Does BSFC convert directly to air/fuel ratio?

[Guest bastaad525]

Just what the topic says I'm wondering if BSFC numbers can be directly translated into a/f ratio numbers. For instance, when you're trying to figure out how much fuel a motor would need (or how big to size your injectors), lets say for a turbo motor with the BSFC of 0.55 (by the way what exactly is this number a measurement of?), is that 0.55 equal to a specific a/f ratio? It would seem to me that the two can be directly linked.

[zuhow]

The performance characteristics of an internal combustion engine can be reported with or without the effect of power reducing friction and other losses. A value of a property (power, fuel consumption, etc) that includes the effect of losses is know as a brake value. (If the effect of losses is removed it’s know as indicated value).

 

 

 

The specific fuel consumption is the fuel usage rate divided by the power generated. Typical units are lbm/h*hp (pounds per hour-horsepower) or kg/kWh (kilograms per kilowatt-hour). The term specific refers to a quantity that is referenced to mass. i.e specific volume would be cubic feet per pound and specific energy would be Btu per pound

 

 

 

It’s probably best to think about BSFC along the lines of wheel fuel consumption. Note that lower values of BSFC are better.

 

 

 

The air/fuel ratio is the ratio of air mass that enters the engine to the mass of consumed fuel. You’re right that the 2 can be directly linked, but not really usefully. If you know the brake hp and you know the fuel mass flow rate (in lbm/h) you can calculate the BSFC:

 

 

 

Fuel mass flow rate= (BSFC) * (BHP)

 

 

 

And if you know the (desired) A/F ratio, you can calculate how much air flow you need. Likewise if you estimate the BSFC and you know how much wheel power you want you can calculate the required fuel mass flow required and then the air mass flow required for your desired A/F ratio. But you wont get an A/F from the BSFC.

 

 

 

A quick search turns up a range of values for BSFC between 0.4 and 0.6 with 0.5 being a good general number.

[Guest bastaad525]

Okay I'm still a little confused.

 

If you want to make X amount of power, you'd want to calculate what size your fuel injectors need be for a given BSFC (0.55 for a turbo was what I read). Now, I would think the air fuel ratio would play directly into this. If I'm making 300 hp, and I want to run a very rich 10:1 air/fuel ratio to be safe at very high boost or whatever, wouldn't the BSFC would be higher than if I was making 300hp at a not-so-sonservative 12:1 a/f ratio? So to even know what injector size you'd want, you'd need to know what air/fuel ratio you'd want to run, to calculate your BSFC. Well okay you wouldn't NEED to... obviously most people aren't that picky, and get injectors that are big enough to give them a little leeway. This is all just for the sake of argument

[zuhow]

You don’t calculate BSFC-you measure it. It is what actually happens as opposed to what the math and engineering say should happen.

 

 

 

BSFC is simply how many lb/h of fuel flow you need to make 1 bhp. If you are running very rich you will have a higher BSFC then if you are running an ideal A/F ratio.

 

Work it this way then- just for the sake of argument assume you’ve measured 400 bhp at the wheels and you’ve measured that your 55lb/h injectors are running at 80% duty cycle there.

 

 

 

Ok 55lb/h * 80%*6 injectors gives 264lb/h

 

In this case the BSFC is (264/400) =0.66

 

 

 

That’s how one gets the BSFC, and it will vary between 2 cars running the same set up (granted, it won’t vary much). And it will naturally vary between different setups.

 

 

 

So then you can work this in reverse, but you have to estimate the BSFC. You can use 0.55 if it makes you happy, but the reality of it is that our estimates are really guesstimates and 0.5 is as good as 0.55 is as good as 0.6, I’d err on the side of caution myself, and stick with 0.6 or higher. All that will happen in that case is that you may over size your injectors a little bit.

