Question about iso-efficiency lines

[tyrannor]

Hello everyone,
I have a question regarding iso efficiency lines which is confusing me a lot.
Please tell me if this is the wrong forum for this question.

Consider the following graph
hxxp://en.wikipedia.org/wiki/File:Brake_specific_fuel_consumption.svg
Please replace hxxp.
I assumed there would be only 1 bmep at each speed of the engine.
How would one achieve the lowest bsfc? How would one operate with the lowest bsfc and also at a particular bmep?

Thanks.

[thorpie]

Please see http://ecomodder.com/forum/showthrea...-got-1466.html

[IamIan]

Originally Posted by tyrannor

I assumed there would be only 1 bmep at each speed of the engine.

1 specific point for each specific combination of engine operating conditions.

Engine speed is one condition , but not the only one.
Engine load is another condition.

If all other factors do not change and you know a specific engine's speed and the load it is under , than a BSFC chart can be an easy way to look up the operating efficiency under those conditions.

If however a specific engine can be changing other operating conditions at the same speed and load , the operating efficiency could still change.

For example, if at the same engine speed and engine load the engine can operate at different ignition timing ... that will effect the combustion characteristics ... or if under the same engine speed and load the engine can operate under different air to fuel ratios , that can also effect the combustion characteristics... with or without turbo active... etc... etc...

Originally Posted by tyrannor

How would one achieve the lowest bsfc?

If you want to design an engine to achieve the best BSFC that is allot of work , and years of study and design.

If you want to achieve the best BSFC for a specific engine , learn about the operating conditions for that engine ... A BSFC chart will show you for one set of conditions ... if the specific engine in question only has one BSFC that makes it much much easier ... if you want to try to improve the BSFC either better peaks or wider peaks , there are lots of modifications you can do to the engine.

Is there a specific engine you have in mind?

Originally Posted by tyrannor

How would one operate with the lowest bsfc and also at a particular bmep?

If the specific engine in question only has one BSFC chart than that chart will show you what engine speed ( RPM ) at that bmep ( load ) will result is the best BSFC.

Just remember it is the load the engine is under at that instant , not what you want it to be or think it should be.

If you make changes / mods to the engine to try and improve or change the BSFC chart ... you could get a new BSFC chart , but producing one usually involves time on a dyno.

[tyrannor]

Thank you both for you replies.

So the way to get different values of bmep at the same speed is by modifying the load?
I must confess I am having a tough time understanding the graph. Let me put the question in another way.

In this graph
hxxp://imageshack.us/photo/my-images/846/powervsspeed.png/

One way to make it operate in the optimal operating region and not along the maximum power line is by adding load? If so, what other factors can be used to make the engine operate at minimum bsfc?

Also I have one more question. My understanding of wide open throttle (WOT) is that maximum air is sucked in. At WOT in a gasoline engine the fuel intake rate must also be maximum. Shouldn't the engine run at it full speed if at WOT then?
If this is true, why are a whole range of speeds specified along with corresponding values for mep and power at WOT in this graph?
hxxp://imageshack.us/photo/my-images/641/woti.png/

[thorpie]

If you are going up a very steep hill you will have WOT at low speed and will not be accelerating. Also, at low speed opening the throttle to wide open results in more power than needed to travel at that speed so the excess power accelerates you.

[tyrannor]

Thanks, am I in the right forum btw? I seem to be asking very amateur questions. Am I better of at some other forum?

Regarding the picture here
hxxp://imageshack.us/photo/my-images/846/powervsspeed.png/

How does one make the engine rotate at the same speed with different power outputs?

[Bill in Houston]

Originally Posted by tyrannor

How does one make the engine rotate at the same speed with different power outputs?

If I am in my car with a 5-speed transmission, I could go up a hill in 5th gear, and mash the gas all the way to the floor, making more power, or take my foot off the gas, making less power, or I could be anywhere in between.

Or, if that doesn't work for ya, driving on flat ground, I could hold the gas with my right foot, and brake with my left. I could press the gas to the floor, and press really hard on the brake, or let off the gas, and brake very little, all while maintaining the same speed.

To jump ahead to where you are going with this, to operate your car at the bmep, you'd need a relatively small engine, and relatively tall gearing (for a 5-speed).

[tyrannor]

Thanks, I think I have a better understanding now.