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

 

[GregLocock]

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



Cheers

Greg Locock

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

 

[JSteve2]

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.

 

[WGJ]

Try Google and Mean Best Timing

Bill

 

[vagman2]

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.

 

[TDIMeister]

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

DING DING DING!!!

 

[vagman2]

??

 

[TDIMeister]

Meaning, you got it right.

 

[dicer]

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

 

[patprimmer]

It could be said that the minumum and maximum and correct are all the same.

Regards
Pat
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[Nemesis4]

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

 

[70btdc]

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.

 

[vagman2]

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.

 

[Tmoose]

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

 

[patprimmer]

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 for tips on use of eng-tips by professional engineers &

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

for site rules

 

[70btdc]

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

 

[SomptingGuy]

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

 

[mattsooty]

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

 

[JSteve2]

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.

 

[mattsooty]

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