Boat/Engine performance on varying ambient temperatures

Discussion in 'Boat Design' started by J.N, Sep 21, 2017.

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J.NJunior Member

Hi all,

I'm a mechanical engineering student in Delft, the Netherlands. Currently I'm researching tugboats and their performance under varying ambient temperatures (& circumstances). I saw some interesting topics on this forum while researching for my thesis.

Right now I'm especially interested in theory and data regarding the performance of a ship's engine in different water & air temperatures. I've done a lot of research so far but can't find anything that provides me solid numbers.

For example: the engine has a power output of 2500 kW at 20 degrees celcius air temp (intake) and 15 degrees celcius water temp (considering HT/LT cooling system). What happens when the ship moves through hotter/colder climate?

I hope there's people here that can help me out or point me in a good direction. Thanks in advance.

Kind regards,

Jeroen

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philSweetSenior Member

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fredrosseUSACE Steam

Diesel engines are sensitive to ambient intake temperature, which impacts performance in two ways.
1. The higher temperature of air before compression also results in slightly higher temperature at the end of compression. This impacts thermodynamic efficiency by a small amount, depending on engine controls. This is a very small reduction of efficiency.
2. The higher temperature of intake air is at a lower density, therefore the mass flow of air is less thru the engine. This is a significant result, depending on the engine and its controls. For a simple naturally aspirated engine the reduction of performance is related to the intake density change. Most engines are rated at ISO conditions of 15C intake temperature, =288K. An increase of intake temperature of 10% (on the absolute temperature scale) = 28.8C higher inlet temperature, implies a 10% reduction of mass flow, hence about 10% less output power. There are several parameters that will influence this approximation, and most engine manufacturers can provide engine performance reduction with increasing intake temperature. High altitude also reduces intake density, but most marine engines operate at sea level (ha ha, an engineer joke)

Turbocharged engines will generally also be influenced in a similar way.

Intercooled engines, cooling the inlet air after the turbocharger and before entering the engine cylinders, will cool the intake air less with increasing intercooler coolant temperature. This will result in higher cylinder inlet temperature, thus less engine mass flow. The coolant for intercoolers can be ambient air, humidified ambient air, or sea water cooling, with most marine applications using sea water cooling.

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Mr EfficiencySenior Member

I recall an old locomotive driver telling me how summer heat would noticeably decrease the output of the turbocharged 4-stroke diesel engines, but on a cold winter night, you could feel the power kick in.

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SamSamSenior Member

I've got nothing much to add here as far as data for a thesis goes, but different air and water temperatures can end up in fog, and my 1960 Rambler never ran better than when it was cool and humid, if not foggy. It ran a whole lot smoother and seemed to have more power. I think it was acting like an octane booster and smoothing out the combustion burn.

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SamSamSenior Member

I wonder if the decrease in hot weather could have been rectified by running the turbo faster...?

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Mr EfficiencySenior Member

I don't know about the possibilities of doing that, I suspect the problem was that the engines were designed in the UK , and maybe not optimized for hot climates, in the way they were set-up. (English Electric, later General Electric (UK))

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J.NJunior Member

Thanks for your insight. For your "2." combined with the fact that marine engines are mostly turbocharged, in what direction do you think the calculation needs to go? I can't wrap my head around it yet.

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J.NJunior Member

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poweraboutSenior Member

When the air is less dense the turbo spins faster, a piece of physics aircraft take advantage of.
I'm not sure what you can do a thesis on this, its all well known and all diesels will come with tables to show the power loss/gain and tugboats AHTS's etc will have ratings on air density and water temp for bollard pull ratings.
PS Humid air is less dense, so less horsepower

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J.NJunior Member

Additional question to all; is there a way to calculate engine shaft power/torque without a meter on the shaft? By calculation on input/output temperatures or something?

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SamSamSenior Member

Not knowing much about them, I thought turbos were mechanically driven by the engine so at x engine rpm the turbo would have x rpm with no variation. I was suggesting that a variable speed mechanism be used so that at x engine rpm the turbo could be sped up or down to compensate for the varying air density.
Humid air is less dense, I guess because the "air" is displaced by water vapor.
WHY IS MOIST AIR LESS DENSE THAN DRY AIR AT SAME TEMPERATURE http://www.theweatherprediction.com/habyhints/260/
But it sure seems like there is more power on a cool foggy day. Perhaps it's because the water vapor gives a more controlled burn, utilizing more of the fuels potential energy. Perhaps it's because
and that introduces hydrogen into the mixture, like turning on the nitro bottles on a street rod.
I dunno.

Also, even though humid air is less dense, cool air is more dense, so maybe they tend to cancel each other out, but you still have a higher octane and hydrogen introduced into the process to give more power.
Again, I dunno.

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W9GFOSenior Member

Maybe on those foggy days you are getting benefits similar to water injection. It would cool the intake charge allowing for higher compression then turn to steam for some extra pressure in the power stroke. Some old aircraft would use water injection to increase power for takeoff.

Nitro bottles add oxygen, when the temp is high enough the oxygen is liberated. In water injection you aren't going to break apart the water molecules to benefit from the hydrogen and oxygen contained within. Breaking apart water molecules takes a lot of energy.

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keroseneSenior Member

SamSam time to read some very basic piece on turbos. They are a combination of a turbine (powered by exhaust) and compressor on same axis.

My dad used to work in high altitude in Ethiopia and the power on behicles was significantly down. Turbo cars suffered much less. Not sure if its a feature of turbos or maybe turbo cars that time had better metering for varied pressures. (Late 80s to early 90s)

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Mr EfficiencySenior Member

Cold and moist air probably helps. The old WW2 fighter planes had water injection, which presumably turned to steam, but if they overdid the water injection ( only recommended for short periods of a few seconds), the engines lost power. It was for emergency use.

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