Quote:
Originally Posted by ztan
Interesting. My calibration is A01G. First second of cranking is full PI, then DI gets added, full DI after around 6-8 seconds. Are later calibrations starting up with DI?
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As I mentioned it will depend on the dynamic compression ratio of the engine during cranking. Almost all engines with VVT go to their end stop positions when no cam dutycycle is applied. Most have a pin to keep them in place. When the cams are at this position the Dynamic compression they create is important.
Most Intake cams are locked at their most retarded position and most exhaust cams are at their most advanced position. I believe this is the safe position for most OEMs. A compression ratio of 5:1 is safe for a hot restart (the incylinder air temperature gets extremely hot in a minute of not running). When the engine is re-started the throttle cannot keep the load under 0.3g/rev because the manifold is at ambient pressure with the engine not running. This means during the first few revolutions the engine is seeing 100% VE (not really because intake cams close late into intake stroke) causing pre-ignition.
Ways around it is to delay fueling by a couple of cranking rpm so allow the hot air to be passed through the cylinder or going lean on the first few pulses.
This is more important on stop start engines and less of an issue with key start since the start quality is less of an issue with normal key start. I mean people do not care of the rpm flare up during a normal key start but dont want their engine to roar into life on a stop start event.
If toyota uses the option to not fire fuel in the first couple of cranking pulses then PFI would work. It will also work if their ECR is 5 thereabouts.
OEMs with pure DI will go for two injections with the second injection just before ignition. This way they can retard spark and get a CAT light off during cold starts. During hot starts this isnt a problem anymore and no real need for DI in my opinion. The thing is we dont know how many levels of strategies exist in the Toyota code. If they just want one strategy across all operating conditions they would choose one option vs if they had a cold strategy vs a hot strategy option