estimate BSFC sweetspot for partial load
estimate BSFC sweetspot for partial load
(OP)
I have a naturally aspirated 2.5L boxer 4-cylinder engine. I would like to estimate brake specific fuel consumption sweetspot for my engine at partial load (say +50% based on MAP readings). I do have a chassis dyno curve which i thought some one could use here to help estimate BSFC from. Basically this is a 16 valve boxer 4 cylinder oversquare engine with variable valve lift (Not timing). I have been told valve lift occurs at high loads and/or higher engine speeds and that one intake valve is always open.
I have attached the dyno graph. Looking at the dyno, the engine has a relatively flat torque curve for a 4 cylinder. Peak torque occurs at 4400rpm and peak horsepower at near 6000rpm. AFR stays stoich until just under 4000rpm. My estimates for BSFC sweet spot are 1500-2000rpm at 80% and 2000-2500 at 60% load, but i'm not sure how valve lift at higher loads affects BSFC.
I have attached the dyno graph. Looking at the dyno, the engine has a relatively flat torque curve for a 4 cylinder. Peak torque occurs at 4400rpm and peak horsepower at near 6000rpm. AFR stays stoich until just under 4000rpm. My estimates for BSFC sweet spot are 1500-2000rpm at 80% and 2000-2500 at 60% load, but i'm not sure how valve lift at higher loads affects BSFC.





RE: estimate BSFC sweetspot for partial load
Didn't Subaru start that option ~ 2005?
Does your car have an instantaneous MPG display option?
If not, there are some phone apps that interface with the OBD port and can display that info.
Or there are more expensive separate meter options.
Then drive the speeds you want and test.
A friend with a new normally aspirated Subaru insists his instantaneous mpg is better in 5th than 6th around 65 mph on the highway in our slightly hilly section of Eastern Massachusetts.
RE: estimate BSFC sweetspot for partial load
RE: estimate BSFC sweetspot for partial load
You also have to decide what criterion to use as a yardstick. Myself, I would use amount of fuel consumed to go an absolute distance, with the proviso that this distance must be sufficient for the slowest accelerating scenario to reach a given cruising speed, which must be the same for all tests.
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: estimate BSFC sweetspot for partial load
Quite why this is so is an interesting question to which I have no pat answer.
However that assumes the torque curve is at optimum af ratio, yours is not. If you fully fueled it lower down you'd get more low down torque
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: estimate BSFC sweetspot for partial load
observations :
Run....Load(%).......Shifts(RPM)....Consumption(L/100km)....Acceleration Time(s)...
A.........60%.............1800rpm.............11.5...........................17...
B.........80%.............2300rpm.............12.0...........................12...
C.........90%.............3000rpm.............11.9...........................9...
consumption is liters per 100km, so lower the better. Load is based on Manifold absolute pressure and NOT throttle.
RE: estimate BSFC sweetspot for partial load
RE: estimate BSFC sweetspot for partial load
RE: estimate BSFC sweetspot for partial load
you can see that BSFC sweet spot is no where near the peak torque
RE: estimate BSFC sweetspot for partial load
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: estimate BSFC sweetspot for partial load
http://ecomodder.com/wiki/index.php/Brake_Specific...
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: estimate BSFC sweetspot for partial load
RE: estimate BSFC sweetspot for partial load
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: estimate BSFC sweetspot for partial load
I wonder about the impact of the fuel use during 20 seconds of a 30 minute real life road trip. I'm guessing throttle control and dRiViNg during the other 29.6 minutes might swamp the results.
I was expecting others to comment on the quoted 12% innaccuracy of a MAF sensor. I'd think that would clobber the A/F ratio and emissions.
I'd repeat the test 30 times using ~ 1/2 throttle (effectively full throttle at low revs and shifting at 2500 or so.
