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MBT (Minimum Best Timing ?)
2

MBT (Minimum Best Timing ?)

MBT (Minimum Best Timing ?)

(OP)
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
 

RE: MBT (Minimum Best Timing ?)

Neither interpretation of MBT that you suggest accords with my experience.

 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: MBT (Minimum Best Timing ?)

Their theory is a bit whacked but their results look like they will be good. The best timing for maximum power in every case is right at TDC (slightly before due to the reality of a finite combustion time), as early or late combustion from TDC both rob realized mechanical power. However, knock and emissions keep you from realizing the maximum power in every case. These guys seem to be thinking about knock, but I'm assuming they are ignoring emissions. In a well-tuned engine developing any kind of power, you have a high enough compression ratio that your timing advance will be knock-limited before you get to a mechanically inefficient advance. Therefore, their scheme looks like it will come pretty close to the maximum realistically achievable power.

At low power, you can advance significantly earlier because you have less material in the cylinder. This is where they go wrong, because they don't advance there. However, the lost potential is minimal because there is little power to achieve there in any case, so it's probably best that they ignore this operating point anyway. In any event, most manufacturers may be advanced near TDC at idle (unless they need the NOx benefit of retarding at that point), so there may be nothing to gain anyway.

RE: MBT (Minimum Best Timing ?)

Try Google and Mean Best Timing

Bill

RE: MBT (Minimum Best Timing ?)

My ununderstanding of MBT is "Minimum advance for Best Torque" That is how an engine should be steady state adjusted(maped)for every load/rpm point .
The advance should be set so the peak cylinder pressure occurs at approx 15deg ATDC. Then you have the best torque.

RE: MBT (Minimum Best Timing ?)

vagman2 (Automotive)    
My ununderstanding of MBT is "Minimum advance for Best Torque"

DING DING DING!!!

RE: MBT (Minimum Best Timing ?)

??

RE: MBT (Minimum Best Timing ?)

Meaning, you got it right.

RE: MBT (Minimum Best Timing ?)

There is no min or max timing at Max brake torque, only the correct timing to achieve it.

RE: MBT (Minimum Best Timing ?)

I agree with dicer - min or max timing etc. makes no sense - only correct timing.

RE: MBT (Minimum Best Timing ?)

Of course there is a minimum and a maximum?  After a certain point, max torque is reached and plateaus.  You can continue to advance timing past this point with no benefit to torque production, only detriment to combustion stability.  Hence minimum.  Minimum is then correct.  maximum is not an operating point at any time...

Some calibration engineers will find MBT and always retard 3deg or so, depending on fuel to maintain a safe margin even still.  

RE: MBT (Minimum Best Timing ?)

Exactly, in real there is always at least few degrees advance range where max.torque stays constant and then falls down. It's good to pick the lowest adv.value for your tune not only for safety but for efficiency too. I mean less advance means lower pressure rise piston has to overcome at TDC so less pumping losses at the end.

RE: MBT (Minimum Best Timing ?)

I think "pumping losses" may be more related to coaxing fluids to  flow into and through holes, passages and slits.

RE: MBT (Minimum Best Timing ?)

I think it depends on whether the engine fuel temperature combination has it's timing limited by torque or knock.

When knock limited, there is a distinct sharp point when knock starts as the timing is advanced. I never tested an engine where the timing was purely limited by torque.

Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
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for site rules
 

RE: MBT (Minimum Best Timing ?)

Yes, but the torque will stay constant/drop off slightly before the knock point with increasing timing.

RE: MBT (Minimum Best Timing ?)

I did an engine running course as part of my introduction to my current employment.  We ran an E6 connected to a manually controlled water brake and plotted out MBT curves by hand.  Then we started fiddling with the CR to make it knock.  Loads of fun.

- Steve

RE: MBT (Minimum Best Timing ?)

I have never found out a comprehensive or indeed perfect explaination for the meaning of MBT, so I wont even pretend to answer that part of your question.....

With spark timing things, as always, are a compromise. If you have an engine that is air limited then yes, wind away the spark until you find the torque peak at any given air load. However, were one to find themselves with a knock limited engine (especially turbocharged and/or out of the stoich region) then the order of the day is in fact LBT/MBT for the best torque. Unless that is you have a close loop EGT control.

In some older gen. systems the open loop spark is always mapped "safe" but with the advent of adaptive knock & EGT control those practices have all but vanished, unless emissions are compromised.

JSteve2 is actually quite wrong suggesting that the best ignition point is at TDC - in the real world gas dynamics etc detract so much from the ideal model that it becomes useless apart from a teaching aid. Judging by his presumption that emissions are a factor at full load says to me that he must work on sit on lawn mowers???

 
In my experience, on high performance engines, knock will always be encountered first. If this is not the case at full load then your VE is too low.

As it actually stands most modelling of spakr sweeps I have done in part load regions the best curve fit for a sweep is an X^2 : X^3 hybrid spline with MBT as the knot point.

Anyway

 

RE: MBT (Minimum Best Timing ?)

No need for insults mattsooty. The statement is quite true as a theoretical matter, which you admit as much. The primary point of the statement, in the "real world", is that once the combustion energy is being developed at TDC, further advances in the timing will begin to work against you. All the real gas dynamics support this as well - you are compressing and expanding combusted gas, increasing heat transfer from the cylinder, etc.

As far as emissions - the engines I work on are about as far from lawnmowers as possible. My guess is that the engines you work on are not in a challenging emissions regulation environment. In the engines I work on emissions are a factor at every operating point, and since full load racks up horsepower quickly, it's even more of a factor there.

RE: MBT (Minimum Best Timing ?)

Well if we are going to get pedantic then I think you are still wrong...if you consider the real world mechanics of a reciprocating gasoline engine - to have total & immediate heat release at TDC (as in the ideal 4 stroke cycle) this will serve nothing more than to flatten bearings and compress conrods with a massive shock load, wasting huge amounts of energy.

I think you will find that the theoretical optimum for MBT is the position of 50% MFB at 8-10 deg TDC, depending on wrist pin offset. Which also ties in pretty well with the centre of area of the NMEP graph on a pCrank plot.

Anyway, I can only guess that a statement like:-

"In the engines I work on emissions are a factor at every operating point"

means that you work on steady state diesels for off highway or something such like because I cant think of a single engine that is tested across its speed/load operating range on a FTP75 or MVEG-B.........

Anyways chill out, we are supposed to be grown ups.

MS

RE: MBT (Minimum Best Timing ?)

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
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for site rules
 

RE: MBT (Minimum Best Timing ?)

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.

RE: MBT (Minimum Best Timing ?)

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.

RE: MBT (Minimum Best Timing ?)

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

RE: MBT (Minimum Best Timing ?)

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

RE: MBT (Minimum Best Timing ?)

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

RE: MBT (Minimum Best Timing ?)

"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.

RE: MBT (Minimum Best Timing ?)

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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: MBT (Minimum Best Timing ?)

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
 

RE: MBT (Minimum Best Timing ?)

FAIL.

RE: MBT (Minimum Best Timing ?)

Oh well at least it preserves the cylinder head bolts!

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: MBT (Minimum Best Timing ?)

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: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
 

RE: MBT (Minimum Best Timing ?)

(OP)
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.

RE: MBT (Minimum Best Timing ?)

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/

 

RE: MBT (Minimum Best Timing ?)

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


MS

RE: MBT (Minimum Best Timing ?)

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

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