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+--- Thread: VNT VGT control theory (/showthread.php?tid=2388)
VNT VGT control theory - Syncro_G - 03-13-2011
I've seen plenty of discussion about getting actuators working both mechanical and electronic on variable geometry turbos.
But little on what the desired behavior would be.
My assumptions:
Vanes closed = increased shaft speed = more boost and more exhaust back pressure
Vanes open = reduced shaft speed = less boost and less exhaust back pressure
Here's my stab at it -
These seem pretty obvious:
- Hard acceleration (WOT) and at or below top boost - vanes closed
(actuator directly connected to vacuum signal)
- Hard acceleration (WOT) and at top boost level - vanes open enough to maintain that boost level
(actuator connected to vacuum signal and moderated by dawes valve)
- Moderate acceleration (not at WOT) - vanes at some tuned point between open and closed (would expect to be balanced with fuel and load in some way)
(actuator position tied to throttle position?)
- No throttle - vanes open
These are probably obvious but tricky to measure and implement:
- High load - vanes closed
(what's the best way to measure load?)
- Low load - vanes open
(again, what's the best way to measure load?)
These I'm just guessing on:
- No throttle and foot on brakes - vanes closed
(turbo brake functionality - could use a vacuum switch with tail light signal)
- Low RPM and low throttle - open vanes for economy?
- High-Speed Cruising - vanes openned a bit for economy
(no idea how to measure this one)
- Low RPM and WOT - open vanes so exhaust can speed up?
- High RPM and WOT - vanes open up to reduce increase in RPM?
I'm sure there are more -
- RPM?
- Rack postion?
- Coolant temperature?
- MAF?
RE: VNT VGT control theory - aaa - 03-13-2011
I'd do RPM, rack position, and exhaust pressure. RPM to avoid surging, rack position to set a boost goal, and EGP to limit how fast you get to that goal, since you don't want too much of it. The desired behavior is your foot controlling the boost level. That way when you're cruising on the highway you wouldn't have the pedal down as much and thus would have less boost and better economy.
RE: VNT VGT control theory - winmutt - 03-14-2011
Speed. Also thinking about an o2 sensor....
RE: VNT VGT control theory - ForcedInduction - 03-14-2011
Nothing is needed beyond boost:throttle. VNT operation needs to be tuned for the minimum boost needed to burn all the fuel at a given load. This is what gives a VNT equipped vehicle its fuel economy and power improvement over a wastegated turbo. A wastegated turbo always produces too much boost for the load demand in steady cruise and part throttle acceleration.
For example, the stock T3 makes about 8psi at 70mph steady cruise while only 2-4psi is useful at that load (the point where EGTs are not reduced any further if boost is increased). Likewise, at 45mph the T3 makes 2-3psi while no boost is needed.
OEM VNT/VGT equipped vehicles have higher cruise boost because they are set to increase exhaust pressure to increase EGR flow. Non-OEM applications like ours aren't concerned with EGR flow so we are free to set the boost for best efficiency and power.
RE: VNT VGT control theory - Syncro_G - 03-14-2011
(03-14-2011, 10:03 AM)ForcedInduction VNT operation needs to be tuned for the minimum boost needed to burn all the fuel at a given load.
So if I understand correctly:
The ideal is to set a boost target based on fuel mix.
Vane control would be regulated (like in FI's system) to meet that boost target
But the question then is how best to set boost to maintain A/F ratio?
FI - in your system, the boost target is set by throttle position. Would it be better to have it set by rack position? and if so, would the difference between throttle and rack be significant?
RE: VNT VGT control theory - ForcedInduction - 03-14-2011
(03-14-2011, 11:25 AM)Syncro_G But the question then is how best to set boost to maintain A/F ratio?The easiest way is by visible smoke.
Quote:Would it be better to have it set by rack position?No difference. Between idle and maximum rpm the rack is controlled directly by the throttle input.
RE: VNT VGT control theory - winmutt - 03-14-2011
(03-14-2011, 11:37 AM)ForcedInduction the rack is controlled directly by the throttle input.Isn't RPM part of that equation as well (in the governor)?
RE: VNT VGT control theory - ConnClark - 03-14-2011
Optimum setting for a VNT/VGT boost controller for both performance and mileage is always (provided you have enough air to burn your fuel) maximum boost for the minimum amount of back pressure. If you have your turbo sized right you will have rpms and rack positions where boost exceeds back pressure. Its in these areas that power and efficiency from the turbo are at their highest.
RE: VNT VGT control theory - aaa - 03-14-2011
Always? Even if boost is greater then EGP, I don't see how that would improve flow thru the engine. For engine efficiency.
RE: VNT VGT control theory - ConnClark - 03-14-2011
(03-14-2011, 07:12 PM)aaa Always? Even if boost is greater then EGP, I don't see how that would improve flow thru the engine. For engine efficiency.
yes always. If boost exceeds exhaust gas pressure it actually helps spin the crank because the force the boost provides pushing the pistons down is greater than the force required to expel the exhaust gases.
