Bukh 38hp

[dave1953]

Can any one give me some advice I have this Engin in my LM30 I have a 210ltr tank I need to know at what rpm how many ltrs an hour it will use I have filled it up to the top so I can see diesel and run for 5 hrs at about 1200 rpm but the fuel gauge has not moved so I'm not sure if it works any help would be much appreciated thanks
Regards Dave

[Sula]

I assume that you are referring to a 36hp Bukh DV36 (three cylinder).  I'm not aware of a 38hp Bukh.

There is a 32hp DV32 (a turbo version of the DV24 two cylinder, I believe), but it is much less common than the DV36.

The fuel consumption of any normal marine engine, including the Bukh, does not depend on the rpm (except indirectly), but overwhelmingly on the load you are applying to the engine.  The maximum fuel consumption for the Bukh DV36 is 8 litres per hour, but in reality it is highly unlikely you will ever consume fuel at that rate for more than a few moments on a boat.  

Note that your boat's engine control lever doesn't operate at all like a car accelerator pedal!  A marine diesel engine control lever only sets the rpm, not the amount of power produced (though the rpm affects the amount of power its could potentially produce). The marine diesel engine's internal automatic governor controls the amount of fuel supplied to the engine to reach the selected rpm, depending on the load applied to it.   

If you were running the engine at 1,200 rpm in neutral it would consume very little fuel (less than 1 litre per hour, I imagine) - the only 'load' it would have is the need to overcome the internal friction in the engine.   I would be unsurprised if the fuel gauge in a 210 litre tank didn't visibly move in several hours.

Even in gear at 1,200 rpm the engine wouldn't consume a lot of fuel.  Although a DV36 is capable of producing 14.75hp at that rpm , your boat wouldn't need that much power to move at the low speed produced by the prop at those low revs, and so the governor would limit the fuel to the engine.  You could probably drive your boat with a 4hp outboard at the sort of speed it would go with your Bukh at 1,200 rpm, so your engine would only be generating that sort of power (plus enough to overcome its internal friction, drive water pumps etc., which would be greater than that of the outboard).

If you were going at that slow speed and then pushed the engine control lever to maximum, the governor would provide the maximum fuel until the engine reached 3,600 rpm, and then reduce the fuel supplied to whatever was required to maintain that speed.  

At a guess, it might take about, say, 20hp to drive your boat in calm conditions at hull speed, so that's what your engine will deliver, and its fuel consumption would relate to.  (With a headwind, your engine power output and fuel consumption would increase, and with a tailwind both would decrease).    

I have seen a 'rule of thumb' of 1 litre per hour per horsepower used.  That's probably as near as you'll ever get as a rough guide for general use, but take it with a pinch of salt - it's not at all accurate.  (Note that at full load the Bukh DV36 does 36hp
on 8 litres per hour, which would work out about 4.75hp per litre per hour, or less than a quarter of a litre per hour per hp used!)

I can't remember what fuel consumption my DV36 actually gave on average in use and on passage in my LM27, but probably a few litres per hour. Perhaps others can provide figures for an LM30 - it wouldn't be much different even with a different make or model of engine.       

As I said, the maximum fuel consumption for the Bukh DV36 is 8 litres per hour.   This will only ever happen if you are not only running the engine at 3,600rpm (because it's only at the speed it is capable of producing the maximum power of 36hp), but also you are putting such a load on it (e.g. heading at hull speed into a strong wind and big sea) that the maximum 36hp is only just able to overcome it (in practice, with such a load, the engine would be dropping below 3,600rpm each time you hit or climbed a wave, and regaining 3,600rpm (and actually using less than max fuel) when you were coming off/surfing down a wave).

Probably more of an answer than you were looking for, but I hope it's informative.

Long story short - Don't panic! Your fuel gauge is not necessarily faulty, you probably used so little fuel it didn't register.  It will take a long time to get through your 210 litres - probably at the very least 36 hours running under load - so don't panic, just keep an eye on the gauge, and only conclude if it continues to show no movement when you have run the engine for a long period under load.  Count your engine hours and do a rough calculation (it will only ever be a rough guide) when you next fill with fuel.  
 

