Brake Specific Fuel Consumption?

[retmil46]

I've been reading thru some of the old threads on getting the best MPG out of one of these beasts. One thing I haven't seen mentioned so far is any data on what the Brake Specific Fuel Consumption (BSFC) is like on these engines, or any mention of a BSFC graph for them.

Basically, a BSFC graph shows, over the RPM range of the engine, how much fuel the engine is burning per each HP generated at that RPM, plotted as something akin to grams/HP. In short, it shows how efficient the engine is at different speeds.

This graph usually plots out resembling a "U" shape. At the bottom of the "U" is a point representing a specific rpm or rpm band, where the engine is using the least amount of fuel per HP generated. This represents the engine's most efficient speed or "sweet spot", usually listed on manufacturer data sheets as "Minimum BSFC".

On older diesels, without electronic injection and all the other modern hardware to manipulate how the engine operates, this point is usually close to if not spot on with the same RPM where max torque is available.

By comparing the BSFC, HP, and Torque graphs, you should be able to come up with an rpm band that is a good compromise between economy and power.

I've a friend down in Lockhart TX that until recently had an '04 Dodge Cummins 2500 4X4 Quad Cab. Looking thru some back issues of the Turbo Diesel Register, we found the engine data graph from Cummins for his particular engine. The BSFC graph indicated that his engine was most efficient in a band from 1800 to 2100 rpm.

His next few trips on the interstate, he tried driving to maintain 2100 rpm, which worked out to 68 mph with his gearing and auto trans. He ended up achieving just shy of 22 MPG. When he speeded up much above this, to 75 mph, or slowed down to below 1800 rpm, outside of this rpm band, his mileage dropped down to 17 to 18 MPG.

I've tried the same experiment with my '05 Jeep CRD on my daily commute, trying to keep the engine at 1800 to 2000 rpm, and it netted a 2 mpg increase, from 23 to 25 MPG.

Another interesting tidbit I've been unable to test concerns EGT vs fuel economy. Reading thru a book on the history of the Cummins family and their company, there was a section discussing testing they did back in the '30's trying to develop an engine to go in family cars and deveop a market for them. One thing they noted was that if they drove their test cars to maintain an EGT of around 600 F, they ended up getting the best fuel economy.

[dannym]

Do a search on Peak Torque