Ignition Mapping - How it works

[boscoe99]

Rodnut is always bringing up "mapping". For those that might be interested, here is a simple chart that shows an ignition timing "map" for one model Yamaha. This is a carbureted model which does not use an intake manifold sensor or a throttle position sensor so that the map is for ignition only. More sophisticated models (fuel injected for instance) might have a another map that addresses how long the fuel injectors are to remain open at various engine RPM's, throttle positions, manifold pressures, engine temperatures, etc.

The computer programmer has coded the engines computer so that at any time the RPM is ABC then the computer will set the timing to XYZ. For instance, if the engine RPM is 2000 then the timing will be 10 degrees before TDC. As the engine speed goes up and down so does the ignition timing.

[zenoahphobic]

Your diagram jumps at a number of points, it is not a smooth curve.
Does this mean the program only alters the timing at selected RPM points?
I always worry when I see graphs presented this way.

[rodbolt17]

and to add as the engine controls get more sophisticated the mapping jumps even more.

once we can add throttle angle and engine load (TPS and MAP) we can add or subtract ign timing at any given RPM and load.

all that ECU does is look at ABC inputs and it responds with XYZ outputs.
it is that simple.

[DennisG01]

Interesting. I've been "generally" aware of ignition timing, but that helps to make it clearer! Now, a follow up question... Why does the ignition timing HAVE to change? Is it because the speed that the piston moves up and down is a variable, whereas the "ignition process" is a constant? Meaning, do you have to start the fuel/air/spark mixture combustion process sooner when the RPM's are higher because otherwise the "ka-boom" would happen to late?

[boscoe99]

Your diagram jumps at a number of points, it is not a smooth curve.
Does this mean the program only alters the timing at selected RPM points?
I always worry when I see graphs presented this way.

I think the graph makes it look "jumpier" that it may really be. From 1000 RPM to 1500 RPM it increases from about 6 degrees to about 8 degrees. From 2000 RPM to 2500 RPM it increases from 10 degrees to about 16 degrees. But, it is a slope no matter what.

What's to worry about? The graph is just a representation of what is going on inside the computer. Just trivia information as I doubt that any mechanic would ever try to measure the actual ignition timing as a function of RPM.

[99yam40]

at least it shows at what point the timing hits max timing so testing can be done not a WOT

[boscoe99]

Interesting. I've been "generally" aware of ignition timing, but that helps to make it clearer! Now, a follow up question... Why does the ignition timing HAVE to change? Is it because the speed that the piston moves up and down is a variable, whereas the "ignition process" is a constant? Meaning, do you have to start the fuel/air/spark mixture combustion process sooner when the RPM's are higher because otherwise the "ka-boom" would happen to late?

Yes.

The spark is timed to ignite the air/fuel mix at a certain time based on RPM. Now the air/fuel mix takes a bit of time to burn and develop maximum pressure. The intent is to have the air/fuel mix burn and create its maximum combustion pressure at so many degrees after TDC.

The faster the piston is moving the sooner the spark has to occur so as to have the maximum combustion pressure happen at the same time after TDC.

Of course it is not as simple as all of this. There are many other variables that have to be taken into consideration.

[DennisG01]

Cool. That makes sense. What would some of the other factors be that need to be considered? Maybe just some of the other "major" factors... no need to go in too much detail for me!

[rodbolt17]

it takes time to burn the fuel.
ignite it to soon and it push's the piston back down.
ignite it to late and it generates excessive heat and melts things and wont make power.

ignite it too soon and it can create spark knock also incorrectly called valve rattle.

then we have to factor in load vs combustion chamber pressures.

most Yamaha micro processors look at the sensor data about 32 times per second.
the ecu can adjust that fast.

[boscoe99]

it takes time to burn the fuel.
ignite it to soon and it push's the piston back down.
ignite it to late and it generates excessive heat and melts things and wont make power.

ignite it too soon and it can create spark knock also incorrectly called valve rattle.

then we have to factor in load vs combustion chamber pressures.

most Yamaha micro processors look at the sensor data about 32 times per second.
the ecu can adjust that fast.

