MBT (Minimum Best Timing ?) 2

[PDMeat]

Hi all. Not a ME but I do appreciate engines and came across some internet forums and posts about "MBT" in regardless to best timing to use to produce maximum power. They were calling it "Minimum Best Timing".

(e.g.)

http://www.ecutek.com/tuning/ignition/

It's been a while since I read over engineering texts on engines on a level that you folks probably deal with every day, but when I was more into it, "MBT" stood for "Maximum Brake Torque". Typically referenced with ignition timing as the timing required to produce maximum torque from an engine.

Are these folks off the rails here or have I just been away from this for too long?

thanks

 

[patprimmer]

JSteve

I also think you are being a bit precious.

Your earlier statement was flawed in several ways and Matty called you on it, albeit in a rather blunt and undiplomatic way.

Think about the lesson and get over the delivery method.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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

for site rules

 

[JSteve2]

I agree with your advice and I encourage you to follow it as well.

My statement was not flawed in several ways, and although it doesn't bother me personally I think it would be a disservice to the purpose of these forums to simply accept a mis-statement.

However, as our primary disagreement has been where we choose to use a fine or broad brush stroke, I'll withdraw. I will think about the lesson.

 

[black2003cobra]

PDMeat - MBT timing = Maximum Brake Torque timing. Knock limit aside, at a given speed and flow rate, it is the spark timing that gives maximum torque, (and minimum bSFC). It doesn't mean maximum (or minimum) spark advance. The word "maximum" refers to torque - not spark.

Basically, if combustion starts too soon, the gas pushes back on the piston as it is coming up in the compression stroke, creating negative work which reduces torque. If the spark is too late, peak cylinder pressure is reduced, and hence, so is expansion work. So there is an optimum spark for maximum torque. If one were to draw out a curve of torque vs advance, (starting out below MBT), as spark is advanced torque will increase until MBT timing is reached, after which point further advance will cause torque to go back down. The top of the curve is somewhat flat, so the percentage increase in tq drops off as spark advance approaches MBT.

In the ideal Otto cycle, MBT timing would be at TDC, as mentioned above. But due to the non-zero burn duration of a real engine, MBT timing is not at TDC as mattsooty mentioned.

I have also heard it referred to as mean best timing, but since MBT depends on engine speed and load, that terminology just doesn't seem to fit, in my opinion.

 

[patprimmer]

BC

That sounds very logical to my feeble brain and fits pretty well with my understanding.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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

for site rules

 

[dicer]

Hmmm, Didn't I say that with less wording?

 

[SomptingGuy]

I checked through my university notes and found the statement:

"Sometimes called Minimum advance for Best Torque"

So if the esteemed ICE profs can't agree, the water does seem somewhat murky.

- Steve

 

[benmpi]

It keeps coming up that "theoretically" MBT would be at TDC... That's pretty far from the truth, like has been said already all you would do is create a huge bending force on the crank with absolutely no torque (ignoring any wrist pin offset). I have never done an extensive study but have heard the peak cylinder pressure should occur around 15ATDC which sounds about right. Since the amount of time it will take to reach peak pressure changes with charge density and the speed at which it must be done changes with RPM MBT will constantly change but will always (in every application I know of) be BTDC

 

[JSteve2]

"Torque" is not mechanical leverage applied to the crank, as seems to be mentioned in a couple of places in this thread, and so I will reluctantly wade back in. Torque is the crank-angle specific amount of work transferred from the combustion event to the crank shaft. The thermodynamically most efficient combustion event is a perfect and complete combustion at TDC, and this is therefore the best possible torque generating event, and therefore generally the most fuel efficient power generating event. This is the truth in theory, and it's not terribly far off in reality either.

In reality, a perfectly realized event like this would, of course, break a number of things. The torque is proportional to the area under the P-V curve that ends up realized as mechanical work at the crankshaft. The force on the bearing is proportional to the cylinder pressure. Therefore, you want to keep the peak pressure capped at some maximum (for other reasons too). The reason you don't want early pressure is due to entropy losses (by having to compress your expanding gas before getting the energy back) and increased heat losses due in part to very high cylinder temperatures. The reason you don't want late pressure is because you are combusting into an expanding volume which will result in a net longer combustion event, lower pressures, and greater thermal losses.

