Looking at 56 Hat's 8V92's Problem??

[MikeP]

"Turbos operate off excess heat energy in the exhaust gas stream."

Only incidentally...Heat alone won't turn a turbo at all; heat a turbo to any temp you want and it won't turn. However, direct cold air from an air compressor against the impellor and off she goes!

What moves the turbo isn't the TEMP of the exhaust gas, it's the velocity and volume of the gas. The turbo could care less if the gasses impinging on the impellor are hot or cold; if they are sufficient, the turbo will spin.

Certainly the hotter the gasses in the combustion chamber, the more they are expanding and the greater the subsequent pressure. But it is not the heat of the gasses that make the turbo turn, it's the pressure of the gasses. The fact that they are HOT is a product of how they are created.

[Trojan]

They will spool only if the primary is designed for low gas flow. If you have low gas flow and low velocity you have no movement. Heat expansion is not the only thing that turns the turbine blades. The expansion adds to the drive force. Without gas flow you have no turbine. The different size of pipes from the inlet to the exhaust lets the gas expand. It just happens to have the turbo blades in expansion chamber. In most cases until the engine is producing a pre designed exhaust flow there is not enough gas velocity to create an expansion to turn the turbo.

BILL

[krush]

The "pressure" of the gas is the result of the heat.

If you take the exhaust pipe from the head and somehow cool the air to ambient before it reaches the turbo, you will build no boost. You just sucked away all the energy that resides in the gas stream.

The kinetic energy associated with the velocity of the gasses moving through the pipe is negligible and is often just ignored when making calculations. The fallacy that the turbine is a windmill that operates off the velocity of the exhaust gasses is a widespread one. Turbines (gaseous type) extract mechanical power from the controlled expansion of gasses. No heat, no expansion. No expansion, no boost.

edit: if ya'll want to have a nice discussion, I can gladly explain the thermodynamics.


edit 2: Bill, if exhaust flow is what made boost, you would build boost at high RPM under no load (eg, in neutral with the throttle wide open)....which is not the case.

[MikeP]

I understand (I think) but I still have some issues...

Sure, if you cool the exh temp to ambient it won't spin the blades because you have reduced the VOLUME of the gasses and, subsequently the pressure as you do anytime you cool a mixture of just about anything.

So in this situation, the heat of combustion creates the expanding gases (pressure) to drive the turbo. But again, in my mind, HEAT per se doesn't drive the turbo, pressure does. To me that's easily proven by the fact that, as I said earlier, you can heat those impeller blades to any temp you want and they won't turn. Put 40PSI of cold air pressure out of an air compressor hose on them and they will.

I understand that the HEAT is what creates the pressure in this situation but since heat alone won't move an impeller and pressure will, it seems to me that it is more correct to say the turbo is driven by the PRESSURE of the exhaust gases, not the HEAT of the exhaust gases.

Lets say we measured the pressure/volume coming from our exhaust manifold at full load and we then had an air compressor generate that same pressure/volume. If we hooked the turbo to either of the sources, engine or compressor outlet wouldn't the boost be exactly the same?

[Avenger]

We've already pretty much beaten this to a pulp:

http://www.samsmarine.com/forums/showthread.php?t=6225

But if you guys want to keep at it, be my guest.

[yachtsmanbill]

Turbos must / most definitely provide something for someone, but I sure am glad the sub's got the Ns !! I could just about imagine what 2 or 4 new turbos would have cost me just to make the thing run, then all that cooler cleaning BS, and oil soaked blankets, and ER temps and on and on. ALL that for a few miles an hour???
YGBSM. ;-) ws

[neosin]

The props people have on these boats are made for getting up to 18kts or so, by putting more pitch in the prop or dia for (holding) you get more water moved for X fuel burned.

You lose your 18kts because you'd overload your engines, BUT you gain more speed at a lower RPM without (or little) extra fuel (it's kinda like putting a higher or lower differential gear in your car, thats what your pitch and dia on a prop do for a boat. ) , if any would be worth if because of the speed you'd pick up with a bigger dia/pitch prop push and holding water better than your smaller (go fast props). I'd bet you could still get 2 - 5 kts over your hull speed without over heating if you just had to. Depends on how you build it

It's works like this

FAST SPEED
Smaller DIA w/ more pitch

HULL SPEED
Larger (i would go biggest you can for holding power in waves) Med. - low pitch.

