Idle tuning, Part 1

Tuning, troubleshooting and the nitty gritty of using rusEFI to make your engine run nicely!
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blundar
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Idle tuning, Part 1

Post by blundar »

It was asked to preserve some of this so... Let me preface this that idle tuning is probably one of the HARDEST parts about making an engine run really well. It's not something like fueling that you can measure with an instrument(s). There's always more than one way to make an engine idle at a given RPM, however, the choices that you make will have a huge impact on stability and robustness of idle.

Idle is a like a three-way see-saw. Fuel, Ignition and air are the three things that can push idle up and down because these are the three main things that control how much torque a motor is making. If you want to have a stable idle, you need to have (somewhat) stable torque output. Idle is the equilibrium state where the LOAD placed on the motor (by friction internally, any accessories like alternator, AC, torque converter/trans pump or even supercharger that are being driven) is equal to the amount of TORQUE the motor is generating. Load and torque equal, net acceleration zero, RPM constant. Get the big picture?
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Re: Idle tuning, Part 1

Post by blundar »

Overall general strategy:

1. You need a solid foundation to build on. Idle is built on top of general motor operation. The quality of your tune in terms of fuel and ignition close to / around idle will have a huge impact on idle quality. Try to get fueling in the region around idle tuned to a CONSISTENT AFR. If AFR changes due to load or RPM close to idle, torque output will vary right along with it.
1a. Battery voltage is typically lowest at idle because it is the lowest RPM the motor operates at, which means the alternator is spinning slowest. It is *extremely* common to have battery voltage move around a lot at idle, especially as electrical loads like blower motors and headlights are turned on and off. Your battery offset tables need to be dialed in well enough that you don't get sudden swings in AFR as voltage changes. This can be really tough but the extra step of creating electrical load deliberately (or even disconnecting the alternator so the car is only running off battery) and tuning for voltage variations WILL improve things a lot. (Note: this is also an argument for good injector data to match a model, but that's another discussion)
1b. Load WILL change around idle. AC is the #1 thing that will drastically change the amount of load on the engine at idle for all engines equipped with it. With an automatic transmission, there is a vastly different load on the engine in park/neutral vs. drive. Typically you need to add IAC duty / air to compensate for the torque required to spin an AC compressor. Again, make sure that your engine doesn't swing AFRs massively when moving from one idle area of operation to another!

2. You need to come up with some idea of where you want the motor to idle. There isn't a "right" answer to this that is independent of the engine - you need to look at factors like flywheel / rotating inertia, accessories driven via belt or gear, noise, personal preference, etc. This can be an iterative process i.e. pick a target, try to make the engine operate at that RPM, revise expectations. As a rule of thumb - higher RPM idling will be easier to maintain.
Say you want to make the motor idle at 800 RPM (lowest point). Task one: turn off or otherwise disable all idle corrections (airflow, fuel, spark) and make the engine idle 50-150RPM HIGHER than your 800 RPM target by only adjusting airflow (IAC position/ETB IAC, throttle stop). This is a good time to make sure that your AFRs are nice and stable - you shouldn't see much of a change as you adjust airflow. Before you enable idle control, you need a solid foundation. For this example, we now have an engine idling 950 RPM with 25 degrees of timing an 1.0 lambda.

3. Enable Idle targeting. Make sure that "RPM upper limit threshold" is GREATER (100-150 RPM minimum) than the RPM. For this example, 1050-1100 RPM would be a good choice for upper threshold.
3a. Once idle controls are enabled, adjust base idle timing / closed loop timing to be SIGNIFICANTLY lower than "normal" timing, figure 10-15 degrees less. Hopefully, the change in timing will drop RPM down from where the engine was at on "normal" timing to the true target, 800RPM. If the engine is still idling too high, whack more timing. At this point, timing should still be FAIRLY constant - not all over the place. AFR should be fairly constant. Throttle/IAC position should be constant.
3b. Once engine is idling at 800 RPM target on lower timing, re-assess. Where is timing? Are you at -10 degrees? Does the engine still sound smooth? if all sounds good, MOVE ON.
3c. If you had to put the motor into a bad place in order to get "what you wanted," revise what you want. There are three things that you can do to reduce the amount of timing you need to whack out: 1. bring your "normal timing" RPM target lower/closer to your final target. 2. Raise your final RPM target closer to your initial settling point 3. raise both targets

