Work (is actually the technical name) is extracted from a pressure temperature/differential across the turbine wheel. Temperature/pressure differences is the reason why a small gasoline motor can spool up a large diesel turbo with relative ease in spite of having 1/3 the displacement of the diesel motor. This is literally one of the fundamental principles of thermodynamics. I'm not saying it's the only factor, just that it IS a factor. If you don't believe that extracting work from the exhaust gas reduces exhaust gas temperature, go through an EGT in the manifold and in the downpipe and test it yourself. It's also the reason leaner exhaust mixtures will spool the turbo up faster. Mass flow is similar to a stoic burn, but egt is higher.Come on man... "work" does not mean making the shaft spin. Go drop your turbo in an oven or furnace and let me know how fast it spins from the heat alone.
For some reason you are trying to make this way too difficult.
In your drawing and description, with a 3-way routing the small turbo exit to the big turbo inlet, the small turbo would have effectively zero force being applied to the shaft. Path of least resistance.. both turbo inlets are on the same manifold...
Spoiler alert: I miss the point and go on a tangent, read to the end to find out where I realize I'm a dipshit and get back on track.Why exactly do you guys run the exhaust from the small turbo into the larger turbo? That will reduce the big turbos exhaust energy. It also results in backpressure. If you have separate exhausts for the small and large turbo you will have the same exhaust response but with less backpressure??
This is a tricky one. On the surface it is correct. But the WG throws a wrench into that. Higher EGT requires LESS pressure to do the same shaft work. Lower temp requires more pressure, which is what you stated in your concern for cooling the exhaust to the large turbo and that adding back pressure. But I thought this was an important distinction to make.Exhaust velocity is dictated by pressure differential. The hotter the temp the higher the pressure = the higher the velocity
That was much more clear and on-target than you thought it was.I think you guys are agreeing on what is driving the turbine but explaining it such different ways you feel like you're disagreeing. For example, you're agreeing that the turbine is pulling heat out of the exhaust, and a lot of it. Otherwise the original question wouldn't exist. The question seems be about the relative weight of each factor, mass flow and heat.
Mass flow is critical. Consider that the turbine housing is there solely for the purpose of directing hot exhaust gasses into the turbine through a NOZZLE, aimed at an appropriate angle to impart maximum energy to the wheel's blades. The nozzle creates the back pressure, which provides the pressure differential to increase exhaust gas velocity through that nozzle (this is the definition of lag, you need to get the nozzle working before you can get the turbine working). Smaller nozzle, higher back pressure sooner, higher velocity sooner, spool sooner. And vice versa. This is a mass flow driven system looking at it from this angle.
Consider also what happens when switching fuels, from those with higher stoichs to those with lower stoichs. When going from gas to E85, turbos spool much quicker. The effect really is quite pronounced. To the point that my current setup (with a ~62mm small turbo) will NOT spool at all on gas, ever. But spools very quickly on E85 (and even quicker still on methanol). In each case EGT usually goes down, particularly if you go to richer mixtures, but spool gets better. The difference is mass flow. With fuels that require more mass fuel flow, total exhaust mass flow (air mass flow plus fuel mass flow) goes up.
Looking at the formula for CTGF, Correct Turbine Gas Flow, there is an exponent that gives more weight to mass flow than to EGT. I'm going from memory here. I won't embarrass myself by trying to go above my paygrade on the thermodynamics and I'll leave people to explore that for themselves.
Now, that said, EGT does have an effect, obviously. And it's very obvious when you do stupid things like run your wastegates to control back pressure instead of boost pressure, or run a back pressure limited turbine side instead of boost control, etc. I've found some interesting things that are obvious in hindsight, but cool to see actually happen. For example, our 2g with it's WG control issue is effectively back pressure limited on the small turbo. I can't make more than 40 psi with it. Yet it would launch with up to 45 psi or so, dropping to the "set" 40 psi a couple seconds into the run which it would hold steady to the end of the run. The reason is the added EGT from the 2 step heating up all of the hotside parts. At the same back pressure, boost goes up a lot just from that extra heat. Enough that I started to preheat the exhaust manifold before the water box and really take advantage of this, leaving with up to 50 psi. I'm tempted to coat and wrap all the hot parts on my car.
The effect of EGT is also obvious when mass flow is held constant and ignition advance is retarded. Boost goes up.
Back to the original assumption, which if I'm finally understanding correctly, is that the loss in EGT, due to cooler small turbine exhaust discharge dilution, completely cancels out the benefits of its mass flow and then some. I don't think you'll find this to be the case. It could be estimated by running both scenarios through the CTGF formula and see which one comes out on top. It would require a few assumptions, but nothing that can't be reasonably estimated from data we have.
I don't know if I understand this well enough to explain it more simply or more correctly than has already been done, but there's my two cents as an untrained layman, for better or worse.
In practice this is not the case. Especially with e85 and methanol, richer can and often will spool faster, up to the point that you're hurting combustion.Work (is actually the technical name) is extracted from a pressure temperature/differential across the turbine wheel. Temperature/pressure differences is the reason why a small gasoline motor can spool up a large diesel turbo with relative ease in spite of having 1/3 the displacement of the diesel motor. This is literally one of the fundamental principles of thermodynamics. I'm not saying it's the only factor, just that it IS a factor. If you don't believe that extracting work from the exhaust gas reduces exhaust gas temperature, go through an EGT in the manifold and in the downpipe and test it yourself. It's also the reason leaner exhaust mixtures will spool the turbo up faster. Mass flow is similar to a stoic burn, but egt is higher.
Some of what you're getting at is already being tested with Kevin's current WG setup, and the system that i'm running. It does not require any special valves though. All we're doing is rerouting the gates to control the parts of the system as a whole, NOT a specific gate dedicated to each turbo.In the drawing, the gate leading in to the large turbo from the manifold could be modulated similar to the small turbo bypass valve on a traditional compound set up. It doesn't have to be an all or nothing.
Plus this whole conversation is a hypothetical. CAN the exhaust routing be optimized farther than it already has been? By rerouting the small turbos exhaust away from the large turbo AFTER the large turbo is online and system mass flow flow and back pressure might be able to be improved through the entire system.
I'm assuming in this drawing, the smallest black squares are both supposed to be giant valves?
This is pretty much what i was trying to say, kevin just did a way better job of it.Kevin yes I followed your posts perfectly. That answers my question. It is more about the massflow than anything else. Apparently the cooler exhaust gives a significant boost in exhaust density and massflow for the E85 and Methanol allowing the turbo to spool up.
Not quite.I'm assuming in this drawing, the smallest black squares are both supposed to be giant valves?
If that's the case, i missed the far right one earlier, and all you did was redraw a standard compound setup with extra steps. The end result is the same IF WE'RE STAYING COMPOUND. Sequential is a different story, but can also be done with just gates, as kevin has done on the 2g.
The end result is that we must have a certain amount of energy put into the shaft in order to get each compressor stage to do what we want. Nothing is going to change that.
Motion is not a requirement for work. You can put 745.7 watt (1 HP) into a light bulb and there is no motion. You could also heat up your leftover dinner in the microwave, the food is hot, yet no motion. Work is being done.Work(aka HP) is equal to moving 550lbs one foot in one second. So yes MOTION is most definitely a requirement.