26 October 2021. Getting the idle better.

26 October 2021. Getting the idle better.

Fuel Pressure – Fixing new parts 🙁

This is the new fuel pressure regulator installed. With the pressure sensor, later this was connected to the Megasquirt ECU.

Fuel pressure regulator and sensor installed.

The first tests went well in that it did not leak, however it became apparent something was not correct with the pressure control and its adjustment, so it had to come off to be taken apart.

Looking at the picture below, you are looking at the inside of the cap with the button in the middle being the bit that pushed on by the external screw to preload the spring (not shown) in there. You will notice the end of the vacuum connection poking in from the right hand side. This stuck in far enough that it was jamming the adjustment and making it impossible to back it off once jammed.

After the excess was machined off and the whole thing reassembled it then worked correctly. It was set to 3.7bar with the engine stopped, at idle with max vacuum this then is pulled down by the vacuum connection to 3.1bar which is as far as I can find out how these injectors should be run.

Oxygen Sensor Calibration

After the fuel pressure regulator was done, the next job was to calibrate the oxygen sensors, I should have done this before now, but better late than never. The instructions from 14Point7 where followed, I don’t know how much if at all this changed the setting on the things but I know now that they are clibrated.

Making the Idle better.

Next was to work on the idle settings and try and get the AFR correct at idle.

There’s a lot of different settings in the ECU that govern this behavior I’m not going to attempt to explain it all, there’s lots of good content out there on YouTube and questions get answered on Megasquirt Forum Introduction – Megasquirt EFI (msextra.com) if you’ve bothered to RTFM first.

Whilst the engine would start and warm up ok the AFR was rich and whenever I tried to turn down the fuel it’d just die. In the end I struck inspiration and thought about changing the injection timing.

26 October 2021. Getting the idle better.
Timing values, circle is 720 degrees, so double some numbers.

Whilst in the live tuning mode, where the program is adjusting the fuel to try and get to the desired AFR, I kept pushing the injection time forward in 10 degree increments, letting the thing settle seeing what the fuel setting had done, then moved another 10 degrees forward until it was clear I’d gone too far. Out of that I’d gone from a timing of 294 degrees to 460 degrees.

I didn’t really understand how this was working, so drew out the diagram above, the first thing that was clear was the initial 294 number was wrong as this was supposed to be at the point that the inlet valves were opening. But the final number is way further round than if I’d just simply corrected that mistake.

Previously I’d aimed at that point to give the fuel time to vaporize off of the back of the inlet valve, however what I think was happening is that with the 9 degrees of valve overlap the fuel mixture was being sucked directly into the exhaust. This left the exhaust mixture rich whilst not leaving enough in the cylinder to run the engine. With the final position the injection doesn’t start until the exhaust valve is shut, and finishes at mid induction stroke when port velocity should be highest. With an approx. 2.5 millisecond injection time that’s about 15 degrees of crank rotation at 700rpm.

With that sorted out the VE numbers went from 55-ish to 43-ish, so a lot less fuel being used at idle and it can sit at 14.7 AFR and do all this sitting nicely at 700rpm idle. I’m very happy with this so far. Having worked out the difference between the calculated VE and the actual VE used for the idle I then applied that as a multiplier to the rest of the VE table so hopefully that is ball park-ish as well when it comes to tuning that.

There’s still a bit more to do as I’m not convinced that the injector dead time is correct, that is the time taken between when the ECU commands the injector to open and it actually being open, this is a constant in the fueling calculation, and shouldn’t make a huge difference if its out by a bit. But I think I can get that a bit better too.

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