Understanding the operation of the Motronics 2.4 ECU

[NoRRmad]

I did a little research, and found lots of sources that agree that there is a "wasted" injector firing. (it's not really wasted; all the fuel does eventually burn in the cylinder, but half of the fuel sits in the throttle body for a bit, until it's joined by a fresh squirt and inhaled. The 1200 engine (and the S1000RR, ) have "multiport" injection, where the injectors operate only on the intake stroke... But not the R1150R.

[CycleRob]

Considering the fact that the O2 sensor is way-way downstream inside a chambered area and it's no wonder the OilHeads surged at part throttle.

About the baro sensor, there may be one built into the Motronic box. I seem to remember a button on one side that appeared to have an air breathing capability. Anybody have one, or one accessible now to look at?

Also, the injector firing with a closed intake valve isn't such a big deal because it all happens so fast, even at an idle.

.

[NoRRmad]

Yeah, browsing around on the subject of fuel injection, I see that there's even a "continuous" fuel injection, where the throttling is done with the pressure, and the injectors run constantly, intake valve open or no. Can't be too bad...

[frankc]

I'm not sure how much valve timing overlap there is on the 1150R (period where both intake & exhaust valves are partially open at the same time for scavenging effect around TDC - end of exhaust & beginning of intake strokes), but my 911 (also a boxer as noted above) has a cam with bit of overlap and a wasted spark ignition (non-OEM). Many 911s run this setup without any problem. It seems even with some overlap the "wasted" plug would fire many degrees before the intake valve opens due to ignition timing advance - even on a twin plug motor (which allows less advance) at low RPM. Too much overlap, and yes, the engine will backfire at low RPM as the exhaust gases begin to enter the intake port and ignite things.

Frank

[Sunbeemer]

This explains why my bike sometimes backfires on a hot restart... fuel trapped in the intake tube is ignited with a "POP"! It's pretty startling...

I'd like to add that this thread has been very instructive...thanks to all for the education.

[boxermania]

NoRRmad and riceburner
I stand corrected, you folks are right, both pistons are in the TDC position, the diference is that one cylinder is on the power stroke (spark plug firing) and the other is completing the exhaust stroke as sweatmark indicated.

I love it when we get folks to contribute and help with the process of understanding what takes place, that being said I think the best description is the one provided by Phang with the diagram he posted.

You can be certain of the following: On the BMW boxer twins with the Motronic 2.4 the injectors fire at the same time and so do the plugs (at least on the 2 plug models) as stated before I don't know if in the 4 plug models the primary and secondary plugs of the same cylinder fire at the same time or there is a small lag between the firing.

CycleRob
I haven't looked inside the Motronic 2.4, but I'm sure that anyone that has installed a chip would have, maybe they can enlighten us. Most ECU's have been potted with the exception of the replaceable chips, so maybe it is a breather for the electronics??? Personally I would be surprised if there is Mass Sensor in the scheme, but I have an open mind.

NoRRmad
Due to the particular speed of the events I couldn't begin to think what is the outcome of the fuel injected into the back of a closed intake valve away from the power stroke or begining to open, depending on the cam overlap as stated by frankc .......the possibilities are endless, the thing one can be certain of is most of it is wasted.....

Fast forward to the "pulsed" fuel injection where each injector is fired at the right time.......as with the R1200 and F800 series.

[riceburner]

For a "wasted injection" setup, the second cylinder would get a squirt of fuel behind closed intake valves

The "wasted squirt" would be onto the back of an intake valve right about the time of combustion of the previous charge - anyone ever tried spraying atomised fuel/air onto a red hot stick??

[NoRRmad]

It's probably not that bad. The intake valve is maybe six inches from the injector (and around a corner.) And, since the intakes get a big cool shot of outside air every other revolution, they're not red hot.

[riceburner]

Yeah, browsing around on the subject of fuel injection, I see that there's even a "continuous" fuel injection, where the throttling is done with the pressure, and the injectors run constantly, intake valve open or no. Can't be too bad...

I suppose that IS the way carburettors work......

[challey]

CycleRob said:

About the baro sensor, there may be one built into the Motronic box. I seem to remember a button on one side that appeared to have an air breathing capability. Anybody have one, or one accessible now to look at?

Here's a picture of the button on the Motronic. My decidely non-technical impression was that it was a "press to reset" affair. It didn't appear to be open in any way to the outside.




Charlie

[Sunbeemer]

Riceburner said:

anyone ever tried spraying atomised fuel/air onto a red hot stick??

I suppose you might get a "BANG!" out of it!
Sounds like a hot restart issue to me...

Charlie, I hope you got that rat's nest figured out...

[iowabeakster]

hey there guys...I gotta disagree...tell me where I am getting this wrong.

Why would the first spray of fuel (closed intake valve) be wasted? It isn't burned instantaneously, yes... But, It is still perfectly viable waiting in the intake. The first spray is burned... not at all wasted. When looking at the phases of an internal cumbustion engine on paper, it's easy to lose sight of what is happening in real life when it is running.

