ringram

Ive been looking trying to find information on bsfc at various loads for the different inductive methods above.

Under WOT I have found a fair amout of info to suggest
NA ~0.45 /lb/hp/hr
SC ~0.60
Turbo ~0.55

Does anyone know the various general bsfc numbers for part throttle of the above apps. Say 50kpa @ 3000rpm?

Now assuming forced induction applications are operating at near to their adiabatic peak efficiency under WOT am I right in assuming that at part throttle all apps will suffer roughly in proportion to the above numbers with the SC suffering the worst?

Therefore given a constant engine displacement (eg 6L LS2) the NA application will always be the most efficient way of obtaining each BHP (even if total output varies between methods)

In which case an efficient and sensible daily driver heads and cam setup will be more efficient than the equivalent mild boost supercharger (say 8psi) on the stock engine?

Personally Ive seen my LS1 with H/C put down more than a non intercooled H112 LS2, in this instance my BSFC would be around 20-30% less?

Would anyone care to comment on this please.

Steve Bryant

iTrader: (1)

Ringram,
I'll comment, but I don't know the answers you are seeking. My comment is that this is just the type of question and discussion topic that this forum is aimed toward. I am now curious to learn more about brake specific fuel consumption (BSFC) myself.

A general thought is that with forced induction, you are using power to drive a compressor. This use of power always comes at a cost in terms of fuel specifics/efficiency. This is one of the reasons that a Roots-based design like the Eaton units have a bypass valve that is actuated by manifold absolute pressure (MAP) versus ambient pressure. Once the mass of the supercharger is spinning, the engine is primarily sustaining the rotational velocity of the supercharger and the supercharger is not being loaded to compress the incoming air. All of this is being done in the name of fuel efficiency. On the other hand, the induction system of the supercharged engine is less effective than that of a normally aspirated engine with properly designed runners. As we know, it's all a compromise that's done in the name of the desired end goal(s). Similar losses are true for turbochargers. Also, with both systems, a lot of potential kinetic energy is being used in a manner that converts it to heat, thus another loss in BSFC.

All my best,

Steve

DanO

First, BSFC varies with rpm and load, so no single figure can be claimed for either.. (its a "map".. not a 'value')

There is also the comparison of BSFC at WOT and BSFC at cruise..

turbodiesel engines at WOT can drop below 200g/kW-hr BSFC (0.33 lb/hp-hr).. This is on its way to becoming a truth for forced induction Gasoline Direct Injection engines as well.

There are so many variables that are included in the BSFC discussion, you would be better off picking up a copy of "Engineering fundamentals of the Internal combustion engine" by John Heywood

As a general rule of thumb, for a given power and torque output (max rated) a smaller displacement forced induction engine will give better Fuel Economy which means that BSFC at cruise is improved.. and may or may not have better WOT BSFC..

If you speaking of Aftermarket performance, they can vary DRAMATICALLY based on calibration, component design, etc...

Here is an example of a BSFC map


one other note.. with performance aftermarket.. everyone is tuning their fuel curve for a SAFE and RICH mixture.. this hurts BSFC dramatically... if you work on determining max EGT's allowable you can minimize your BSFC by decreasing your enrichment by as much as possible

gametech

iTrader: (1)

The only comments I can offer to this would be that the constraints of your question would give the advantage to one design over another. By keeping displacement constant and assuming a set manifold vac and rpm you would seem to bias the result toward NA. However, what if we kept HP constant, but let displacement vary at said vacuum and rpm. By juggling the full combination of variables, I believe the results could at some point be skewed to favor every combo from a BSFC standpoint. This is one reason a lot of econo-style cars use tiny FI motors, rather than NA motors large enough to produce the equivalent peak power. These same constraints explain why trucks use exactly the opposite. The very narrow "normal" operating conditions of the motor in a particular vehicle weigh heavily on which approach is used.

ringram

Yes I have this It didnt appear to have a clear explanation of this specific query. Though perhaps if my differential calculus was any good I could have worked it all out myself.. DanO do you have maps like that for each of the above applications for the same displacement engine with the same BMEP axis? Thats the sort of thing Im after.

I agree that given a specific power output its possible to build say a turbo engine of smaller displacement where the output exists in the area of peak efficiency and that in this case its possible this application would deliver the best bsfc.

Perhaps we can work a more specific example. An LS2 with a peak horsepower requirement of 530bhp. Which application will have the most efficient production of power over the widest range of operating loads?

Forced induction increases volumetric efficiency at the expense of mechanical (increased paracitic loads) and thermal efficiency (higher temps and lower compression). I would like to know more about the relationship between these tradeoff's.

As for your comment on aftermarket tuning rich. I agree, there are civil liability reasons for this. Its very noticeable over here. I know of top notch tuners using WOT mixes of 13.4:1 in fact TonyM at AFR mentions in one of his latests posts that they ran 13.2:1 under WOT over a certain range for optimal power. Often guys quote 12.8:1 more for safety than power. That represents a 5% difference just in fuel, let alone the reduction in power the richer mixture brings. However for the purposes of this question I would like to assume that AFR and all other variables such as ring tension, exhaust tuning etc are optimised for each application so we can just concentrate on the bsfc effects of the induction system itself.

Maybe the variables are too broad?

ringram

Ok, nobody wants to answer

How about a confirmation of this then. "A naturally aspirated 6L LS2 with head/cam/intake modifications to 530bhp is more efficient over the widest load range (lowest bsfc) in comparison to turbo or supercharging"

DanO

I'd say the opposite... a smaller camshaft with boost will beat a large camshaft NA... especially for part throttle!!

ringram

Including SC?
This book seems to suggest that bsfc which is already worse in an SC than NA gets even worse again at part throttle http://books.google.com/books?id=pkM...icCvQREyCIl5fI

A Stock engine with FI is just forcing air into an inefficient engine.
Surely an engine with good intake, cam, heads etc is more, mechanically and thermodynamically than a stock engine with SC. Plus volumetric increases too, maybe not as much as with the SC. If you chose more lift rather than more overlap or duration on teh cam that should also help NA
Sure volumetric is better with SC but you lose out on mechanical by having to drive the supercharger and heat the air.

