Fuel flow measurement on small diesel engines

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rogerthebodger

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I know there has been some discussion on measuring fuel flow to small mechanical injection diesel engines in the past and there have revolved around the small paddle wheel flow meters that give pulse rates dependent on flow but tend to be very inaccurate or will not register at very low flow rates.

I tried out the RS one but could not get it to work with my main engine let alone my farymann diesel generator.

Looking around for a differential vacuum gauge for my secondary fuel filter, I came across a differential pressure chip. Over kill for a fuel filter but thinking back to my fluid mechanics the use of a venturi driving one of there chips could give the accuracy needed at very flow rates. If this was linked to a micro or PIC it could drive some display or give some message on a serial line (RS232/422)

This is the chip I looked at

http://www.ebay.com/itm/Breakout-Bo...589?pt=LH_DefaultDomain_0&hash=item485919d195

This one is -2 to + 2 Kpa -03 to + 0.3 psi.

My engine has a maximum HP is about 63hp which would have an approximate max fuel consumption of 13-14 litres/hour (0.0038 litres/second)

back of the *** packet calculations I think a venturi 6mm major dia with a throat dia of 2 mm with diesel density at 900 Kg/m3 would give a differential pressure of 2 Kpa with a flow rate of 0.0065 lit/s (23.4 litres / hour)

Of cause we would need 2 for each engine for flow and return and the flow rates for high return engines wound be larger, mine has a very low rate of return and my farymann has non even from the injector as there is no pipe on the injector top bleed connection.


Any one got any comments on my theory and does the collective wisdom think this would work.
 
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I have no idea but I would be interested in your results as I am planning to develop an engine monitoring system and have been looking at the flow meters with the same issues you have had. It may well be that I need to rely on an assumption based on revs and average fuel usage to get the results I need. It is a pity
 
PCUK said:
Mecdine made one. If you can find one they were very accurate and not expensive.
Click to expand...

Do you have any details as they don't seem to have a web site. I have only found a physical address and telephone number and it not like I can just pop round to see then unlike when I used to live in the UK
 
I'm not sure how well the pressure sensor would work. I imagine not very well, since the fuel flow is not a linear flow, but a pulsating one and may at times even reverse a small bit.

The only accurate[1] one I've seen was the Navman DIESEL 3200, which used positive displacement flow meters[2] in supply and return lines. These enclose the fuel passing through into "parcels" of a fixed and known size, then count the number of parcels going through each meter. That way, it cannot get confused by pulsating or reverse flows.

[1] It was accurate when it worked. Sadly it is now broken and no longer made and there doesn't seem to be any equivalent.
[2] https://en.wikipedia.org/wiki/Positive_displacement_meter

Edit: Found a picture from when I took one of them apart:
IMG_0993.JPG
 
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Off the cuff I would say that air flow is directly related to the RPM. However, the fuel consumption is related to the load on the engine, so I don't think that the consumption at, say, 2500RPM would be the same when the engine is under load (head wind, tide flow, etc.) as when motoring at the same RPM in flat water.
 
PuffTheMagicDragon said:
Off the cuff I would say that air flow is directly related to the RPM. However, the fuel consumption is related to the load on the engine, so I don't think that the consumption at, say, 2500RPM would be the same when the engine is under load (head wind, tide flow, etc.) as when motoring at the same RPM in flat water.
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I don't think your engine is in the slightest bit interested in the tide.
 
PuffTheMagicDragon said:
No, mine wouldn't be, although there are places not very far from here where the current can be pretty impressive. Yours would be if your boat is struggling against the flow.
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As I said, your boat is not interested in what the tide is doing. It's speed will be slower over the ground, but it will have no effect on your engine.
 
Forget boats.
This thing started with the relationship between air flow and fuel flow (as in consumption). I said that air flow is related to engine RPM. I don't think that keeping an engine running at a set RPM is independent of the load on that engine; I would imagine that an engine that is pulling a load while running at 'x' RPM would consume more fuel if pulling twice that load and keeping the same 'x' RPM.
 
PuffTheMagicDragon said:
Forget boats.
This thing started with the relationship between air flow and fuel flow (as in consumption). I said that air flow is related to engine RPM. I don't think that keeping an engine running at a set RPM is independent of the load on that engine; I would imagine that an engine that is pulling a load while running at 'x' RPM would consume more fuel if pulling twice that load and keeping the same 'x' RPM.
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Yes, but it was you who said,

"Off the cuff I would say that air flow is directly related to the RPM. However, the fuel consumption is related to the load on the engine, so I don't think that the consumption at, say, 2500RPM would be the same when the engine is under load (head wind, tide flow, etc.) as when motoring at the same RPM in flat water.
Read more at http://www.ybw.com/forums/showthrea...t-on-small-diesel-engines#fseJLdjVL3MsUYfi.99
 
ghostlymoron said:
How is it measured on cars?
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I believe on modern fuel injected cars the fuel flow is directly proportional to the number of pulses and duration opf the pulses of current to the injectors. So fairly easily produced by the computer. hence many cars will give you a readout of fule flow in my case litres per hour when idling and litres per hunderd kms when moving. My problem is the latter is always too high. (car drinks like a fish 4l heavy body lots of cold short distance driving not so bad at 100km/h) olewill
 
Yngmar said:
I'm not sure how well the pressure sensor would work. I imagine not very well, since the fuel flow is not a linear flow, but a pulsating one and may at times even reverse a small bit.

The only accurate[1] one I've seen was the Navman DIESEL 3200, which used positive displacement flow meters[2] in supply and return lines. These enclose the fuel passing through into "parcels" of a fixed and known size, then count the number of parcels going through each meter. That way, it cannot get confused by pulsating or reverse flows.

[1] It was accurate when it worked. Sadly it is now broken and no longer made and there doesn't seem to be any equivalent.
[2] https://en.wikipedia.org/wiki/Positive_displacement_meter

Edit: Found a picture from when I took one of them apart:
IMG_0993.JPG
Click to expand...

I had one of those fitted and I still have it lying around bur they only work on high power diesels as used in inboard diesel power boats. would not work on my 63 hp main inboard engine.
 
William_H said:
I believe on modern fuel injected cars the fuel flow is directly proportional to the number of pulses and duration opf the pulses of current to the injectors. So fairly easily produced by the computer. hence many cars will give you a readout of fule flow in my case litres per hour when idling and litres per hunderd kms when moving. My problem is the latter is always too high. (car drinks like a fish 4l heavy body lots of cold short distance driving not so bad at 100km/h) olewill
Click to expand...

You are correct its the same with my common rail diesel in my Toyota 4x4 it has a consumption meter on the dash which takes data from the electronic diesel injection system.

Regarding the relationship of fuel flow to air flow. A constant speed diesel generator will tend to have a constant air flow as the speed there for the air sucked in at each cycle would be the same but as the load increases from the alternator the governor moves the injector pump rack to allow more fuel in to maintain the set speed. So in this case the fuel flow to air intake flow cannot be constant.
 
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