wastegating facts

EAFMan said:
I've read this thread through and am still a little confused about the debate.
I followed it through the point that the faster the engine turns the more load on the compressor of the turbo.
If I understand correctly, the debate started with the statement that when the wastegate opens, especially too soon, the TIP pressure exceeds the compressor outlet pressure. i.e. the -dp. I don't deal with compressor maps every day so my understanding is limited so bear with me.

The wastegate controls the pressure ratio of the outlet to inlet of the compressor as long as it hasn't opened beyond its control range. Providing that I am still operating the compressor at a reasonably compressor ratio, I may sacrifice compressor efficiency as I increase the load (rpm) but I shouldn't end up with a higher TIP than compressor outlet pressure. Nor would I end up at a negative pressure as the wastegate would close, driving the compressor harder to increase the pressure ratio back to its setpoint.

As far as actual TIP pressures, opening the waste gate should reduce the rate of increase of TIP as the engine RPM increases. The model of this in my head is having a resistor in parallel with a potentiometer. The wastegate being the potentiometer. The sum of the parrallel resistance is less than either of the individual resitances.

In my truck, I have enlarged the waste gate passage on my primary to reduce the drive pressure seen by the engine at high Hp and RPM's. The intention is to shift the restriction down to the secondary turbo. I hope that my understanding isn't all screwed up. (please note I did not say anything about what pressure the gate will be set at). I don't have any maps of my primary turbo at higher inlet pressures.

Please help me understand if I am wrong.
Click to expand...

The pot/resistor annalogy "almost works". However, as you lower the resistance on the pot, what happens to current as it passes through the resistor? in order to "almost" mimmic the fluid system we are discussing, Power consumed by the resistor needs to remain constant, I^2xR = Constant

On a single turbo application, JWT was describing what happens on the non wastgated side of a split scroll housing.

Now on a sequential set (or a single with wastgating on all six cylinders), that kinda goes out the window as most sets have a wastgate that vents all six cylinders.

If engine RPM remains constant, but load increases, boost and TIP stabilize to a constant pressure as the wastgate opens.

If Engine RPM rises, (regardless of load) Boost and TIP increase due to the fact that as the engine rpm rises, it wants more air. To supply this air, the turbo speeds up. To speed up the turbo, this requires more mass flow OR, a higher exhaust enthalpy (higher temp) at the turbine inlet. Both happen at the same time while the wastagte is opening.

wastgates don't suddenly pop open, they are progressive

The rise in TIP is the minor player, the rise in exhaust enthalpy (available thermal energy) and increasing mass flow are the major players.

note: the rise in mass flow is governed by the rise in TIP

A wastgate limits the ammout of mass flow (exhaust gas flow) through the turbine by bypassing it around the turbine. This actually has little to do with TIP since TIP at a specific load is already predetermined by the turbine and housing geometry, not the wastgate.

I am sure I will have some dissagreements on this , but I am standing by it...the math and standard engineering practice backs it up.
 
Last edited:
one more thing...if it were possible to raise the exhaust temp high enough (maintaining mass flow constant and dissregarding material failure) it would be possible to extract enough energy by the turbine to significantly limit (reduce?) TIP.

But those temps are so high (>2400F), not only would the engine fail, but the turbine would also fail.
 
Diesel Freak said:
of the options you currently have, it's the cheapest!!
Click to expand...

Yea very true...do you think itd help the bark? really im just wondering cuz either way im gonna put it in...pretty sure. i really don't need to be overspinning the turbo and like you said or someone did about the hot air (is that true? wats the limit you think?)
 
An increase in exhaust temperature will slow down the rate of pressure increase, but the pressure at the turbine will still increase eventually creating a negative delta pressure when the waste gate is opened during acceleration of the truck. If you are taking pressure readings make sure you take them on both exhaust ports.
 
JWT said:
An increase in exhaust temperature will slow down the rate of pressure increase, but the pressure at the turbine will still increase eventually creating a negative delta pressure when the waste gate is opened during acceleration of the truck. If you are taking pressure readings make sure you take them on both exhaust ports.
Click to expand...

