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Lean spool for turbo freaks
Posted: Sun May 13, 2018 11:18 am
by ptuomov
Here’s some thinking out loud about air-fuel ratios for the RPM ranges in which the turbo is running at low speed and the wastage is closed. One of the high priority goals in those rpms is to get the turbo to spin faster. I’m thinking thru this idea that Mitsubishi uses to spool the turbo faster: running the engine pretty lean in the spool-up region.
Is the right way to think about this that it’s a trade off between torque to the wheels and energy to the turbine? If so, then the optimal AFR has to be between lean best torque AFR and the maximum exhaust gas temperature AFR. For a typical car, the LBT AFR is probably 13.2 or something and maximum EGT AFR is around 14.7. So the best guess optimal spool AFT would be around 14.0 or so.
Does this make sense? I think it does. However, my observation is that most tuners run the engine significantly fatter even before the turbines have spooled to the wastage opening point. Who’s wrong?
Re: Lean spool for turbo freaks
Posted: Sun May 13, 2018 12:20 pm
by peejay
I have always heard to run rich when trying to spool the turbo, because you can easily increase the exhaust mass that way.
Re: Lean spool for turbo freaks
Posted: Sun May 13, 2018 12:28 pm
by Scotthatch
peejay wrote: ↑Sun May 13, 2018 12:20 pm
I have always heard to run rich when trying to spool the turbo, because you can easily increase the exhaust mass that way.
This is the general understanding with the turbo guys ....
Also limiting pipe size on the hot side to force the velocity up...
Or opening the ex valve a little bit sooner to dump more energy into the turbine sooner .... you need to balance this with not overdriving the turbine at full tilt
Re: Lean spool for turbo freaks
Posted: Sun May 13, 2018 1:10 pm
by ptuomov
peejay wrote: ↑Sun May 13, 2018 12:20 pm
I have always heard to run rich when trying to spool the turbo, because you can easily increase the exhaust mass that way.
So running it like steam turbine? The extra fuel won’t burn because there’s no oxygen, but it does convert heat to pressure. The turbine runs on both heat and pressure, so someone would have to explain to me like to a fifth grader how the thermodynamics of that strategy work out.
Re: Lean spool for turbo freaks
Posted: Sun May 13, 2018 1:28 pm
by B Original
Scotthatch wrote: ↑Sun May 13, 2018 12:28 pm
peejay wrote: ↑Sun May 13, 2018 12:20 pm
I have always heard to run rich when trying to spool the turbo, because you can easily increase the exhaust mass that way.
This is the general understanding with the turbo guys ....
Also limiting pipe size on the hot side to force the velocity up...
Or opening the ex valve a little bit sooner to dump more energy into the turbine sooner .... you need to balance this with not overdriving the turbine at full tilt
VVT really comes into its own with a turbo application I would imagine. Isn't heat and pressure both happening at the same time call something like thermal expansion. So I would think the go will be closer to 12.5 to 12.7 to 1 to during spool up?
Re: Lean spool for turbo freaks
Posted: Sun May 13, 2018 1:30 pm
by peejay
ptuomov wrote: ↑Sun May 13, 2018 1:10 pm
peejay wrote: ↑Sun May 13, 2018 12:20 pm
I have always heard to run rich when trying to spool the turbo, because you can easily increase the exhaust mass that way.
So running it like steam turbine? The extra fuel won’t burn because there’s no oxygen, but it does convert heat to pressure. The turbine runs on both heat and pressure, so someone would have to explain to me like to a fifth grader how the thermodynamics of that strategy work out.
I think the theory isn't so much thermodynamic as much as it is more mass in means more mass out. It is easy to add 10%+ more fuel.
Haven't tried it myself, though...
Re: Lean spool for turbo freaks
Posted: Sun May 13, 2018 1:49 pm
by Scotthatch
Fuel will keep burning even in the ex pipe after the port
Re: Lean spool for turbo freaks
Posted: Sun May 13, 2018 2:13 pm
by ptuomov
Scotthatch wrote: ↑Sun May 13, 2018 1:49 pm
Fuel will keep burning even in the ex pipe after the port
Sure but humor me and assume complete combustion such that there isn’t enough oxygen and thus the rich side of lambda = 1 cant be burned. It ends up as vaporized fuel in the exhaust gas that then drives the turbine.
Re: Lean spool for turbo freaks
Posted: Sun May 13, 2018 2:25 pm
by ptuomov
peejay wrote: ↑Sun May 13, 2018 1:30 pm
ptuomov wrote: ↑Sun May 13, 2018 1:10 pm
peejay wrote: ↑Sun May 13, 2018 12:20 pm
I have always heard to run rich when trying to spool the turbo, because you can easily increase the exhaust mass that way.
