bsfc vs torque vs HP 1

[carnage1]

Is there a genral rule of thumb for peak bsfc? I know it is best at wide open throttle, but what about engine rpm? I am certain it would depend on the motor and the cam, but is there a relative curve that could be roughly plotted based on torque and HP curves?

 

[GregLocock]

It is not best at full throttle. At least, not for a normal production SI gasoline engine.

It is usually best at the rpm of maximum torque, or close to it, and typically at about 60-90% of that torque.

You won't find many bsfc maps on the www, there's a generic one in the Bosch blue book.

p27 of the following has one

me.queensu.ca/courses/MECH435/ 2.%20Engine%20Performance.ppt

note that it doesn't quite agree with what I wrote above!





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[EngJW]

Bsfc curve generally is an upside down torque curve.

 

[JKart]

Greg is 100% right! I have an old factory power/torque/bsfc graph (1972), max power at 6krpm, max torque at 4krpm and lowest bsfc at 2500rpm.
JimK.

 

[crysta1c1ear]

Maximums (and minimums) occur when the derivative is zero, ie curves are flat at the top - if they weren't flat at the top then you could just follow the slope upwards and get to a higher value.

torque

The peak in the torque curve is generally determined by airflow. When cylinders are filling well, more fuel can be burned than if they are filling badly. There are factors which will allow an engine to breath better at higher RPM and there are factors which will enable an engine to breath better at lower RPM. So ignoring things other than breathing, the peak in the torque curve would come when the breathing advantages from higher RPM (eg less valve overlap blowback) equal the disadvantages of higher RPM (eg trying to push air faster and faster out of an exhaust valve and it maybe eventually choking).

BSFC

Foot on the floor?

Unused oxygen isn't going to bother fuel consumption much, but unused fuel is. So although a stoichiometric AFR ratio might be theoretically ideal for producing power, clearly fuel consumption will suffer if there is unburnt fuel somewhere in the cylinder and unused oxygen elsewhere. So one might expect maximum BSFC to occur with a slightly lean mixture, to ensure all the fuel is burnt.

Another reason that a lean mixture might be better can be explained by looking at where the heat goes from combustion.
Heat going into the argon in the air gives the atoms kinetic energy. Heat going into the nitrogen and oxygen in the air makes the molecules spin, as well as giving them kinetic energy, since they have a linear shape. The byproducts of combustion, CO2 and H2O have three atoms per molecule, and so they vibrate as well as spinning and bouncing around. So, although we need the heat from the combustion, there are disadvantages to actually having the products of the combustion.

max BSFC at MAX torque RPM?

There are other RPM related losses in an engine, not just those associated with pumping air. For example there are thermal losses. The longer hot air spends in a cylinder, the more time it has to heat the cylinder and be lost that way. The longer cold air spends in a cylinder, the more time it has to heat up and oppose compression. So low RPM can be inefficient for thermal reasons, particularly in small cylinders: half dimensions and you get 1/4 the surface area but only 1/8 the volume so surface area is comparatively high for small volume cylinders.

That's was a low RPM efficiency issue.
A high RPM efficiency issue is the fact that the hot air is chasing a receding piston. Just as a wind on a surface exerts a pressure - stagnation pressure - air molecules hitting a receding piston are going to exert less force on the piston than you'd get from just considering 'cylinder pressure'.

So as I see it, maximum BSFC is a more complicated issue than maximum torque and results from balancing lots of factors, not just the scantily covered examples above, but others like piston friction, power needed to run accessories, etc.

Foot on the floor, again!

As RPM increases, so does the way to get efficient power. As explained above, the best fuel consumption comes from running lean. As RPM increases and air gets harder to pump through the engine, the advantages of running lean start to be outweighed by disadvantages of pumping the extra gases.

So the engine might run most efficiently at 60%-90% of WOT giving a particular amount of power, but if you need full power, the most efficient way to get it will be at 100% WOT (since that's the only way to get it). I'm getting off-topic now though, as these last two paragraphs are no longer related to maximum BSFC.

 

[GregLocock]

Thanks for posting that. Note that some SI engines, and many diesel engines, are deliberately underfueled (I think they call it) - that is, at the rpm for optimum bsfc they do not have the fuelling to reach the optimum - in the example you have posted this would be a max bmep of say 10 bar limited by fuelling.

One of the first things I do with a map is to plot contours of equal power, so I can read off what my preferred operating speed would be for a given demand power.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[schmidtj86]

It's also a matter of spark timing/flame speed/knock limits, fuel vaporization, and the friction losses due to RPM (slinging all the oil, moving all the parts, etc...). The later the timing -required- for MBT, the higher the BSFC is. On the engine I'm currently working on, it's peaking (minimum) at 2800 RPM and 75% of max torque available at that RPM, with max WOT torque at 3400 RPM, and max power at 5600 RPM.