General Air Fuel Ratio Effect on Engine Performance

[SA]

These curves are general rule of thumb for spark ignited engines. This kind of data is typically gathered under controlled laboratory conditions at a fixed timing advance and locked throttle position. It should give you a pretty good idea of what happens to the engine as you vary AFR. The curve for Specific Fuel Consumption is Brake Specific Fuel Consumption where 100% is BSFC at the point of maximum power.  MBT Timing means Maximum Brake Torque Timing.

[dre99gsx]

Middle of "best power range" could be 12.5

Top of "best economy range" could be 14.5

[justinn]

Top of "best economy range" could be 14.5

not exactly.... on the Supra, I found this to be from 16-16.5:1, but every car is different and it has alot to do with the design and efficiency of the motor.

Justin
Protech-Performance

[SA]

not exactly.... on the Supra, I found this to be from 16-16.5:1, but every car is different and it has alot to do with the design and efficiency of the motor.

Well said and exactly why there are no actual AFR values on the x-axis of the plots above.

[UpInTheLex4]

I thought MBT meant mean best torque.  

I don't understand the bottom of the graph, the fuel consumption and specific fuel consumption part.

Can someone explain what the graph is doing? and what the difference is between fuel consumption and specific fuel consumption

[SA]

If the start of the combustion process is progressively advanced before TC, the compression stroke work transfer (which is from the piston to the cylinder gases) increases.  If the end of the combustion process is progressively delayed by retarding the spark timing, the peak cylinder pressure occurs later in the expansion stroke and is reduced in magnitude.  These changes reduce the expansion stroke work transfer from the cylinder gases to the piston.  The optimum timing which gives the maximum brake torque - called maximum brake torque or MBT timing occurs when the magnitudes of these two opposing trends just offset each other.  Timing which is advanced or retarded from this optimum gives lower torque.  Detonation limits really don't have anything to do with this.  MBT timing defines the optimum point (maximum brake torque) for peak cylinder pressure relative to crank angle.

SA

[hackish]

I just did some searches and I find conflicts as to what MBT actually stands for. Some say it's Minimum Best Timing and some say it's Minimum (timing for ) Best Torque. In any event same stuff, different name.

SA is correct in that MBT is not technically related to detonation but in practise MBT generally occurs a few degrees before knock. This applies to tuning street cars on pump gas, as octane increases, the gap between these two widens.

-Michael

[supra87t3/t4]

With pump gas in my supra for example, the timing at wich I would hypothetically make the most torque (MBT?) is more advanced than the timing where I start getting knock (knock limit). This is what I was tought is called "knock limited". As a contrast on a restricted naturally aspirated motorcycle engine on 100 unleaded, timing can be advanced well past peak torque output without knock. In this case MBT is clearly defined, but what is it in the first case, the timing where my engine actually makes best torque on that fuel or the timing that I can't get to because it is past the knock threshold?

[SA]

With pump gas in my supra for example, the timing at wich I would hypothetically make the most torque (MBT?) is more advanced than the timing where I start getting knock (knock limit). This is what I was tought is called "knock limited". As a contrast on a restricted naturally aspirated motorcycle engine on 100 unleaded, timing can be advanced well past peak torque output without knock. In this case MBT is clearly defined, but what is it in the first case, the timing where my engine actually makes best torque on that fuel or the timing that I can't get to because it is past the knock threshold?

From a technical standpoint, MBT timing has nothing to do with your detonation limit.  The definition is in my post above.  Knock is a completely seperate combustion phenomenon.  If you can only run 20 degrees of timing under certain conditions because of limiting detonation...that's your limit.  It has nothing to do with MBT timing.  To test to determine MBT timing, the engine can't be limited by detonation.  You have to be able to start at a retarded timing setting and continue to advance the timing (under a constant operating condition) until torque peaks then falls off.  The timing advance that corresponds to peak torque is defined as MBT timing.

These terms are not generally applied to tests run on a chassis dynamometer and certainly not on the street or race track.  These are laboratory only concepts.

SA

[Powertrip]

Middle of "best power range" could be 12.5

Top of "best economy range" could be 14.5

Is this correct? I thought best power was typically produced at 11.5.

[Erich Moraga]

Middle of "best power range" could be 12.5

Top of "best economy range" could be 14.5

Is this correct? I thought best power was typically produced at 11.5.

The quote says "could be 12.5" and it's also said in other areas that this can vary from car to car.  That said, I personally run in the 11.5:1 range on 93 octane and make a little too much power  

-Erich

[BLKMGK]

Much of this depends on the motor. 14.7:1 is a perfect burn - no fuel left - but you need additional fuel for detonation assistance. 12.5:1 is generally considered optimal for a N/A motor, forced induction generally goes richer for the additional detonation insurance. If you have higher octane you can generally go leaner as well but past 14.7:1 you will certainly not make more power but may gain economy at part throttle. Lean burn motors may be an exception to the general rules of course.

[WadZii]

does anyone have this saved.. they deleted the picture

[SB]

I think I found the original picture... reattached to first post.