2012-06-25

Fuel injector voltage compensation

I've noticed that the engine sound changes when the cooling fan kicks in. Not a big deal, I thought, but now I understand it is something worth a bit of attention.

Injector "dead time" is the time between pulse and fuel is delivered. What makes it a little bit complicated is that the dead time is not only voltage dependent but also fuel pressure dependent.

When the fan kicks in, or the headlights are lit, the battery voltage is dropped. The ECU have a voltage compensation map that add some time to the fuel map. If that map isn't correct the engine rpm may drop, causing the alternator to generate less current, even less fuel, and the engine could eventually stall. If we use closed loop lambda control the ECU would stable the engine within a second, but what would be written to the long term fuel map?

In my case the compensation map set up was not correct for my injectors.

The easiest way to measure injector dead time is to have the engine running and measure the lambda. Then remove the alternator control cable to drop the voltage about 2v, and see if the lambda changes. Adjust the map until the lambda reading is as before the alternator was disabled.

A more accurate way is to start the engine, remove the battery and connect a variable voltage source, and then disable the alternator and measure lambda at different voltage levels and adjust the map accordingly.

Or, you could do as I did and Google the injector's specs.


Bosch "Green giant" injectors at 4 bar, MBE 9A4:



It worked.

2012-06-21

Mantorp

Very windy, sunny day. The wind kept the top speed down on the long straight.

Used 13" Radical slicks. Started with ~20 psi hot, but it got slippery a couple of turns if I pushed a bit too hard. Raised to ~22 psi after a talk with the Radical guys. It certainly improved but I think I could go even higher.

Inertia switch released twice on the straight! Then I short wired it...

Experienced some mid corner oversteer when pushing hard. Maybe the rear suspension bottoms out? Next time I'll investigate this.

I've always had problems with fuel surge in long right hand corners when having about 1/4 of fuel, but it was even more obvious now. The fact it is in the right hand corners doesn't make sense to me, since the fuel pump is on the left side of the tank. Here is an interesting thread on Blatchat.



Best time 1:24.3!!! That is almost two seconds faster since last visit!

I need a better microphone to my GoPro.

2012-06-05

Inertial switch




Who said highway driving isn't exciting?

Fortunately it was only the inertial switch that cut off the fuel pump. Never happened before and took a while to figure out standing roadside with oncoming darkness and decreasing battery power.

A press on the top and I was back on the road.


Inertial switch