basic turbo question.

[monomer]

Real basic. right down to how they properly function...



I've gotten into an argument with a gasser turbo fiend. He stats that exhaust temp does not matter in regards to turbo spool-up, He believes it's just by flow (and states I could easily spool something like a 57-trim t3/t4.)


From what I've read - it's all about convection currents. expanding HOT gases.





And then we got on the debate of how a rear-mounted turbo works...

[zukgod1]

From what I've read - it's all about convection currents. expanding HOT gases.

You are correct.
The gases are still expanding as they exit and that's what causes the turbine to spin, thus the compressor spins and compresses more air than the engine would normally require thus boost/

[Smokey Eddy]

Likely a bit of both. I think it would be wrong to say that exhaust flow doesn't spin the turbine but its largely expanding gasses, i agree.

[the caveman]

there is a reason that turbos are said to create power from heat, not exhaust. something i'm sure to do with thermodynamics. although i would think the obvious thing is how much flow of exhaust air is pushing on the impeller there is  more to it that meets the eye.

[jtanguay]

Real basic. right down to how they properly function...



I've gotten into an argument with a gasser turbo fiend. He stats that exhaust temp does not matter in regards to turbo spool-up, He believes it's just by flow (and states I could easily spool something like a 57-trim t3/t4.)


From what I've read - it's all about convection currents. expanding HOT gases.





And then we got on the debate of how a rear-mounted turbo works...

i could be wrong, but this is how i believe it to work.  the combustion process in an engine still occurs after the power stroke in the exhaust manifold & exhaust piping.  so the turbo acts as a restriction, keeping the exhaust gas in there for a bit longer to take advantage of the expanding gases, which in turn create the pressure.  this is why having zero backpressure after the turbo is a good idea... so that more of the hot expanding gases can get through  :wink:

i bet you could easily spool a  57-trim t3/t4, but you will need to increase the fueling... and also increase the size of the exhaust piping (to help reduce EGT's)

[molgrips]

If it were just the flow of exhaust gas spinning turbine, then since this flow would have to be generated by the piston pushing the exhaust gas out of the cylinder, it would actually be a pure mechanical load on the crank and hence a turbo would not be any more efficient than a supercharger.

The exhuast gasses are still burning when they exit the cylinder and hence still expanding - which spins the turbo much more than if it were just being pushed out by the piston.  This is why you see flames when you have an engine with a really short pipe, like a WW2 aeroplane engine

It might be possible to build a really efficient engine that allowed all the fuel to burn in the cylinder and hence use all its energy, but it would take a really long stroke and you'd encounter diminishing returns - the last bit of expansion would be really weak and hence not contribute much to powering the car.

Indicentally the gas engine in the Toyota Prius has a longer power stroke than compression stroke, so that you harvest more of the expansion at a cost of reduced power (for the size of the cylinder).  This makes it somewhere between a normal gas engine and a diesel engine in terms of efficiency.  So if you fitted a turbo (it has been done, I read about it) you'd never get the same boost as you would with a normal engine.  It also makes the engine remarkably quiet.

[andy2]

Well 935racer's t3/t4 on the franken engine was bigger so I'd say that the 57 trim(.63 A/R turbine) would work.However It will be no good for daily driving at all and will likely start to make boost at 3500+ rpm.The k24's compressor inducer has a 36mm inducer and the 57 trim t3/t4 has a 50mm inducer as an example.

I would try a 50 trim T3 as Its plenty big enough for a 1.6 or 1.9 with a 42.5mm inducer and .48a/r turbine.The 50 trim turbo is still bigger than the merc 3L diesel turbocharger.You could also try the merc's turbo as its slightly smaller at 45 trim (40mm inducer).


Edit:Unless a 1.6 or 1.9 is reving to gasser like rpm then a t3/t4 will not work well.

[flapjack]

Any remote turbo I've seen has had all the exhaust tubing up to the turbo wrapped.  Seems like they want to keep the exhaust gas hot as possible when it hits the turbine.  I don't think they're still burning though, and yet the remote turbo works well enough for people to bother with them.

the gas engine in the Toyota Prius has a longer power stroke than compression stroke

?? I must be reading that wrong. What do you mean?

[monomer]

Any remote turbo I've seen has had all the exhaust tubing up to the turbo wrapped.  Seems like they want to keep the exhaust gas hot as possible when it hits the turbine.  I don't think they're still burning though, and yet the remote turbo works well enough for people to bother with them.

the gas engine in the Toyota Prius has a longer power stroke than compression stroke

?? I must be reading that wrong. What do you mean?

Most racers also wrap the header in fiberglass.  Keeps heat out the bay also.



Then they tried the fact that diesel is twice the compression - "thats 415 PSI flowing out the exhaust"


I chuckled. "Man, those VW guys must be crazy then....)

[molgrips]

molgrips wrote:
the gas engine in the Toyota Prius has a longer power stroke than compression stroke


?? I must be reading that wrong. What do you mean?

A bloke called Atkinson patented the Atkinson cycle engine (as opposed to Diesel or Otto) which had a complicated series of linkages so that the piston did all four strokes in a single crank revolution.  By varying the lengths of the linkages you could get the power  stroke (and consequently the exhaust stroke, obviously) to be longer than the intake and compression strokes.  This lets the gas expand more and hence is more efficient.

Well that's no good for a car engine, so what Toyota do is set the cams up to leave the intake valve open for a little longer past BDC, so that some of the air/fuel mix goes back out of the cylinder on the compression stroke without being compressed.  The upshot of that is that the compression stroke is effectively shorter than the expansion.  The engine is effectively a 1.3l with 1.5l worth of expansion.

I really wish they made a diesel version of that damn car.  It'd be spectacular.

[mk2diesel]

isn't that ... the intake stays open past BDC and the flow of intake air cause a ram effect to over fill the cylinder ... increasing the amount of charged air in the cylinder and simulaing a larger stroke ( but only at certain speeds , depending on the tuned length of the intake and swept cylinder) ....

the old Dodge RAM big blocks used the same technology back in the 50's and 60's  ...

[molgrips]

Hmm, interesting idea mk2diesel.. I would expect though that Dodge probably had the intake valve open only slightly after BDC - to achieve the effect you mention it'd have to shut BEFORE the incoming mass of air had time to change direction.  Probably a huge mass of air coming into that dodge engine too which would end up creating a larger pressure wave as it changed direction.

For the Prius we are talking about a much smaller mass of air incoming and the valve is left open long enough for the pressure wave to build and air to start actually leaving the cylinder.  I do wonder where that air actually goes tho...  In 20 years when my Prius is old and knackered, maybe I'll rip it apart and start modding it

[arb]

isn't that ... the intake stays open past BDC and the flow of intake air cause a ram effect to over fill the cylinder ... increasing the amount of charged air in the cylinder and simulaing a larger stroke ( but only at certain speeds , depending on the tuned length of the intake and swept cylinder) ....

the old Dodge RAM big blocks used the same technology back in the 50's and 60's  ...

This is all old Hat for any small block Chevy racer - its called your Cam grind. You can get just about any combination of lift, duration, over lap, and timing you want. If you can't find it in a catalog, there are shops with blank cams that will grind what ever you want.

I wonder if there are any of these shops that would grin a cam for us? :-D