You could play with pitch to find that sweet spot for 1200-1300 rpms for hull speed, maybe even a little over. My hull speed on my 44TC is about 8.6kts, but i do very well at 9kts in deep water, deeper than 12 feet i get 9.5-9.8kts out of 1300-1400 rpm if i change my props to HULL speed props i would get 9kts with less fuel burn because DDs are built to be under load. When I'm at hull speed my DDs will burn more fuel per mile than if i put a little load on them, say 9-10kts... just depends on a few things like, wind... current, etc.

i'm not gonna get into all the details of how a 2 stroke or 4 stroke diesel works but on your 2strokers DDs mine being 330ish-hp 8v71s it's always WIDE OPEN throttle your just controlling how much fuel is put into the monster. there is no "throttle body valve controlling air flow".

It's built wide open, you pour more fuel in, it makes more horse power... Injector size being your limiting factor. IN theory you could just keep adding more fuel and keep making more power until you reach the point the crank/rods can't handle the load and BOOOM! lol ... don't do it however... when rods come out the side of the block it's nasty... trust me I've done it.

The point is, DDs your 8v71 and other flavors like to have some load on them at least. You'll find if you change your props to hull speed props, running 1/2 - 2 kts over your hull speed will give you the best fuel economy using DDs... other engines will be a little diff... but DDs must have X% load to become efficient to a given RPM & speed ...

I hope i put that all correct... i confuse it myself sometimes... I can care less about going 18kts with fuel at $2.20 gal for me... 8-10kts is just perfect... If i'm wrong on something guys point it out to me... this is what i've picked up after talking to DD people, prop people, people people, green people, old people, young people, and my dad... but he's just nuts anyways...

I'm no expert, but isn't this advice backwards? I thought to lower the load on an engine, you need to make it reach over max rpm (50 to 100) by decreasing pitch or diameter. I thought any engine that doesn't reach max rpm is overloaded throughout it's rpm band, not just at the top.......

Or did I read this wrong...?

[krush]

But again, in my mind, HEAT per se doesn't drive the turbo, pressure does.

I understand that the HEAT is what creates the pressure in this situation but since heat alone won't move an impeller and pressure will, it seems to me that it is more correct to say the turbo is driven by the PRESSURE of the exhaust gases, not the HEAT of the exhaust gases.

We do agree! A turbo does operate off of a pressure differential (not velocity). This pressure differential is caused DIRECTLY by heat. I'm just being an anal engineer and a fundamentalist I want people to get it right so they can show us cocky engineers up!

They beauty of the whole thing is we can calculate the power output of the turbo merely based on MASSFLOW (of the gas) and the change in temperature...with a couple safe assumptions thrown in there. This is why I say and people that really know turbos say that HEAT is what spins a turbo...semantical, maybe.

So fundamentally, the heat energy in the exhaust creates a pressure differential (ideal gas law) and the turbine does an isentropic expansion of these gasses and spins a shaft. Moving a step ahead, we can say the hot gasses cause a pressure increase and this pressure is what spins the turbo...both are correct, but the former is less ambiguous. Note that no where in there is the word velocity hehe.

[krush]

We've already pretty much beaten this to a pulp:

http://www.samsmarine.com/forums/showthread.php?t=6225

But if you guys want to keep at it, be my guest.

Trust me, I've had this discussion MANY times hahah. But I always bring the big ammunition to the table (formulas, cycle analysis, etc). Sadly, people loose interest when you trump their thinking with the laws of thermodynamics Are there any other mechanical engineers in the house? (FYI, I really "learned" most of this stuff on my own...classes just teach students to do book problems).

[Genesis]

Thermodynamics is one of the most useful sciences to understand.

Once you "get it", 99% of the BS you hear spouted about this or that "great idea" goes right in one ear and out the other, as it violates one of those laws - and they ain't suggestions.