Engine/Transmission (1994 - 1998) Stupid question - turbo spool-up

[Darel]

OK, this is probably going to sounds pretty dumb to you guys who really understand turbos but here goes. Other than the obvious wear-and-tear issues, why can't we keep our turbo spooled-up all the time? My thought is this: an engine-driven air compressor feeds into a fitting in the exhaust side of the turbo housing. The compressed air (pick your PSI, I haven't thought that much into it yet) "blows" on the blades of the turbo when there's no exhaust pressure, keeping it spinning all the time. Maybe mount a sensor to cut off the air when there's exhaust pressure? Yes? No? I'm not about to go drilling into my turbo just for fun. If this was viable I'm sure someone would have thought of it already, but I figured why not ask the stupid question?

Embarrassingly,

Darel

 

[bighammer]

It is spinning all the time. You can hear it at idle especially on the straight piped trucks. It's just not going fast enough to create any boost pressure. It would take an incredible amount of air to keep it spinning that fast.



Maybe you blow the compressed air into the manifold under low-boost acceleration? Not sure how the turbo would like that though.

 

[illflem]

It would take a pretty big compressor that would probably take more power to run than you would gain.

 

[Darel]

So what kind of pressure does the exhaust put out? I didn't think it was that much, optherwise it would feel like a rifle shot coming out of the tailpipe (if you could keep an exhaust on it).

D

 

[fest3er]

It is counterproductive to feed vastly more air into the engine than it needs to burn the fuel injected into it. Among other things, it will increase NOx (from the extra heat).

If you want to pre-spool your turbo, take a 120 PSI compressor, attach a 'blower' nozzle and blow air onto the intake air vanes at, say, 75-90 degrees (that is, nearly perpindicular to the vane).

Hmmm. I guess I'm going to have to go to Tom's dyno event Saturday and test my theory.

Fest3er

 

[dalexander]

fest3er,



Turbos do not work according to the velocity of the exhaust gases, they are driven more by the heat of the exhaust. Not sure exactly how but someone with a background in thermal dynamics would be able to explain it. That is the reason they place the turbo as close to the motor as possible, the closer you get the hotter the exhaust gases. I'm sure someone will have more info on the subject.



Dennis

 

[baby.driver]

Nope, the turbo is placed close to the engine to utilize exhaust gasses that are STILL EXPANDING.

It is pressure and velocity that drive the turbo, it just happens that the heat is in that place also.



Jay

 

[dalexander]

baby. driver,



Expansion of the exhaust gasses do play a part, but it is mostly the heat of the exhaust than the velocity. The original post was about using an air stream to keep the turbo spooled-up, and my reply was that without the extreme heat produced by combustion you will not get any appreciable boost. Most references to the advantages of turbos is that they take advantage of waste exhaust heat to improve the thermal efficiency of the internal combustion engine. Heat plays a big part in turbo boost. It's been 10 years since I did any turbo system research I'll have to look for my notes to provide specifics.



Thanks, Dennis

 

[fest3er]

Increasing the temperature of the exhaust gas should increase its pressure, which should increase the velocity of the gas flow, thus helping spin the turbo faster.

Exhaust gas (in a stock engine) can get up to 20 PSI or so. I would think it is a combination of the gas velocity and mass that drives the turbo. Under normal, clean-burning operation, given a specific boost pressure, the mass of the exhaust gas should remain the same, cold or hot. But when cold, the velocity will be a bit slower, whereas when hot it will move faster. Thus the same mass moving faster will have more momentum to turn the turbo.

However, the turbo is designed to efficiently flow low pressure gas from its intake through to its outlet, whilst extracting the maximum amount of energy from it. My idea of directing a low-mass, high-velocity stream of 120 PSI air onto the turbine blades at a near right angle is not based on efficient flow. It's based on the concept of using momentum to spin the turbo. Efficient flow of this stream is not part of the equation; rather,. maximum transfer of energy from the stream to the turbo is the main thing. It should work because the velocity is so much greater, its momentum may be similar to that of lower-pressure, higher-mass exhaust gas, thus it should impart similar energy to the turbo.