RE: VNT VGT control theory - ForcedInduction - 03-14-2011
(03-14-2011, 06:48 PM)ConnClark Optimum setting for a VNT/VGT boost controller for both performance and mileage is always (provided you have enough air to burn your fuel) maximum boost for the minimum amount of back pressure.Nope. Producing more boost than necessary needlessly restricts exhaust flow.
Quote:If boost exceeds exhaust gas pressure it actually helps spin the crank because the force the boost provides pushing the pistons down is greater than the force required to expel the exhaust gases.False. The engine's volumetric efficiency prevents the boost air from pushing on the cylinder in any amount.
RE: VNT VGT control theory - Syncro_G - 03-14-2011
I would think that if boost exceeded exhaust pressure, that the turbo would slow down pretty fast to equalize the two.
RE: VNT VGT control theory - ForcedInduction - 03-15-2011
Nope. EMP can be up to 3:1 with boost without problems.
RE: VNT VGT control theory - 300D50 - 03-15-2011
(03-14-2011, 11:18 PM)Syncro_G I would think that if boost exceeded exhaust pressure, that the turbo would slow down pretty fast to equalize the two.
A turbo feeds mostly on heat, not flow, so you can have those odd effects.
RE: VNT VGT control theory - ForcedInduction - 03-15-2011
(03-15-2011, 12:40 AM)300D50 A turbo feeds mostly on heat, not flow, so you can have those odd effects.
Incorrect. A turbo is a pump. The turbine operates on the pressure differential between the inlet and outlet as well as the aerodynamic energy of the exhaust velocity. The gas velocity is what VNT/VGT turbos can directly alter.
The only role heat plays is expanding the exhaust gas, increasing efficiency by increasing the volume of the same mass.
Drive down the road and put your hand out the window. Now change the angle of your hand to the wind. That difference in force pushing on your hand is the same effect a VNT mechanism has on the turbine.
A wastegate turbo controls the velocity through its fixed turbine by venting gasses and reducing the pressure differential between the inlet and outlet (reducing the velocity through its fixed nozzle). In the analogy above, it would be like keeping your hand fixed at the same angle and decreasing your driving speed instead to reduce the force against your hand.
Even the closest thing to a real "heat engine", a Stirling engine, and still operates on fluid pressure differential.
The only "true" heat engine we have on our cars are the brakes.
RE: VNT VGT control theory - winmutt - 03-15-2011
The only way I can see to measure stoich is an o2 sensor. But where do you put it? Pre turbo? Post turbo? Both?
RE: VNT VGT control theory - ConnClark - 03-15-2011
(03-14-2011, 10:01 PM)ForcedInduction(03-14-2011, 06:48 PM)ConnClark Optimum setting for a VNT/VGT boost controller for both performance and mileage is always (provided you have enough air to burn your fuel) maximum boost for the minimum amount of back pressure.Nope. Producing more boost than necessary needlessly restricts exhaust
Not according to Physics, Thermodynamics, and the SAE http://www.erc.wisc.edu/people/faculty/Reitz%20SAE%20Index/Reitz%20SAE%20Documents/Reitz%20SAE%20Documents/1999-01-0840.pdf
Excess air improves BSFC which means more power output at the crank for the same amount of fuel burned.
Quote:incorrect , Volumetric efficiency will reduce the amount of force pushing on the piston but not prevent it.( Okay if you have a volumetric efficiency of 0% it will stop it but an engine like that won't function)Quote:If boost exceeds exhaust gas pressure it actually helps spin the crank because the force the boost provides pushing the pistons down is greater than the force required to expel the exhaust gases.False. The engine's volumetric efficiency prevents the boost air from pushing on the cylinder in any amount.
RE: VNT VGT control theory - ForcedInduction - 03-15-2011
(03-15-2011, 01:18 PM)winmutt But where do you put it?It doesn't matter. The exhaust before and after the turbo is the same.
(03-15-2011, 02:23 PM)ConnClark Excess air improves BSFC which means more power output at the crank for the same amount of fuel burned.Thank you for exactly proving my point that engines need the "minimum boost needed to burn all the fuel at a given load."
Quote:Volumetric efficiency will reduce the amount of force pushing on the piston but not prevent it.No. Boost just reduces the pumping losses on the intake stoke. Very few 4-stroke engines ever see more than 100% VE, and a 2-valve uniflow engine is definitely not one of them.
RE: VNT VGT control theory - RustyLugNut - 03-15-2011
'(03-15-2011, 02:23 PM)ConnClark Excess air improves BSFC which means more power output at the crank for the same amount of fuel burned.Thank you for exactly proving my point that engines need the "minimum boost needed to burn all the fuel at a given load."