[Brian & Glenda]

Click on the link below, and select the DV36-RME from the list and you'll see the Power Rating, Torque and Fuel Consumption curves for this engine. The values for the SME would likely be identical as it is the same engine, just a different drive system.

The trouble with the original VDO float gauge, is that it does not consider the shape of the tank. It simply thinks that if the float is in the middle, the tank must be half full. Given the irregular shape of the tank, there is in fact less fuel per inch at the bottom inch of the tank than there is in one inch of depth at the mid-point or top of the tank.

I removed my fuel tank inspection cover (with its float switch and pick up lines attached), drilled a hole into it and epoxied a 3/4" PVC fitting with screw cap into the top of my fuel tank inspection cover, such that I could dip the tank whenever I wished to do so. (My VDO float switch fuel gauge wasn't working which prompted the desire to dip the tank, and although I replaced the gauge ... I still like to have the manual dip verification option available.)

At that time I made some crude estimates of tank volume for various depths of fuel given the shape of the tank in our 1983 LM32 with DV36-SME and its 200 litre tank. I used average end area calculations at the forward and aft ends of the tank, and near the midpoint of the tank where the side of the tank changes alignment as it meets the hull of the boat. I'm still calibrating these estimates of the volume of fuel on hand for various depths of fuel by tracking the change in depth of fuel for every 10 litres of fuel added each time I refuel the boat. My figures appear to be fairly close, but I never have had less than half a tank in the boat, and this is a time consuming proposition as I only fuel the boat about once per year. I far prefer to sail rather than motor, but I also like to know such things as how much fuel I have on board!

By my calculations for my LM32's tank, it takes:

~15 litres to fill the tank to 2" depth. (The inaccessible depth / volume of fuel.)
~55 litres yields 6" depth of fuel in the tank, or just a wee bit more than 1/4 tank, yet this is 1/3 of the total tank depth.
~102 litres measures 10" depth of fuel in the tank, or about 1/2 tank.
~151 litres measures 14" depth of fuel in the tank, or about 3/4 tank.
The tank is said to hold 200 litres in total, and it's 18" tall for a full tank.

Of course these measures will only be applicable for an LM32, unless the LM30 uses the identical shape tank.


Perhaps more relevant to your question ... sorry I digressed ... perhaps someone may find the above info of value...


I have also completed some calculations for hours of operation remaining at various RPM based on these figures, and the volume of fuel remaining in the tank

Here are some of those figures:

Remember that the fuel pick-up tube does not sit at the very bottom of the tank. Our boat has a mesh screen for the first ~2" of the pick-up tube. That means that the last 2" of fuel cannot be accessed. By my crude calculations, I estimate that about 8% of the total volume of the tank is therefore inaccessible. Given tank slop in a sea, even with baffles in the tank ... I would begin to fret about air getting sucked into the fuel uptake pipe if the tank got below 5" or so of fuel remaining. That is ~22% of my total tank volume.

At 1500 RPM ... the Bukh graph says the engine uses 2.8 litres per hour, so a full tank (200 l) would give me 66 hours of operation if I could use all of the fuel in my tank, so let's say 25% less or about 50 hours if I wish to leave the bottom 5" of fuel in my tank. That RPM moves my LM32 along at about 5 kn. 50 hours at 5 knots = 250 nm of travel. (To which you must add or subtract allowances for currents and wind etc.)

At 2100 RPM, the Bukh graph says the engine uses 3.8 litres per hour, so a full tank would give me 48 hours of operation if I could use all of the fuel in my tank, so let's say 25% less or about 36 hours if I wish to leave 5" of fuel in my tank. That RPM moves my boat along at about 6 kn. 36 hours at 6 knots = 216 nm of travel. (To which you must add or subtract allowances for currents and wind etc.)

I believe that these figures are fairly accurate, but have no fuel flow monitoring meter to confirm.

Here is the link:
www.bukh.pl/bukh-brochure-datasheet/?lang=en

Hope that helps...

Brian.

[Sula]

The graphs Brian refers to do not attempt to state what he takes them to mean. Bukh DOES NOT say 'at 1,500 rpm . . . the engine uses 2.8 litres per hour'.

It is impossible to state categorically how much fuel a marine diesel will use at a given rpm, because it depends primarily on the LOAD, not on the engine speed.

