conversion of 8V71 Turbo to NA

[ssreese1500]

I have a set of Detroit 8V71 Turbo Engines that run at a max of 50% of capacity for most of their lives. This is an electrical generating application.

Obviously, the carbon build up eats the turbo seals long before they should, requiring a lot of maintenance.

Have any of you done the conversion of removing the turbo with out changing out all the cylinder kits? The non-turbo pistons have a 19:1 compression ratio versus 17:1 for the turbo engines.

I was thinking I could reduce the size of the injectors to a minimum and advance the injector timing maybe 1 to 2 degrees and make this work without the expense of a whole new set of cylinder kits and not increase my gallons per hour significantly.

thoughts? suggestions? tell me I am an idiot?

 

[catserveng]

Yeah, it's possible, we used to do those kinds of reworks in our used equipment department. You will not get the non-turbo horsepower rating with the lower compression pistons. And you will lose some fuel economy, probably around 5-7% based on my experience. Advancing the timing can help a little.

Is this a certified vessel? Sometimes USCG or ABS get a little upset if you change the rating of an installed piece of equipment like a ships set.

Depending on your applcation and home port, you may want to see if there are any program funds to repower you gen sets with EPA compliant engines. In CA Carl Moyer funds were used on a number of vessels, and I have heard there are federal funds available in some areas.

Hope that helps.

 

[Bribyk]

This may be dumb but, could you not just run one generator package at 100% load at a time? You could swap to the other at oil change or service intervals and you'd have a backup plus reduced fuel consumption.

 

[ssreese1500]

sorry, I wasnt very clear on that point. I only run one generator at a time, and still only get 50% load on that one.

 

[TDIMeister]

If maximizing fuel efficiency is your prerogative, the best course given the engines you have is to study the RPM/BMEP/BSFC map if you can obtain one and see where you're operating on the map.

Going to NA from an originally turbocharged engine is generally disadvantageous thermodynamically, and you may find your fuel savings to be benign if not actually negative.

Heywood's The Two-Stroke Cycle Engine: Its Development, Operation, and Design would be a good read for any ideas that you could get. If you're operating near the best-point BSFC at rated load, and you're operating strictly at 50% at the same RPM, one possibility is just disabling one-bank of cylinders and all of its ancilliaries. Is it a single turbo? If so, matching will be affected...

Diesel engine efficiency is not as strong a function of pumping losses like in SI engines due to intake throttling. It's mostly due to friction losses as well as, in the case of the 2-stroke Diesels, parasitic losses from driving the scavenging blower (that you cannot remove completely, but maybe you can try to underdrive it somewhat). Short of going through all the friction and parasitic losses, disabling one bank seems to me to be the most sensible idea, or leaving the engine completely alone. I'm sure you don't assume to cut your fuel consumption by any more that a very small percent no matter what you do to a given engine. For a given load, the BSFC will be decisive, and you will be lucky to come up with a few percentage points at the same load-point short of some heavy re-engineering.

 

[IceStationZebra]

My original thought was you should switch to a 6v71 or maybe even a 4-71, but figured that you probably wouldn't see the payback. The 6 would probably be easier to retrofit than the 4. And if this is on a boat the 6 will have less vibration. Since this is for a generator I assume that all you need is a standard SAE bellhousing which should be available for either one.

ISZ

 

[greenblur]

you'll be fine & probably better off with 17:1.just use injectors and injector height setting for the na motor.you'll use less fuel & lower the thermal & mechanical load on the engine without the turbo.you'll also increase reliability/lower maintence.The na motor is alot more reliable.I love them 2-stroke detroits but since it's a 2-stroke it naturally has a higher thermal load on the pistons,rings & valves then a 4 stroke.so the weak link tends to be the rings on them detroits and the thermal load
from turbocharging really make that weak link weaker.Mostly ever turbocharged 6-71/6v92/8v71/8v92 i rebuilt was just because the rings cracked.

 

[TDIMeister]

Is thermal load an issue when the engine never runs over 50%?

