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Old 12-04-2020, 11:38   #1
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Asymmetric Fuel Burn - Volvo D2-55F

My cat has a pair of D2-55F Volvo diesels. Port is original 2009 with about 2,810 hours, Starboard is newer 2014 with about 990 hours. Each side is a standalone system with its own fuel tank and return line to that tank.

We frequently run one engine at a time, and if I need to charge of course run just one. I've been running the engines mostly by the fuel gauges - keeping an equal amount of fuel on each side. I know the gauges are basically correct, because we've been putting an equal amount of fuel into each tank when we refuel.

Now 10 months later, on essentially equal amounts of fuel (give or take 5-10 gallons AT MOST), the port engine has run 230 hours and the starboard engine has run 200 hours.

As a broad average we burn about 1 gallon an hour across idling, motoring, charging, etc., etc. So starboard is consistently burning roughly 10% more fuel.

Exhausts looks identical - no sign of a dribbling injector or other similar. The engines behave identically at power - same RPM gives same boat speed.

So - ideas???
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Old 12-04-2020, 12:15   #2
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Re: Asymmetric Fuel Burn - Volvo D2-55F

My guess would be slightly different reading tachs, it doesn’t take much to get 10% difference in fuel consumption, or slightly different pitched props.
10% is sort of within the scatter factor if you will. It could be two slightly different things that add up to 10%.
However if it’s been a long time since you have had your injectors serviced, it couldn’t hurt.
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Old 13-04-2020, 16:19   #3
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Re: Asymmetric Fuel Burn - Volvo D2-55F

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Originally Posted by a64pilot View Post
My guess would be slightly different reading tachs, it doesn’t take much to get 10% difference in fuel consumption, or slightly different pitched props.
10% is sort of within the scatter factor if you will. It could be two slightly different things that add up to 10%.
However if it’s been a long time since you have had your injectors serviced, it couldn’t hurt.
Thanks for the ideas.

We have digital tachs, though I haven't researched how they're driven - but presumably less likely to be inaccurate compared to analog dials?

The props are nominally identical, and they are the same age - the engine is newer not the drive/prop. Doesn't mean they can't be a little different, but if more than a little it would show in RPM/boat speed.

I've only had the boat 16 months, and I haven't touched the injectors. I have maintenance records on the newer engine from new, no injector work done. Same for older engine in the same time period, but I don't know what happened before that.

I guess eventually I'll get the props serviced and returned to spec off the boat, and eventually I'll service the injectors too - so maybe I'll find out down the line.
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Old 13-04-2020, 17:23   #4
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Re: Asymmetric Fuel Burn - Volvo D2-55F

Digital is very often just the display and your not alone with assuming that something digital, that displays to for example .01V is accurate to .01 V, but most often that is not the case.
Very often digital displays have analog sources and therefore are not really digital at all.
Many engines use the alternator frequency to drive the tachometer, something that I wouldn’t have believed unless I saw it, to me it’s so simple to use a Hall effect sensor to count the teeth on a flywheel for instance and be very accurate, why in the world would they cheap out and use a belt driven appliance as an RPM source?
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Old 13-04-2020, 17:37   #5
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Re: Asymmetric Fuel Burn - Volvo D2-55F

This is an obvious question/answer so pardon me for asking but do you have a generator that draws off just one tank?
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Old 13-04-2020, 17:42   #6
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Re: Asymmetric Fuel Burn - Volvo D2-55F

> We frequently run one engine at a time, and if I need to charge of course run just one.
Do you always charge off the same engine, or do you alternate?
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Old 13-04-2020, 17:55   #7
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Re: Asymmetric Fuel Burn - Volvo D2-55F

Also , do you have do you have fridge compressor or water maker run off one engine, do they have the same alternators.
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Old 13-04-2020, 17:59   #8
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Re: Asymmetric Fuel Burn - Volvo D2-55F

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Originally Posted by a64pilot View Post
Digital is very often just the display and your not alone with assuming that something digital, that displays to for example .01V is accurate to .01 V, but most often that is not the case.
Very often digital displays have analog sources and therefore are not really digital at all.
Many engines use the alternator frequency to drive the tachometer, something that I wouldn’t have believed unless I saw it, to me it’s so simple to use a Hall effect sensor to count the teeth on a flywheel for instance and be very accurate, why in the world would they cheap out and use a belt driven appliance as an RPM source?
D2-55 engines use an inductive pickup mounted in the flywheel housing counting machined slots on the flywheel to drive the tachometer.
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Old 13-04-2020, 18:00   #9
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Re: Asymmetric Fuel Burn - Volvo D2-55F

Also on a nice flat calm day, run one engine at 2500 rpm and note the water speed then switch it off and run the other and note the water speed. At same time note the amps going into the batteries under each scenario.
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Old 27-04-2020, 11:45   #10
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Re: Asymmetric Fuel Burn - Volvo D2-55F

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Originally Posted by DougR View Post
D2-55 engines use an inductive pickup mounted in the flywheel housing counting machined slots on the flywheel to drive the tachometer.
DougR
Thanks DougR for that - so probably not a tach issue.

As for the questions asked by others, a few of the answers are in my original post. The answer that covers the other questions is - the installations are identical port and starboard, and we have no generator.
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