thermal efficiency

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thermal efficiency

Postby pem » Thu Feb 09, 2012 12:30 pm

Does anybody have thermodynamic explanation for a better efficiency with a lower scavenge air temperature in slow speed engine. Theorical thermal efficiency of diesel cycle is only a function of the compression ratio and of the maximum cylinder pressure. What about the relationship between air inlet temperatue and specific fuel consumption on real working cycle.
What is the right scavenge temperature at low load, partial load and full load.

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Re: thermal efficiency

Postby Madzng » Fri Feb 10, 2012 1:02 am

Hi Pem,

The colder the air the more dense it becomes, resulting in a higher scavenge, PCOMP and PMAX pressures.

A rule of thumb figure is a 10 degree (C) increase in scavenge temperature will result in a 1 - 1.5 g/KWhr SFOC increase.

The lower the air temperature the better for normal operation, a lower air temperature results in a higher density of air, as mentioned above but it also results in a lower quantity of moisture being carried into the cylinder. Moisture in the air (and fuel) will take engergy away from the engine as the mositure/water is converted to steam.

At low load there is a lot more problems with cold corrosion where the acidic products of combustion will condense on the cool (relative) liner walls and destroy the liner hard phase leaving only the softer materials behind.

This cold corrosion results in a higher wear rate and can, in some cases, turn a liner which is expected to last 80 to 100 000 hours into scrap after only 10 000 hours.

Generally at low load you need to worry more about cold corrosion than SFOC, and in real terms at low load there is not so much fuel to save. At normal loads a lower temperature wll result in better SFOC, and at high loads the lower the better but care must be taken not to exceed the limits on PMAX.

It can be very difficult to control the scavenge air temperature, depending how well your engine has been designed and maintained. Fouling of coolers, CW temperature settings etc,.. turbocharger air inlet and also the air outlet temp can all have a far bigger influences on scavenage air temperature than the engineers operating the plant on most ships.

On vessels using the waste heat in an economiser for steam production, the lower scavenge temperaures will result in a lower exhaust energy being available for steam production. Careful consideration is needed to decide where the greatest fuel saving is. Is a higher SFOC on the engine more cost efficient than running the oil fired boiler to to top steam production etc....

The attached file should give you more/better info.
Ambient temperature conditions on Main Engine Operation.pdf
(248.86 KiB) Downloaded 943 times

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Big Pete
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Re: thermal efficiency

Postby Big Pete » Fri Feb 10, 2012 4:07 am

Good info from Madzng, he has covered most things,

The Carnot cycle, (Google it) is the most efficient thermodynamic cycle, and all other theoretical and practical cycles are compared to this.
The Carnot efficiency is based on the difference between the air temperature at the engine inlet and exhaust divided by the exhaust temperature. (T4 -T1)/T4.
The turbocharger and air cooler are all part of the engine cycle, so you should take the air temperature at the TC inlet not after the air cooler.
Therefore, Mathematically, to acheve 100% efficiency the air inlet temperature would have to zero absolute, (MINUS 273 Celsius)

So with a typical Engine Room air temperature of about 30 C and a maximum exhaust temperature of about 430C due to materials limitations, (say 300K & 700 K respectively)
The maximum theoretical efficiency works out about ((700-300)/700) or 57%

JK should be able to do a bit better with her ships in the frozen North. Though she may find that the high density of the air overloads the turbochargers. Obviously the rest of us can not cool the air inlet temperature, running a giant A/C plant to cool the air intake wont do anything for our overall efficiency! Although warships always have fresh air supplies ducted from outside the ER directly into boiler and engine intakes, as part of their NBCD measures, but the presure drop in the ducting probably balances out any gain from the reduced temperatures.
We will just have to wait for better materials that will allow higher exhaust gas temperatures, and hope to get some of the waste heat out of the exhaust gases after the turbocharger.