RE: estimate BSFC sweetspot for partial load
RE: estimate BSFC sweetspot for partial load
basically higher the efficiency colour the better gas mileage. What i found was that at lower RPMS higher loads are beneficial, but as RPMs climb load must fall too. It might have some thing to do with the variable valve LIFT system (i-AVLS). Anyway during one run we managed to shift at low enough RPMS without logging or kicking in the valve lift and that's when we scored one of our best runs. each run was approximately 800 meters long. Beyond 3500rpm we found the efficiency to drop significantly no matter what the engine load was. Mixture was consistently enriched above 3500rpm. Sweet spot seemed to be 2100rpm with 6 in.hg of vacuum or approximately 30kw/45hp held steady at 1500-2500rpm.
thanks for the help.
RE: estimate BSFC sweetspot for partial load
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: estimate BSFC sweetspot for partial load
RE: estimate BSFC sweetspot for partial load
RE: estimate BSFC sweetspot for partial load
Tmoose, the acceleration rates are fine...35kW and shifting at 2500rpm allows me to flow with the traffic if not faster. For highway cruising i don't use BSFC data unless i'm pulse and gliding. Cruising at say 65mph requires 13kW which is no where near the 35kw sweetspot. But BSFC is not a helpful tool for cruising on a relatively flat surface. It does help in a sense that lower cruising RPM will benefit fuel economy due to lower friction but higher load in general means higher throttle which is not exactly what you want when cruising. For cruising I try to minimize speed and load to reduce drag on the freeway. Updated chart:
RE: estimate BSFC sweetspot for partial load
I'm sure the charts shown are for normal engines that cannot run at the torque peak at WOT without enrichment and do not include any throttling.
BTW, throttling by restricted valving is still throttling. If pressure in the cylinder before compression is held below atmospheric, that's throttling. Yet valve throttling is indeed more efficient than plenum or intake tract throttling. Why?
RE: estimate BSFC sweetspot for partial load
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: estimate BSFC sweetspot for partial load
RE: estimate BSFC sweetspot for partial load
RE: estimate BSFC sweetspot for partial load
RE: estimate BSFC sweetspot for partial load
I'm not sure if I am missing something but BSFC is not generally measured transienttly. It is a figure that is quoted from steady state testing and involves the use of fuel weighers of some other means of accurately measuring fuel consumption.
I think that I know what you are trying to achieve but I am not sure this is the best way of going about it, nor do I think that BSFC is the metric you want to be measuring/optimising.
What is it exactly that you are trying to do/measure?
MS
RE: estimate BSFC sweetspot for partial load
RE: estimate BSFC sweetspot for partial load
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: estimate BSFC sweetspot for partial load
Again, enrichment is the fly in the ointment of efficient operation, meaning that best fuel efficiency does not necessarily align with best thermal efficiency.
RE: estimate BSFC sweetspot for partial load
here is one for a vtec honda engine.
RE: estimate BSFC sweetspot for partial load
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: estimate BSFC sweetspot for partial load
Just like the Subaru it's a boxer engine with oversquare bore/stroke ratio and 4500rpm peak torque. Porsches claiming a 4000rpm BSFC sweetspot. again this is a full throttle BSFC which doesn't mean anything in the real world.
RE: estimate BSFC sweetspot for partial load
Power would have to be estimated via an input of the road load coefficients of the vehicle.
For a given cruising speed requirement & tolerance, a 'pulse and glide' strategy could be devised to minimise fuel consumption, and indicated to the driver by a device connected to the OBD port.
Regards, Ian
RE: estimate BSFC sweetspot for partial load
RE: estimate BSFC sweetspot for partial load
I'd build a map of injector pulse width vs MAF vs rpm, or MAP or throttle opening instead of MAF.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: estimate BSFC sweetspot for partial load
I have one of those ELM327 USB scan tools, which use a virtual COM port. Looks like the simplest way forward would be to snoop on the serial traffic between the non-logging scan tool and the laptop, and log that.
Regards, Ian