How much energy would such a stream produce? I don't yet know. It might only be enough to produce 10 PSI boost. It might be enough to produce 30 PSI boost. I'll find out Saturday if there's a volunteer handy. Or I'll experiment on my own truck!

Fest3er

 

[Darel]

Fest3er,

That was exactly what i had in mind with my original post. Basically "blowing" on the turbo vanes. I would hope to only get about 10 psi or so, because just from watching my boost gauge that's right about where the power starts to come on. I understand the idea of the exhaust gas heat and expansion, though. I originally thought it was just the pressure of the gases flowing out of the engine that spun the turbo.

The other thing is, you don't think pre-pressurizing the manifold would keep the exhaust gases from being scavenged from the cylinder, do you?

Maybe a little electric motor on the turbine...

D

 

[Blakers]

Darel,



I believe some diesel locomotives use your idea. I think they have combination turbochargers/superchargers that are gear driven at periods where there is not enough exhaust gas to drive the turbines for good boost. They employ overriding clutches that allow the turbines to spin faster than the gear train when the exhaust gas is providing high pressure. Thus the boost can remain high even at low exhaust gas pressures.



Blake

 

[T. Baker]

It has been done. Turbos take a lot of energy to make boost which they give back by making the engine more efficient and powerful. I have been told that they take one third of an engines output. A 200 hp engine is using 66 HP to drive the turbo. There ain't no way you can get an electric motor to do that. And it is real hard to get enough compressed air to do it either unless you put a giant air tank in the bed. Here is where I got this info. Turbo info.

Remember that as far as exhaust gases driving the turbo; heat velocity and pressure are all directly related. Mount the turbo too far away and the exhaust cools and contracts thus loosing pressure causing a loss of velocity. Mount the turbo close and you keep the heat and velocity and pressure. A turbo doesn't take high pressure to run it but it does use a lot of volume. The problem with compressed air is it is hard to get a lot of volume and have high pressure.

 

[Blakers]

Originally posted by T. Baker

I have been told that they take one third of an engines output. A 200 hp engine is using 66 HP to drive the turbo. There ain't no way you can get an electric motor to do that.

The one third used to power the turbo would mostly be lost by the engine to the exhaust anyway - the turbo runs on this "free" energy, putting energy back into the engine in the form of compressed air.



A pretty good deal.



Blake

 

[Diesel Freak]

1. The energy required to turn a turbocharger is not free.



2. Keeping the turbo spooled all the time even when the engine is not under load is not beneficial. Too much boost will actually lower fuel economy.



3. Energy consumed by a turbo diesel can be roughly devided into thirds... 1/3 to the crankshaft, 1/3 out the exhaust pipe, and 1/3 to the radiator.



4. Exhaust Gas Enthalpy is what is required to turn the turbocharger. It is a combination of Internal Energy or heat, and the actual mass flow rate of the exhaust.



On a Turbocharged engine the exhaust must travel into the turbine housing where it is accelerated to a high velocity by the housing scroll. Exhaust manufold pressure rises as a result of this. This pressure is what is commonly refered to as turbo "drive pressure". On a stock engine it does not normally exceed 25 PSI... this is why I said that the energy to turn a turbocharger is not free... the engine now has to overcome the restriction of the small turbine housing scroll.

 

[zjones]

Why not just use a supercharger to get you to 10+ psi. then when the turbo catches up have a clutch disengage the charger to keep from losing top end HP?? Seems like you could use a much larger turbo (lower egt's ect. ) this way without line lag. ??.

 

[T. Baker]

Zjones,

Detroit diesels have this system on the turbo models. They have to have the blower to keep them running when the turbo isn't making boost. The reason it is not done on others is because superchargers are really expensive and you have to have room for it + all the piping and special valves to bypass the supercharger when it is not working.

 

[Diesel Freak]

and because the Detroit's with blowers are two cycle..... that is the main reason.