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Old 01-17-2016, 10:58 AM   #21
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Turbos and all the goodies added to a NA are to increase the power.

This requires an increase in fuel burn.

An engine is considered to be able to burn a certain number of gal of fuel before its worn enough for a rebuild.

Burn fuel at 3 times the basic rate and the engine life in hours will be at least 3x shorter

If you need plenty of power to operate a sport fish the goodies will give it to you.

If you operate a displacement boat there is little need for the cost and complexity of turbos or anything else.

Just install a DD that operates at 60%+ of rated HP or more and enjoy the insane number of hours these NA engines can operate.

Happily the 1936 design allows some selection of rated HP by changing injectors or timing,if the vessel service changes.
Turbos do not increase fuel burn. If you have a choice between a TA engine and and a NA engine to make say 300hp, the TA will burn less fuel doing the same job.

There's a good reason all modern industrial, commercial and ship engines are turbocharged with charge air cooling. Some ship engines run both high boost, up over 40psi, and log over 100,000hrs. Their goal is efficiency. Ferries, fishing trawlers, CG cutters, tugboats, etc... All turbo engines unless engines are from way-back.

In a trawler that only needs say 40hp on the shaft, an NA might be the best choice as the turbo engine optimized for that duty really does not exist on the market. Maybe a VW tdi running 2000rpm at high boost, but who wants such a tiny beast pushing a big slow boat.
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Old 01-17-2016, 11:12 AM   #22
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Turbos do not increase fuel burn. If you have a choice between a TA engine and and a NA engine to make say 300hp, the TA will burn less fuel doing the same job.
OK Sky, you're right on this one. A turbo engine will not burn more fuel (at idle) but once you start extracting that extra horse power offered by the turbo, you pay for it in fuel costs and accelerated engine wear.

The reason why more engines are equipped with turbos are mainly due to environmental regulations requiring a more complete burn of the fuel. The secondary reason is you get more horsepower out of the same sized engine with nearly the same weight.

As my father used to say, if a candle burns twice as bright, it lasts half as long.
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Old 01-17-2016, 12:06 PM   #23
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OK Sky, you're right on this one. A turbo engine will not burn more fuel (at idle) but once you start extracting that extra horse power offered by the turbo, you pay for it in fuel costs and accelerated engine wear.

The reason why more engines are equipped with turbos are mainly due to environmental regulations requiring a more complete burn of the fuel. The secondary reason is you get more horsepower out of the same sized engine with nearly the same weight.

As my father used to say, if a candle burns twice as bright, it lasts half as long.
You didn't really read what I posted. If you need 300hp down the shaft, and have a choice between NA and TA engines to do it, the TA will burn less fuel.

And no, turbos have nothing to do a more complete burn of the fuel. Both NA and TA engines properly designed, in good shape, and operating in the middle of their map burn virtually all the fuel, cleanly.
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Old 01-17-2016, 02:53 PM   #24
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OK, I thought I was with you until the argument came up about a turbocharger not necessarily using more fuel.

So, using a Cat 3208TA 375 hp for example. I was under the impression that the turbocharger "kicks in" at a certain rpm. So, am I wrong thinking that if I am underway at a lower rpm (before the turbocharger "kicks in"), that I would be using the same gph as a 3208na at that same speed. I know I am probably trying to get simple explanations for complex issues, so I will hope you will bear with me.

And your discussion about "hp down the shaft" threw me for another loop. How do you determine how much hp one needs down his shaft. Sounds like I am trying to be funny, but I'm serious.

My experience has been with Cat 3208na They would move my boat at about 9 knots at 2100 rpm and would consume 10 gph. If I needed to get off the water due to weather or other issues, I could punch them up to 2450 rpm and gain about 2 knots, but use about 16 gph. Can anyone hazard a guess how a 3208TA 375 hp Cat would have affected those numbers?
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Old 01-17-2016, 03:18 PM   #25
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A turbo is progressive. At idle, it idles too, whirring along at 20,000 RPM or so.

As you rev up the engine, more exhaust gases are created which spins up the turbo and crams more air into the engine.

You decide how much HP you need with the throttle. Faster = More HP.

Your peak economy would be about the point of peak torque on the engine.

Stu
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Old 01-17-2016, 03:39 PM   #26
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Thank you for helping me understand this better. I had incorrect assumptions for sure.
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Old 01-18-2016, 07:37 AM   #27
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"So, am I wrong thinking that if I am underway at a lower rpm (before the turbocharger "kicks in"), that I would be using the same gph as a 3208na at that same speed."

IF the engine is being operated at a slow speed , where there is NO turbo boost the turbo becomes another restriction in the exhaust and will increase fuel burn.

Additionally if the compression has been lowered (compared to the NA) with no boost the efficiency will also be harmed.

All engines have a set of BMEP curves which hopefully the NA that selected the engine understood , and the owner operates as it was designed.

The hassle today is the lack of real industrial diesels in the 100HP range that have been marinized., and are common enough to obtain parts for.
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Old 01-18-2016, 08:21 AM   #28
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Is the Caterpillar 3054B still in production? That's a low power NA platform that's completely marinized.
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Old 01-18-2016, 09:53 AM   #29
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The Cummins 6B series of engines gives you the ability to compare fuel consumption for a turbocharged and NA engine at the same horsepower.