Also if SC or Turbo was the most efficient wouldnt all vehicles be using it?

The Manalishi

iTrader: (3)

BSFC my thoughts. The "perfect air fuel mix is 14.2:1. If a NA engine were to be tuned to this it would theoretically make the best HP number and be the most efficient at the same time. The problem occurs when you add boost. This is no longer a safe A/F because of the added heat from the air being compressed. Turbos make this worse. Intercoolers and aftercoolers are a band aid but they can't keep up. If you could get a boosted motor to live at 14.2:1 you would gain even more efficiency and HP but probably not over the NA application.

This all assumes you could...
1. Maintain 14.2:1 A/F
2. Maintain the same heat though the entire operating range.
3. Maintain the same friction throughout the enire operating range.
4. Always have the same air density, altitude and humidity.
5. Ensure the same load was applied in the same manner everytime its used.
There are several other variables that would have to be controlled to show what you are asking if I understand the question correctly.

My thinking is that BSFC is skewed by heat, friction and just enviromental issues to be a true measure of what an engine will really perform like across the entire spectrum. Too many variables. If you could control all of the variables you could get a nice tidy little graph to show the numbers perhaps.

DanO

They will be in the future! the reason that all vehicles arent is because of cost!!! just type in ford Ecoboost into google or read this..

http://www.autobloggreen.com/2008/01...njection-engi/

basically.. its only the begining for downsized boosted vehicles.. all manufactuers will have boosting in one form or another..

RednGold86Z

Supercharger cannot help economy, almost by definition.

VE isn't important at all, but the means to get VE can be (such as non restrictive intake and exhaust).

Friction is king. Pumping losses are Queens, Fuel burn efficiency (i.e. lean and quick) is, uhh, Jacks.

Assuming GASOLINE...
Maybe the best way to pose this question is how to get 500 hp with best daily driving fuel economy - <2 Liter, ~15:1 CR (or variable even), TURBO with variable vanes, Super High pressure GDI (no end gas to knock, and lean burn stratified charge cruise, no manifold or wall wetting), Valvetronic - no throttle body.
Edit: Now that I've read the above link, I see Ford is on track with that already.
Quote:
"Therefore given a constant engine displacement (eg 6L LS2) the NA application will always be the most efficient way of obtaining each BHP (even if total output varies between methods)"

In most cases, at WOT, probably true (just BSFC going on there, and it's pretty common knowledge). But, Turbo would probably make most power, and probably still have best daily driving economy (modest cam, modest RPM range, thus modest gearing).

DanO

Please.. help me understand how a turbo can help and a supercharger cannot?

turbo's are an exhaust restriction, even at part throttle.. thus taking energy.. a supercharger has a bypass and the only energy imparted is overcoming friction to keep it spinning.. which at cruise is less than 0.5hp and with a few improvments can be less than that

The benefit comes from downsizing (reduced friction, reduced pumping loss at cruise, etc..)... in both cases.. turbo and supercharged. and then the boosting device is there to increase maximum power output

either way.. for a given max power output, part throttle fuel economy will be better with a smaller boosted engine than it will be with a larger NA engine

RednGold86Z

I guess I was assuming similar builds (cam) when I made that statement, as it is always some efficiency loss at all loads. I also didn't say a turbo (in a direct comparison) would be better (read on).

Without downsizing and without newer technologies (like direct injection), N/A is usually best for economy. Charging usually requires either higher octane or lower compression than a similar N/A engine, with old technologies, both can put a downsized engine of similar output at a disadvantage. http://www.deq.state.or.us/aq/orlev/...40904attC5.pdf

With newer technologies, getting more power from a smaller engine with turbocharging surely should have better economy than a larger one N/A. You could argue that a supercharger is also similar to a turbo, but I would bet it would be harder to get it quite as good as variable vane turbo (although s/c is a simpler package to design, and might be more reliable long term).

DanO

That article focused specifically on spark knock.. HOWEVER, with new technology and future technology.. spark knock limitation is significantly reduced. Technologies like Direct injection, egr, etc.. will enable the spark ignition engine to be downsized and increase fuel economy.

that article specifically states the limitations of port fuel injection engines with todays technology.. once we can hit MBT timing at every condition.. you will definately notice a benefit to downsizing and boosting.. direct injection is the first step..

also if you look at the % of fuel cost increase by upgrading to premium, its not as significant as it once was. Typically preimum fuel is a $0.20 price increase over 87 octane.. with 3 dollars per gallon its only a 6.7% cost increase versus a 10% at 2 dollars per gallon.. so as fuel prices increase and the "premium cost" isnt impacted, it will almost be pointless to argue.

71 chevy

iTrader: (18)

I think the turbo at same CID and same power level would have better bsfc than the NA car

1. at part throttle, the turbo might be under little to no boost, so its acting like a stocker and making little power.

the heads cam, high comp engine will make more hp at lower rpms, which burns more fuel

loser187

I think the cam is the biggest drawback to a N/A performance engine due to big overlap at low engine speeds killing power.

I wonder if there is a big difference in a long stroke vs. short stroke engine? Higher compression with tight overlap cam would sound like a overall winner, but not make the best HP.

Maybe a goal like: most efficient 500hp engine would clarify things??

ringram

L99 vvt ftw!