B.s.f.c. Bs.....?

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pishta

pishta

I know I'm right....
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This is a well known formula for deducing fuel requirements for a certain power level..but I don't get it.

"...In most cases a naturally aspirated engine will have a B.S.F.C of .50. This means that the engine will use .50 lbs. of fuel per hour for each horsepower it produces. Turbocharged engines will want to be at .60 lbs. per hour or higher..."

Now correct me if Im wrong: gallon of gas is just more than 6 lbs. So a slant is 'bout 100HP for formula sake.. it should use 50 lbs of fuel per hour, or 50/6= 8.33 gallons per hour..? At 60 mph, that is about 7.2 mpg? what am I missing.....Seems the BSFC would be more like 3X that .50 figure estimating on a good day it gets 21 mpg on the highway....I like math but I like it more when it works.....as described. And if it doesn't, well tell us WTF.
 
I would say that formula is BS because its all in how your engine is set up. You can always lean your carb out and get way more mpg compared to the power loss.
 
Don't forget that Holley 4bbls and Holley 2bbls are rated at different test vacuum readings. Carter used to be flowed "wet", Holley "dry," so they were not equivalent. I have no idea how various 1bbl carbs are flowed, nor if the Ed carbs are flowed like the old Carters.
 
I think cruise HP is under 40, for most cars, so that puts mpg at about 20.

One of the best resources for automotive formulas and data is the Bosch automotive handbook. It was a great help in the development of my efi system. Another good book is the Heywood book Internal Combustion Engine Fundamentals. It is old, but done on stationary engines, lots of good data.
 
I agree with Kit on this. Not being an engineer, if the slant develops 100 HP @4500 rpm, you don't usually drive around at 4500 rpm. Lets say you drive at 1500 rpm and only develop 1/3 the max HP, or about 30.
In your equation, 30 HP would use 15 # of gas over 60 miles, or about 2 1/2 gal. The difference between this 2 1/2 gal and what you actually use (maybe 3 1/2 gal for a difference of 1 gal) would be used for other "work" like generating electricity and lost to heat out the exhaust pipe.
Again, not an engineer; just my random thoughts.
C
 
.50 lb of fuel per hour per horsepower?
so a 100 hp 350 Chevy (That's the max that they'll run reliably for a whole hour at, I think)

I'm sure that it's an hour in my math, but this is what I see...
 

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I may be off base, but the only way I have ever seen this formula used is to determine the amount of volume needed to run a high horsepower motor. If you have a 600hp motor and a 20 gallon per hour pump, you starve the motor. I have a Clay Smith 160 gph on my car, so I am ready for E85 and the excessive volume of fuel it will use.
 
Fuel requirement increases with the quantity of intake air. Normal cruise is not at full throttle, and is likely below 35% throttle. RPM and air flow contribute to fuel use.

I built in a feature of adding injector pulse widths over a moving time window for real-time fuel consumption display. The fuel pressure regulator maintains near constant differential pressure at injector.
 
OK, so this is a max effort estimation on what you are to build your fuel system to? That makes more sense.

OK, this makes more sense now. This chart shows what Kit is saying, cruise RPM/power is way down...Diesel shown
CumminsBSFCChart.jpg
 
You guys aren't considering the fact that this is for wide-open throttle only... It's a way to determine the maximum efficiency of the engine when it's working the hardest. Obviously nobody drives around at WOT all the time so you can't directly compute realistic gas mileage figures from the BSFC.
 
Pishta, you have the relationship inverted. At 21 mpg, the BSFC would be 1/3 that at WOT (not 3x). The only way to use 100 HP steadily at 60 mph (without accelerating) is to drive out of the LA Basin on I-5 or I-15 where you climb continuously to 4500 ft. We did that last Sunday in our 2002 T&C 3.8L (close to a slant) which has an instant mileage indicator. I recall ~8 mpg vs the normal 23 mpg on the flatlands.

In both cases, the fuel controller keeps the air/fuel ratio almost the same. What is different is that when cruising on the flatlands the throttle plate restricts the inlet air, dropping manifold pressure. The pistons still draw in about the same volumetric flow, but the mass flow is 1/3 due to the lower pressure. When climbing, with WOT, the manifold pressure gets close to atmospheric pressure.

A diesel is quite different. It always runs WOT (no throttle plate). You inject more fuel to increase horsepower. Diesels don't rely on spark ignition. The high compression (~20:1) heats the air enough to ignite each fuel droplet, regardless of how many droplets are there.
 
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