Variable geometry turbo

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wsimpso1

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If there is one thing I've learnt about diesels over the last couple of days it is to forget everything I know about petrol engines. Thanks Bilski and PMD, you have increased my understanding 100 fold, along with a load of other reading I've done.

I also received a reply from Serge. He says at take-off to have a maximum pressure which does not make you lose engine revs at take-off throttle, so no more air than needed. At cruise, you do the same thing, set the cruise revs then reduce the turbo pressure to just above where revs decrease. So from this I gather I would find a setting (distributor pump and boost) that gives maximum HP (little smoke) that can be used for take-off at sea level (most of Australia). When flying I would reduce throttle (a misnomer) to cruise speed then back off the turbo until it starts to decrease in revs. As altitude increases, I dial in more boost keeping an eye on revs and EGT. Have I got that right?

I guess there will also be a setting I could use to 'set and forget' that will give me enough boost for adequate take-off power and OK in cruise, one that can be used by other pilots without special instruction, similar to a non-variable turbo.

BTW, my basic medical doesn't allow cruising at altitudes above 10,000 feet in Oz and our highest airport is 4000ft with a vast majority being less than half that. though it can get very hot.

Mark
Hmm, so you are willing to manually control the vane setting and fuel flow setting while watching the manifold pressure? The human engineering guy in me rebels at the thought. For a while, Piper sold a turbocharged Cherokee variant (Arrow, IO-360, retract gear) with a fixed wastegate and you trimmed power with the throttle (gas engine, has a throttle plate, can adjust manifold pressure with it) to keep manifold pressure in bounds. If you got sloppy, you overboosted the engine or went short on power. It made the pilot look at the power settings, which might be OK when at cruise, but it sure complicates things during the go part of a rejected landing. And it was only one lever to trim power.

You will have both a fuel lever and a manifold pressure lever, and it sounds like you will have to manipulate both. You might be well served in this area by putting in three adjustable detents in each linkage, one for take-off power, one for climb power, and one in the middle of cruise. Mark the manifold pressure gauge similarly. Then during high workload events, you can just go to the appropriate detents. When you get settled down a little, you can check manifold pressure and trim the manifold pressure and fuel flow settings.

This all still sounds like a human factor nightmare - Imagine another airplane intrudes on the runway while you are a quarter mile out. You have to transition from approach to climb configuration of the airplane, the fuel flow, and the manifold pressure. Many folks have their hands full just getting the airframe changed and cobbing the throttle. Maybe you can, when you retard the fuel lever on final, shove the manifold pressure lever forward counting one, two, three clicks, so that advancing fuel puts you there. Or maybe that forces enough power that descent and land becomes a long floaty thing... I do not know, but you should definitely find out.

I again emphasize that instead of making and flying something that is already known, you seem to be entering the arena of invention, with both benefits and risks.

Billski
 

Hephaestus

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Even the turbine guys trained on multiple levers get it wrong.

Like the Basler that "landed" on the lake this past winter... Crew pushed the wrong levers. Gear up slide across the lake.

Keep it stupid simple is a good expression for a lot of reasons.
 

azevedoflyer

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I really like a single lever differential strokes/ boost gage concept. That is the way I would go.
Will add a few nuggets to look for:
1- Keep gas temperature at TC inlet at 700-750 deg C. for longevity.
2- make sure your engine has a well cooled exhaust valve seat area. Temps higher than 800-820 deg. C at seat face will warp and yield the parent metal ( Al possibly). Remember that Aluminium alloys working temperature are no higher than 300-350 C.
3 - the vital piece of it all, the piston, is capable of sustained operation in the range of 0.50-0.55 kW/cm2 thermal load.
That is so IF adequate piston cooling is provided i.e. about 4-6 liter/kW.h per cylinder.
These little details behind you, go play with your boost pressures but leave yourself a safety valve, kind of a Limp-Home capability, that opens the vanes fully, returning your engine to a NA mode.
Cheers,
😎
 

