Diesel Efficiency vs. Gas Efficiency

[Northwester]

Doing research for repower options, I've noticed that fuel consumption curves for marine diesel engines seem to be readily available from manufacturers. Whereas, marine gas engine manufacturers don't very often provide such information, if ever.

I would like to compare gas engine fuel consumption in terms of grams/kwhr or some other standard measure.

Where can I find some numbers to compare different gas engine offerings available in the market today. I would like to compare all the different GM marine engines to determine which is most efficient at a modest cruising RPM:

3.0l
4.3l
5.0l
5.7l
6.2l
7.4l
8.1l

Is such an analysis possible?

[Guest62110524]

we were taught as diesel apprentices that the diesel has a mech efficiecy of 35% roughly and gasoline much less, thought it was 15% , but things have changed, the modern diesel is now much more efficient so google up mechanical efficiancy of internal combustion engines
until electronic injection came for gas, then it was all a bit hit and miss, no such things as accurate fuel rate delivery,for ships cross head 2 cycle engines, they just seem to refine the things more and more so the consumption grams K/w hr comes down and down

[speedboats]

This would be hard to determine as fuel consumption will vary depending on the load.

This means things aside from RPM.

An engine swinging say a 19* prop at 4500rpm will be working harder than the same engine swinging a 17* prop at the same rpm. Working harder means higher fuel consumption.

[daiquiri]

I'll take the long way round to explain, hopefully it won't get too messy...

Modern small diesel engines' efficiency arrives to 0.40-0.41 and more, whilst the gasoline engines' efficiency hardly go over 0.27-0.28 (based on fuel mass). So, their efficiencies based on fuel masses are related by the formula:
(1) Efficiency_mass_gasoline = 0.68 * Efficiency_mass_diesel

Gasoline's and diesel's densities are 0.74 kg/l and 0.84 kg/l, respectively. So their efficiencies based on fuel volume will be proportionally smaller:
(2) Efficiency_volume_gasoline = 0.62 * Efficiency_volume_diesel

Now, if you analyse the declared data for various diesel engines, you'll find out that their approximate fuel consumption at max power can be calculated with the formula (in litres per hour):
(3) FC_diesel = 0.19 x HPmax

Knowing the relation between diesel and gasoline's efficiencies (the formula n.2 above), the formula for gasoline engine fuel consumption will be, approximately:
(4) FC_gasoline = 0.30 x HPmax

You can find some power and consumption curves for marine gasoline engines here:
http://www.bpm-marine.it/english/engines.htm

Take their V12 620S model as an example.
You will find out that at max power it delivers 630 HP and consumes 219 gr/HP/h = 138 kg/h of gasoline.
The energy content of gasoline is 46 MJ/kg ( http://en.wikipedia.org/wiki/Fuel_efficiency ), so the raw input power from the fuel is 1762 kW = 2361 HP.
Therefore, this engine's efficiency is 630 HP / 2361 HP= 0.27 which confirms the efficiencies I've assumed at the beginning .

Now, back to those 138 kg/h of gasoline consumption. It corresponds to 186 l/h (litres per hour). If you try to calculate it from the formula (4) you'll find:
FC= 0.3 * 630 HP = 189 l/h
which confirms the validity of the formula used.

Of course, there will be differences between various engine manufacturers, but they will be very small. Both diesel and gasoline engines technology have reached a point of developement where it has become very difficult to make such a big progress to substantially change those numbers.

Now, that analysis is for max. RPM and max power. If you need to analyse reduced (cruise) RPMs, there you have a problem which is called ECU (Engine Control Unit - find more here: http://en.wikipedia.org/wiki/Engine_control_unit ).
It is a programmable electronic control system which controls the engine behaviour at various regimes, ambient conditions, fuel characteristics etc. It can be programmed (mapped) to control and regulate the engine in nearly any way desired and therefore is a big unknown variable for any partial-RPMs analysis, unless you have equipement for reading or re-mapping the ECU.
So you can't perform that kind of analysis without direct help and data about your engine's current ECU mapping from the manufacturer.

[Ad Hoc]

As a rough guide, a diesel is about 200~210 g/kWh, a GT is around 270~300g/kWh.

Main difference is that as you pull back the revs of a diesel, the fuel consumption changes, ie gets better. Whereas with a GT, not much change, the fuel consumption graph is not exactly horizontal, but certainly not anywhere near the same as a diesel.

[apex1]

As a rough guide, a diesel is about 200~210 g/kWh, a GT is around 270~300g/kWh.

Main difference is that as you pull back the revs of a diesel, the fuel consumption changes, ie gets better. Whereas with a GT, not much change, the fuel consumption graph is not exactly horizontal, but certainly not anywhere near the same as a diesel.