That wasn't his point. ConnClark's original statement in it's entirety concerned the singular point on a boost/VE map where the turbo's positive boost on the piston head during the intake downstroke compensates for the negative work loss (pumping loss) during the upward exhaust stoke. Both the intake and the exhaust path have ratings for VE. If both are equal, then the boost differential becomes the overriding factor in the equation. You are correct in that an excessively restricted exhaust path will cause negative work ( work loss), but if the system is well designed, that should not be the case. Even if your have far more air introduced into the combustion chamber than there is fuel to burn, there is a small gain in power from the increased mass of the working fluid. There will be a reduction in combustion temperature from the increase of the working fluid mass (heat capacity), but the increased mass at some point can increase power at the flywheel. IF you cap off the EGR as most of you do, you will also get a secondary heat source from the N2/O2 reaction into NOx (an exothermic reaction) that makes use of the excess air available.
Quote:' Volumetric efficiency will reduce the amount of force pushing on the piston but not prevent it.No. Boost just reduces the pumping losses on the intake stoke. Very few 4-stroke engines ever see more than 100% VE, and a 2-valve uniflow engine is definitely not one of them.
Many NA tuned port engines match or exceed the 100% VE point at intake resonance. If your turbo system can cram in twice as much air, your VE just hit 200%. The well designed turbo system takes the lost energy in the exhaust and uses part of it to ELIMINATE the pumping losses in the intake tract. Otherwise, you would not have the extra air to over fuel and gain power.
RE: VNT VGT control theory - ConnClark - 03-15-2011
(03-15-2011, 02:59 PM)ForcedInduction(03-15-2011, 02:23 PM)ConnClark Excess air improves BSFC which means more power output at the crank for the same amount of fuel burned.Thank you for exactly proving my point that engines need the "minimum boost needed to burn all the fuel at a given load."
From the paper.
At ordinary boost level the BSFC was 231g per kW/hr and had 0.107g of particulate matter per kW/h. So at normal boost 230.893g of fuel was burned per kW/hr of work output.
At the high boost level the BSFC was 212g per KW/hr and had 0.089g of particulate matter per KW/h. So at the high boost level 211.911g of fuel was burned per kW/hr of work output.
That means you get the exact same energy output by burning almost 19 less grams of fuel at the high boost level.
Exactly how does this prove your point?
RE: VNT VGT control theory - ForcedInduction - 03-21-2011
(03-15-2011, 06:08 PM)RustyLugNut Many NA tuned port engines match or exceed the 100% VE point at intake resonance.How many OM6xx engines got that feature?
Quote: If your turbo system can cram in twice as much air, your VE just hit 200%.No. Resonance changes with density.
Quote:Exactly how does this prove your point?By showing engines operate most efficiently with minimum boost.
On ConnClark's idea of running only 1:1, that could be done mechanically with only a double check valve (DC-4). Two problems with that;
1- It would cause slow spooling since it won't allow high drive pressure during turbine acceleration.
2-It would allow exhaust into the vacuum system, pretty much necessitating either a filter or a relay valve.
A relay valve brings up two more problems;
1- Relay valves (R-12 or R-6) have a 2-5.5psi cracking pressure, meaning drive pressure would always be at least that pressure under load.
2- A relay valve would require a pressurized air source greater than the boost pressure.
RE: VNT VGT control theory - ConnClark - 03-22-2011
(03-21-2011, 11:29 PM)ForcedInductionI think the point was it is possible to exceed 100% volumetric efficiency.(03-15-2011, 06:08 PM)RustyLugNut Many NA tuned port engines match or exceed the 100% VE point at intake resonance.How many OM6xx engines got that feature?
Quote:if a piston and cylinder at BDC and displace one liter and hold 1.1839 grams of air at zero rpm with a valve open and you can cram 2.3678 grams of air in the cylinder at some rpm, that is defined as 200% volumetric efficiency regardless of resonance.Quote: If your turbo system can cram in twice as much air, your VE just hit 200%.No. Resonance changes with density.
Quote:But the paper shows that the engine runs most efficiently with excess boost.Quote:Exactly how does this prove your point?By showing engines operate most efficiently with minimum boost.
Quote:On ConnClark's idea of running only 1:1, that could be done mechanically with only a double check valve (DC-4). Two problems with that;
1- It would cause slow spooling since it won't allow high drive pressure during turbine acceleration.
2-It would allow exhaust into the vacuum system, pretty much necessitating either a filter or a relay valve.
A relay valve brings up two more problems;
1- Relay valves (R-12 or R-6) have a 2-5.5psi cracking pressure, meaning drive pressure would always be at least that pressure under load.
2- A relay valve would require a pressurized air source greater than the boost pressure.
Actually I said running the turbo to generate the most amount of boost for the least amount of turbine pressure drop, provided you have enough air to burn the fuel. That is not a fixed ratio. Also I don't think it could be done mechanically with a reasonable amount of hardware.
RE: VNT VGT control theory - ForcedInduction - 03-22-2011
(03-22-2011, 05:42 PM)ConnClark Also I don't think it could be done mechanically with a reasonable amount of hardware.
I just showed it can be done with only 1-4 pieces of hardware.