Mcmark

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Mark, from some of the planes I’ve flown and some of the reading I’ve done to the turbo VW type 1 I built, the options are yours to choose. The AT-6 has a supercharger that the pilot manages the boost with the throttle handle. The Sonex turbo is set up the same as the pilot is responsible for how much boost is generated by throttle position.
The wastegate can be set with different springs for it to “pop off” when it hits a magic number that you set. Using the spring method will require you to manage the throttle position. It will be set so you can have max boost at altitude to either maintain atmospheric or to have boost at alt.
There should be plenty of data to give you a fair idea of what your max boost should be.
With the diesel, even small cc, should give plenty of grunt at sea level with little or no boost. They’re mostly torque.
Good luck.
Mark
 

Puggo1

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hi,
thanks for your information on your DV6 plane. I am an automotive engineer but know very little about diesels, so I've approached your dilemma from basic principles. I too will have the same dilemma when I build my engine.
The DV6 engine has a variable vane turbo to increase low rpm torque which is not required in your aircraft. The DV6 is rated at 130hp 320Nm with multi pulse electronic injection for maximum efficiency in a car. It has oil cooled pistons and has been installed in millions of vehicles. So basically at your 95hp it should be reliable. There's 2 caviats to that 1. adequate cooling monitored by temperature gauge and 2. don't exceed the crankshaft/conrod torque limit. Your engine prop set-up is unlikely to do that.
If your propeller is rated for 95hp at 3800rpm, then I would simply lock the turbo vanes at the maximum angle open (lowest boost pressure) and adjust the mechanical injection pump to achieve ~3400rpm on WOT ground runs. When flying the rpm will increase to the desired 3800rpm.
There's been a few negative comments about adding a separate control over turbo boost. Personally, I'd add a vernier cable as it may enable the engine to achieve higher efficiency during cruise by closing the vane angle and increasing boost. With regard to multi-lever operation, there's minimal risk on a diesel providing all levers are set to fully pushed in on takeoff. Conversely there's plenty of spark engines fitted with carburettor heat and yet they still make them.
Keep us posted on your developments.
 

AdrianS

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I hope someone can enlighten me:
At lower atmospheric pressure, the input air is less dense, so the turbine has to spin faster to produce the same boost.

Could this cause the turbine to over speed?
 

PMD

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I hope someone can enlighten me:
At lower atmospheric pressure, the input air is less dense, so the turbine has to spin faster to produce the same boost.

Could this cause the turbine to over speed?
yes, it COULD depending on how the turbo is spec'd out, how much pressure drop from air filter, exhaust system, etc. Most of all, a VNT with vanes slammed shut (on edit: closed to their smallest apeture setting) at altitude and its original small compressor would be highest at risk - thus why I recommend setting a fixed level of boost and a more efficient compressor wheel (or larger compressor). The answer lies in the maps provided by the turbocharger manufacturer.
 
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PMD

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With the diesel, even small cc, should give plenty of grunt at sea level with little or no boost. They’re mostly torque.
Important to realize that EVERY engine produces the same amount of torque at the same cylinder pressure (given same bore/stroke). Diesels are little more capable than gassers when normally aspirated. Modern diesels are far more about the turbocharger followed by injection details. The reason they can produce prodigious torque at very low RPM is that the engines do not aspirate a charge, so have no risk of detonation (OK, technically very, very low risk as detonation CAN take place during injection event, but that is very easy to design around) to can use a very large amount of boost. In diesels, you can make any amount of power you desire at any engine speed by providing the required amount of fuel and then giving it more than enough air (can be WAY lean of stoichiometric, in fact really want to be). The limits are only the mechanical strength and cooling capacity of the engine. Old school diesels ran at low RPM because their injection systems were so crude they couldn't manage high speeds and most of such engines were very large. All ancient history with modern materials and technologies.
 
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wsimpso1

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Mark, from some of the planes I’ve flown and some of the reading I’ve done to the turbo VW type 1 I built, the options are yours to choose. The AT-6 has a supercharger that the pilot manages the boost with the throttle handle. The Sonex turbo is set up the same as the pilot is responsible for how much boost is generated by throttle position.
The wastegate can be set with different springs for it to “pop off” when it hits a magic number that you set. Using the spring method will require you to manage the throttle position. It will be set so you can have max boost at altitude to either maintain atmospheric or to have boost at alt.
There should be plenty of data to give you a fair idea of what your max boost should be.
With the diesel, even small cc, should give plenty of grunt at sea level with little or no boost. They’re mostly torque.
Good luck.
Mark
Two things to know about what is being said above.

