estimate BSFC sweetspot for partial load 1

[pickler]

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.

 

[140Airpower]

I think Greg is right. Peak efficiency should occur at the torque peak, but without enrichment. AFAIK most engines at WOT REQUIRE enrichment at the torque peak to suppress detonation because they are designed to use up all the octane rating of the fuel. Reducing the charge pressure just a little by just a little throttling will take you out of enrichment.
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?

 

[Lou Scannon]

Why is because, ideally at least, valve throttling is not imposing a pressure drop as charge flows across the valve, it is an on-off event (akin to Miller Cycle), so little energy is wasted pumping charge past the valve.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[140Airpower]

hemi, good point, but valve throttling techniques include cracking open the valves rather than ultra short duration at higher lifts. The latter is closer to the on-off, "chopper" type, flow control. The former is closer to butterfly or other high restriction flow control. I guess you can argue that even cracked open valves still have an element of the chopper function.

 

[pickler]

One of the intake valves is in my NA Subaru engine is always in high lift mode to promote swirl as part of the variable Lift feature. I think this is why throttling doesn't affect it much. There are newer continuous variable valve Lift technologies that basically eliminate pumping losses almost entirely.

 

[pickler]

valves in my NA Subaru...*

 

[mattsooty]

Pickler,

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

 

[pickler]

BSFC is measure of power developed vs fuel burnt. In short i want to find the optimum BSFC for my engine. So far from experiments best acceleration is achieved when engine is developing 50hp. This translates to about ~7in.Hg of vacuum (3/4 engine load) at say 2000rpm . My fuel economy has thus far increased by %5 accelerating this way.

 

[Lou Scannon]

Fascinating. I'd be interested to learn how you made this determination in a statistically significant manner.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[140Airpower]

Good question hemi. Another is what is the overall fuel mileage for a trip of a given distance. BSFC is an instantaneous measure. The duration of an acceleration phase counts as well. 3/4 power acceleration (or "part throttle") has long been the recommendation, but shorter duration full throttle acceleration has been claimed to be better overall.
Again, enrichment is the fly in the ointment of efficient operation, meaning that best fuel efficiency does not necessarily align with best thermal efficiency.

 

[pickler]

as i said above I did try full throttle but it had significant negatives effects on fuel mileage even though my AFR was not enriched unless i went over 3500rpm. Also at high load i-AVLS switches from fuel economy cams to high output ones, similar to honda's i-VTEC.

here is one for a vtec honda engine.

 

[Lou Scannon]

That's a very plausible diagram, and for a fairly small engine I presume, based on the peak torque of only 90Nm. If I were attempting to accelerate with optimum fuel economy with that application, I would install a vacuum gauge and endeavour to keep vacuum in the vicinity of 10"Hg while accelerating, at the same time upshifting no higher than 3500rpm. In fact, that's pretty good generic advice for any similar spark-ignited application, non-boosted at least. Nothing much has changed. If one could produce such a diagram for an old-fashioned carbureted engine, the general map would be very similar, perhaps with contour values shifted up to some degree.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[pickler]

I would think that lower displacement usually has lower friction due to smaller stroke and lower number of cylinders, so the BSFC sweet spot would be up higher in the diagram? As for carbureted I have no clue, there are very little charts out there on older vehicles that take friction into account. The Subaru 2.5L engine which has a oversquare design and coated pistons should have lower friction than most other engines on the market, so in theory I was expecting higher RPMs (in practice enrichment and friction loss does not allow this). Just for fun, here is a full throttle BSFC for a mechanically fuel injected 2.2L 1971 Porsche 911E:

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.

 

[murpia]

Is anyone aware of any software that could collect and process the necessary data to produce a BSFC map using an OBD connection?

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

 

[pickler]

tools like scangauge 2 do exactly that but do not log the information. Also the reported horsepower numbers are grossly inaccurate at high loads. The only way would be a BSFC capable chassis dyno.

 

[GregLocock]

Ian- Your vhicle model could be a bit better than that, since you also know acceleration. You don't know wind and gradients.

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

http://eng-tips.com/market.cfm?

 

[murpia]

I didn't have much luck Googling for a cheap / free OBD port data logger. That surprised me a bit as I thought it would be a common thing to want and someone would have produced a freeware version by now.

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