Higher EGT'S equal more boost?

[NIsaacs]

On the 12 valve Cummins engines, Cummins literature says they built the head with no water passages near the exhaust runners/ports for less heat exchange to the radiator and for better turbo performance. That the higher the EGT'S the more boost the turbo generates due to the heat causing closer tolerances on the turbine side. On my two 12 valves I have seen this, so I know it's true. However, with the 24 valves and the 6. 7, is this still true? With computers, waste gates and variable turbos I would think this design would no longer work other then the heat exchange to the radiator.



Comments?



Nick

 

[MChrist]

Interesting, I've never heard that before about the 12 valves. The hotter the EGT, the higher the boost due to heat causing tighter tolerances on the turbine side of the turbo? I make no claims to know or have any experience on the 12 valves, but that sounds interesting.

 

[Gonzo 1066]

Heat is what drives the turbo, The turbo is basically a small heat engine. If heat is rejected into coolant in the exhaust passages it is a loss in many ways. I would assume that this design feature is not exclusive to Cummins engines.

 

[SRadke]

This is common to all turbocharged engines.

 

[NIsaacs]

This is common to all turbocharged engines.

Thanks for the info, I did not know this but it makes sense. I guess I thought it was a Cummins exclusive and another reason to brag on them



Nick

 

[MChrist]

"due to the heat causing tighter tolerances on the turbine side. "

That's the part that makes me say "huh"?
Where is this written? And which tolerance tightens? Exhaust turbine to housing? Compressor turbine to housing?
I'll need more evidence before I can buy into that one.
Since these trucks have waste gates, and they open at a certain psi, you wouldn't boost past that point, tighter tolerance or not. (unless 12 valves don't have WGs). Even then, I have a hard time believing that there is such an expansion of the turbine as to cause a noticeable increase in boost, if any.
Please direct me to your source.
Thanks

 

[AH64ID]

It makes sense for sure, but on a WG truck you won't see the increase in boost but the turbine will be more efficient as the tolerances tighten up.

 

[MChrist]

Which tolerances? And how much expansion are we talking about? I'm still having a hard time with that one...

 

[AH64ID]

The turbine to housing tolerances. The turbine gets more heat and expands more than the housing, thus the gap gets smaller between the turbine and housing and you have greater efficiency.

 

[MChrist]

That sounds more theoretical than practical. Is it even measurable? I cruise around at 500-600 degrees. At full boost, my EGT gets to 1100. You are saying the impeller grows between those two temps? By how much?
Not trying to ruffle feathers, just finding it hard to swallow.
Thanks

 

[FFL8N]

I'm sure its true. I know iron will grow almost 1 foot for every 100 feet with a 1000 degrree temp change. (That is until it softens and sags

 

[rbattelle]

The growth of a turbine due to heat is a well-established science. The only question is whether the cold-running clearance is so far away from the housing that a difference in turbine inlet temperature between a cooled exhaust and an uncooled exhaust makes a measurable difference.

So the first question is, how much hotter is the exhaust without the extra water jackets? My guess is probably not more than a couple hundred degrees.

If that's right, how much will the turbine and housing grow in moving from, say, 1000°F to 1200°F? If the cold running clearance is tight enough, it could be a significant closure at higher temperatures.

However, you have to figure that Cummins wants to avoid the turbine rubbing the housing at all costs. That suggests large clearances, even at maximum inlet temperature.

We don't have enough information to dismiss the idea that it is turbine growth relative to the housing that improves the turbine performance. But as others have mentioned, the waste gate will ensure that any extra power available to the turbine will be dumped overboard once the waste gate opens.

Of course, higher heat means more turbine power. Gonzo put it nicely when he said the turbo is a heat engine (of course, so is the reciprocating engine). The first law of thermodynamics tells us that heat=power. It's that simple. So one could argue that higher turbine inlet temperature would build boost faster, since more power is available.

To answer the original question, higher EGT could mean more boost, except that the waste gate prevents this. Higher EGT probably will yield faster spoolup, so in that sense it's "more boost per unit time" until the WG opens.

-Ryan

 

[NIsaacs]

Good reading Ryan, thanks. As usual, I wasn't specific enough in my original post. I don't always type whats in my head



On the "early 12 valves" pre W/G turbos, the boost goes up with heat, as much as 3 psi. The exhaust sound changes as well with stacks. When you first start into a big hill with cool (700 deg. ) EGT's and you put your foot into it, you can hear the pulse or crack of the cylinders firing. Then as the EGT goes up the boost goes up and the sound goes away and now is just a steady roar without the pulse sound. So something is growing in the turbo.



I have some old literature on A, B and C series Cummins that I recall about the head on the B-series. If I remember right, it was not mentioned on the A and C so I was thinking it was just the B. I will try to find it in my stacks of old junk and read it again.



Nick

 

[MChrist]

Interesting, I've never actually heard of that. It looks like aluminum expands 2. 6 times more than mild steel, so it would expand, but I thought the cool side was aluminum and the exhaust side was steel. I really don't know what material they are, though.
Anybody have dimensions of exhaust impeller and inside of exhaust housing?