First is that with a gas engine, you have a throttle valve that you do not have in a Diesel. Close down the throttle valve, air flow and pressure below the throttle drops, the compressor works its pressure ratio on the air it is getting, and manifold pressure is reduced. With less air in, the exhaust flow drops too, the turbine speed drops and the system free floats to a new equilibrium. With no throttle valve in a Diesel, you control only fuel. And if the manifold pressure is not regulated, you have to move a control

Old school is a mechanical pressure controller and your power lever plus a fuel rack calibrated to always be lean and keep EGT's in bounds. New school is a sensor loop and ECU calibrated to do the same thing. In both cases the control reads manifold pressure, adjusts either a wastegate or the variable geometry pieces to move manifold pressure.

Second thing is a wastegate is a device that adjusts how much of the exhaust manifold gases go through the turbine, and how much goes around. This allows adjusting manifold pressure while being set up to avoid overspeed of the turbine. The gadget that Mcmark seems to be mentioning that pops off is NOT a wastegate, it is a blow-off valve. It is a valve held on a seat with a spring and it keeps manifold pressure from going over a certain value by blowing off intake manifold air into the atmosphere. In some applications, it is useful for keeping manifold pressure in bounds. Bad indeed to have it operate much if downstream of a carburetor (blowing off fuel-air mixture) or your air mass sensor (excess fuel to the engine). Some racing classes specify the blowoff valve and its settings, keeping everyone at the same max boost.

Billski
 

PMD

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To elaborate on Billski's post for the OP's purpose: The VNT does not use a wastegate to limit turbine inlet pressure, it instead uses a set of vanes that change the velocity and angle of incidence of exhaust gasses impinging upon the exhaust turbine. The never "close" but go down to a minimum setting that give maximum power to the compressor wheel (max boost) and open to limit same to a much lower level. Airplanes are for all practical purposes constant speed engines, and really don't need all of that fancy control, just design limit of two critical conditions so as not to exceed turbo RPM and engine EGT (also a turbocharger limiting factor - tolerance of the exhaust wheel for gas temp). The first limit is takeoff power - that would be an EGT or mechanical strength of something limit. You would want to define a maximum "closing" vane setting for that - and the second is high altitude climb power (METO power) where once again, you would need to find the minimum vane opening (maximum closing) that hits max turbo RPM. The first you evaluate by EGT, since the turbo is designed for this condition (given rest of engine in design limits of OEM, which the OP's engine is NOT!!) and the second you refer to compressor map to see what design limits are available with that air density (at specified cruise power - again, this needs to be done by testing as the OP has different injection hardware). Since he plans fairly low altitude in Oz, and only intends modest power from the engine, the OEM turbo is probably well suited to find a fixed vane setting and just forget about the rest and accept the slight compromise at one of the two critical setpoints.

Where these things get really critical is from those idiots (such as myself) who want to make WAY more power from their diesel. As I said, you can make any power at any speed...but you WILL find the cooling and/or mechanical strength limits at some point. Staying down in the OEM rated power range there is a fair amount of safety margin from the original hardware, so just a matter of some testing and setting to be home free (OK, NOTHING is "free" in aviation, but you get my drift.)
 

PMD

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I hope someone can enlighten me:
At lower atmospheric pressure, the input air is less dense, so the turbine has to spin faster to produce the same boost. Could this cause the turbine to over speed?
Let me appologize for not giving you a complete answer first time around: the big deal with turbochargers at altitude on diesel (Compression Ignition) vs. gasoline (i.e. Spark Ignition) is that on a gasser, the power you can make falls off with MAP and you can restore it with forced induction up to "critical altitude" (i.e. when wastegate is fully closed - as you must limit MAP at lower pressures to prevent detonation). On a diesel, there are no such limits, so at the target altitude on this thread (around 10k) the fuel system of the OPs engine is mechanically controlled and can give takeoff power level of fuel delivery...that will burn nice and clean IF you can give it enough air. Problem is: will the turbo overspeed when trying to keep enough MAP to keep enough air to fully burn that fuel? The VVT vanes can be closed down to do that, but you must be aware if the compressor is designed to load the turbine to below limit RPM to achieve that MAP.