 

[greenblur]

Thermal loading is not the issue in this case but
Any time you lower heat/thermal load you lower wear/maintence & increase reliablity.

even at 20% load the turbo motor has higher thermal load compared to the na motor

 

[TDIMeister]

I completely agree with you. What was not completely clear from the original posted was the main reason he wanted to do this.

On the one hand he mentioned the maintenance issue of carbon buildup on the turbo seals, and on the other hand gallons per hour was mentioned.

On the issue of maintenance, it's a relative no-brainer: the fewer parts there are, the less opportunities for something to go wrong or require maintenance. However, as I advised, abnormal carbon buildup could be symptomatic of other problems, and attention should be paid to oil consumption and black/blue smoke out the exhaust, and lube oil selection could be influential as well.

On the issue of fuel economy, removing the turbocharger is usually thermodynamically disadvantageous. One would be mistaken to think that removing the turbo for operating at part-load will be better for BSFC without exception. At very low-load, this is true, but almost everywhere else having a turbocharger improves BSFC. More could possibly be gained by systematically reducing the source of frictional- and pumping losses in the engine, and in a 2-stroke Diesel, primary focus should be upon the scavenging blower.

 

[greenblur]

I think his main concern was the carbon build up
The lower exhaust temp & slower turbo speed allows carbon to build up.I used to do marine diesel engine repair and one boat i use to work on had that problem.The boat used to have four 700hp 8v92's but they replaced them with four 800hp Duetz v12's and with the extra torque/hp there wasn't much load on the motors.

 

[ssreese1500]

Thanks for all the valuable info. The reason that I wanted to do this is because the applications for the engines now dont require nearly the kw that the system was designed for.

I would even doubt that with the low loads that the engine is producing enough heat in the exhaust to even make having a turbo worthwhile. My experience with larger turbocharged engines has shown that running them for long periods of time with significantly reduced load will cause failures in the seals due to carbon build up. I figured if I dont need the turbo, what is the cheapest way to eliminate it without seeing a drastic reduction in fuel economy?

Of course I could change out to a smaller engine, but that is a cost i did not want to incur, just as i did not want to incur the cost of changing out all 8 cylinder kits. The posts about the smaller size and reduced stroke of the injectors coupled with a slight timing advance is what we had figured on having to do, but it is good to have the confirmation from you guys that have done it. Thanks for all your help, if we make the conversion I will let you know how it goes.

 

[jomor]

Seems that you are wet stacking due to too much fuel at low load. The easiest/cheapest thing to do may be to put smaller injectors in. Detroit Diesel has a lot of info on sizes and ratings etc. You don't appear to be urgent so just a trial on one may work for you.
Good luck and let us know how it turns out.

 

[greenblur]

ssreese if you need some info on 2-stroke detroits there's a shop in lakewood,new jersey called jersey shore diesel(732-961-1880) They do marine diesel & some heavy duty trucks.most of there marine work is on 2-stroke detroits it's there specialty.They also have have detroit parts in stock.

i'm not sure if your worried about fuel economy if so you shouldn't be cause i think you'll see a nice improvement in fuel economy

 

[rmw]

Do you have a boost gage on the generator set(s)? Your answer may be there. If not you may want to add one.

If your turbochargers are making any contribution at all, it will be reflected in your boost pressure readings.

I suspect, having operated both the NA 8V71 and the 8V92TTA Detriot engine on automotive equipment that at your generation loads you have precious little if any boost due to your turbochargers. I can remember that at low power (but full speed) situations, the turbos didn't produce much boost and I had to be careful when I added fuel to do it slowly until about 5 PSI boost was reached and then I could mash on the foot feed. Anything before 5 PSIg just produced smoke (your problem.)

On the other hand, with the 8V71's, any fuel added belwo 1800 rpm (goverened up to 21-2200 rpm) would just make them smoke too, but I had the largest injectors I could in it.

If I am wrong, then you have a hard decision because they are contributing accordingly to your BSFC (that is to say the lowering thereof.)

If I am right, you might find that your turbos are doing not much other than contributing to your maintenance problems. You have an engine set up to be able to put a lot if fuel in thinking that it will be at full power and hence full boost, but you aren't operating there.

If you change, the comments above regarding changing timing and injectors is important to consider.

rmw