 

 

 

So assume BSFC is 0.6

 

Calculate the required fuel flow rate for a desired power output- say 300bhp

 

Fuel mass flow rate= (BSFC) * (BHP)

 

=(0.6 lb/h*bhp) * (300bhp)

 

=180 lb/h of fuel flow (divided by 6 injectors gives 30 lb/hr)

 

don’t forget to factor in duty cycle: that 30lb/h is required, so you would need a bigger injector (30/0.8= 37.5lb/h or more would be a good choice)

 

 

 

now decide on your A/F, lets call it 10.5

 

calculate the air mass flow rate

 

10.5=(A lb/h air flow)/(180 lb/h fuel flow)

 

10.5=(A)/(180)

 

A= 1890 lb/h airflow ( call it 14ft^3/lb dry air and you get about 440cfm at atmospheric pressure)

 

 

 

Like wise you could use 12

 

And you would come up with 2160lb/h

 

 

 

BSFC isn’t some thing you really have direct control over, it’s just a very quick and dirty measure of efficiency, basically you take the final net power output (after all losses in the drivetrain and whatnot) and simply divide the fuel mass flow rate by that number to get BSFC. Don’t get hung up on this number, but you can use it to guesstimate component performance requirements for fuel and air delivery.

 

 

 

Or you can use it to guesstimate max power output

 

Assume a NA setup with a head that can flow 250cfm (pulled this number from my arse)

 

250 cfm is about 18lb/ min or 1070 lb/h

 

so for a 11:1 A/F you would need 97lb/h of fuel (divided by 6 gives 16lb/h per injector)

 

and you would expect a power output on the order of (again assume BSFC of 0.6)

 

Fuel mass flow rate/ (BSFC) = (BHP)

 

(97/0.6)= 160 bhp

[Guest bastaad525]

cool see that's what I was getting at.

 

The final question from me then, is a turbo-motor specific one. As I have said, I have seen it mentioned on these boards (I believe quoted directly from Corky Bell's Maximum Boost book) that the ideal BSFC for a turbo engine was 0.55. That wasn't my number I didn't even know what BSFC was.

 

But my question is, what air/fuel ratio would that BSFC roughly equate to? If I am putting out 300 flywheel HP at a BSFC of 0.55, how much fuel am I using (in lb/hr or cc per min), how much air by volume, and what would that make air/fuel wise?

[zuhow]

The final question from me then, is a turbo-motor specific one. As I have said, I have seen it mentioned on these boards (I believe quoted directly from Corky Bell's Maximum Boost book) that the ideal BSFC for a turbo engine was 0.55. That wasn't my number I didn't even know what BSFC was.

 

 

 

Just to be clear: The ideal BSFC of ANY engine is 0.00

 

In that case your engine would make power without consuming any fuel!!! A lower BSCF is always better because you use less fuel.

 

But my question is, what air/fuel ratio would that BSFC roughly equate to?

 

 

 

YOU CAN’T GET THERE FROM HERE BROTHER! (I hope that doens’t come out wrong,- I’m not shouting at you….)

 

 

 

If I am putting out 300 flywheel HP at a BSFC of 0.55, how much fuel am I using (in lb/hr or cc per min), how much air by volume, and what would that make air/fuel wise?

 

 

 

I’ll cut you some slack, but you should plug the 0.55 into the calcs I laid out for you, and you’d see that with 300 hp and 0.55 BSFC you’d need about 165lb/h. You then have to DECIDE what A/F you WANT and then calculate the amount of air mass flow to reach it.

 

 

 

I’m thinking you’re not quite understanding the brake part of BSFC. It takes into account all the losses, so asking about what’s going on at the flywheel makes me thing you’re still a little unclear. Maybe I’m not so good at explaining the concept….

 

 

 

There is no ‘backward’ connection from BSFC to A/F, but there is a connection ‘forward’ from the A/F to the BSFC. You CAN’T determine the A/F from the BSFC, but the A/F is part of what goes into measuring the BSFC

[Guest bastaad525]

okay I think I get it now

 

Sorry I DID miss some details from your reply, I was reading this right after I got home from work in the morning and was pretty tired. But it does make a bit more sense now, and did fully answer all my questions. Well... mostly .