Trying to keep this as simple as possible.
 

dog

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The production numbers of the engine in question were quoted in the millions,so it is very likely that the boy racers are busy, and have determined how much power for how long one will yield. They will have also long lists of alternate bolt on parts, like turbos, dyno results of there efforts, data on egt and boost,fueling, and more.

It could be a quick way to get some data that is outside of the original design specs and maybe a turbo set up that is simpler than the stock one with the variable vanes and proven at much higher boost and turbo impeller speeds.

I think that aftermarket turbos can have interchangable intake "rings" that will limit boost. Also many turbo rebuilding services advertise that they balance there turbos to much higher than stock values and speeds.
 
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Markproa

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Thanks everyone, all very helpful. Today I installed a control cable to my turbo and intend to do a whole load of ground testing. I suspect I'll be finding an ideal "set" position for take-off that will do for most flying however it will be interesting to experiment with increased boost at higher altitudes. I don't see this increasing workload to any significant extent, I don't have carb heat, constant speed prop or mixture to worry about.
 

PMD

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Thanks everyone, all very helpful. Today I installed a control cable to my turbo and intend to do a whole load of ground testing. I suspect I'll be finding an ideal "set" position for take-off that will do for most flying however it will be interesting to experiment with increased boost at higher altitudes. I don't see this increasing workload to any significant extent, I don't have carb heat, constant speed prop or mixture to worry about.
I hope you have a vernier control (as travel is very small) or did you do some kind of lever at turbo end to reduce cable travel to vane control travel? EGT and MAP available? What might be nice if you can't see exhaust exit would be an opacity meter (as "adequate air" can be defined by clean burn). Unlike mixture adjustment, you won't be able to find a peak with dropoff of prop RPM on either side, as it is not so much power that will change, but efficiency of burning the fuel you send. What injectors are you using and what version/size of VE pump?
 

Markproa

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I have a lever next to my throttle lever which was I originally intended for pitch trim (now controlled with a servo). I have arranged it to have plenty of control swing with very little movement at the cable and a tension spring at the turbo to remove any slop in the cable and a breakage will default fully open vanes. Do you think I should restrict the actuator movement to only half as I probably won't need the vanes closed off a great deal?
Yes, I will have EGT and MAP available. Is EGT to be measured before or after the turbo? The MAP is measured just prior to the inlet manifold, after the intercooler.
Where would I get an opacity meter?
Injectors Bosch KBAL70P46.
Distributor pump - Bosch VE R127
 

Chris Matheny

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Do you have a compressor map for the turbo? As has been said at high altitude you may well be above the safe pressure ratio for it. This is especially true for the small stock turbos they use to make boost at very low RPM that cars use in takeoff.
 

PMD

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Mark: Usually, you want to get TIT (turbine inlet temperature) for EGT as what you are worried about is melting down the exhaust turbine wheel. EGT drops quite a bit after the exhaust valve, so max values from "experts" vary quite a bit from both experience or ignorance. I would ask your turbo manufacturer for THEIR limit, and stay a good bit away if you can.
MAP is interesting from an engineering perspective, but for real world testing only needs to be a reference value that you can use in normal flight. I wouldn't worry about a mechanical stop on the vanes, just start everything full open and close to get your indicated results. While you are speaking with turbo OEM, they might be able to give you some reference numbers for pressure differential max at your spec'd max altitude (actually absolute pressure at that alt./temp). Opacity meters are for diesel emissions testing, so not panel mount, but there are portable units. In my "day job" client's world, we use a LOT of test equipment, and with 20 odd branches, it is not always where we need it when we need it, so we rent a lot. Not sure if you can rent a meter, but worth a try. Governments, high end shops and probably trade schools will have them. Once you are done testing you will never need it again. In a pinch, use the old eyeball method, that means an HD camera for inflight pointing at exhaust dump.
 
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