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Tip in enrichment for PI but less or no effect on DI?

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Hi, I have a curious question about tip in enrichment, since The port injection engines will result in temporary lean AFR causing by sharp throttle opening,(because some of the injection is topped on fuel film instead of making it to combustion chamber), but this should have less or even no effect on Direct Injection engines, due to it injects the fuel directly into cylinder with or without intake valve open and the mixture is happing inside the cylinder which means the full amount of fuel is well mix with air inside the cylinder for combustion phase, therefore the measured AFR should be close or on target right?🤔

There is still a fuel film effect in place in a DI engine, it isn't as great as with PI, but its still noticeable and needs to be tuned for optimum behavior of the engines responsiveness.

Some deep reading on Fuel Film in DI engines.

Thanks for taking time to answering my question with useful info provided too!

much appreciated!

I've never experienced a need for transient fuel enrichment on a DI engine, but transient airflow calculation error causes transient air/fuel error, and I think sometimes we get used to fixing that on the fueling side of things rather than the airflow side.

A quick skim of the article Stephen linked seems it's a discussion of fuel film on the piston resulting in particulate emissions, which is a significant emissions concern on gas and diesel DI engines, rather than a need for transient accel enrich to support wall wetting related to engine response.

Incorrect cylinder air calculation in transients is both a DI and PI engine concern. One common cause is engines operating based on mass air flow sensors use assumed transfer delays between the sensor and intake port, which can become compromised if piping length and/or volume or other factors are changed via new parts, if the sensor is moved, etc. This causes error in transient airflow calculation. What you see when you log the car is the result monitored air/fuel ratio error from the wideband air/fuel sensor. While you could fudge that by altering fueling, the actual issue and proper fix to avoid misrepresenting load, is not altering transient fueling, it's better transient airflow calculation, which will get lambda back on target and improve engine response.

In some applications, switching from MAF to SD based calibration avoids this issue because your point of measurement is so much closer to the intake port.

In some applications, altering the transient airflow modeling system is possible i.e. 15-21 Subaru WRX.

If you don't have access to those options, but do have transient fuel values you can adjust, sometimes we have to do the best we can with what's available in the software in front of us. As long as we understand what's happening, why we had to make the compromise, and account for load being fudged which may skew ignition timing etc., I totally understand the get it done approach.

Hi, thank you for sharing precious experience, not sure if I understand correctly from your previous post, transient airflow error that you mentioned cause by any new parts or modification on intake with MAF sensor solely or both MAF and MAP sensors, due to the airflow differences than previous, therefore to fix this error, we’ll need to recalibrate the MAF or both MAF and MAP table for the correct fuel and ignition timing, correct?

cheers!

Steven,

Generally I have to adjust both.

When a modern vehicle is totally stock, I expect it to run well as is, but as certain physical modifications are made, they can effect the efficiency of the engine system, which generally requires adjustment of the airflow model whether it's based on MAF, MAP, or both. Also, sometimes calibration changes can also move engine operation outside of the window which has been fully mapped in the stock calibration, without changing any physical parts. For example, requesting more airflow than stock might induce this.

While an efficiency change requiring a change in speed density/MAP related efficiency values may seem more obvious based on HPAs explanation of speed density tuning, I find efficiency changes often require adjustment of MAF based airflow evaluation as well.

I try to be careful when explaining this, because we should strive to use MAF scaling to best represent the air mass the engine receives, but in practice, if you give an engine the same air mass, but change what the engine does with that air mass, the result air/fuel ratio and load can change, and MAF rescaling is sometimes the best or only option to get fuel trims, result fueling, back on targets.

Hopefully this example will help. Let's say we have a totally stock vehicle, stock intake system, stock MAF, stock MAF housing. Fuel trims are minimal, fuel targets are being hit, so the OEM has done a great job. Now let's say we install a new set of camshafts which change engine efficiency, impact pumping through the engine. Just because you supply the engine with the same air mass, get the same MAF sensor reading, does not mean the engine will operate the same as when it was stock. The physical changes will cause a different result, and getting fuel trims back near zero may require MAF scaling if the change can't be accounted for in a better way.

The change doesn't have to be as dramatic as a camshaft change for this to be required either. For example, I've seen a turbo inlet hose massively improve compressor efficiency, requiring a significant shift in MAF scaling to get fueling back on target. On the exhaust side, most factory gas engines won't have exhaust backpressure sensors, so improvements on the exhaust side can cause a big change in engine efficiency that will not show up in a MAF reading on the intake side, so MAF rescaling may be needed for even an exhaust change. I know this likely seems counterintuitive, and in an ideal world I always update the portion of the model that actually represents reality, but when an exhaust change causes an air fuel change, and you're working with a ROM that has no exhaust model, MAF scaling often ends up being where you have to account for the change.

Please let me know if this makes more sense.

Hi Mike,

After reading your last post several times, and this is how I think for example, an none modified turbo engine equipped with variable valve timing cam, fitted both MAF and MAP sensors, and on stock mode, stock boost, let’s say at 80kpa point with 0 degree cam timing, and the engine has been already calibrated to correct or targeted fueling and ignition timing, but after we start to altering the cam timing, the Volumetric Efficiency would start to be different at the same point of the MAF sensor or MAP sensor reading at 80kpa, thus needs further rescaling or calibration to meet our target AFR?

as for the larger housing on turbo, I think it’s the same effect for larger diameter pipe would does too right? Same flow pressure but more air pass through? However from the HPA’s EFI course mentioned this would reduce the resolution of the table cell, I think that’s why we need to rescaling the sensor reading to get the right fueling.

please correct me if anything is wrong, much appreciated!

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