I have just sailed on a ship with a B & W slow speed engine, when I joined a service Engineer was on board to inspect the engine, he pronounced that all 6 pistons and liners would have to be replaced due to scuffing damage and liner wear. He put this down to running the engine at too low a charge air temperature. Investigation into the History of the ship showed that the sea water tubes of the charge air cooler had been heavily choked and the ship had been running with high charge air temperatures without any problems. In dry dock a new charge air cooler was fitted and afterwards the charge air temperature was run at 40C, running in the Gulf and South China sea, with high humidity, the condensation was washing off the cylinder lubrication, within a few months the Engineers started to notice problems with broken piston rings, and within a year the engine was wrecked. The owners were not amused.
Most Engine makers warn that charge air temperature should be maintained sufficiently high to avoid condensation in the charge air manifold, in Tropical conditions that can mean 50C or higher.

I agree with Mazdng that at full load the charge air temperature should be kept to the minimum permitted by the engine builder, but be aware that excessively cold air temperatures increase thermal stresses inside the engine and can cause cracking of cylinder heads, liners and pistons. Be aware of the dangers he and I have mentioned.
At part load, efficiency can be increased by raising the the charge air temperature. This will raise the exhaust temperature and therefore the ratio between the air inlet and exhaust temperatures. It can also help improve combustion and improve the efficiency of the conversion of the potential chemical energy in the fuel into heat energy. It will also make more energy available for recovery in the economiser, if fitted.

I am not sure about lowering the charge air temperature to reduce the amount of water going into the engine, water vapour in the air will not absorb any latent heat of evaporation because it is already a vapour. Cooling the charge air may cause some vapour in the air to condense into water droplets which will then absorb heat in the engine when they evaporate, as well as destroying the cylinder lubrication and causing cold corrosion. Unfortunatly, efficient co alescing filters to remove all the water droplets from the air would cause a pressure drop in the charge air and reduce the efficiency. The only solution to all three problems is to raise the charge air temperature.
Charge air temperature control is a big problem, on older ships with Sea Water cooled air coolers. Modern Medium speed Engines tend to have 2 stage coolers, using Jacket water to preheat the air at low load and as a first stage cooler at high load and LT water for second stage cooling at high load. I am not sure if the latest slow speed engines do this, but it is definitly the way to go, precise control of the optimum temperature throughout the power range. I suppose you could even integrate charge air humidity control, to keep the charge air above its Dew Point.

One of the problems with Sea Water cooling is that throttling the SW flow will raise the mean scavenge air temperature, but some of the air flow is cooled almost to the SW inlet temperature causing condensation even if the mean or average temperature is above the Dew Point. It is better to raise the mean scavenge temperature by recirculating the SW to raise its temperature rather than throttling the SW outlet valve or by passing the cooler.

It is always better to ask a stupid question than to do a stupid thing.

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The Dieselduck
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Re: thermal efficiency

Postby The Dieselduck » Fri Feb 10, 2012 12:34 pm

jazzus, if that not a good enough answer, I don't know what is...

Thanks Madzng and Big Pete, excellent learning for me.
Martin Leduc
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Martin's Marine Engineering Page

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Re: thermal efficiency

Postby JK » Fri Feb 10, 2012 1:09 pm

The cool thing about the Arctic (haha) is the lack of humidity, so condensation is not a problem in the turbo.
Now overloading because of the cold is a problem and why the turbos should draw directly from the spaces not from the outside.
If you draw directly from the outside, the cold air passes through the turbo and hits the charge air heater ( not cooler) in this case, the compressed air expands and causes the turbo to "flutter" alarmingly. Give the cold air a 5 deck drop through trunking to the turbo inlet and the trunking does interesting things when the turbo "flutters"

Yes, thank you Madzing and Big Pete for the educational posts.

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Re: thermal efficiency

Postby offshoresnipe » Thu Mar 08, 2012 6:08 pm

Thanks Big Pete and Madzng, boy keep it coming.

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