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Practical Reflash Tuning: Open Loop vs Closed Loop

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Open Loop vs Closed Loop


00:00 - One of the key aspects to become familiar with when re-flashing a stock ECU is that the ECU will spend a large portion of its time operating in what's known as closed loop mode.
00:12 To understand what's happening, we first need to understand what the terms open-loop and closed-loop mean.
00:20 This all refers to the way the ECU controls the fuel delivery and comes about due to the OE manufacturers necessity to meet emissions compliance, as well as the desire to produce an engine with low fuel consumption.
00:36 While the basic fuel calculation should get the air-fuel ratio very close to target, there's still a variety of conditions that can add up to result in a small discrepancy between the target and measured air-fuel ratio.
00:53 These errors are a big deal for OE manufacturers and they employ a closed-loop fuel control strategy to ensure any error is quickly corrected.
01:03 The closed-loop fuel control strategy takes input from the lambda sensors or oxygen sensors that are fitted to the exhaust system, and these report on the actual measured air-fuel ratio.
01:17 If the reported air-fuel ratio doesn't match the target, a trim is applied to correct the air-fuel ratio and achieve target.
01:27 These fuel trims may consist of both short-term trims, which act on any instantaneous error, as well as long-term trims, which are updated more slowly based on the average short-term trims.
01:41 To explain how this works, let's say that an engine is operating with a relatively constant short-term trim of plus 5%.
01:51 This means that the measured air-fuel ratio is leaner than the target, and the ECU is trimming an additional 5% fuel in to correct this error.
02:02 If the short-term trim constantly sets a plus 5% over the course of several minutes, the long-term fuel trims would gradually increment plus 5% in order to bring the short-term fuel trims back to zero.
02:18 The long-term fuel trims typically are stored in non-volatile memory, which means that the trims aren't reset when the car is shut off.
02:28 This means that the next time the engine operates in that load and RPM range, the long-term fuel trim will already be active and the air-fuel ratio should already be on target.
02:41 Often these trims will be broken down into different ranges of engine operation such as low, mid, and high to better allow the trims to correct changing errors across the engine's operating envelope.
02:55 The ECU will operate in closed-loop mode at idle, cruise, and light load, and during this operation, the ECU will be commanding a stoichiometric air-fuel ratio to minimise emissions.
03:09 To be more specific, the ECU will actually purposely swing the air-fuel ratio slightly richer and slightly leaner than stoichiometric as this is essential to the effective operation of the catalytic converter.
03:24 Of course the engine can't operate at a stoichiometric air-fuel ratio all the time.
03:30 When the driver is commanding maximum power, the ECU will transition into open-loop mode where the ECU ignores the oxygen sensors and instead decides on the correct fuel delivery based on the fuel maps and measured load.
03:47 When it comes to tuning the fuel delivery, often our lives are a lot easier as in many cases, we can focus our fuel tuning only on the open-loop aspect of engine operation.
03:59 This is quite different from tuning an aftermarket standalone ECU where we need to tune every single site in the fuel table.
04:08 It's important to understand when the engine is operating in open-loop or closed-loop modes is when the engine is operating in closed-loop mode, it will be targeting a stoichiometric AFR, and hence if we attempt to make changes to the target air-fuel ratio while the engine is in closed-loop mode, these changes won't be effective.
04:30 While traditionally, OE engines have used narrow-band oxygen sensors in their exhausts, which are only effective around the stoichiometric air-fuel ratio, it's becoming more common for late model engines to be fitted with wide-band oxygen sensors, and this may offer the potential to extend the closed-loop operation throughout the entire range of engine operation in some circumstances.
04:56 The combined short-term and long-term fuel trims when the engine is operating in closed-loop mode can give us a good idea of how accurate the MAF scaling, injector scaling or VE table is.
05:11 If these parameters are correct, the total value of the trims should be very close to zero.
05:17 Any time we're seeing trims in steady-state operation exceeding approximately plus or minus 5%, this would indicate we have some work to do in the calibration.
05:29 We also need to be careful of the effect of the long term fuel trims on our open-loop operation.
05:36 If the closed-loop trims are doing a significant amount of work to correct the air-fuel ratio in closed-loop operation, the long-term trims will usually remain in effect when the ECU transitions to open-loop operation.
05:51 This can end up affecting the air-fuel ratio under open-loop, as the long-term trims can take several hours or sometimes even several days to update.
06:02 This means that while the air-fuel ratio might be perfect while the engine's being tuned on the dyno, after some amount of driving and learning, the long-term trims may mean we actually end up a long way from our target.
06:16 For this reason, my personal preference is to disable the long-term trims where possible and only rely on the short-term trims during closed-loop operation.

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