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Transient throttle or acceleration enrichment is a way to model the fuel bubble that's stuck on the intake / valves and doesn't make it into cylinder. So when throttle opened sharply, extra fuel will need to be injected to replace this bubble

Direct injection engines inject fuel directly into the engine, so is there no more need to model this bubble?

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    @Ruslan - While I can't spell out exactly how (yet), you still need to tune for transient conditions. While the fuel wall is part of the problem with transient, you also have to consider the other parts of it with throttle open/close, where you'll get the rush of air when you open the throttle or a vacuum spike when closing the throttle. Without tuning for this, you'll get lean and rich spikes depending on which way you're going. Jan 11, 2023 at 15:09
  • If direct injection Is part of the electronic fuel injection (EFI) system, the the engine computer was programmed for all weather starting, taking into account severe cold temps by enriching fuel and leaning out automatically based on coolant temps, map, throttle position and O2 sensor. EFI systems eliminated pumping the gas pedal for virtually instant starting in all weather conditions as long as periodic maintenance (fuel filter, spark plugs, oil and filter, air filter, etc) are performed.
    – F Dryer
    Jan 12, 2023 at 22:01

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