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Author Topic:   Brake Specific Fuel Consumption

jimh posted 01-29-2006 01:17 PM ET (US)
The amount of fuel which an engine consumes is rated by its BRAKE SPECIFIC FUEL CONSUMPTION (BSFC). In the U.S. this is generally computed in dimensions of pounds of fuel per horsepower per hour. For most internal combustion engines the BSFC will be in the range of 0.5 to 0.6. This estimate fits well with the old rule of thumb that an engine will burn HP/10 gallons per hour.
Here is an example with a 90-HP engine. When developing 90-HP, how much fuel will it burn? Assume a BSFC of 0.55 and gasoline at 6.25 lbs/gallon:
90 x 0.55 = 49.5 pounds of fuel burned per hour
49.5 x 1/6.25 = 7.92 gallons per hour

The fuel efficiency will tend to peak at highest engine speeds. At near wide open throttle the BSFC will be closer to a value of 0.5.
The BSFC tends to be the same for similar engines. Really huge diesel engines have reported BSFC values in the 0.35 range.
More reading:
http://www.epi-eng.com/ET-ThermlEff.htm
Also see:
http://www.prime-mover.org/Engines/GArticles/bsfc.html
In this article the author suggests the following values of BSFC for various types of engines:
0.40-0.48 = Fuel injected four-stroke gasoline automobile engines
0.48-0.60 = Carburetor four-stroke automobile engines
0.55 or more = Two stroke gasoline engines (not direct-injection)
Also note that an engine does not maintain a fixed BSFC over its entire range of operating speeds. The best BSFC generally comes around 3/4-throttle.

jimh posted 02-01-2006 01:12 AM ET (US)
The diesel engine shown in this article
http://www.bath.ac.uk/~ccsshb/12cyl/
has a BSFC of 0.278 pounds per horsepower per hour
jimh posted 08-11-2007 10:18 AM ET (US)
Brake Specific Fuel Consumption (BSFC) for Two-stroke Loop-Scavenged Outboards
The estimate of brake specific fuel consumption for two-stroke engines ranges from 0.55 to as high as 0.8 pounds of fuel per horsepower per hour. I have a 1992 Evinrude V6 loop-scavenged 225-HP two-stroke which is in good tune and for which I have some fuel consumption data. From this I try to estimate my engine's BSFC.
The Data
At wide open throttle we assume the engine is producing 225-HP. We then consider the fuel consumption in gallons per hour, and calculate the BSFC from:
Gallons/Hour / 225-HP = Gallons/Hour/Horsepower

The gallons-per-hour data is extracted from our records of miles-per-gallon using:
Miles/Gallon / Miles/Hour = Gallons/Hour