Although the above figures are about right (as a rule of thumb), I totally have to disagree with the latter statement! Its wrong!
The Diesel engines efficiency to which the 200g rule applies is provided in a very, very narrow band of revs only! Usually at a setting around 80%. If you go above or below this optimal setting, the (specific) consumption increases dramatically! That unfortunately applies especially for the very low rev / low load band, where consumption can increase to 370g kw and above!
The gas guzzler has not such problem with low rev / low load consumption.

Regards
Richard

[FAST FRED]

The Diesel engines efficiency to which the 200g rule applies is provided in a very, very narrow band of revs only! Usually at a setting around 80%. If you go above or below this optimal setting, the (specific) consumption increases dramatically! That unfortunately applies especially for the very low rev / low load band, where consumption can increase to 370g kw and above!
The gas guzzler has not such problem with low rev / low load consumption.

This is the hardest problem for one attempting to powerr a cruiser.

Sure the Mfg will show a useless Prop HP curve , and might publish a graph of HP aviliable at different rpm

BUT the real tool to efficiently set up a vessel is a FUEL MAP , which are guarded inhouse.

A fuel map looks like a bulls eye or series of clouds , with the most efficient operation in the bulls eye and every ring outside is lower efficiency.

FF

[apex1]

BUT the real tool to efficiently set up a vessel is a FUEL MAP , which are guarded inhouse.
FF

right

[Northwester]

Thanks for all the information and discussion. What I am trying to accomplish with my research is to mate the most fuel efficient engine with the resistance curve for my small planing hull vessel.

I'm working with classic deep-V Bertram 25' hull which weighs about 5500 lbs. fully laden. The deadrise for the planing portion of the hull aft is about 24 degrees. The beam of the planing section is about 8.0 feet. If anyone has published a good resistance curve for this boat, please point me to it. From my own observations, a cruising speed of 22-25 knots seems to be the best.

In a perfect world, I would want to know how many KW of power needed to drive the vessel at the target cruising speed (where the resistance is at it's least point) and then find an engine that will produce that power while consuming the least amount of fuel in term of g/KWHR.

Am I on the correct track?

[marshmat]

BUT the real tool to efficiently set up a vessel is a FUEL MAP , which are guarded inhouse.

A fuel map looks like a bulls eye or series of clouds , with the most efficient operation in the bulls eye and every ring outside is lower efficiency.
And it's flat out stupid of the manufacturers to hide these things.
They're fairly simple to read, there's no proprietary information and you can't reverse-engineer the motor from a fuel map. The reason for making the fuel maps (or performance maps) in the first place- basically just curves of constant specific fuel consumption, plotted against rpm and bmep (or torque)- is so that the guy responsible for selecting and setting up the engine can get it running in the conditions it's best at. So why are they so hard to find?
If any sketchy 3rd party wants to reverse engineer the thing, all they have to do is buy one and get out the micrometer and ECU scanner. Performance curves and fuel maps exist only to help set the thing up at its best operating point.

[Ad Hoc]

whenever I've request these maps, I've always been supplied them, for verification on sea trials.

[marshmat]

I think what Fred and I are saying is that performance maps should be easily available when trying to select an engine in the first place. To be fair, I haven't yet had reason to heckle manufacturers to provide them- but if I were selling engines, I'd put the performance maps on the website right along with the tech specs, dimensional drawings, etc.

[daiquiri]

That's all correct, but, like I said before, while the fuel consumption at max power is pretty much limited by the mechanics and internal fluid dynamics of the engine (and is quite comparable between different engines and related by formulas I've written above), the partial-load behaviour can be varied pretty much by ECU programming strategy.
So the fuel maps will vary significantly between various engines and various manufacturers. It will vary significantly even for the same engine with different ECU mapping. That makes a generalized analysis for partial engine loads nearly impossible.
So, as I see it, Northwester could do something like this:
- decide which engine manufacturer he wants to use
- phone them and ask if they will send him performance maps for different engines he wants to analyse
- get back here with those maps and so we'll be able to give him an advice for those selected engines.
;)

[Northwester]

I've selected an engine for discussion. Cummins 6BTA 330HP@2800 RPM.

I've attached a PDF of the manufacturers marine performance curve.

[Northwester]

I've also attached estimated performance from boat diesel calculator.

[Northwester]

Finally, I've attached a Boat Magazine reprinted article which contains a sterndrive and inboard v-drive performance curves. From a review of the sterndrive curve it appears you need to burn 14.3 US Gallons/hour of gasoline with twin inline 160hp mercruiser engines running 15x21 props with 1.7 ratio to attain desired cruising speed of 22 knots or thereabouts.

