a gt35 (600 hp rated) cant possibly pass more exhaust than what can get past the 12 ports / valves in the 2j , but something like a gt4788 (1500hp rated) i could easily see outflowing the stock ports.
im not arguing for arguements sake here , just interested.

Same here. I am genuinely interested and intrigued by this subject, and tend to agree with your theory. That is what I was getting at before. The GT35 will still work better with a decent port job, but the bigfuckoff will probably NEED a port job to work right, other wise it will be choked.

[Billzilla]

Guess what? The link says that it works via the expansion of air caused by heat.

So go and paint one side of your turbine blades black and the other white.

[2JZR31]

So go and paint one side of your turbine blades black and the other white.

I'm getting the spanners and liquid paper out as we speak. :p

[2JZR31]

Energy can be changed in form easily tho, heat energy can be change to kinetic energy - that is the process that causes the turbine to spin.

I want to see someone explain this without using expansion (or rather lack of contraction, which is what happens if the exhaust gas is allowed to cool) Because if you use expansion, you are talking about flow. There is no way I can see heat being turned directly into measurable work in a turbine besides via the expansion of exhaust gas.

[BigMuz]

I think the heat retaining aspect is a minor improvement. The improvement would surely come about from the ease at which the engine can push exhaust gas out. If it is trying to get rid of a kilo of air per exhaust stroke through a 10mm hole (effectively) then it will lose a metric shitload of energy via the crankshaft. If you open the exhaust completely it will evacuate teh cylinder with almost no work taken from the crank. Look at a cylinder as an air pump- if you restrict the output of a compressor it works hard just trying to empty itself. Why would an engine be any different?

If it was only about energy then the biggest damn ports you can make would be best surely? That would be the most efficient way to evacuate a cylinder, no?

I think this is why porting makes less difference on a turbo engine than on a n/a. I know tk disagrees strongly with that, and I will happily eat my words if this exercise shows that to be wrong.

Also remember that the exhaust pressure is likely to be double the boost pressure, so you are emptying the cylinder into a manifold that is already at maybe 60psi and chock a block with screaming hot air.

[TK]

The heat retaining aspect is not minor, due to the restriction of the turbo the increase in flow capacity of the ex system is not as effective as it would be on an NA engine, yet results on a dyno from other engines (mainly WRX) show that porting can have a dramatic effect on spool time & outright output - the only viable explanation is heat retention.
Common practice on a turbo engine is to make the ex manifold out of thick wall tube & either ceramic coat or heat wrap it - to keep heat in the pipes & thereby reduce spool time, it's a well known phenomenon.
Another bit of evidence that it's the heat difference that makes the biggest impact is that a turbo engine with a ported head will run cooler - not noticable in use unless the cooling system is not up to scratch, but on the dyno it can be seen.

To think of it another way consider the basic internal combustion engine, it is what is known as a 'heat engine' - what it basically does is convert chemical potential energy into heat energy & then convert that heat energy into kinetic energy (in the form of rotational motion).
A turbo does much the same thing, it converts heat energy in the exhaust into rotational motion.
It is actually the air rushing past the turbine that does the work on the turbine, but the energy for that work comes from the heat of the ex gas.
I need someone who's studied physics more than me to explain it.

If reduced resistance to ex flow in the ex port is not important, then why do you need multi-valve heads - a single exhaust valve should suffice considering the biggest restriction is the turbo.

TK

[GTSBoy]

This is either very hard to grasp or very easy.

1) Heat is a process, it is not a "thing". Stop talking about heat as if it exists as a thing.

2) What you guys have for the most part been meaning when you say "heat" is "enthalpy". Enthalpy is just a fancy physics word for energy. It is more correct to use enthalpy when describing the energy contained in our exhaust gases when we are talking about that part of the energy that relates to temperature, because that part of the energy is fairly independant of the energy in the gas that stored in the form of pressure. Whilst adding pressure (as a process) does add temperature and therefore does add enthalpy, we will ignore that aspect for the moment because there is no usch proces going on in the exhaust port or manfiold.

3) A turbine extracts energy from a flowing gas. Becuase it does it on a continuous basis, we can say that there is a rate at which the energy is being pulled out. The rate of moving energy is power. So the turbine is a power extraction/transfer device. Drives the compressor, which needs power to do it's work. Basic enough right?

4) The energy that is transfered to the shaft by the turbine is extracted from the flowing gases. The gases undergo an expansion across the turbine. This is a complex change of thermodynamic state for the gases. They are expanded, which reduces the pressure, increasing the volume flow rate (after the turbine). This is not an adiabatic process. The energy change in the gases is transfered to the turbine. The gases cool substantially and some of the pure thermal energy goes to the turbine also, but only by being "converted" to pressure first. It doesn't actually happen that way, but the idea is worth remembering anyway.

4) So what about the port job? Well, the important thing to maximise power available in the gases is to keep the total energy up. Not having small flow paths is good because it reduces the simple frictional losses of the gases flowing against the walls (and internal friction in the gases) which we are all familiar with. That reduces the amount of pressure lost in the gases, and hence increases the pressure available at the turbine. I am not so sure that the increased port size and improved flow geometry do much to keep the temperature up also. I suspect that the change in temperature in the gases as a result of a port job is super important. It is certainly true that a bigger port will have more surface area and so will actually have more potential to lose heat from the flow (just from increase contact area) however the reduced superficial velocity and turbulence level might well compensate for this by decreasing the convective heat transfer coefficient in the port. Whatever happens in that aspect, the reduced pressure losses alone will actually keep the temperature higher anyway.

5) Remember that the turbine has a nozzle, and the nozzle is probably not much bigger than a single exhaust port anyway. Now you know where the real restriction to flow is in a turbo exhaust. Cleaning up the ports etc just gets rid of all the other losses and focuses the restriction where it should be, at the turbine nozzle. There is no point making the piston work harder to push the gases out through the exhaust port than is absolutely necessary. Doing so just robs power from the crankshaft. You just want the piston to provide the power to overcome the static pressure in the exhaust manifold, not the static pressure plus the frictional losses in a small/badly shaped exhaust port.

Does this help?

[2JZR31]

Makes 100% sense to me. Specially this part.

The gases cool substantially and some of the pure thermal energy goes to the turbine also, but only by being "converted" to pressure first. It doesn't actually happen that way, but the idea is worth remembering anyway.

.....


Rusty.

turbos work on a differential, ie in the case of pressure, high pressure on one side and low(er) on the other.

why would heat be any different?

those of you talking about putting the turbo in an oven, are you intentionally being stupid?

Just raising the point that heat alone does nothing to spin the turbine, and only secondary effects of heat do the work. Therefore flow is the more important factor than heat since flow is the primary factor, and heat is a secondary factor which effects flow.

You think heat differential will spin a turbine? Try heating one side of it up and see what happens. You will never get it to spin until the heat diferential causes some sort of flow through the turbine. So the point is the main thing that directly spins the turbine is flow. Heat just affects pressure, and therefore flow.

[TK]

Cheers GTSBoy, I was hoping you'd chime in & put it all in words that make sense.

TK

[2JZR31]

I never said heat does nothing. I have been saying heat effects pressure and flow as a secondary effect. So its wrong to say "stop thinking in terms of flow, and start thinking in terms of heat". Since flow matters more than heat, and heat only matters becuase it effects flow.