Had an interesting idea today and wanted to soundboard it.
Could we see decreased RPM required for spool on a turbocharger by putting a de laval nozzle right before entry to the turbine inlet?
Where this idea came from:
I was reading Street Turbo Charging by Mark Warner and he mentioned that, "The most important thing is to get the turbo spooling as soon and with as little backpressure as possible", and "a turbine wheel operates on the principle of momentum transfer from the exhaust gas stream to the turbine wheel. Momentum is the product of mass multiplied by velocity. To maximize turbocharger performance, we want to maximize both the mass flow rate and the velocity of the exhaust gas stream as it passes through the turbine"(Street Turbo Charging PG 124-125).
Seems to me that a de laval nozzle would be perfect for maintaining mass flow rate and increasing exhaust gas velocity.
Would also have to calculate what size nozzle would be needed to prevent the nozzle from acting as a venturi tube rather than a de laval nozzle/ if the exhaust speeds are event fast enough for this to begin with.
In the middle of running the numbers, but it seems that for this to work the exhaust has to reach mach 1 at the choke point. If it doesnt got transonic, it wont go supersonic and speed up during the divergent point.
In the middle of running the numbers, but it seems that for this to work the exhaust has to reach mach 1 at the choke point. If it doesnt got transonic, it wont go supersonic and speed up during the divergent point.
In the middle of running the numbers, but it seems that for this to work the exhaust has to reach mach 1 at the choke point. If it doesnt got transonic, it wont go supersonic and speed up during the divergent point.
Had an interesting idea today and wanted to soundboard it.
Could we see decreased RPM required for spool on a turbocharger by putting a de laval nozzle right before entry to the turbine inlet?
Where this idea came from:
I was reading Street Turbo Charging by Mark Warner and he mentioned that, "The most important thing is to get the turbo spooling as soon and with as little backpressure as possible", and "a turbine wheel operates on the principle of momentum transfer from the exhaust gas stream to the turbine wheel. Momentum is the product of mass multiplied by velocity. To maximize turbocharger performance, we want to maximize both the mass flow rate and the velocity of the exhaust gas stream as it passes through the turbine"(Street Turbo Charging PG 124-125).
Seems to me that a de laval nozzle would be perfect for maintaining mass flow rate and increasing exhaust gas velocity.
No this will not work because the turbine inlet housing is already an optimally-designed nozzle. Think of what the path from the turbine inlet flange to the turbine wheel looks like. If you put another nozzle before the turbine inlet, you will just add a small flow loss to the overall path.
I agree that adding a nozzle will, under most cases, just introduces a flow loss. However, if the exhaust speeds are high enough, and the choke point is sized appropriately, along with the pressure difference before and after the nozzle being high enough, then there is no question that this will result in a gas reaching the turbine with a higher momentum than it would have otherwise.
It's beginning to look to me that the issue is what rpm range will these conditions be true for? With the 1zz fe only having 1.8l of displacement, every two revolutions, the exhaust speeds might just not be high enough in the first place. though, admittedly, I still havent sat down to crunch the numbers completly yet. Ive been having trouble calculating a figure for the initial exhaust speed.
The reason I am sure that this method can be used to produce a higher turbine rpm per volume flowed is because the usa navy uses the exact same method to accelerate steam in their nuclear reactors, right before it hits the turbines.
As for compressor housing being optimally shaped, I agree that they are. But, if you just used the final diameter of the inside of the housing as the de laval outlet diameter, and considered the housing to be part of the nozzle, you could probably make it work.
Also, I'm pretty sure that the choked cfm of exhaust gases is gonna be a really low number with what the input speeds are coming off the engine so this might all be moot haha. Because even of the parameters are met and the gases do go supersonic as they hit the turbine, it doesn't really matter of the engine cant breath.
OK! The numbers are in! All of the following is assuming 6500rpm, a t28 flange, and a 500F exhaust temperature at the manifold.
The choke would have to be .323 in in diameter to achieve mach1 at 500F (1028.86mph@500F). This would allow for a maximum flow rate of 97.258L/s through the choke. Our engine produces 97.5L/s at this rpm. So, we will be choked at a rate of ~.25L/s, before taking into account choking due to the turbo.
Our exhaust gas speed through a t28 flange is~118.603 mph. Speed of sound at 500F is 1028.86. We are delivering approximately 8.7 times the momentum (ignoring the .2% choke) BEFORE THE SUPERSONIC ACCELERATION!
Now, in reality the goalposts will keep sliding as the manifold heats up but still.
Now, as for the usability, obviously you dont want a bad restriction that isnt good until 6500 rpm. The cross section required at lower rpm will be smaller with a smaller max flow rate so more choking.
@ria You can only possibly get a net benefit from a De Laval nozzle when choked, and if you size it based on steady flow you will be beyond choking during all of the pressure pulsations. Exhaust flow is not steady-state, not nearly! And the turbine does a really good job of utilizing the pressure pulsations. A Delaval nozzle operating near the steady-state choke flow point will effectively filter out all of the pressure pulsations from reaching the turbine, and this will hurt the power output of the turbine.
Even if the pressure pulsation benefit was really small (it isn't) once you get to choke flow (as you are going up in power), you hit a hard limit to mass-flow to the turbine and beyond that point, backpressure rises rapidly hurting engine efficiency due to increased pumping work and worse knock resistance. And below the choke point, all you accomplish is adding an extra small flow loss to the path from the cylinders to the turbine. I will repeat: the turbine housing is already an optimally-designed nozzle that delivers the best velocity vs. speed characteristic to the turbine blades. A nozzle at the turbine housing is too far away from the turbine blades to do any good.
If you want more power, you can get a turbine housing with a different A/R, or a turbocharger with a different turbine and/or compressor wheel. Turbines and compressors come in different trims as well (for fine-tuning).
It is not correct to think of the turbine as a momentum conversion device, it is an energy conversion device based on the mass-flow and temperature and pressure difference before and after the turbine. You can't just look at velocity.
Also, 500F at high power is way off. It is more like 1400 to 1700 F.
Hahaha yeah you're probably right. Im gonna put this idea to bed. It was fun to run the numbers though. If i ever get a riding lawn mower I might mess around with this just to get some data.
Unrelated: I was reading about a carbureted drag car that used a cd nozzle to provide a more even fuel/ air mix by placing it between the carb and intake. Basically using it to swirl the mix and more evenly distribute the fuel. That doesn't really apply to us though, just interesting.