If you imagine the pressure generated as a skewed bell curve, and recall that pressure generated before TDC is extremely costly (see above) then you will see that a cost function can place the center of mass of the bell curve onto TDC and then begin shifting it right until the inefficiency before TDC is just about equal to the inefficiency after TDC and that will position you very close to the best realizable timing. You may still have a maximum cylinder pressure limit that pushes you slightly farther right, but for most operating points of the engine probably not. For example, an engine that achieves max torque at 10 DBTDC at 50% load will probably have to retard that a few degrees at full load to avoid exceeding peak cylinder pressure.

Finally, even if you get all that worked out, you still have to check emissions. Even though my esteemed colleague indicates that engine manufacturers do not check every operating point for emissions, the fact is that they do (to the extent possible), and especially at peak torque and power. The reason for this is that the engine must operate away from the test points in substantially the same way that it operates at the test points, and the peak points are the first points the regulating body will ask about, although anything is fair game. Therefore the peak torque points will certainly be checked and may well have some adjustments due to emissions.

To the point of the OP - it's a pretty good system to advance the timing until you reach the torque plateau, and then stop. Further advances yield little in the way of extra torque (thus the plateau) and greatly increase cylinder pressures (and therefore mechanical loads) and emissions. Even after you do this, you may have to further tweak due to emissions.

 

[GregLocock]

My edition of Heywood says he means Max Brake Torque, but notes that Minimum advance for Best Torque is a previous usage.

Perhaps we should just admit that MBT is the spark timing beyond which we do not wish to advance, and leave the derivation of the initialism to the crossword enthusiasts.



Cheers

Greg Locock

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

 

[BillClark]

All I can say is I have spent countless hours on the dyno using combustion pressure analysis to understand what is best and I beleive generaly speeking you have to strive for moving peek cylinder pressure out (retarded) as far as possible to take advantage of a more favorable rod/crank angle to see mechanical advantage really shine. Attached is a combustion cycle and when you do the math you start to understand what a pitty it is to have all that pressure so close to TDC

 

[TDIMeister]

FAIL.

 

[GregLocock]

Oh well at least it preserves the cylinder head bolts!

Cheers

Greg Locock

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

 

[patprimmer]

You have to maximise the average pressure during the power stroke without exceeding the detonation point or the point where maximum pressure compromises the structural integrity of the engine. In reality this creates a complex system of compromises.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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

for site rules

 

[PDMeat]

Thanks gents. I've been away for a while but I'm back.

GregLocock: I have the heywood book "I.C.E. Fundementals" and that's where I was used to reading about MBT but I didn't recall the sentence you quoted. Thanks for pointing that out though.

BC2003: Thanks for that as well. I used to race mustangs back in the 90s. I'm with you on the explanation for MBT.

The "minimum best timing" sounded hokey to me. It seemed like someone was making up words to try and describe the acronym after having forgotten what it stood for. After rearranging the words a bit to get "minimum timing required for best torque" that sounds a lot better.

In the end it appears it's more of an opinion thing but it's interesting re Greg's comment where Heywood referred to MBT as "the minimum advance for best torque" as a previous reference no longer used as often as the "max brake torque" wording.

 

[marcusaurailius]

Hi all, one thing that most people are missing , is that the best timing at one particular rpm site, will not be the same at a site further up the rev range

Ie, little throttle opening , at 4000+ revs, will get away with alot of advance

but a lot of throttle openning, IE WOT, will need a lot less

also when the engine, is at its most efficiant, due to a number of things, , but at this speed/throttle openning, the timing can be almost TDC, where as 1000rpm more, at same throttle position, it may well be 25 or more degs BTDC

this has been bourne out by the ability to ..map.. an engine with a laptop and a screen, to see exactly where the ..best.. spark will occur

and if set to ..auto map.. then the timing is set by the comp, using a variety of sensors ,.

and is very different for a lot of different speed /load ranges,

also heat ,and air temp play a big part in it too,.


So I would say that there is no such thing as MBT,

as it alters almost constantly, on a car engine,.


IE, 80 degs water, 20 degs air, at 4000 rpm, wot = 25 degs btdc

105 water, 60 degs air , wot =16 btdc

so how can one set the the mbt , when it changes constantly,
with air/water temp, throttle angle,

ALL at the same revs !!!!!!


MBT on a static engine/generator ,where every thing is running just right , then ..yes..,

regards Marcus



One does nae have to know how some thing works, to know that it is nae working right

http://s545.photobucket.com/albums/hh369/marcusaurailius/

 

[mattsooty]

Oh dear..............


MS

 

[black2003cobra]

Yes, how did we miss those things. Er wait...

 

[patprimmer]

I thought I alluded to that on 22/08 and several others also commented at some point.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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

for site rules