Using data from my records of previous test runs, I calculate the BSFC as follows:
SPEED MILEAGE FUEL FLOW G/H/HP x 6.152 LB/GAL BSFC
41.1-MPH 1.8-MPG 22.8-GPH 0.101 0.62
40.0-MPG 1.9-MPG 21.1-GPH 0.093 0.57
41.3-MPG 1.85-MPG 22.3-GPH 0.099 0.61
Averaging the results gives a BSFC of 0.60
The Results
The results are in good agreement with the rule of "horsepower-divided-by-ten" will show the fuel flow in gallons per hour. They are also in agreement with the proposed range of BSFC for a two-stroke loop-scavenged engine.
Direct-Injection Two-Stroke
There has been considerable improvement in the BSFC for two-stroke engines which use direct injection such as the E-TEC and OptiMax. Owners of E-TEC or OptiMax engines are invited to submit data which provides the fuel flow in gallons-per-hour at wide open throttle and the horsepower of the motor. From this we can calculate the BSFC for these direct-injection two-stroke motors.
jimh posted 08-11-2007 11:11 AM ET (US)
Comparing my engine's BSFC of 0.60 with the estimates given above for a fuel-injected four-stroke engine of 0.40 to 0.48, and taking a middle value of 0.44, it appears that there is potential for considerable fuel savings.
0.6 - 0.44 = 0.16
0.16/.6 = 0.26 or a 26-percent improvement
This improvement would be at wide-open throttle. Based on my experience, the loop scavenged two-stroke has an even worse BSFC at low speeds, and at idle or below 1,500-RPM the BSFC is probably even higher than at wide-open-throttle. The direct-injection two-strokes tend to have very much better fuel economy at idle and low speeds, so the potential for improvement is probably much greater than the 26-percent implied by the comparison of the wide-open-throttle figures.
If judging the potential for fuel savings, a figure of 26-percent is probably very conservative, and it is anticipated that actual fuel savings might be significantly higher, more like the 50-percent reported by enthusiastic owners of new outboard motors.
Peter posted 08-11-2007 01:06 PM ET (US)
At WOT, each of my 2002 Evinrude 225 Fichts consume approximately 22 GPH. Various reports over the years suggest that each is producing about 237 HP at WOT. Thus Gallons per hour/HP is 0.093. BFSC is 0.57, which is essentially identical to your carbureted loop charged 2-stroke.
4-stroke outboards really do not do any better at WOT. In fact, certain low displacement pressurized 4-stroke outboards are worse.
The fuel efficiency advantages of 4-strokes and DFI 2-strokes occur at engine speeds that are less than WOT. When comparing fuel efficiency advantages in terms of percentages, the ICOMIA duty cycle is really the best way to compare potential fuel savings. Typically, a DFI 2-stroke or 4-stroke will show a 35 to 50 percent savings over a conventional 2-stroke under that duty cycle because under that duty cycle, approximately 50 to 60 percent of the operating time is spent under half throttle where fuel savings are the greatest.
lordswork2007 posted 08-13-2007 11:42 PM ET (US)
For comparison, a 160 hp air cooled fuel injected 4 stroke Lycoming airplane engine, at 75% power (120 hp) burns approximately 8.5 gallons per hour for a SFC of .435. At 100% (takeoff) power, however, the fuel consumption almost doubles, to 16 gph, for an sfc of .61. The increase in sfc is due to enrichment of the fuel mixture to lower exhaust gas temperature and to provide additional cooling. At 160 hp, reducing the fuel flow below about 13 gph would result in damagine exhaust gas temperatures over 1700 deg F.
Aircraft engines are interesting test beds because the amount of fuel can be controlled independently. The 8.6 gph figure for 120 hp is the result of setting the fuel flow to correspond with a desired exhaust gas temperature. There is some controversy about this subject, but the factory manual suggests leaning until peak, then enriching until the egt rises 25 deg F, which results in about 8.5 gph at 120 hp. The engine will almost one gph leaner without misfire, but EGT's are near peak, and the peak is near 1600 deg F. Leaner than about 7.5 gph and the engine becomes rough and the power falls off steeply. At about 6.5 gph the lean edge of stoichometry is reached and combustion ceases. All of the above assumes a manifold pressure of about 25" and an RPM of about 2400, which together when the fuel flow is correct, produce 120 hp.
I have contended before that 4 stroke outboards must most likely enrich at full throttle to protect their exhaust valves. Perhaps that is programmed into their engine management systems. Carburated autos did the same thing with a full throttle enrichment valve.

AZdave posted 08-14-2007 02:32 AM ET (US)
Lower specific fuel consumption can also be found in diesels of modest size. Nebraska did detailed testing on farm tractors over a number of years. A John-Deere 730 put out 56.7 HP on 3.15 gal/hr. They also report specific fuel cnsumpion of 0.38 - 0.39. These are the old two cylinder models fom the late 1950's, and are very heavy. That's actually an asset in tractors, as it improves traction. As usual, other factors can win out over fuel economy. These tractors had hand clutches. My Dad got rid of his J-D before I started to drive. He had a number of persons driving tractors and wanted all the clutches to work the same. Dave