Dry weight of vessel is given @ 4300 lbs. Actual tested weight is probably closer to 5300 lbs.

I would assume that the fuel consumption number can converted through one of the above formulas to estimate actual power produced for each engine @ 3000 RPM.

In the absence of proper resistance curve for hull, this is the best information I have on hand for my vessel.

[CDK]

The Diesel engines efficiency to which the 200g rule applies is provided in a very, very narrow band of revs only! Usually at a setting around 80%. If you go above or below this optimal setting, the (specific) consumption increases dramatically! That unfortunately applies especially for the very low rev / low load band, where consumption can increase to 370g kw and above!
The gas guzzler has not such problem with low rev / low load consumption.
FF

This is NOT correct. It may apply to Detroit diesels of Caterpillars, but not to smaller and more modern types.
I have studied datasheets from Steyr and Volkswagen Marine, with and without turbo charger. Both supply complete graphs for torque, HP output and specific consumption.

The point of highest efficiency always coincides with the maximum torque, with only a minor increase in consumption at higher rpm. If the specific consumption is almost 200 g/KW/h, it stays within a margin of only +10% for the whole usable bandwidth. Only near idle and near max. permissible rpm the consumption rises sharply, which is to be expected.
Near idle the mixture is too lean to burn completely, at max. rpm (that is several 100s rpm above maximum output) the combustion time exceeds the duration of the output stroke, causing a sudden rise in exhaust temperature.

Because at that point both torque and HP curves are falling steeply, this rpm range cannot be reached with a marine engine with a matched gearbox and prop, only with a road vehicle going downhill.

[apex1]

This is NOT correct. It may apply to Detroit diesels of Caterpillars, but not to smaller and more modern types.
I have studied datasheets from Steyr and Volkswagen Marine, with and without turbo charger. Both supply complete graphs for torque, HP output and specific consumption.
The point of highest efficiency always coincides with the maximum torque, with only a minor increase in consumption at higher rpm. If the specific consumption is almost 200 g/KW/h, it stays within a margin of only +10% for the whole usable bandwidth. Only near idle and near max. permissible rpm the consumption rises sharply, which is to be expected.
Near idle the mixture is too lean to burn completely, at max. rpm (that is several 100s rpm above maximum output) the combustion time exceeds the duration of the output stroke, causing a sudden rise in exhaust temperature.
Because at that point both torque and HP curves are falling steeply, this rpm range cannot be reached with a marine engine with a matched gearbox and prop, only with a road vehicle going downhill.

It IS absolutely correct, and it was my statement.

Sorry, we are mixing apple and orange here.
Your statement is correct (even for the oldest Diesel) if we are talking about load! And I mentioned that as stated above. In every day life, in marine use, it is valid with a CPP only.
The average drivetrain is a gear, fixed prop system and these load the engine sufficiently only in a very narrow band of rev´s! Above and below this point the specific consumption increases noticeable to dramatically, no matter if it´s a VW, Steyr or MTU. That is just a fact.
The petrol engine does´nt have such problem.

For my fellow members interested in a deeper insight into marine propulsion efficiency I´ll provide some links. Start here:
http://www.proboat-digital.com/proboat/20070607/
goto "content" then page 82 "Hybrid Marine Power" read the article in the following issues too.
And if ever possible, affordable and senseful (not for runabouts) install a CPP with a Diesel! Your engine manufacturer will hate you, you´ll never buy a new one!

Regards
Richard

[mydauphin]

I am not a Marine engineer or even a real mechanic but I have been working on boats and engines for 35 years. So my observation are based on practical knowledge and observation.
Gasoline engines are more efficient somewhere between peak torque and hp curves. They have a wider power range and their efficient/speed range varies accordingly. Lets say 3500 to 5000 rpm. With gearing to reduce rpms at prop to lets say 1500 rpm and the props are smaller.
On a diesel, the rpm range may be 1800 to 2200 rpm, at the torque maximum. At lower rpm the engines may use less fuel but engine is overloaded, same as higher. More fuel consumption no more speed. Propeller efficiency drops also. So in a Diesel propeller, gearing, and matching to hull are much more important because of narrower power range.

Diesels are torque engines, big slow propellers, properly matched to gears.

Now lets say even in the old Detroit two strokes, which I love, 653 will rev nicely, and generates torque much higher rpm than 671 or 692. Every engine is different. But these old engine work on displacement not rpms to generate torque. Newer engines are higher RPM, less rotating mass, consequently greatly reduce engine life, but better power to weight ratio.

To me I like slow, reliable diesels perhaps not as efficient but still better MPG than gas.

