Diesel Engine Fuel Consumption 3

[Singhnishesh]

Greetings everyone!

I am trying to work out the fuel consumption of a ferry that uses a diesel engine (Detroit Diesel 8V92Ta). I have attached the fuel consumption chart in this post. The idling rpm of the engine is around 200 rpm but the minimum rpm given in the fuel consumption chart is given to be 1200 rpm. The problem is the fuel consumption is provided for an rpm range from 1200-2000 rpm. What about values below 1200 rpm?

Thank you for your help

 

[enginesrus]

The idle RPM is 400 for some applications and 500 for others. Where did you get 200?

 

[Singhnishesh]

I found the data published in one of the reports published by Transport NSW. Considering the idle speed is 400-500 rpm, how do I find the Fuel usage at that rpm when the chart starts at 1200 rpm? Thanks

 

[TugboatEng]

It won't work regardless. The chart is fuel consumption at maximum continuous rating. Your ferry operates on the propeller curve which means the engine will be operating significantly below the maximum continuous rating at all speeds below maximum rated speed if your propellers are pitched correctly.

For an engine that size the idle fuel consumption should be around 2 gallons per hour.

 

[LittleInch]

If you're operating at 4 to 500 rom, your engine is far too big and will be very inefficient.

Why so slow?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Singhnishesh]

So, my problem statement is that I have the required power output as a function of time that the engine has to output, I have the corresponding RPM as well for the cycle.


I want to calculate the corresponding fuel consumption for that cycle. The engine in use is the Detroit Diesel Engine 8V92Ta (chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/

https://www.dieselrebuildkits.com/wp-content/uploads/2018/02/8v92-specs.pdf).

Any suggestions or help with this is greatly appreciated.

 

[LittleInch]

Your problem is unclear. If you have RPM, you have a chart of consumption so what's the issue?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Singhnishesh]

Thank you for your reply @LittleInch. The chart provided in the data sheet is only for range of 1300-2000 rpm. What about lower rpms? For example, the engine idles around 600 rpm. The chart does not cover the entire range of rpms that the engine operates in.

 

[LittleInch]

Tug has given you this information.

In any event consumption is determined by many factors and hence a chart can only give you a guide.

The engine should be operating in the range above 1200 rpm. That's a pretty decent sized engine so should be operated properly. Engines which are too big for their duty have poor economy, increased risk of coking and deposits and should be replaced if the power demand is too low.

Actual usage is the only way to get clear numbers.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BrianPetersen]

The answer to this question might help us help you: "Why do you need to know this?"

Related question, "Why do you need to figure this out on paper, rather than asking the actual operator what is really happening?"

 

[Singhnishesh]

The answer to your question Brian Peterson is because I conducted power requirement for an operating ferry. I aim on calculating the fuel consumption of the trip. The power calculations shows that at certain points the engine rpm dips below 1200 rpm. It is at those points where I cannot calculate the fuel usage.

Appreciate your efforts!

 

[3DDave]

The fuel consumption for the trip will be more affected by wind and water currents than what exact RPM the engine is running.

 

[TugboatEng]

If you make some assumptions you can make an estimate.

You can build a propeller curve:

power = (propeller constant) x (rpm³)

P=C(N³)

C= 435/2100³

Use this and the assume the BSFC number from your chart is correct even at the lower power for each RPM point. This will give you a ballpark number for fuel consumption. The numbers will be more accurate at loads above 30% or so as the BSFC curve for typical engines is very flat at higher loads.

 

[BrianPetersen]

... because I conducted power requirement for an operating ferry. I aim on calculating the fuel consumption of the trip.

And why not ask the operator how much they're using, instead of relying on a calculation that is sure to have significant uncertainty in it?

Obviously I'm missing something here. It's an existing engine in an existing application and actual field experience ought to be available, thus no longer requiring a calculation. What am I missing?

I can think of some reasons why it might not be available, but which reason it might be would require guessing on my part, and I don't want to guess, so I'm asking.

 

[gruntguru]

@TugboatEng. What is happening at 30% load on your chart where diesel BSFC drops to 50 g/kW.hr?

Whoops - just realised it is a trendline issue.

je suis charlie

 

[GregLocock]

The dangers of curve fitting! That's an overfitting error. In fact it's a dodgy graph, at 0% load by definition BSFC is infinite.

You get much better idea of the reason for the shape of that graph if you look at the curves of IMEP FMEP and PMEP vs load.

Cheers

Greg Locock


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