fuel consumption of pump coupled to dieselmotor 1

[26april50]

Hi,
I like to consult you about the following problem.

An irrigation pump in Mozambique is directly coupled to a dieselmotor LISTER TR2 (1500 rpm). In the Operation point of the pump (Q=60 l/s; Hman= 7 m, pumpefficiency = 70%; 1500 rpm)) I calculated the necessary power of the motor as 7.7 kW (included 20% extra power for continuous operation). However, according to the power curve of the motor (Power output-rpm) the motor delivers 11 kW at 1500 rpm. So, in the operation point the motor is somewhat oversized.

How can I correctly estimate the diesel consumption per hour of this motorpump?
As: Specific dieselconsumption*7.7 kW
or as: Specific dieselconsumption* 11 kW?

in which: specific dieselconsumption = 0.3 litres/kW/hour

Beforehand, thank you for your answer

Adrian van den Dries
irrigation engineer

 

[TD2K]

The 11 kW capacity of the diesel should be thought of as the maximum power output it can deliver. If the load (your pump) only needs 7.7 kW, that's what it will deliver and consume fuel for.

It's similar to your car. While it might have a 100 kW engine, when you are idling at a stop light, the engine isn't still delivering 100 kW.

Granted, at partial load the specific fuel consumption might vary from the value you have at full load but it should be a reasonable starting point.

I'm not sure why you are adding 20% for 'continuous operation', that doesn't seem to make sense to me.

On the other hand, I might be all wet in which case someone here is sure to correct me.

 

[26april50]

Hi TD2K
I suppose that the additional 20% for "continuous operation" is no more than a safety factor. It is recommended in all texts on pump and motor design.


"While it might have a 100 kW engine, when you are idling at a stop light, the engine isn't still delivering 100 kW".

That's true, when it is idling it gives hardly power at all but it still will consume fuel. How much? And how do you calculate that?

 

[quark]

BKW and IKW of an engine can't be calculated w.r.to load but experimentally derived by a brake dynamometer(incase of diesel engines). The engine manufacturer can give you this data.

Continuous operation doesn't make sense to me either. In the obsense of 1.2 FOS, the pump bkw is 5.89 kW and this is about 55% of loading. I would rather consider the specific fuel consumption any where between 0.4 to 0.5. The best SFC at 100% load is, generally, 0.26 to 0.3 lit/(kW-hr).

Your fuel consumption should always be based on load as suggested by TD2K.

 

[Artisi]

If you have a full engine performance curve for this engine Lister TR2 (fairly old unit by now)it normally shows a number of power/speed curves (maybe 2 or 3). When driving a centrifugal pump you HAVE to use the continuous power curve which will the the lower curve drawn. There is no need to add 20% to your pump power calculation.

As for the amount of fuel used it can be calculated from the curve which gives you the lbs(of fuel)/hp(being used)/hr(of operation). From memory (now a bit hazy) it is probably about 0.35 - 0.4lbs / hp / hr. Of course this is more than likely changed to metric terms - so I leave it you to convert.

If you have the full engine curve and can scan and post it somewhere on the net - and I can access it I can calculate the fuel consumption for you.

Naresuan University
Phitsanulok
Thailand

 

[26april50]

Thank you very much for the messages and comments.

If I understand well your comments, I could resume the conclusion as follows:

hourly diesel consumption = specific diesel consumption* actual motor load (5.89 KW)

but the specific diesel consumption (liter/kW/hour) is higher (estimated 0.4 to 0.5 instead of 0.3) with partial load (5.89 kW) than at full load (11 kW).

Adrian van den Dries
Irrigation engineer

 

[waross]

There are a great many interacting variables in fuel consumption. Fuel consumption is generally determined experimentally by the manufacturer.
Use the published curves.

Plus 20%.
When you are sizing a motor for a pump or a generator as well as some other loads, it is normal to add a 20% safety factor. Part of the reason for this is that the engine power output may decrease due to aging, altitude and/or poor fuel. In the case of reduced power output the oversized engine will be able to use the 20% reserve before it starts losing speed.

Manufacturers power curves:
The energy of diesel fuel is closely related to the specific gravity of the fuel.
Read the fine print on the manufacturers fuel consumption curves to ensure that the fuel used for testing is a reasonable specific gravity.
Some years ago I was comparing the fuel consumption of various diesel generators with various power plants.
most of the engines had very comparable fuel consumption at comparable loading. One engine manufacturer was head and shoulders above the others for fuel economy.
Then I read the fine print. They were testing with fuel of greater than normal specific gravity. When the fuel consumption was corrected for specific gravity, the fuel consumption was comparable with the other engines.
Moral, try to get an appreciationm of the specific gravity of the fuel available locally, and adjust the manufacturers fuel consumption charts accordingly.
Adjusting for specific gravity.
It is my understanding that if you convert gallons or liters to lbs. or kilograms (mass) at the specific gravity on the published curves, and then convert the mass back to gallons or liters at the specific gravity of the fuel available, you will have an accurate consumption figure with the actual fuel used.
respectfully

 

[Artisi]

I cannot see any good reason for adding an additional 20% to the calculated power required for the pump. If you are using the engine continous rating curve, this rates the engine for continuous use 24 hrs / day and already allows for overload from short durations.
Would you add 20% to a 1000 kw engine - I would hardly think so as it could easily put you into a far more expensive engine configuration or even into a different engine.

One of the worst things you can do to a diesel engine is to run it at a constant speed at low load - this causes glazing of the piston rings and cylinder bores leading to increased lubricating oil consumption. It is far better to operate diesel engine right on the limit of it's continuous rating than on a light load.

Plus if you are sizing an engine for operation and other than sea level or in a high ambient temperature or extememly dusty conditions etc - you should be guided by the manufacturer or the published data for that particular engine - don't guess.

And sure, the specific gravity of the fuel and the quality of the fuel governs the fuel consumption.

Naresuan University
Phitsanulok
Thailand

 

[Artisi]

To clarify my comment re adding 20% safety margin-, after establishing the power needed for the pump at its duty point and a review of the whole pump / engine system including for any upset changes to the power requirement, there is no need to add any further safety margin. Bear in mind that with a diesel drive you have the benefit of speed variation allowing you to balance engine speed with the required pump output.

Naresuan University
Phitsanulok
Thailand

 

[BigInch]

The 20% is for tank sizing.