Boost question 8

[panelman]

Guys, might sound a stupid question but stay with me

can someone explain what causes a turbo to generate boost, more specifically...

Subaru Impreza, 4000 rpm standing still generates no boost, same rpm when driving and it's scalded cat time

Is it something the ecu does?

Cheers

 

[Wilsonman02]

"the gas will have to flow at an increased velocity"....isnt that what you want? I've seen countless high horsepower cars with wrapped down pipes. And most turbo kits come with heat blankets that cover the turbine and upper part of downpipe, to increase heat and gas veolicty.
And if you cram more oxygen and fuel into a cylnder, then the more exhaust is going to come out, which is why they use nitrous to spool a turbo.

 

[SBBlue]

The engine has to work harder to get the gas to flow at an increased velocity.

The "heat blankets" do benefit the turbo -- if they are used to minimize heat loss prior to the turbine. If anything, you would be better off cooling the exhaust system distal to the turbine.


I won't argue that they use nitrous to spool a turbo.

But that's not "loading" the engine. If anything, it's unloading the engine, since this is being done with the engine in neutral or the clutch engaged.

 

[Wilsonman02]

They dont really use heat blankets before the turbine, they are used ON the turbine and top of the downpipe, the exhaust gas exiting a turbo can be 2000-3000ft per second. You want to keep that speed up. Thats why they also heat wrap Dump tubes off of wastegates. I dont want to get into an argument, but there has to be a reason why some of the highest horsepower street cars continously do this...

 

[GregLocock]

"the exhaust gas exiting a turbo can be 2000-3000ft per second."

Rubbish.

Absolute rubbish.

Stop making numbers up. Some of us do this stuff for a living.



Cheers

Greg Locock

 

[Wilsonman02]

Sorry my numbers got mixed up with something else, but forget those numbers, the rest is correct.

 

[SBBlue]

Okay, I'll bite again. If some of the highest horsepower street cars use insulation on the exhaust pipe. . . . .

Do any car manufacturers do this? Do any turbocharger manufacturers recommend this? Is then something that is done in F1 racing? I would think that F1 racers would certainly do this if there was any advantage in doing so.

 

[Wilsonman02]

The Ft per second is 200-300ft not 2000-3000, my bad.
Car manufactures dont do a lot of things performance enthusiast do because its unreliable and takes a lot more maintence to up keep, something consumers don't want. And F1 hasn't used turbo's since 1988.

 

[ruggedscot]

COST COST COST

Manufacturers wont lag a pipe as it ocsts too much and also if there is a need for this they will seel it to you for a greatly increased price under a motorsport brand....

The lagging does work though - and the reason that they dont lag the exhaust is that it will be bigger than the inlet to the turbo to allow more flow and less back pressure.

exit the turbo you have to maintain the heat energy in the gases so you lag to stop the heat getting out - this ensures that the turbo gets the hottest gasses possible. The pipe after the turbo is big to drop the back pressure and allow faster exit of gasses. No need to lag as it is exiting and does not need to keep heat, effects are there, maybe small but in a race everything counts.

and winning is the name of the game - manufactures will do try anything that gives them an edge.

Rugged

 

[al1]

In commercial power generation, the turbines --- both gas and steam --- the hp power changes significantly with backpressure. Turbochargers work off the same principal in that the pressure differential is what make the turbine wheel work more or less efficiently. Whether this pressure comes from high-pressure heat or low heat volume flow, it’s all about pressure differential. Just think if both the intake and the exhaust had near the same pressure, obviously there would be little work done no matter how high the pressure and/or how hot. When you keep the exhaust hot by thermo rapping it, you defeat the intent. In some of the commercial application, they even go to the trouble and cost of inducing a vacuum on the exhaust side.
AL

 

[GregLocock]

One reason OEMs might insulate the downpipe after the turbo is to shorten the light-off time for the cats. Incidentally you may not be able to see the insulation being used - we often use double tube downpipes with an airgap between the tubes.

Has anybody got any idea of the typical temperature profile ahead of, and after, the turbo at full power?


Cheers

Greg Locock

 

[1fasttsi]

>"Okay, I'll bite. How is it possible to load an engine to >it's "full extent" if it's not connected to a load (ie, in >neutral)? "
>Well to my knowledge its not possible to load an engine to >its full extent in neutral, but if your peak boost is set >to 20psi, and u hold it at rev limiter you can still build >8,10, or even higher psi, which makes for one hell of a >launch. Also many guys use a small shot of nitrous to load >the engine and create more exhaust and to spool the turbo >further before a launch.

I'll throw some more info into this one. If you want to throw a phantom load onto a combustion engine with multiple cylinders you want to fire every other cylinder. This makes the engine work harder and you now can build boost while just sitting without moving. On automatic cars (sissies) its the same principal as brake loading the engine. Its actually called a "stutter box" and used primarily for drag racing. Although I do use mine on the street too

 

[patprimmer]

Some other reasons for insulating exhaust pipes might be to prevent the carpet being melted and heat under the bonnet getting so high it causes problems like vapour lock, paint blistering, electrical wire insulation and rubber hose degradation.

My experience is that:-

Insulating the pipes with lagging accelerates corrosion considerably.

Insulating the manifold between the turbo and the head gives a slight increase in response rate.

Reducing back pressure in the exhaust system after the turbo noticeably improves response rate.

Nitrous oxide injection at full throttle then progressively reducing the nitrous shot as boost comes on effectively eliminates lag altogether.

Free revving an engine applies no load other than pumping losses, and some friction and inertia of the moving parts. These loads are so low, that the engine will over rev or hit a limiter within about 1 second or less.

By loading up an engine by a brief acceleration run (like a burn out) or by restraining applied power with the brakes (like stalling the torque converter) the turbo can be spooled up. Inertia will keep the turbo spinning fairly fast, but the throttle will keep boost out of the manifold. Once the throttle is opened, the boost will be instantaneous, to the level produced by that rpm of the turbo.

The mechanism of nitrous overcoming lag is twofold:-

1) The nitrous shot is a substitute way of producing the sort of power levels normally only obtained with boost, as the nitrous is a chemical supercharger, so it compensates for rather than eliminating lag.

2) Once the nitrous starts to burn, it makes more exhaust gas, which then has a higher pressure and temperature. This extra exhaust then spools the turbo more quickly, thereby reducing lag. This quickly makes the nitrous unnecessary, and maybe even undesirable as to high a boost and nitrous together might cause damage to the engine.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[patprimmer]

I think the use of inflammatory, argumentative, rude, impolite or unprofessional language, like calling people who have different taste or opinion "sissies" is against forum rules.

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Wilsonman02]

"Has anybody got any idea of the typical temperature profile ahead of, and after, the turbo at full power?"
Most EGT gauges at full throttle on a turbo car will read anywhere from 1200-1700 degree's F. Depending on the AF ratio. I believe cruising is around 800-1000 degree's. But i dont have an EGT Gauge in my car yet. So I have watched one go up and down with throttle.

 

[carnage1]

Some other reasons for insulating exhaust pipes might be to prevent the carpet being melted and heat under the bonnet getting so high it causes problems like vapour lock, paint blistering, electrical wire insulation and rubber hose degradation.

patprimer got it right but left out one thing in the list.
the transmission. take an e4od turbo ford diesel, remove the insulation coming down after the turbo and you will be at your local trans shop in a month or less, and yes we will say not nice things about your intelegence as we repair it.