Is this still regarding a devided housing with a wastgate on only one port (aka, holset)?

On the new schwitzer stuff with dual gates, it really should not matter...right?

I have noticed when tuning my truck I can't tell when the gates open by watching the drive pressure and boost gauges. However, I can tell by the noise....the 50mm gate dumps straight to atmo (by the passenger side frame rail) brrrrrrrrrrraaaaaaaaaaaaaaaaaaaaaap. Drive pressure is always about 5psi lower. How do you explain that?

Dont let my sig mislead you. The HX40 on my truck has a non-wastgated 16cm housing (from a H1C turbo machined to fit the HX40 turbine) It is wastgated by a single 38mm external gate plumbed to both ports on the scroll.
 
Last edited:
Joey89 said:
Yea very true...do you think itd help the bark? really im just wondering cuz either way im gonna put it in...pretty sure. i really don't need to be overspinning the turbo and like you said or someone did about the hot air (is that true? wats the limit you think?)
Click to expand...

well, you need to set a target power level and target use for the truck...then size the turbo accordingly.

or detune and drive like a human...not an ape. :poke:
 
So if I have 5k worth of rpms I should throw each stage progressively to keep the HP rise consistent through the power band, not one big WHACK!
.75 second between each stage?
 
Diesel Freak said:
well, you need to set a target power level and target use for the truck...then size the turbo accordingly.

or detune and drive like a human...not an ape. :poke:
Click to expand...

Okay yea i have been and for now i think i got it where i want to...actually where i can afford to have it not where i want it. so ill just put that on and be done probly. i have college next year so i cant afford to much.
 
Joey89 said:
Okay yea i have been and for now i think i got it where i want to...actually where i can afford to have it not where i want it. so ill just put that on and be done probly. i have college next year so i cant afford to much.
Click to expand...

I highly recomend school over bombing the truck!!!!!! Doing both at the same time really eats the funds!!!
 
Red Stroke said:
So if I have 5k worth of rpms I should throw each stage progressively to keep the HP rise consistent through the power band, not one big WHACK!
.75 second between each stage?
Click to expand...

I keep telling you Adam, a progressive stage controller .:poke:
 
JWT and Diesel Freak,
Let me make sure that I am on the same page as you. When you say negative delta pressure, you mean the difference between TIP and Compressor Outlet Pressure. i.e TIP = 50 and Boost = 40 psi = -10psi delta pressure.

Diesel Freak
When looking at the two resistors theory it is parallel to flow coefficients in valves and piping. Constant = I^2R of turbine + I^2R of wastegate.

The energy driving the turbine does not neccesarily remain constant. It doesn't care how much energy, only that it has enough to maintain the outlet to inlet pressure ratio set by the actuator. The wastegate flows not only the mass of the air but also the enthalpy of the gas.

I am going on a limb here as I don't have my books readily avail, but the enthalpy extracted from the turbine is related to the delta pressure across the turbine.


As the engine rpm increases under constant load, the increased demand on air will cause the gate to close slightly to maintain the pressure ratio which means it will increase the portion of energy flowing past the turbine. TIP will increase but Boost pressure will remain constant (Mass flow increases). I think we are almost in agreement here.

At constant RPM the air demand remains constant. If the load increases, the enthalpy of the gas will increase. This will result in the wastegate opening more. As far as the compressor is concerned in this scenario, it has no idea of load. ( I think we agree here )

What I still can't figure is, (again assuming my understanding of negative delta p as stated above), how do does that happen. Without going to extremes and blocking the wastegate open at 2 psi of boost or something ridiculous as that. If we are reasonable with how soon we open the gate, the only thing we loose is potential efficiency. In other word the Tip and boost become closer to each other.
I could use the same argument for opening the gate to late. I may never achieve good efficiency and be off of the turbo map with a potential grenade. Definate negative delta pressure.

This will be interesting to measure. I am putting taps into my manifold and between my turbos to be able to see this very thing.
 