So in summary it is the efficiency of installation , not engine.

[apex1]

On a diesel, the rpm range may be 1800 to 2200 rpm, at the torque maximum. At lower rpm the engines may use less fuel but engine is overloaded, same as higher.

Sorry to correct you, at lower rpm the Diesel is (sometimes extremely) underloaded! That increases BSFC noticeable.

So in summary it is the efficiency of installation , not engine.

I really do´nt like to contradict you, I assume you just mixed up the overload at higher rpm with the underload at lower.
I agree with your statement: efficiency of installation

Regards
Richard

[mydauphin]

I really do´nt like to contradict you, I assume you just mixed up the overload at higher rpm with the underload at lower.
I agree with your statement: efficiency of installation

Regards
Richard

Your right in your statement about underload...That was typo on my part. However, running diesel at too low rpm starts acting like overload and actually can put a lot of strain on engine and will cause it to wear it out just like overload.

[apex1]

However, running diesel at too low rpm starts acting like overload and actually can put a lot of strain on engine and will cause it to wear it out just like overload.

Naturally !;)

[marshmat]

Moral of the story: Put the wrong prop on your diesel, then idle it a lot, so you can replace it more often, which keeps the engine builders in business :D

Looking at Northwester's comments earlier on the Cummins 330B:
As is typical on data sheets, Cummins gave a rated power curve, a full load torque curve, a 'standard prop' curve (I hate these) and a fuel consumption curve. That's enough data to select an engine that'll propel the boat. But if you're actually trying to choose the ideal engine, gear ratio, etc. for a particular application, a performance map / fuel map is ideal. It is a contour graph, with rpm (and in this case piston speed) on one axis and bmep or torque on the other; the curves are contour lines of constant brake specific fuel consumption (here in grams per kilowatt hour). See the example below. (This graph is a textbook example for a 4-cylinder 4-stroke, each different ECU mapping on each different engine produces its own map. The calculations are my own and are just rough ones, so please watch out for typos.)

Aside:

bmep (brake mean effective pressure) can be thought of as a way of stating torque, ie. load, relative to displacement. The relationship is
bmep = ( 2 pi n T ) / V
for torque T and displacement V (units must be consistent: eg. T in newton-metres and V in cubic metres works for bmep in pascals). n is 2 for a 4-stroke and 1 for a 2-stroke.

Remember that power is just torque times engine speed:
W. = (2 pi) N T
for W. in kW, N in revolutions/second, T in newton-metres.


Back to how to read the charts...

For most applications, where you're interested more in efficiency than in sheer power, you want to select the engine and gearbox so that the engine is operating at the 'bulls-eye' in the middle of the chart- the minimum brake specific fuel consumption (in grams per kilowatt hour, on this one). On the example below, that means this particular engine is best at 2500 rpm, turning a load that gives bmep = 500 kPa. It's a 2.0 litre 4-stroke, so (see formulae above) it is producing about 79.6 N.m of torque and 20.8 kW of power. (Or, 59 ft.lb and 28 hp.) That's its most efficient operating point, and is where you'd try to get it at cruising speed. This being a small car engine, probably geared to turn about 2500 rpm at highway speed (at which point the combined aerodynamic drag, tire resistance and auxiliary loads of a small car will take about 25-30 hp to overcome), it is very well matched to its application.

Note from the "max bmep" curve that this engine peaks at bmep = 900 kPa @ 4000 rpm. That's 143 N.m and 60 kW, which is what will be quoted in its sales brochure. From that same "max bmep" curve, we get 850 kPa at the 2500 rpm we found earlier to be the most efficient point- ie, 135 N.m and 35 kW. These are the values the manufacturer's "max torque" and "max power" curves take at this rpm- note how far away they are from the actual point of lowest specific fuel consumption. If you were to load this engine to near its maximum rating all the time, you'd be somewhere above 335 g/kWh, ie. about one-third more fuel used per horsepower than if you kept it in its ideal range. Underloading it all the time is even worse, just look at the specific fuel consumption numbers for low load.

You can sort-of guess at the actual location of the best operating range from the curves the manufacturers usually provide, keeping in mind the rules of thumb already mentioned by other folks. But, as I hope this example has described, picking out the best operating point is a heck of a lot easier with the proper graphs....

(Guys- feel free to correct my math if you see any errors.)

[apex1]

Moral of the story: Put the wrong prop on your diesel, then idle it a lot, so you can replace it more often, which keeps the engine builders in business :D
My recommendation.:p


(Guys- feel free to correct my math if you see any errors.)

Sh....i...t.... once in a lifetime one is invited to do that, and then... nothing to complain!
Life must be awful, nobody tries again.