The data sheets and performance curves for the 120 hp 6B and the 370 hp 6BTA are available on Boatdiesel's library. I picked 50 hp as a reasonable slow speed trawler cruising hp to compare. As best as I can read the curve and as you might appreciate the 6BTA curve is hard to read at 50 hp, the NA engine uses 3 gph and the highly turbocharged 6BTA uses 3.5 gph.


Interestingly the NA engine produces its 50 hp (propeller curve) at about 2,200 rpm and the 6BTA produces it at about 1,600 rpm. So the turbo is definitely at work at that low rpm. You can see the torque curve head upward sharply at about 1,400 rpm to confirm this.


The factors at work here are lower compression for the turbocharged engine requiring more fuel but it operates at a lower rpm to produce that power than the NA engine which should reduce parasitic losses. I suspect that the exhaust back pressure from the turbo is mostly balanced out by the boost it produces.


The difference should be more dramatic at lower rpms and power.


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Old 01-18-2016, 11:27 AM   #30
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David- I found a curve for a 330 6BTA CPL 1975 that has the tabulated prop curve burn data. Got it from Boatdiesel. Using the 2.7 exponent, which is what they used, you can calc hp. Got 50hp at 2.8gph at 1400. Easier than trying to pick numbers off a curve like they have for the 6B 115. Anyway, that comes out to 18hp/gph which is not too shabby running an engine at way low power.

Also, that engine uses the Bosch P7100 pump which is known for very "crisp" injection which is good for economy. The CAV pump in my opinion is a bit sloppy and it tends to show up in the burn numbers. The 6B and 6BT are fine engines, but when I do the burn rate calcs, if I can get the raw data, they are decent but not stellar.
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Old 01-18-2016, 01:03 PM   #31
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OK, I thought I was with you until the argument came up about a turbocharger not necessarily using more fuel.

So, using a Cat 3208TA 375 hp for example. I was under the impression that the turbocharger "kicks in" at a certain rpm. So, am I wrong thinking that if I am underway at a lower rpm (before the turbocharger "kicks in"), that I would be using the same gph as a 3208na at that same speed. I know I am probably trying to get simple explanations for complex issues, so I will hope you will bear with me.

And your discussion about "hp down the shaft" threw me for another loop. How do you determine how much hp one needs down his shaft. Sounds like I am trying to be funny, but I'm serious.

My experience has been with Cat 3208na They would move my boat at about 9 knots at 2100 rpm and would consume 10 gph. If I needed to get off the water due to weather or other issues, I could punch them up to 2450 rpm and gain about 2 knots, but use about 16 gph. Can anyone hazard a guess how a 3208TA 375 hp Cat would have affected those numbers?
Just realize in theory, turbochargers do not "kick in" at a certain RPM. They kick in at a certain load. It is heat that drives a turbocharger(not exhaust flow)....heat is energy. To give you an idea, an engine at idle will not drive up the boost because it is not loaded. Just to clarify things.

And Ski was just using 300hp arbitrarily. Don't let his "down the shaft" terminology throw you off. He just used 300hp as an example.

Now the main reason why turbocharged aftercooled/intercooled engines are more efficient in his example is because of the charge air volume and temperature. You are actually extracting more energy out of a parcel of fuel than you are in an NA engine. It really is as simple as that. And THAT IS THE REASON why turbodiesels dominate the current modern market. They are more efficient...hence they burn cleaner...hence less pollution.
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Old 01-18-2016, 01:30 PM   #32
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It is heat that drives a turbocharger(not exhaust flow)....heat is energy. To give you an idea, an engine at idle will not drive up the boost because it is not loaded. Just to clarify things.
OK, how does heat drive a turbocharger?

Exhaust pressure (due to engine combustion) is what spins the exhaust turbine which is close coupled to the intake compressor turbine. I know exhaust systems heat up based on fuel consumption = loading, but that isn't just heat.

you can heat up a turbine with a propane torch and it won't move... It is solely exhaust pressure, caused by the engine combustion (which generates heat and expansion of the gases) which are fed to the turbo.

Are we all agreeing in our own way?

English can be so ambiguous sometimes.

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Old 01-18-2016, 01:54 PM   #33
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I did it your way using the 370 hp engine data sheet and got the same result as you. My old eyes or parallax fooled me. So the NA and highly turbocharged Cummins engines both use the same fuel at low hp. Makes better sense to me.


Stu:


Combustion gas volume exiting the engine is influenced by these variables: the pounds of air sucked or pushed into the engine, the pounds of fuel burned and the temperature of the exhaust gas as it exits.


The chemical energy in a pound of fuel roughly is converted into 1/3 kinetic (rotational) energy, 1/3 exhaust gas temperature and 1/3 jacket cooling and convection radiant losses from the block.


So more fuel means more heat in the exhaust gas which means more volume of exhaust gas. PV=NRT, the ideal gas law. P=pressure, V=volume, N= ideal gas, N= number of moles which is related to pounds, R=ideal gas constant (sort of a fudge factor).


