Purpose of High rpm Low Torque Engines

[RonL]

Originally Posted by rxcomposite

It is typically called a "marine gear" because it has only two ratios. Forward and reverse. Gonzo just shortened it to gear. Some marine gears are available with 2 speed forward. Not all marine gears have 1:1 ratio for reverse.

"gearbox" is commonly used in the automotive world. It just get carried over to the marine world.

Thanks, I had noticed the intent of gonzo's comment, after my post was made.

You are generous.

[Yellowjacket]

Originally Posted by powerabout

Guessing, but I assume they cannot meet emission regs either..anyone add to that?

Turbines are exceptionally clean burning. The excess air that is present in the cycle provides very complete combustion. Essentially all of the fuel is burned and there are virtually no unburned hydrocarbons or carbon monoxide. High combustion temperatures can result in production of oxides of nitrogen, but modern lean burn combustion systems have made this an non-issue. A good gas turbine will have oxides of nitrogen emissions in single digits (below 10 ppm) and some are as low a 5 ppm.

Where diesels require after treatment (which is expensive and maintenance intensive) turbines need none of that. While many marine diesel engines are unrestricted in their emissions, a large diesel capable of meeting Tier III standards is going to need a urea system to reduce emissions, and that is expensive. A reciprocating engine running on diesel fuel is a smudge pot compared to a modern turbine.

[Submarine Tom]

Yellow,

While we're on the topic:

Why are turbines (exhaust) so stinky?

-Tom

[Yellowjacket]

Originally Posted by Submarine Tom

Yellow,

While we're on the topic:

Why are turbines (exhaust) so stinky?

-Tom

Turbines stink at idle if the fuel atomizaton isn't done properly, but most of it is a result of the fuel. Sulphur and other residual elements in the fuel cause a lot of the smell. As you power up, so long as the combustor is efficient they don't have much smell. Certainly less smell than a diesel running on the same fuel. Older engines (like JT-3's and J79's) are pretty dirty, and do emit smoke and stink to high heaven because of unburned hydrocarbons. When fuel got expensive in the mid to late 70's turbines started getting a lot cleaner.

Depending on the fuel, turbines also have higher emissions on startup. That is because you have to get a combustable mixture in the air stream before it lights off and combustion is poor as the engine is coming up to idle. That results in a good bit of unburned fuel going out the exhaust before it gets up to idle. That is also one reason that turbines fell out of favor with the auto companines. It was difficult to control startup emissions and that is where 90% of the exhaust emissions from a typical engine happen.

[Submarine Tom]

Well that makes a lot of sense as they (Single and Twin turbo-prop Otters) idle on taxi here all the way across the harbour and if you're down wind, pee-yew.

Thanks Yellow and OP for the hi-jack.

-Tom

[DCockey]

Originally Posted by Steve W

Ok Fred,i think got it, this stuff is great info for the mathematically challenged like myself. So,using 7.13lbs as the weight of a gal of diesel and using actual fuel consumption figures from a recent road trip from Minnesota to Virginia and back, on the way out with 2 people at a 70mph average we achieved 51mpg so 1.18gph,8.41lbs of fuel using about 23.4hp. Now on the return trip we were towing a 19ft beach cat, not real heavy,maybe 800 - 900lbs but significant windage. We achieved 30mpg through the mountains of west Virginia so 2gph, 14.26lbs/hr, using close to 40hp. interestingly we used slightly more fuel for the rest of the trip even though not many hills but a lot of cross and headwinds. Do these figures look about right?
Steve.

How did you determine you were "using about 23.4hp" and "using close to 40hp"?

[Steve W]

DCockey, I just worked backwards from FastFreds #71 post. 1.18gph @ 7.13lbs/gal = 8.41lbs which divided by 0.36lbs/hr/hp gave me 23.36hp, i dont know if this is correct,which is why i was asking. I thought this was a good example for me to try the math since we ran light in one direction and under load the other way.
Steve

[powerabout]

Originally Posted by Yellowjacket

Turbines are exceptionally clean burning. The excess air that is present in the cycle provides very complete combustion. Essentially all of the fuel is burned and there are virtually no unburned hydrocarbons or carbon monoxide. High combustion temperatures can result in production of oxides of nitrogen, but modern lean burn combustion systems have made this an non-issue. A good gas turbine will have oxides of nitrogen emissions in single digits (below 10 ppm) and some are as low a 5 ppm.

Where diesels require after treatment (which is expensive and maintenance intensive) turbines need none of that. While many marine diesel engines are unrestricted in their emissions, a large diesel capable of meeting Tier III standards is going to need a urea system to reduce emissions, and that is expensive. A reciprocating engine running on diesel fuel is a smudge pot compared to a modern turbine.

Thanks for the info.
Do any turbines in any use have to meet emissions and who controls that?

[Joakim]

Originally Posted by powerabout

Do any turbines in any use have to meet emissions and who controls that?

The same turbines are used in power plants, which have very strict emissions controls, at least in EU. They have to be measured for particles, NOx, SO2 etc. Modern turbines meet these without additional equipment like electrostatic precipitator (ESP) or flue gas desulfurization (FGD), which many other types of power plant need in order to meet the emissions control.

[gonzo]

Reformulated fuels also have a huge influence of how clean the emissions are.