The sprays are metered, so that the between the two sprays COMBINED, they equal the ideal (about 15:1) air/fuel ratio. Plus gasoline is highly volatile, after being sprayed, it isn't stuck to the walls or valves in a liquid state...it is a gas. The whole purpose of a carb or fuel injection is to create this gaseous mixture. The size of the sprays are varied (by all those inputs Boxermania stated) so that this air/fuel mixture is correct. If there was all this "wasted" gas in the intake...it would "flood" the engine and stall, like in days of yore.

Now like Rob said, all this happens very quickly...at 1200 RPM's (near idle) each cylinder fires 600X a minute equaling 10X per second. This basically means a near continuous air flow through the intake system. This near continuous air flow needs a near continuous fuel flow. At 3600 RPM each cylinder is firing... 30X per second! at 575cc each intake...there is in excess of 17L of air/fuel moving through the intake... per second.

[boxermania]

iowabeakster wrote
The sprays are metered, so that the between the two sprays COMBINED, they equal the ideal (about 15:1) air/fuel ratio.

That is an interesting concept that requires additional thought as one pulse of the injector is into a moving column of air and the other one is not.......I'm getting a headache, need some nourishment.

[riceburner]

hey there guys...I gotta disagree...tell me where I am getting this wrong.

Why would the first spray of fuel (closed intake valve) be wasted? It isn't burned instantaneously, yes... But, It is still perfectly viable waiting in the intake. The first spray is burned... not at all wasted. When looking at the phases of an internal cumbustion engine on paper, it's easy to lose sight of what is happening in real life when it is running.

The sprays are metered, so that the between the two sprays COMBINED, they equal the ideal (about 15:1) air/fuel ratio. Plus gasoline is highly volatile, after being sprayed, it isn't stuck to the walls or valves in a liquid state...it is a gas. The whole purpose of a carb or fuel injection is to create this gaseous mixture. The size of the sprays are varied (by all those inputs Boxermania stated) so that this air/fuel mixture is correct. If there was all this "wasted" gas in the intake...it would "flood" the engine and stall, like in days of yore.

Now like Rob said, all this happens very quickly...at 1200 RPM's (near idle) each cylinder fires 600X a minute equaling 10X per second. This basically means a near continuous air flow through the intake system. This near continuous air flow needs a near continuous fuel flow. At 3600 RPM each cylinder is firing... 30X per second! at 575cc each intake...there is in excess of 17L of air/fuel moving through the intake... per second.

The perfect stochiometric ratio is something like 13.9:1 air/fuel (IIRC) - and strictly speaking the fuel/air mixture in the manifold isn't a gas - it's more like what you get from an aerosol spray - fine drops of liquid suspended in the gaseous air.

[NoRRmad]

Yup. And carburetors have all sorts of tricks inside to generate turbulent airflow so that the large drops blown off the needle are agitated enough to break up into smaller and smaller drops. Injectors, on the other hand, break up the flow with high pressure through small orifices, and the resulting high-energy collisions between droplets. Fuel enters the throttle body already aerosolized. I don't see why that fog of fuel and air can't hang around for a milisecond, until it's sucked through the intake valve.

Tricky.

[Sunbeemer]

Ok, I get it: Tic...Tic...BOOM

[AirForceDirt]

I'll throw my minor understanding of the 4 spark model's ignition timing to the fray, feel free to correct me, I'm still learning.

From what I understand, the dual spark engine only fires the second plug (thinking one cylinder at a time here) if the fuel isn't being burned properly. I know it doesn't fire all the time as I've heard it ticking occasionally (hook up an MP3 player to the for power and use a set of conventional plug-in type earbuds, under certain circumstances you can hear a second plug firing). When it does fire, it fires at the same time as the main plug.

This then begs the question: If on the 04+ R's, without the CCP to connect the O2 sensor to the motronic chip, how does the computer know to fire the second plug? This has led me to believe that CCP does more than just connect the O2 sensor. There must be more to it. Unless the computer has some other way of knowing if its not burning all the fuel with just one plug.

Thoughts?

[boxermania]

AirForceDirt writes
From what I understand, the dual spark engine only fires the second plug (thinking one cylinder at a time here) if the fuel isn't being burned properly.

Interesting concept, but not possible. There isn't anything that I know of that can detect a poorly burning mixture fast enough to activate a second spark plug. The firing/expansion of the combusted gases takes place in about 80 degrees (including timing advance) of crank rotation once the piston begins to travel down the cylinder most mixture energy has been released.

The O2 sensor is a mile away, literally speaking, and is a narrow band design, quite lazy in it's response (1 sec typ.)

[AirForceDirt]

So when does the second plug fire? Or is it always firing? Then whats the extra ticking noise at low revs and full open throttle?

I understand what you mean about the O2 sensor being so far away. I still think that, even without the ccp, its still active.

Looked up the power commander for the R1150....and the reason why its so damned expensive: Its a wide band (unlike, apparently, the normal sport bike PCs) which makes it much more versatile. Not sure if its worth $400 for what it'd do. It might help if one were to install the Boxer Boost cams from Wunderlich.

[NoRRmad]

Then whats the extra ticking noise at low revs and full open throttle?

Uh...

Knock?