At idle or even higher rpms at no load, there is very little fuel injected. The air/fuel ratio is very high, maybe as much as 100:1. Remember the 1/3 rule of thumb above. If kinetic energy is very low (just parasitic losses) then it doesn't take much fuel to make it so the temperature will be low. But at high hp loading the pounds of air and fuel go up and the temperature goes up, therefore the exhaust gas volume is very high.


That's all the physics my tired brain can handle today. Let's talk about neat places to anchor out now.


David
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Old 01-18-2016, 03:47 PM   #34
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Turbines work off volume flow rate and pressure differential.

Using the ideal gas law, most engines have about three times the volume of exhaust gas leaving the engine compared to air going in.

Temp has to be expressed in absolute terms, so I'll show my age and use Rankine:

Inlet air 70F + 460= 530R
Exhaust gas 1000F + 460= 1460R

So all else being equal in the ideal gas equation, exh volume 2.75 times intake air volume. Plus a little bit more from combustion products, so call it three.

What's nice about that is the boost can then be a good bit higher than the exhaust manifold pressure. Can't be three times as the turbine and compressor both have inefficiencies that add up.

It's true heat does not spin the turbine, it is flow volume and delta p that spin it. But heat that gas up and it works much better!!!
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Old 01-18-2016, 03:54 PM   #35
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I have overhauled a turbo or two, and knew that regardless of the temperature, it wouldn't spin.

You guys are getting into the laws of thermodynamics and that kind of thing which I sort of grasp, but it has little to do with the mechanical side of things.

or perhaps my medication needs to be adjusted again

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Old 01-18-2016, 05:09 PM   #36
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OK, so I thought I was walking through Harvard Square looking for a lobster-roll and I wandered into a room of professors discussing thermodynamics. I hope there isn't a final test on the details of your discussion, but you have been very helpful in correcting some of my misconceptions.

I am certainly less frightened by turbochargers. I am a little more aware of the positive boost that can be achieved if the electrical nurget is placed in front of the inter-cooler. I am being a smart a-- here, but it really has been very helpful. Thank you, sincerely.
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Old 01-18-2016, 10:15 PM   #37
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I will maintain my initial statement!!! Turbochargers are driven by thermal energy...ie HEAT!!! Does E=MC2 ring a bell???? It is free "wasted" energy that would otherwise exit the exhaust pipe. Those with boost gauges please tell me the boost pressure with the transmission in neutral at a particular RPM versus a loaded engine at the same RPM. The boost pressure will not be the same....not even close!!!! And Stu, to answer your question versus a blow torch on a turbine wheel....if you were able to contain that heat in a confined chamber without burning everything, then, yes, you will get boost. But it takes a lot more than just aiming a blowtorch at a turbine wheel. I found this on youtube. I think it does a good job of explaining how a turbo works.
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Old 01-18-2016, 10:28 PM   #38
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Thanks Mr. Baker,

I know how they work and have had a couple on the workbench (30 years ago). It was the thermodynamics lesson that threw me for a loop. I know engines are 'heat' engines, and the combustion causes expansion and drives the piston down which is translated to the crankshaft through the connecting rod, etc...

What I was trying to say was it isn't the 'heat' that causes it to spin the exhaust side turbine. The heat causes 3-4 x expansion and that expansion of exhaust gases cause the turbine to spin.

It is like looking up at a windmill in the midwest on a windless day. Despite the sun shining on that windmill, it won't turn unless there is wind. it isn't HEAT that makes it turn. That would be a thermal pile.

Ok, I know that sun shines and causes irregular heating / warming of the earth's crust which causes high and low pressure air zones which tend to cause windmills to turn... ok, back to heat

Too late for me to keep this up guys!

Enjoy the lesson in thermodymanics without me.
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Old 01-18-2016, 10:35 PM   #39
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Thanks Mr. Baker,



What I was trying to say was it isn't the 'heat' that causes it to spin the exhaust side turbine. The heat causes 3-4 x expansion and that expansion of exhaust gases cause the turbine to spin.



Too late for me to keep this up guys!

Enjoy the lesson in thermodymanics without me.
What you are trying to say is exactly what I was trying to say....the HEAT causes the 3-4x expansion.....if that heat is not there, THE BOOST DOES NOT HAPPEN!!!!.....PERIOD!!!!! Your analogies(windmills and blowtorches) are not relevant!!!!...because the heat in your analogies are not contained and directed!!!! Heat is energy!!!! And it is THAT ENERGY that you are capturing in the exhaust gases to spin the turbine!!!!

Ps....your windmill analogy might be a little more relevant. Wind is caused by the uneven heating of the Earth's surface.
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Old 01-18-2016, 10:56 PM   #40
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I found this on youtube. I think it does a good job of explaining how a turbo works.
That's a great little demo, Baker.Thanks.
It shows an intercooler at the 2:50-3:00 mark and, going back to my original starting post, we can call that an aftercooler as well, right?

Learned a bunch more than I asked for in this thread.
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