16 replies to this topic

[GSX-R]

Hello,


Does anybody can provide me brake specific fuel consumptions curves according unit displacement ?

Regards

[Greg Locock]

Better than that, wander over to www.lesoft.co.uk and download their free, fully functional, single cylinder engine simulator. It gives you BSFC curves.

Not that i really understood your question.

[GSX-R]

Thank you Greg, i've already got this software a few weeks ago and i plan to test it about this subject, good idea.

If anyone else has any exprimental curves...

Regards

GSX-R

[Greg Locock]

http://www.hotrod.co...660/index3.html

try googling for bsfc curves

I still don't understand what you are after to be honest, a bsfc curve by itself doesn't tell you much.

[GSX-R]

I'm looking for a graph where i can see many BSFC curves (at WOT) corresponding to differents mono cylinder displacements.

for instance :

300cc BSFC
400cc BSFC
...
1000cc BSFC


using the same parameters as stroke/bore ratio.

You see ?

[Greg Locock]

Ah OK.

You may find an SAE paper on that - try searching on optimum cylinder size or something. I don't know if the Lotus software is clever enough to produce valid differences based on cylinder size, their heat transfer model would have to be pretty good.

When I was involved in engines there was a rule of thumb that 300-400 cc per cylinder was the optimum, at the time. However that was before the introduction of CFD and so on to engine design.

Also bear in mind that for production cars part throttle BSFC is more important than full throttle, and the two don't track especially well. The speed at which optimum bsfc does tend to track across throttle openings, but that is not an ironclad rule either.

Are you interested in power to weight ratio, full throttle BSFC, part throttle BSFC, torque spread, power to volume ratio, cost, emissions,specific output, or what? For what size vehicle? For what duty cycle? for what gearbox?

[GSX-R]

Sorry to disappoint you, it's just to enhance my understanding of the piston engine.

I'm interested in piston engines theory in general, not only for automotive. So i'm also interested into experiments results especially about efficiency.

I've learned the more unit cylinder you have the less the BSFC you have due to better volume/surface ratio, i would like to get some kind of scale beetween a 100cc and a 1000cc.

I know in four strokes CI engine we can go as far as 300.000 cc at least with a BSFC as 140g/kW/h

My concern is SI engines but i'm also interested into CI engines especially small engines using small cylinders like Smart CDI engines (233cc unit).

Regards

[McGuire]

Wellll.....BSFC is really no more than the the inverse product of the engine's total net efficiencies. Improve one or more efficiencies in a given range, there you will lift the BSFC curve. Assuming ideal carburetion, the BSFC curve will mirror the mep and torque curves (or when inverted, track them). So whatever can be said about the mep and torque curves of a specific engine configuration will also tend to follow for its BSFC curve (disregarding carburetion considerations). I have to admit I don't understand exactly what you are asking.

[GSX-R]

I've tested it into Lotus sim engine and it gives me for a typical 4 stroke engine the minima following values :

50cc : 275 g/kWh
100 : 267
200 : 261
300 : 259
400 : 257
500 : 257
1.000 : 256


I'm gonna test for a CI engine.

Regards

[Earthling]

Does that take into account the increase in knock sensitivity from increasing cylinder bore? If not then then I would have thought that efficiency would have increased a bit faster over 400cc/cyl....

[GSX-R]

I've used the basic model.

Numbers above are result using a constant stroke/bore ratio=1.

[panic]

IDK what you're trying to establish. Yes, the thermal efficiency goes up (and fuel consumption down) with larger cylinder due to lower heat loss from surface/volume ratio.
However, power per unit of displacement goes down quite rapidly as cylinder size increases.

[GSX-R]

less surface/volume also implies less friction / HP
Of course power/displ is less

[Greg Locock]

Isn't that just because the valve area only increases with bore^2, so in effect you are throttling the larger engine?

[J. Edlund]

The problem when making large cylinder for the otto cycle is that the flame path tend become long and thereby increase the time for combustion which reduce the thermodynamic efficiency of the engine among others. Knock also tend to become a problem.

A large engine also tend to have a high BSFC during part throttle due to large pumping losses.

Of course, if we use the diesel cycle instead these problems go away.

There is also a paper about the turbocharged Honda V6 F1 engine where some effects on BSFC was mentioned, for example so did BSFC decrease when boost was increased or when fuel or charge air temperature was increased.

[GSX-R]

Reply to Greg :

In my example, i've made the valve size to follow the bore size.

Regards

[Greg Locock]

So you are increasing the valve diameter faster than the % increase in bore diameter? OK. That's that theory out of the window, but is not usually a practical approach