Computers wait for what to them is an eternity for something to happen. A few milliseconds to a computer is nothing. Let's say a computer tells something to happen for 5 milliseconds every second. Sounds impossible and difficult to understand to most humanoids. But to the computer that is less difficult than me pushing and holding a switch for 5 seconds out of every minute.

Now with respect to the map making things look jumpy, for all I know the computer is making ignition changes to the tenth of a degree (maybe less) based on RPM changes of 10 or maybe less. Changes are probably a lot smoother and faster than they appear.

[zenoahphobic]

Now with respect to the map making things look jumpy, for all I know the computer is making ignition changes to the tenth of a degree (maybe less) based on RPM changes of 10 or maybe less. Changes are probably a lot smoother and faster than they appear.

So you somewhat agree with me that the graph is not a true graph.
somebody simply probably plotted the graph at values each 500rpm not realizing , it does not relate to an ECU's switching ability or how often it receives data.

The graph also begs the question about what is happening to the "set" ignition timing between 2500 and 3500 rpm, arguably the most useful range.

Designing ignition timing has always been of some contention (and its importance overrated). Apart from a basic need for the engine to have some advanced commencement of the burning process, ignition timing changeability is more for pollution control purposes. Earlier engines only had one setting, later they went to only two settings (changed curve once at a certain point to less severe) and this was totally acceptable for many decades. Recent decades, as it would seem here, ignition timing is changing at a number of points (without adequate explanation). However the constituent of the fuel used, the turbulence and compression has a greater affect on the timing of the burning process rather than the initial electrical ignition.

The argument that it needs to be adjusted because of peak pressure timing is contentious. As in this case (and common elsewhere), why then does it stop increasing at 4500 when it appears the engine can run on over 6000 rpm? I would argue ideally peak pressure should occur just after TDC, but there needs to be consideration for the thermal and structural well being of the engine in this regard.

[rodbolt17]

and I have to vehemently disagree with your assessment of ign timing.
ign timing has EVERYTHING to do with a motors(articulated rod internal combustion) ability to make power.
early inline 4's and some straight 8's had not only an accelerator they also used a magneto control lever.
the operator had to actually interact with the motor.

then came centrifugal advance distributors that stayed for many years.

then came more modern electronically con*****ed advance based on rpm.
now advance is electronically con*****ed based on many engine variables.

it is why you will see 38*BTDC on some engines based on engine load.

[99yam40]

don't forget the vacuum con*****ed timing that went along with the centrifugal

[rodbolt17]

do you understand why vacuum advance was added to some engines?

[boscoe99]

So you somewhat agree with me that the graph is not a true graph.
somebody simply probably plotted the graph at values each 500rpm not realizing , it does not relate to an ECU's switching ability or how often it receives data.

The graph also begs the question about what is happening to the "set" ignition timing between 2500 and 3500 rpm, arguably the most useful range.

Designing ignition timing has always been of some contention (and its importance overrated). Apart from a basic need for the engine to have some advanced commencement of the burning process, ignition timing changeability is more for pollution control purposes. Earlier engines only had one setting, later they went to only two settings (changed curve once at a certain point to less severe) and this was totally acceptable for many decades. Recent decades, as it would seem here, ignition timing is changing at a number of points (without adequate explanation). However the constituent of the fuel used, the turbulence and compression has a greater affect on the timing of the burning process rather than the initial electrical ignition.

The argument that it needs to be adjusted because of peak pressure timing is contentious. As in this case (and common elsewhere), why then does it stop increasing at 4500 when it appears the engine can run on over 6000 rpm? I would argue ideally peak pressure should occur just after TDC, but there needs to be consideration for the thermal and structural well being of the engine in this regard.

My guess, and just a guess, would be that it is detonation related. Structural well being of the model as you note.

No knock sensor on this motor. 87 octane gasoline specified. If it makes its rated HP at 5500 RPM then there is no need for any further ignition timing advancement. Reliability trumps more HP.