Engine Overrun

Hi there. Can you explain further what is engine overrun in general and what Andre was meaning when he spoke about engine overrun in this context?

Also, a couple of questions about the Load(pressure) against RPM graph at 10:08

1) Is it based on a forced induction engine? I cant understand why the Full Load portion is below the regions of Maximum and High Boost.

2) What is Transition F/I?

3) On factory settings, during the cruise rpm range, is it set stock to operate at 1.0 lambda or is it advised, when tuning for power, to run at 1.0 lambda? (11.55)

Are there any disadvantages of running slightly richer at cruise on NA and FI engines other than the drop in fuel economy?

Do spark plugs stop working when the foot is lifted off the acc pedal?

Is it possible to switch off the fuel injectors on a factory ecu or does that depend on what driver that particular ecu has available?

Over run is the area of the map where the throttle is fully closed and the engine is slowing down. Consider where you step off the throttle while in 4th gear at 5000 rpm - The engine will slow down until you get back on the throttle. this is over run. Under these conditions the injectors are often disabled to help with fuel economy and emissions.

I'm sorry but I don't know which specific video you're referencing here. Let me know but I'll do my best to answer:

2. Transition in a forced induction engine is the period where we move from cruise operation towards full throttle where the boost is still building up.

3. Normally for cruise we tune for lambda 1.0 but under full power we may be in the region of 0.75-0.85 - every engine is different.

4. If you run too rich then you risk fouling spark plugs. Beyond that though a slight rich mixture would just result in a loss of economy.

5. It's normally fuel that is cut off, not spark during over run operation

6. The injectors on a factory ECU do normally switch off when in over run as explained. Beyond this however, no, you can't turn the injectors off.

Hi Andre, the video is "Choosing the right AFR" under "EFI Fundamentals"

Thanks for the answer buddy.

5. So is there any point in the engine operation where the spark plugs stop firing?

6. Alright, do you have more control over the injectors on a standalone ecu in terms of whether you can switch them on or off or change the length of their duty cycle? i.e. increase/decrease the operation pulsewidth

The spark plugs may stop firing when a rev limit is active, assuming you're using an ignition cut limiter? There's really no reason to stop the spark event though other than this situation.

I wouldn't say a standalone ECU gives more control over the injectors, it's just a different process for making the changes. There's very little that can't be done these days on some of the popular OE ECUs.