So running it like steam turbine? The extra fuel won’t burn because there’s no oxygen, but it does convert heat to pressure. The turbine runs on both heat and pressure, so someone would have to explain to me like to a fifth grader how the thermodynamics of that strategy work out.
I think the theory isn't so much thermodynamic as much as it is more mass in means more mass out. It is easy to add 10%+ more fuel.
Haven't tried it myself, though...
So the extra power of rich mixture would be coming from the power of the fuel pump? I don’t think that is enough to move the needle.
I’m thinking that this may be a situation where we’re running something like a Brayton cycle with stoichiometric AFR and full combustion and then adding a phase-change Rankine cycle aspect to it by vaporizing and not burning the excess fuel. Anyone got the thermodynamics down well enough to help me with this one?
Re: Lean spool for turbo freaks
Posted: Sun May 13, 2018 3:53 pm
by ptuomov
So I’m thinking that the excess fuel works the same way as water injection into the combustion chamber or exhaust manifold. Kind of like early B-52 bombers had water injection into the turbine system? Add a Rankine element to the Brayton cycle?
Re: Lean spool for turbo freaks
Posted: Sun May 13, 2018 5:03 pm
by Amilcar
ptuomov wrote: ↑Sun May 13, 2018 11:18 am
Here’s some thinking out loud about air-fuel ratios for the RPM ranges in which the turbo is running at low speed and the wastage is closed. One of the high priority goals in those rpms is to get the turbo to spin faster. I’m thinking thru this idea that Mitsubishi uses to spool the turbo faster: running the engine pretty lean in the spool-up region.
Is the right way to think about this that it’s a trade off between torque to the wheels and energy to the turbine? If so, then the optimal AFR has to be between lean best torque AFR and the maximum exhaust gas temperature AFR. For a typical car, the LBT AFR is probably 13.2 or something and maximum EGT AFR is around 14.7. So the best guess optimal spool AFT would be around 14.0 or so.
Does this make sense? I think it does. However, my observation is that most tuners run the engine significantly fatter even before the turbines have spooled to the wastage opening point. Who’s wrong?
For what purpose of use you´re talking about here?
Anti-lag is found on most aftermarket ECm`s.
Lot`s more fuel and pulled timing out is the way it works, but it`s fully programmable.
Its just matter of what suits better your application.
Re: Lean spool for turbo freaks
Posted: Sun May 13, 2018 5:28 pm
by user-23911
Lean spool?
There's more than one way to do it.
If you run extra rich (11s) when you don't need to , you kill the power output.
So keep it lean, normal lean, in the 12s or 13s......pull back the ignition timing a bit.
You lose a bit of power but the boost goes up quicker.
Everything has a downside, the downside to this sort of thing is failed turbos.
Re: Lean spool for turbo freaks
Posted: Sun May 13, 2018 5:41 pm
by ptuomov
joe 90 wrote: ↑Sun May 13, 2018 5:28 pm
Lean spool?
There's more than one way to do it.
If you run extra rich (11s) when you don't need to , you kill the power output.
So keep it lean, normal lean, in the 12s or 13s......pull back the ignition timing a bit.
You lose a bit of power but the boost goes up quicker.
Everything has a downside, the downside to this sort of thing is failed turbos.
So for say Mitsubishi EVO X, what’s the best AFR to run at wot before the wastage opens?
Re: Lean spool for turbo freaks
Posted: Sun May 13, 2018 6:40 pm
by ptuomov
exhaustgases wrote: ↑Sun May 13, 2018 5:52 pm
On rpm governed industrial diesels I've been around the turbo spools up more with load meaning more heat from more fuel, the flow volume of exhaust remains the same sort of yes there is extra from the boost. More heat would be more expansion or pressure wouldn't it?
Diesels are a little different.
For gasoline engines that are usually air and not fuel limited, the burn is fast and there usually aren’t a huge amount of unburned oxygen left after the turbine outlet. That’s the case I’m interested in.
Re: Lean spool for turbo freaks
Posted: Sun May 13, 2018 10:39 pm
by peejay
joe 90 wrote: ↑Sun May 13, 2018 5:28 pm
Lean spool?
There's more than one way to do it.
If you run extra rich (11s) when you don't need to , you kill the power output.
So keep it lean, normal lean, in the 12s or 13s......
Modern turbocars run stoich in boost, sometimes up to 15-17psi.
I didn't really believe it either until I saw the numbers. There is a LOT of attention paid to cooling system dynamics. Ford has engines with three-zone cooling systems that can focus on specific parts of the head!