4. Enable CLOSED LOOP Idle spark. RPM error vs. timing change. Note: THIS IS AFTER using idle timing to get damn close. If you want to have a stable idle, you do NOT want closed loop idle making big changes all the time! Closed loop idle is necessary for long term stability. Closed loop idle timing adjustment is the #1 thing that will KEEP your engine continuing to idle when something changes. Running the engine with significantly less timing (step 3) gives this strategy room to work! Proporitional heavy, Integral a bit, Differential very little
4a. Putting boundaries on timing (overall minimum, overall maximum) is a VERY good idea. You don't want to let closed loop idle run away. -5 degree minimum and normal timing +5degrees is a good place to start.

5. Enable closed loop idle air. This is the BIG GUNS. You need to be very, very, very mindful of the big guns. SMALL changes in airflow will make a huge difference. More than that, the speed at which an electronic throttle body or idle valve can move is like slow motion compared to the speed at which spark control can make a difference. For closed loop air control to make a difference, it needs to be used very, very, carefully. You MUST have a deadband (area where nothing happens) around your target to allow "lesser" corrections to work. If Proportional is used at all, make damn sure the gains are low and there is a significant deadband. Integral dominates, again with a healthy deadband. Derivative, not so much.
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Re: Idle tuning, Part 1

Post by blundar »

Addendums for Special cases.

if using a very light flywheel or torque converter, adjust the "normal" timing initial target HIGHER and pull down further with timing. Applying a higher idle entry can help too. Prepositioning / "dashpot" can also help prevent RPMs dropping too fast.

if using large camshafts or ITBs, you will NEED to look at your camshaft profile, do a little math and time your EOI (End Of Injection) really carefully.
The math is looking at duration and centerline of intake cam in order to figure out where in 720deg the valve is open vs. closed, where intake valve opening event occurs. Both very large cam lots of overlap and ITBs create situation where there isn't a lot of vacuum to draw fuel into engine.

EOI requirements are going to change dramatically with engine temperature. Cold engines have cold intake valves. Spraying fuel on a cold intake valve does nothing. Adjusting EOI to make use of the instantaneous vacuum afforded by an intake valve opening makes more sense. (Disclaimer: IMHO) Once engine has warmed to a reasonable temperature, standard recommendation of timing EOI to sit idle fuel on top of (hot) closed intake valve to aid in fuel vaporization applies.

ITBs are a bit of a special case, especially when the injectors are facing AWAY from the intake valve. Especially in this case, vacuum immediately after intake valve is just about the only way to make sure fuel gets in the cylinder. They typically do better with *SOI* around the intake valve opening so that the instantaneous vacuum will draw fuel into the engine. To avoid having to refactor idle, look at total injector MS at idle, RPM at idle, do a little math and place EOI such that SOI happens just right.

Code design reminder: with variable cam timing where intake cam is moving: remember to adjust EOI to remain constant w/r to intake valve opening
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Re: Idle tuning, Part 1

Post by blundar »

Automatic transmission special notes:

Air changes slowly.
There is a HUGE difference between PN and D airflow with an automatic transmssion.
Once drive has been engaged, the torque converter and pump have to be spun. This is a much larger load on the engine.