 

I'll play with the equations a bit, I want to see what BSFC I'm running, seeing as how I know my injector size (stock 260's), power level (240rw or approx 280 flywheel), and a/f ratio (varies by rpm but averages 11:1). I want to see if it matches up with the recommended 0.55.

[BrandonsZ]

So then I clearly get your calcs, great job with that, you sound like a pro.

 

The computer will adjust the A/F using the O2 sensor (assuming you are using FI) and the jet size will deturmine A/F ratio if you use a carb. So you can set the A/F ratio that way, that's easy.

 

Assuming you have a carb, you can spec the carb by CFM, so that's a no brainer. (assuming your heads let at least that much through as well)

 

But with Fuel injection, you're only concerned with CFM of dry air so there must be curves or empirical data to support CFM ratings for different throttle body sizes assuming the intake and heads can support the throttle body size.

 

Or am I way off in the bushes?

[zuhow]

BrandonsZ, I think you're right on . I was thinking more along the forced induction lines. As we all know, getting more fuel into the engine is the easy part, it's getting more air into the engine that's the trick.

 

I guess you'd have to know what kind of flow you were getting on the NA set up (either carb or FI) but you should be able to SWAG (Scientific Wild A** Guess) that. Or as you say, 'empirical data' sounds better that way.....

 

I'm really interested to see what Bastaad comes up with ( if you get screwy numbers because of the units let me know if I can help you out) . I'm guessing that he'll come up over 0.55 but i dunno, let's see!

[zuhow]

Bastaad

 

I've got to spend a little more time at work today since I left early the last two days and I'm putting that to use for you, plus I'm dying to see what happens:

 

let's call your bhp 240 ponies (at the wheels)

you're using stock injectors right? those are 265cc or 25.2lb/h (so says that great turbo link that I dont feel like looking up the address to put in here, but you all know it)

 

Ok, on to the meat of it:

 

25.2 lb/h * 6 injectors is 151.2 lb/h ----this is 100% duty cycle *more on this later

151.2 * 0.9 = 138.1 lb/h ---90% DC

151.2 * 0.8 = 121 lb/h ----80%DC

 

if we take:

 

(121 lb/h) /(240 bhp) =0.50 BSFC * at 80%DC

(138.1)/ (240) = 0.58 at 90%

(151.2)/ (240) = 0.63 at 100%

 

*so which number is it? You're using the RRFPR correct? Any idea what pressure it's going to? If you (or someone else) can give me an estimate of that it would help.

 

You're clearly flowing more fuel than the 80% duty cycle at stock fuel pressure. How much more, I've got no idea. Hopefully you or someone else here (there's a lot of guys smarter than me here, come on, I know one of you knows this...) can give us a better idea

 

You're certainly in the ballpark of 0.6 BSFC, give or take. We need a little more data to really nail it down. Do you have any way to "borrow" or otherwise appropriate an ocilliscope and find out just how long those injectors are open and tell me what fuel pressure you're running at peakpower?

 

My gut tells me you're probably pushing 10-15% more fuel because of the RRFPR and operating at about 80-90% duty cycle. that's pure speculation, mind you.

in that case you be looking at a BSFC of about:

 

(121 * 1.1 ) [10% more fuel flow] / (240) = 0.55 on the low end (80% DC)

(138 * 1.15) [15% more fuel] / (240) = 0.66 on the high end (90% DC)

 

so it seems 0.6 BSFC is a good number to use for the L28ET

[Guest bastaad525]

at wide open throttle and full boost, my fuel pressure hits about 70 psi. That's at 13-14psi of boost.

 

Relatively, w/o the RRFPR, my fuel pressure would hit in the 50-55psi range. So it's not that big of an increase.

 

And I think I've heard it mentioned on these boards a few times that the stock EFI doesn't run the injectors at more than 80 or 80% duty cycle.

 

Anyways it was enough to bring air/fuel rations into the 10:1 to 12:1 range. It varies pretty wildly, running extra rich at boost onset but leaning out gradually as RPM's climb.