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EFI Tuning Fundamentals: Intake Air Temperature Sensor

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Intake Air Temperature Sensor

04.48

00:00 - We've just covered the main load input to the ECU but there are also a range of additional sensors I'd consider to be critical to the operation of any EFI system so let's discuss these now.
00:11 One of the biggest advantages of EFI is its ability to provide a very accurate and consistent tune regardless of atmospheric conditions.
00:19 The reason this is possible is because the ECU is able to monitor these ever-changing conditions.
00:25 One of the most critical parameters we need to measure to do this is the intake air temperature monitored by an intake air temp sensor or IAT sensor for short.
00:36 As the air temperature changes, so does the air density.
00:40 As we saw earlier in the course, hot air has a lower density, and contains less oxygen hence, less fuel is needed to maintain a constant air/fuel ratio.
00:50 On the other hand, cold air is denser, containing more oxygen and thus requiring additional fuel.
00:56 The intake air temp sensor provides air temperature information to the ECU, which can then be referenced by compensation tables.
01:04 An intake air temp sensor is a type of resistor known as a thermistor.
01:09 These are a two-wire sensor and its resistance changes with relation to air temperature.
01:15 Most intake air temperature sensors for automotive use are what's referred to as a negative temperature coefficient sensor, which means that as the temperature increases the resistance of the sensor decreases.
01:29 Where the intake air temp sensor is mounted is also important in order to give the ECU a realistic measure of the actual temperature of the air entering the engine.
01:39 What we really want to measure is the temperature of the air as it passes the intake valves and enters the cylinder.
01:45 We refer to this as the inlet charge temperature.
01:48 Often intake air temp sensors are fitted into the intake manifold which might sound ideal, but in my own experience, this can actually create a few problems.
01:58 While at high air flow values, the sensor will give an accurate measure of the true inlet charge temperature.
02:04 The intake manifold can also be prone to heat soaking at low speed or when the engine is shut off.
02:10 This results in excessively high readings on a hot restart or when idling in traffic.
02:16 In this situation, the intake air temperature sensor is being influenced by the heat of the intake manifold rather than the actual temperature of the air and this affects the accuracy of the air/fuel ratio.
02:27 The common result is an engine that runs excessively lean and responds erratically for a few minutes after a hot restart.
02:35 I personally find the best compromise is to fit the intake air temperature sensor into the intake pipe just before the throttle body.
02:43 This will work for both naturally aspirated or turbocharged engines, and avoids most of the heat soak related problems associated with mounting the sensor in the intake manifold.
02:54 If you're tuning a factory ECU that uses a MAF sensor, then the MAF sensor will have an integral IAT sensor fitted to it so that it can correctly measure the mass airflow.
03:04 Since the mass airflow sensor is directly measuring the mass of air entering the engine, the ECU can accurately control the air/fuel ratio as air temperature varies.
03:15 But there may be other compensations we want to make to the tune based off inlet charge temperature.
03:20 And this may be harder with the IAT sensor mounted in the MAF sensor housing.
03:25 While the air temp from the MAF sensor will probably be similar to the inlet charge temperature in a naturally aspirated engine, in a turbo or supercharged engine, the inlet charge temperature will have been heated when the air's compressed and then perhaps cooled somewhat by an intercooler so it may be dramatically different to what's being read at the MAF sensor.
03:47 For this reason, in some MAF equipped engines, a secondary air temp sensor will also be installed in the inlet manifold so that the ECU can adjust aspects of the tune as required based on the actual inlet charge temperature.
04:00 This may be used for targeting a richer air/fuel ratio or removing ignition timing if the inlet manifold air temperature became excessive.
04:09 To summarise this module, EFI's ability to maintain a consistent tune, irrespective of changing atmospheric conditions is one of its major selling points.
04:19 One of the key sensors that's critical for the EFI system to be able to do this is the intake air temperature sensor.
04:26 This is because as the air temperature changes, so does the density of the air and this in turn affects the amount of oxygen inside the cylinders.