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Author	Topic: Brake Specific Fuel Consumption
jimh	posted 01-29-2006 01:17 PM ET (US) The amount of fuel which an engine consumes is rated by its BRAKE SPECIFIC FUEL CONSUMPTION (BSFC). In the U.S. this is generally computed in dimensions of pounds of fuel per horsepower per hour. For most internal combustion engines the BSFC will be in the range of 0.5 to 0.6. This estimate fits well with the old rule of thumb that an engine will burn HP/10 gallons per hour. Here is an example with a 90-HP engine. When developing 90-HP, how much fuel will it burn? Assume a BSFC of 0.55 and gasoline at 6.25 lbs/gallon: 90 x 0.55 = 49.5 pounds of fuel burned per hour 49.5 x 1/6.25 = 7.92 gallons per hour The fuel efficiency will tend to peak at highest engine speeds. At near wide open throttle the BSFC will be closer to a value of 0.5. The BSFC tends to be the same for similar engines. Really huge diesel engines have reported BSFC values in the 0.35 range. More reading: http://www.epi-eng.com/ET-ThermlEff.htm Also see: http://www.prime-mover.org/Engines/GArticles/bsfc.html In this article the author suggests the following values of BSFC for various types of engines: 0.40-0.48 = Fuel injected four-stroke gasoline automobile engines 0.48-0.60 = Carburetor four-stroke automobile engines 0.55 or more = Two stroke gasoline engines (not direct-injection) Also note that an engine does not maintain a fixed BSFC over its entire range of operating speeds. The best BSFC generally comes around 3/4-throttle.
jimh	posted 02-01-2006 01:12 AM ET (US) The diesel engine shown in this article http://www.bath.ac.uk/~ccsshb/12cyl/ has a BSFC of 0.278 pounds per horsepower per hour
jimh	posted 08-11-2007 10:18 AM ET (US) Brake Specific Fuel Consumption (BSFC) for Two-stroke Loop-Scavenged Outboards The estimate of brake specific fuel consumption for two-stroke engines ranges from 0.55 to as high as 0.8 pounds of fuel per horsepower per hour. I have a 1992 Evinrude V6 loop-scavenged 225-HP two-stroke which is in good tune and for which I have some fuel consumption data. From this I try to estimate my engine's BSFC. The Data At wide open throttle we assume the engine is producing 225-HP. We then consider the fuel consumption in gallons per hour, and calculate the BSFC from: Gallons/Hour / 225-HP = Gallons/Hour/Horsepower The gallons-per-hour data is extracted from our records of miles-per-gallon using: Miles/Gallon / Miles/Hour = Gallons/Hour Using data from my records of previous test runs, I calculate the BSFC as follows: SPEED MILEAGE FUEL FLOW G/H/HP x 6.152 LB/GAL BSFC 41.1-MPH 1.8-MPG 22.8-GPH 0.101 0.62 40.0-MPG 1.9-MPG 21.1-GPH 0.093 0.57 41.3-MPG 1.85-MPG 22.3-GPH 0.099 0.61 Averaging the results gives a BSFC of 0.60 The Results The results are in good agreement with the rule of "horsepower-divided-by-ten" will show the fuel flow in gallons per hour. They are also in agreement with the proposed range of BSFC for a two-stroke loop-scavenged engine. Direct-Injection Two-Stroke There has been considerable improvement in the BSFC for two-stroke engines which use direct injection such as the E-TEC and OptiMax. Owners of E-TEC or OptiMax engines are invited to submit data which provides the fuel flow in gallons-per-hour at wide open throttle and the horsepower of the motor. From this we can calculate the BSFC for these direct-injection two-stroke motors.
