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Old 03-11-2011, 12:10 AM   #1
Eklipse113
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how does an ECU work?

Since I feel like a dunce to be doing something to my car (ECU tuning) and not fully understanding how it works, I was wondering if anyone in the know could do some lengthy explaining or point me to some good websites on how ECU's work.

All this AFR, fuel pressure, spark advancement stuff... I sort of get the idea, but I did just find a couple good things at
http://www.automotivehelper.com/topic681567.htm
and http://aeromotiveinc.com/2010/01/adv...fuel-pressure/ .

After finding that stuff I found it pretty interesting in itself, so now I really wanna know what it's all about.

So... anyone able to fill in the gaps for me, and tell me more about how an ECU works and how adjusting all these parameters affects it?


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Old 03-11-2011, 07:10 AM   #2
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the ECU is a computer, it read inputs from sensors in order to control the outputs. They pretty much work like any other computer around. You can change the parameters in the ECU so that it lengthens the pulse width to the fuel injectors under certain conditions. the AFR is read by the o2 sensor, that signal tells the ECU what pulse width to send to the injector. it also uses the MAF to sense the temp and amount of air going in the engine. then changes the fuel for that too.

the correct response to your question would be a 500 page book.
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Old 03-11-2011, 08:23 AM   #3
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Moved to "ECU Tuning Chat"

I think the guys in here will be a better help, hopefully Tom can chime in and help you out.
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Old 03-11-2011, 11:51 AM   #4
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I was looking for an explanation somewhere between " the ecu is a computer" and "pulse width". I more or less would like an introduction to the lingo.
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Old 03-11-2011, 12:30 PM   #5
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oh ok. Pluse with modulation also called PWM or duty cycle is a way to control a component. It is basically a digital signal, On and off. Pulse width/ duty cycle is the measure of time that the component is on compared to off. So say you want your light on as bright as it can go. The power to the bulb might be 111111, on all the time, duty cycle of 100%. But, now its to bright, you want it dim you would send it 101010, on of on off on off, 50% duty cycle. Because of the speed at which the signal is sent, it appears the bulb is on constantly with 50% of its light output. You can use this same type of signal in almost any of the ECU outputs. this is a basic description it can be very complex.
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Old 03-11-2011, 07:35 PM   #6
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The engine needs the right amount of fuel and the spark at the right time. Sensors measure your throttle position, air flow, engine rpm, etc. The factory programmer develops a table of approximate values for air fuel ratio(AFR) and spark timing. Based on the airflow the ECU pulses the injectors. The O2 sensor helps the ECU fine tune the AFR and injector pulses. If you open the throttle, the engine needs more fuel before the mass air flow (MAF) sensor can measure it, so the ECU uses the rate of throttle movement to add fuel. As the rpm increases, the spark must occur earlier as the piston is moving faster, so the ECU advances the timing. If the engine is lightly loaded (rpm high for the throttle position), the ECU advances the timing more to increase efficiency. If the engine starts knocking the knock sensor tells the ECU and it retards the timing. A tuner manipulates the tables for fuel and timing and changes multipliers and correction factors for the inputs and outputs to increase the power by taking advantage of an alteration to the engine that the original programmer didn't have to account for like an exhaust header or a cold air intake.
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Old 03-12-2011, 02:52 AM   #7
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http://books.google.com/books?id=X1E...ed=0CFoQ6AEwAg

Buy that book. It covers everything EEC-IV and is very informative. The downside is our cars use a EEC-V so some of the book is dated. But that doesn't matter much because the basic premise and functions are the same.


In a nutshell the ECU has inputs from,
Mass Air Flow Sensor (MAFS)
Air Intake Temperature Sensor (AIT)
Heated Exhaust Gas Oxygen Sensor (HEGO)
Cylinder Heat Temperature Sensor (CHT) or for the SVT, Engine Coolant Temperature Sensor (ECT)
Crank Position Sensor (CKP)
Camshaft Position Sensor (CPS)
Clutch Peddle Position Sensor (CPPS?) or for an automatic trans, Transmission Range Sensor (no clue on abbreviation for it)
Vehicle Speed Sensor (VSS)
Throttle Position Sensor (TPS)

The outputs are pretty simple,
Fuel Injectors (PWM)
Fuel Pump (PWM)
Ignition Coil
Intake Manifold Runner Control (IMRC), SVT only
Cam Shaft Phaser, SVT only

There are a bunch more outputs that act like switches, things like the A/C compressor, the cooling fans, mostly creature comforts.


The ECU takes voltages or resistances from the inputs, looks them up in tables and makes the appropriate changes to fuel trims and ignition timing.
The tables are based on a few situations such as Wide Open Throttle or Closed Throttle. From the tables it knows that at say 50% throttle opening and 3,000 RPM the engine can ingest 300 Cubic Feet of Air per Minute, it can then calculate how much fuel will be needed to hit the target Air Fuel Ratio of 14.68 (air) : 1 (fuel), or 300/14.68 = 20 parts fuel ... should actually be in units of Mass and not Volume but you get the idea.

Pretty much if you can understand a spreadsheet you can understand look-up tables, same concept.
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Old 03-17-2011, 01:19 AM   #8
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Thank you guys so much for the explanations. Right at the level of detail I needed. I'll look into that book too. I tried to give you all rep (if you care) for it but the forum says I can't give rep to those posts, no idea why.

What's the business between closed loop and open loop? As I understand it a cruise at constant RPM is closed, where the STFT and LTFT are slowly adjusted towards each other to achieve stoichiometric ratio. And WOT would be open loop, as well as the initial cold-start up, correct? How does the engine handle everything in open vs. closed?
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Old 03-17-2011, 04:26 PM   #9
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The open loop I'm not positive on.
I'd always assumed it used the O2 sensor and shot for a predesignated AFR. But I've been told that's not the case. Apparently the O2 sensor isn't used during WOT/open loop. The confusing part is when you tune open loop you tune for a target Lambda. So the only thing I can come up with is that target Lambda translates to a specific STFT and injector pulsewidth. If that's right or how it works I haven't the foggiest.

Cold start is open loop. There are both functions of time running and engine coolant temp that determine when it goes to closed loop idle control.

There isn't anything between open loop and closed. Well there is a 'ramp' that smooths one into the next but it's only to smooth the transition between the two.
What you're thinking, I think, is idle/closed throttle, part throttle/cruise and wide open throttle. Idle and part throttle are closed loop for the most part while WOT is always open loop. Part throttle can be open loop if the 'load' exceeds a preset percentage and/or throttle angle, both are about 1/2 throttle stock.
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