Strategy for tuning:
1. Map out airflow required for idle for (low target -100) to (high target + 100) in Park/Neutral
2. Map out airflow required for idle for (low target -100) to (high target) in Drive
3. Determine final low target in gear. Note how much airflow required for idle operation
4. Look at the RPM vs. airflow in park neutral. Find the RPM that has the same amount of air required as the low RPM. This will generally be 50-250 RPM *higher* than the RPM in drive. Set the target RPM in Neutral to this RPM.
5. Rinse and repeat steps 3 + 4 for the target idle vs. temp curve

The name of the game here is that you want to choose different targets in drive and neutral that have the same airflow requirements so that the idle valve/ETB does NOT need to move when transitioning from drive to neutral.
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Re: Idle tuning, Part 1

Post by blundar »

With cable throttle, the throttle stop is an important factor affecting the minimum amount of air that can go through the throttle. When it closes, the throttle rests against the stop, but that doesn't mean that it restricts airflow completely. It's generally a good idea to adjust the throttle stop so that when the engine is idling fully warmed up at its lowest RPM (i.e. lowest airflow requirements) the Idle Air Control (IAC) is between 20% and 40% duty. This gives the idle valve plenty of room to open to increase idle but still leaves room to close it to bring revs down quickly.

With ETB, the IAC is modelled by using a range of ETB motion for idle. There is a parameter which determines the range between 0% IAC (ETB commanded filly closed - note some air will still move through the ETB) and 100% IAC (ETB positioned at the "Electronic throttle idle range(%) location). Essentially, you can choose how big of an IAC you want to have by changing the range. It's generally good form to set the maximum idle ETB to 2-2.5X the actual position it ends up at idling fully warm (lowest airflow requirements)
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Re: Idle tuning, Part 1

Post by AndreyB »

Say we have https://rusefi.com/online/view.php?msq=526

I assume you would need logs and maybe a video to make suggestions on how to improve that tune in terms of idle?
Very limited telepathic abilities - please post logs & tunes where appropriate - http://rusefi.com/s/questions

Always looking for C/C++/Java/PHP developers! Please help us see https://rusefi.com/s/howtocontribute
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Re: Idle tuning, Part 1

Post by Dron_Gus »

This should be copied to wiki. Perfect explanation.
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Re: Idle tuning, Part 1

Post by MHTSOS »

This is pure gold. I predict many people uploading revised tunes of their cars on rusefi online in the coming days.

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Re: Idle tuning, Part 1

Post by stayros32001 »

blundar wrote:
Mon May 17, 2021 2:44 am

if using large camshafts or ITBs, you will NEED to look at your camshaft profile, do a little math and time your EOI (End Of Injection) really carefully.
The math is looking at duration and centerline of intake cam in order to figure out where in 720deg the valve is open vs. closed, where intake valve opening event occurs. Both very large cam lots of overlap and ITBs create situation where there isn't a lot of vacuum to draw fuel into engine.
I am on the paper build status due to missing connectors and parts, but I have my MRE.
I was looking for the EOI settings but I can not find it.
Maybe I am blind. Can anyone help me?
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Re: Idle tuning, Part 1

Post by mck1117 »

Injection phase in the fuel menu:
image.png
image.png (41.95 KiB) Viewed 20376 times
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Re: Idle tuning, Part 1

Post by stayros32001 »

I have seen this, but it is not self explanatory!
you set the open or the close time?

I was looking something more like:
Duration, Overlap, Lift
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Re: Idle tuning, Part 1

Post by mck1117 »

stayros32001 wrote:
Fri Apr 15, 2022 12:57 am
I have seen this, but it is not self explanatory!
you set the open or the close time?

I was looking something more like:
Duration, Overlap, Lift
That table sets the angle at which your injections end. Or, as blundar called it above, EOI = End Of Injection angle.

The values in that table are degrees after top dead center ignition. So 0 degrees means injection completes at the same time that the spark fires. -360 means injection completes 360 degrees before the spark fires, somewhere near when the intake valve is starting to open. A good starting point is -400 degrees, just before the intake valve is starting to open.
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