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Old 10-18-2010, 10:18 AM   #1
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Fuel burn VS load

<h3>Found this on a site , and although its not "marine diesels" (usually marinized* truck engines) the principals are the same.</h3>
Notice the position in the graph for max hp is not the same as the region for lowest fuel burn.

"hp.performance map which shows an engine's BSFC over the entire load (BMEP) and speed (N) range"

The REAL thing to learn from this graph is at the bottom of the graph where minor HP costs BOODLES of fuel , for very little power.

This shows that it can be really expensive to have a bigger engine than necessary.

IF these fuel maps were avilable for the common marine conversions , it would be really EZ to set up a boat for best economy , and retain extra power for adverse headwinds or seas.<h3></h3><h3>Specific Fuel Consumption</h3> <h4>By Sean Kelly</h4>
Specific fuel consumption is based on the torque delivered by the engine in respect to the fuel mass flow delivered to the engine. Measured after all parasitic engine losses is brake specific fuel consumption [BSFC] and measuring specific fuel consumption based on the in-cylinder pressures (ability of the pressure to do work) is indicated specific fuel consumption [ISFC].

BSFC is directly proportional to engine volume, EG: as engine volume goes down so does BSFC. This is due to the heat losses from the end gas to the cylinder walls. The heat loss drops the thermal efficiency of the engine. This can be observed in the cylinder surface area to cylinder volume ratio, which increases with cylinder bore B:

Every engine has a minimum on the plot of BSFC versus Engine speed N.

Higher engine speeds produce a high BSFC (more fuel consumption for the same torque) because of rising friction losses in the engine. Higher friction losses reduce brake torque [d(Wb)/dt], which increases BSFC [eqn 1. above].

Lower engine speeds produce a higher BSFC (more fuel consumption for the same torque) because of increased time for the heat transfer from the working fluid to the cylinder walls. This reduces [d(W<sub>i</sub>)/dt] ], which increases BSFC.

Compression ratio r<sub>c</sub> is inversely proportional to BSFC due to higher thermal efficiency. That is, higher compression ratio yields lower specific fuel consumption.

Above is a performance map which shows an engine's BSFC over the entire load (BMEP) and speed (N) range. A dynamometer is used to calculate brake torque and fuel flow is entered into the computation to calculate brake specific fuel consumption. This map only incorporates the wide-open throttle condition, but adding a third independent axis of throttle position can create a 3D performance map; it is noted that throttling directly affects efficiency via an inverse proportion, and maximum efficiency and minimum BSFC is realized at 100% throttle.

BMEP is brake mean effective pressure in the cylinder and is the scale of measurement for an engine's potential to produce torque.

N = engine speed, revolutions/minute

T = torque, N*m

V<sub>d</sub> = Displacement, liters


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Old 10-18-2010, 06:34 PM   #2
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RE: Fuel burn VS load

I agree ?

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Old 10-18-2010, 06:42 PM   #3
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RE: Fuel burn VS load

What he said.
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Old 10-18-2010, 07:21 PM   #4
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Fuel burn VS load

Geez FF, you have outdone yourself again.

I don't know where you find this stuff but let's start with the basics.

The date that guy published is based on a spark ignition engine. The "throttle" should have been a tipoff. Diesel engines are not throttled and that has considerable impact on power and part load part throttle BSFC.

"BSFC is directly proportional to engine volume, EG: as engine volume goes down so does BSFC."

What absolute horse pucky. The guy doesn't have a clue what his is saying. The "volume" (what engine literate folks call displacement) of an engine is not linked to BSFC.

For example, the largest engine on the planet has a bore of 960mm and a stroke of 2500mm and delivers a* BSFC of 170 grams per kilowatt hour.*

A little CAT 3056 with a bore of 100mm and stroke of 127mm delivers a BSFC of 280 grams per kilowatt hour.

And BSFC is based on power, not torque. That is about as fundamental as it gets and if the guy missed that then it's good he publishes on the net because it ain't worth putting on paper.

As far as your fear mongering regarding fuel burn at lower power (and we are talking about power since with a fixed pitch propeller, speed and power are linked) conside the big Wartsila above, that 170 g/kWh is at 80 percent rated power. At 60 percent the BSFC is 160.3 g/kWh.

Oh, and that little CAT, the BSFC of* 281 g/kWh occurs at 2600 rpm and 125 hp. If we reduce power to 58hp at 1000 rpm - the BSFC increases* to 481, it nearly doubles but the fuel burn is still only 10 percent of full power. You really need to reread that bit I posted not too long ago about the reality of reduced power fuel burn vs your fantasty.

You would be better served to find a more reliable source. Either that or learn enough to recognize when you are looking at garbage.

-- Edited by RickB on Monday 18th of October 2010 08:35:54 PM
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Old 10-18-2010, 08:19 PM   #5
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RE: Fuel burn VS load

I'd rather weave a knitting needle though my nasal passages than read this twice.
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Old 10-18-2010, 08:35 PM   #6
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RE: Fuel burn VS load

Delfin wrote:

I'd rather weave a knitting needle though my nasal passages than read this twice.
you actually made it all the way through the first time???

You are a better man than I!


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