Diesel Efficiency vs. Gas Efficiency

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.

 

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

 

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.

 

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

 

Underload/Overload

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.

 

Naturally !

 

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

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.)

 

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.