Nice scope trace of MAP variation

[Fred]

This is from a single cylinder engine over 720 degrees.



Pretty cool trace :-)

Fred.

[jbelanger]

Now, which point should be used for SD computations? And does it change with RPM and load?

Jean

[Fred]

I'd be inclined to go for the lowest pressure point because if you let the engine cycle last an infinite time it would taper back to 100kPa eventually. The lowest point represents the restriction to flow which is effectively what you want to know. You could argue that you should take the highest point as it gives you what is in the engine when the valve closes too. The latter might be a more accurate representation of real VE, but the former will guarantee a larger range of values and corresponding better resolution.

Whether it moves with RPM doesn't matter much provided that it is consistent at any given RPM. The tune can take care of such shifts in scale due to timing and delay and resonance. With respect to load, of course the shape will change, at near WOT it will be a small dip in pressure at intake opening and then flattening out to 100kPa. At closed throttle it will be similar but flatten out to a lower pressure value. On a nice setup the WOT pressure should barely change/not change at all but that is resonance dependent.

What are your thoughts? Don't tell me you are going to start the 20 questions game too LOL.

Fred.

[jbelanger]

Those were just 2 questions that popped into my head when I saw the scope trace and I don't have any answer for either so would be interested to know if someone has answers based on some theory or better yet on real experimentation. However, as you say as long as this is consistent and repeatable, it should not be an issue because you can compensate with the tuning table(s) (as long as your sampling is also consistent).

Of course the signal will change in shape with RPM and load. What I meant is will the position of the ideal sampling point change with different RPM and load. Also, I think that the "correct" sampled value will become important when we start talking about model-based fueling. Then you'll want measurements that more precisely represent the actual VE and pressure condition in the cylinder. That's not for right now but it's one thing (among many others) to think about.

Jean

[Fred]

If the optimum point is the lowest pressure reading then it will probably be some fixed offset from the real event and thus move in timing slightly with RPM. If the curved flattish region of that plot is the low pressure (which I assume it is) then you have quite a bit of area to aim for.

Also, I think that the "correct" sampled value will become important when we start talking about model-based fueling.

Where is the practical benefit in this? Am I blind to not see it, or is there none there? I know gearhead and myself both think the model based stuff is largely a "for the fun of doing it" sort of thing rather than something that will deliver a real world performance benefit.

For me building the car is for the fun in driving it afterwards, ditto this is to make the car run well and go fast so I can enjoy it the most. Same when I build a speaker or amp, it's for the result. If the result doesn't differ much then I don't see the point from a boring practical perspective. Definitely something for the new advanced section anyway :-)

Fred.

[jharvey]

I'm curious about the system, was it boosted or natural asperated? It appears it was a 12 tooth 1 missing wheel, so about 3.5k RPM? Do you expect a MAP of 1V to equal X PSI (bar, ect)? I'm sure it was constant load / RPM right, so not changing the fuel, throttel, ect.

I picture the air flow is probably porpotiant to the average curve. I'd claim average at about 2.1V. Can you do another at say 1K RPM, 2K RPM, ect?

[Fred]

Do you expect a MAP of 1V to equal X PSI (bar, ect)?

Yeah, I'd expect that as any normal map sensor is like that. As for the rest, no idea.

I picture the air flow is probably porpotiant to the average curve.

The curve isn't an air flow curve though, it's a pressure curve. When using VE and pressure to estimate your cylinder filling what you really want is the pressure found in the cyilnder after the valve closes. The manifold pressure is an approximation to that because cylinder filling will always be close to the same as manifold pressure. Thus what you probably want for VE accuracy reasons is the highest pressure seen which correlates roughly to when the valve closed and therefore the pressure of the cylinder.

I'd claim average at about 2.1V. Can you do another at say 1K RPM, 2K RPM, ect?

Nope, I stole the image outright :-)

The only real requirement is that it is noise free and has a good range such that each incremental change of ADC reading is a small one. If you want best range, lowest pressure is best, if you want most accurate VE then highest pressure is best.

It certainly would be interesting to scope that signal on a multi cylinder engine under a variety of conditions.

Fred.

[nitrousnrg]

Hi!
I'm gonna see that signal in my car.. i wasn't aware of such variations. Couldn't it be removed with analog filters?

I only used Bosch's recommended circuit to filter that signal, you may find it useful -it's really basic-. It also helps you to diagnose malfunctions.

http://www.bosch.com.au/content/languag ... cation.pdf
(4th page)

Marcos

[Fred]

Just sample the signal at the same rate as the injection pulses or ignition signals in a timed way and it should be pretty consistent. It's OK with a normal throttle anyway, the issue comes when you have ITBs.

Fred.