Inconsistent flow rate measurements of an engine diesel fuel

[AEW]

TL;DR Summary

Inconsistent volumetric flow rate measurements of an engine diesel fuel due to change of the fluid temperature between supply and return fuel lines.

I am measuring the volumetric flow rate of a diesel fuel using "Turbines" flow meter, both in the supply and return fuel lines of an industrial diesel engine (https://www.cat.com/en_US/products/new/power-systems/industrial/industrial-diesel-engines/18398034.html – coupled with a generator for power production), in an attempt to measure the total fuel consumption by the engine. However, the fuel temperature in the return line is higher than the fuel temperature in the supply line, thus, the viscosity and density of the return fuel are lower than that of the supply fuel. Due to this change in fuel viscosity/density, I obtain erroneous flow rate measurements; sometimes negative fuel consumption!

Therefore, I was wondering if there is a way to compensate for the temperature/viscosity/density change of the diesel fuel in order to obtain consistent and accurate flow rate measurements.

Thank you for any idea you could provide.

 

[jrmichler]

Some thoughts:
1) Are you using the correct model flowmeters? Their smallest flowmeter is rated 0.3 to 3 GPM. The maximum fuel consumption of that engine is about 1.5 GPM, so the maximum flow in the supply line will be that much plus the amount of return flow. Flowmeters, especially turbine meters, are most accurate when near maximum rated flow.

2) Is the minimum flow rate within the specified range of your flowmeter? The spec sheet states +/- 1.0% standard accuracy. A typical specification with that wording really means that the accuracy is +/- 1.0% of full scale, so a meter operating at minimum flow rate of 10% of full flow can have error 10% of actual flow and still be within specification. If the actual flow is lower than the minimum flow rate, the flowmeter will still give a reading, but the accuracy and repeatability will be such that the reading will be useless.

3) The linked flowmeter spec sheet does not mention viscosity or specific gravity. Have you had the flowmeter manufacturer confirm that their meter is accurate over the full flow range with your liquid? You know the range of temperatures in your fuel, therefore you also know the range of fuel viscosities. What is the variation of the K-factor over that viscosity range? The manufacturer should know. If not, you will need to do your own calibration tests.

Therefore, I was wondering if there is a way to compensate for the temperature/viscosity/density change of the diesel fuel in order to obtain consistent and accurate flow rate measurements.

Yes, there is. You need to do calibration tests over the full range of temperature / viscosity / density. Then add a calibration constant to the reported fuel flow rate.

 

[berkeman]

Summary: Inconsistent volumetric flow rate measurements of an engine diesel fuel due to change of the fluid temperature between supply and return fuel lines.

both in the supply and return fuel lines of an industrial diesel engine

Just curious (and sorry if too far off-topic), but why would there be a fuel return line from an engine? Is that common in industrial diesel engines?

 

[AEW]

Yes, this is common. The idea is to measure the fuel consumption by subtracting the return fuel flow rate from the supply fuel flow rate, in order to monitor the fuel consumption in general.

 

[berkeman]

Interesting. Why does some of the fuel bypass combustion?

 

[AEW]

Sorry, but I'm not sure I understand your question.

 

[Dullard]

berkeman:
In most modern fuel systems, the fuel pump is shunt-regulated. A pressure regulator at the fuel rail 'returns' whatever fuel is required to maintain the required pressure. The capacity of the pump (at least) slightly exceeds the maximum required fuel flow - there is always some 'dumped' fuel. A 'constant' fuel pressure makes fuel injectors a lot easier to implement.

 

[berkeman]

In most modern fuel systems, the fuel pump is shunt-regulated.

Huh, thanks. I learn something new every day here at PF! My MX bikes were pre-fuel-injection, so I never would have seen two tubes from the engine to the tank.

https://i.pinimg.com/originals/5e/3f/7a/5e3f7a60e652e6387e9024db9fcb03e5.jpg

 

[Dullard]

Glad to help. I suspect that you could teach me a lot more that I can teach you...

 

[Dullard]

BTW:

My Fz-1 has 4x 37mm Mikuni carbs. I like FI, but...

 

[Averagesupernova]

Returning fuel to the tank also warms the fuel up. This helps prevent the fuel from gelling up in cold weather. One downside is the fuel filter(s) likely need to be serviced more often, but of course the upside is that the tank stays cleaner.
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If I'm not mistaken, fuel is now typically taken off of the very bottom of the tank so everything including water is drawn into the fuel system and taken care of. When the water in fuel indicator comes on you can be sure that the last place you filled the tank was the source of the water. Nothing foreign sloshes around on the bottom of the tank very long.
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I'll do a little digging later to provide a link to a fairly good training video for the 7.3 litre international fuel system on the diesels used in Ford pickup trucks. Later engines are very similar.

 

[Baluncore]

Just curious (and sorry if too far off-topic), but why would there be a fuel return line from an engine? Is that common in industrial diesel engines?

Air in the fuel system is self-bleeding if the top of the injection pump inlet reservoir, mechanical fuel injectors, and the ullage at the top of the fuel filter, are connected through a line back to the fuel tank.

The piston clearance in a mechanical fuel injector allows some bleed of fuel into the spring chamber, that will eventually block the injection action, if not vented through the return line to the tank.

Due to this change in fuel viscosity/density, I obtain erroneous flow rate measurements; sometimes negative fuel consumption!

Numerically subtracting the fuel return-line flow from the fuel flow will generate noise in the data due to high return flow with low fuel consumption. It is also possible that air in the lines will upset the fuel volume measurements.

 

[Averagesupernova]

The videos are specific to the 7.3 but modern diesel engines follow the same basic scheme. The second video is about the oil system but don't mistake it for just an education on lubing the engine. These diesels use high pressure oil to activate the injectors.

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