On a turbo like the Schwitzer that is wastegated on both ports the pressure will still rise
But the driving load will be on both ports and all 6 cylinders instead of just 3 and 1 port
 
EAFMan said:
JWT and Diesel Freak,
Let me make sure that I am on the same page as you. When you say negative delta pressure, you mean the difference between TIP and Compressor Outlet Pressure. i.e TIP = 50 and Boost = 40 psi = -10psi delta pressure.

Diesel Freak
When looking at the two resistors theory it is parallel to flow coefficients in valves and piping. Constant = I^2R of turbine + I^2R of wastegate.

The energy driving the turbine does not neccesarily remain constant. It doesn't care how much energy, only that it has enough to maintain the outlet to inlet pressure ratio set by the actuator. The wastegate flows not only the mass of the air but also the enthalpy of the gas.

I am going on a limb here as I don't have my books readily avail, but the enthalpy extracted from the turbine is related to the delta pressure across the turbine.
Click to expand...

No. Look at the change in temp vs. the change in pressure. The change in temp is HUGE compared to the change in pressure. Gas energy (enthalpy) has nothing to do with the wastgate, and everything to do with the conditions within the manifold which BTW is a direct result of what happens within the combustion chanber. I related the resistor analogy to the POWER generated by the turbine. Not the head loss coefficients of the components.

Think power, not resistance.

EAFMan said:
As the engine rpm increases under constant load, the increased demand on air will cause the gate to close slightly to maintain the pressure ratio which means it will increase the portion of energy flowing past the turbine. TIP will increase but Boost pressure will remain constant (Mass flow increases). I think we are almost in agreement here.
Click to expand...

almost , constant load would cause gas temps to either remain constant or drop as RPM went up. Boost would drop off and the gate would close. TIP may infact go down.

Look at it this way...on a hill, pull it in 4th and 5th gears at the same road speed. What happens to EGT and boost? Power produced by the engine is the same in both cases as you are pulling against gravity at the same rate.

EAFMan said:
At constant RPM the air demand remains constant. If the load increases, the enthalpy of the gas will increase. This will result in the wastegate opening more. As far as the compressor is concerned in this scenario, it has no idea of load. ( I think we agree here )
Click to expand...

almost.... as load increases, fueling rate increases as a result...enthalpy (temp) and mass flow rise, and yes, the gate opens to keep the compressor at a constant rpm.

EAFMan said:
What I still can't figure is, (again assuming my understanding of negative delta p as stated above), how do does that happen. Without going to extremes and blocking the wastegate open at 2 psi of boost or something ridiculous as that. If we are reasonable with how soon we open the gate, the only thing we loose is potential efficiency. In other word the Tip and boost become closer to each other.
I could use the same argument for opening the gate to late. I may never achieve good efficiency and be off of the turbo map with a potential grenade. Definate negative delta pressure.

This will be interesting to measure. I am putting taps into my manifold and between my turbos to be able to see this very thing.
Click to expand...

This is the heart of the discussion. On a dual port wastgate, this will not happen. Once the valve opens, pressure will either drop or momentarily remain constant. It only rises as RPM increases

On a single port gate which only vents 3 cylinders, the non-vented three see a rise when compared to the vented three cylinders.
 
Last edited:
JWT said:
On a turbo like the Schwitzer that is wastegated on both ports the pressure will still rise
But the driving load will be on both ports and all 6 cylinders instead of just 3 and 1 port
Click to expand...

That is what I thought. So, it does not matter which side of the division you put the gauge.
 
GRotman said:
I keep telling you Adam, a progressive stage controller .:poke:
Click to expand...
Definitely. With progressive you can ramp in the spray using PWM to control the solenoids. Set it up faster for the dyno and then slower on the street/strip. Plus you have the advantage of ramping with boost signal with a NX mini-controller.
 
Now you have done it. Going to have to dig my books out to remove the cob webs out of my brain. I understand what JWT is saying in regards to a single wastegate.

As I said I don't deal with this everyday, and actually when I do it is with vacuums.

In some brief reading Temperature equates to a higher velocity for a given mass flow and Enthalpy is the combination of both the Internal Energy of the gas plus the P*V H=Q+PV. Unfortunatley it has been 20+ years since I have read this material in detail. Good reading and learning.
 
You must log in or register to reply here.
Back
Top