jimh	posted 08-11-2007 11:11 AM ET (US) Comparing my engine's BSFC of 0.60 with the estimates given above for a fuel-injected four-stroke engine of 0.40 to 0.48, and taking a middle value of 0.44, it appears that there is potential for considerable fuel savings. 0.6 - 0.44 = 0.16 0.16/.6 = 0.26 or a 26-percent improvement This improvement would be at wide-open throttle. Based on my experience, the loop scavenged two-stroke has an even worse BSFC at low speeds, and at idle or below 1,500-RPM the BSFC is probably even higher than at wide-open-throttle. The direct-injection two-strokes tend to have very much better fuel economy at idle and low speeds, so the potential for improvement is probably much greater than the 26-percent implied by the comparison of the wide-open-throttle figures. If judging the potential for fuel savings, a figure of 26-percent is probably very conservative, and it is anticipated that actual fuel savings might be significantly higher, more like the 50-percent reported by enthusiastic owners of new outboard motors.
Peter	posted 08-11-2007 01:06 PM ET (US) At WOT, each of my 2002 Evinrude 225 Fichts consume approximately 22 GPH. Various reports over the years suggest that each is producing about 237 HP at WOT. Thus Gallons per hour/HP is 0.093. BFSC is 0.57, which is essentially identical to your carbureted loop charged 2-stroke. 4-stroke outboards really do not do any better at WOT. In fact, certain low displacement pressurized 4-stroke outboards are worse. The fuel efficiency advantages of 4-strokes and DFI 2-strokes occur at engine speeds that are less than WOT. When comparing fuel efficiency advantages in terms of percentages, the ICOMIA duty cycle is really the best way to compare potential fuel savings. Typically, a DFI 2-stroke or 4-stroke will show a 35 to 50 percent savings over a conventional 2-stroke under that duty cycle because under that duty cycle, approximately 50 to 60 percent of the operating time is spent under half throttle where fuel savings are the greatest.
lordswork2007	posted 08-13-2007 11:42 PM ET (US) For comparison, a 160 hp air cooled fuel injected 4 stroke Lycoming airplane engine, at 75% power (120 hp) burns approximately 8.5 gallons per hour for a SFC of .435. At 100% (takeoff) power, however, the fuel consumption almost doubles, to 16 gph, for an sfc of .61. The increase in sfc is due to enrichment of the fuel mixture to lower exhaust gas temperature and to provide additional cooling. At 160 hp, reducing the fuel flow below about 13 gph would result in damagine exhaust gas temperatures over 1700 deg F. Aircraft engines are interesting test beds because the amount of fuel can be controlled independently. The 8.6 gph figure for 120 hp is the result of setting the fuel flow to correspond with a desired exhaust gas temperature. There is some controversy about this subject, but the factory manual suggests leaning until peak, then enriching until the egt rises 25 deg F, which results in about 8.5 gph at 120 hp. The engine will almost one gph leaner without misfire, but EGT's are near peak, and the peak is near 1600 deg F. Leaner than about 7.5 gph and the engine becomes rough and the power falls off steeply. At about 6.5 gph the lean edge of stoichometry is reached and combustion ceases. All of the above assumes a manifold pressure of about 25" and an RPM of about 2400, which together when the fuel flow is correct, produce 120 hp. I have contended before that 4 stroke outboards must most likely enrich at full throttle to protect their exhaust valves. Perhaps that is programmed into their engine management systems. Carburated autos did the same thing with a full throttle enrichment valve.
AZdave	posted 08-14-2007 02:32 AM ET (US) Lower specific fuel consumption can also be found in diesels of modest size. Nebraska did detailed testing on farm tractors over a number of years. A John-Deere 730 put out 56.7 HP on 3.15 gal/hr. They also report specific fuel cnsumpion of 0.38 - 0.39. These are the old two cylinder models fom the late 1950's, and are very heavy. That's actually an asset in tractors, as it improves traction. As usual, other factors can win out over fuel economy. These tractors had hand clutches. My Dad got rid of his J-D before I started to drive. He had a number of persons driving tractors and wanted all the clutches to work the same. Dave