The Bukh fuel consumption figures quoted by Brian are the MAXIMUM POTENTIAL fuel consumption at those speeds, and relate to the MAXIMUM POTENTIAL power available at those speeds, which is shown in the closely related accompanying power and torque graphs in the Bukh brochure. Note that the consumption graph is marked not in grams per rpm, but grams per kWh. (An equivalent figure for grams per hp can be easily calculated, or an equivalent graph produced.)

If you run the engine at those speeds - lets say the 2,100rpm Brian mentions - in neutral it will not consume anything like the 3.8 litres per hour Brian quotes. It will only be producing something under 2hp to overcome the friction in the engine, the load on the water and oil pumps, and any load you have placed on the alternator.

If you then select forward gear the engine speed will dip very slightly and then quickly recover to the 2,100rpm you had originally set with the engine speed control lever (which is NOT a throttle!) as the governor increases the fuel supply to the engine to overcome the resistance of the boat moving through the water at the speed achieved by the prop turning at that (engine/gearbox) dependent speed, in order that the engine maintains the rpm you set. It will only provide just enough fuel to give just enough power to maintain that engine speed. In practice that will be somewhat below the maximum power (and hence fuel consumption) that the engine is capable of generating at that rpm.

If anyone doubts my explanation, perhaps they could explain
- what they think the engine speed control lever does;
- what they think the fuel governor on the engine does;
- and if you run the engine in neutral at, say, 3,600rpm, where do they think the 36hp it will supposedly be generating is going.

[Brian & Glenda]

I stand corrected. The chart I provided a link to, is not the same chart as I have on my boat that I based my fuel consumption calculations on some time ago. It looks very similar, but it is not the same. I errored in providing that link. The fuel consumption figures that I gave, were based on a chart that remains on my boat, that I believe gives typical fuel consumption for various RPM when the boat is in motion.

Perhaps Sula, you could delete my post, or at least the bottom half that discusses the fuel consumption, and the referenced link.

You know though ... I have never concerned myself with fuel consumption at idle, and I've never had the need to run my boat at 3600 RPM in neutral! I have only ever concerned myself with fuel consumption while underway. If the referenced chart is as you suggest the basis for maximum fuel consumption ... then as you suggest ... you'll most likely be able to go further than my figures indicate ... but I've found my real world experience to be close to the figures I provided, based on the few occasions that I have compared my hour meter readings and the change in the level of fuel in the tank, as determined by dipping the tank. As I said ... I don't motor a lot, so rarely have occasion to determine my true fuel consumption. More data is needed to give a definitive statement on same... and I should have stated that more clearly in my earlier submission.

Brian.



Thanks,
Brian.

[Sula]

Brian,

I wasn't disputing your reporting of the fuel consumption you have experienced, just correcting your interpretation of Bukh's figures which completely ignored the explanation I had gone to some trouble to provide in the previous post.

To reiterate - fuel consumption in a marine diesel is not a function of engine speed, but of the load placed on the engine. The 'throttle' lever on a boat is no such thing. Unlike a car accelerator pedal, it does not change the amount of fuel provided to the engine. It merely sets the revs. The (automatic) governor provides the amount of fuel required to reach and then maintain that engine speed.  Put less load on the engine, and it will not speed up (the governor will just provide less fuel); put more load on the engine and it will not slow down (the governor will just provide more fuel) unless it reaches the point where the engine cannot generate enough power at those revs to meet the load - the only relevance of revs to fuel consumption! - which should never happen (except perhaps in extreme weather) if the propellor is correctly sized and pitched for the boat, engine and gearbox ratio combination. 

The relevance of running the engine in neutral is twofold -
1) the OP was trying to understand why he had seen no change in the fuel gauge when he was running his engine, and if he was doing so in neutral this would have used radically less fuel than running at the same engine speeds under load, and likely explained why it hadn't registered on the fuel gauge; and
2) I was giving examples of running the engine in neutral at high revs to emphasise the point that there is no direct connection between engine revs and fuel consumption.  

[dave1953]

Thanks for all your help so under load if i was to sa at most 3 ltrs per hour then if i run under load for 4 hours i should put say 12 ltrs in i will put 10 in today and some more tomorrow till i can see fuel in the filler thanks Dave