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EFI Tuning Fundamentals: Throttle Position Sensor

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Throttle Position Sensor

05.06

00:00 - The next critical sensor we're gonna look at is the throttle position sensor.
00:04 This is a pretty straightforward sensor that tells the ECU the position of the throttle plate.
00:08 Older designs use a variable potentiometer, that can present reliability issues as it ages.
00:15 In particular, they're sensitive to contamination and vibration.
00:19 More common in recent years is the contactless throttle position sensor, which offers far better reliability.
00:25 The throttle position sensor has three jobs to perform.
00:29 First, it's used to signal to the ECU when acceleration enrichment is required, based on the rate of change of the signal, or how fast the throttle is being opened or closed.
00:39 Secondly, it can be used to tell the ECU when the throttle is closed.
00:43 And thirdly, in some scenarios, we may actually use the throttle position sensor as the main load input, instead of the MAF sensor or the MAP sensor that we've already discussed.
00:53 Let's look at these three scenarios.
00:56 First, we have acceleration enrichment, which is covered in detail in a separate module later in this course, since it's quite a complex function.
01:05 While you'll see in that module that the actual requirement for acceleration enrichment is due to the changing size of the fuel film on the port wall, since this is influenced, in part, by rapid throttle movement, basing acceleration enrichment on the rate of change of throttle position is a reasonably effective way of dealing with what is a complex situation.
01:26 I'm using the term "acceleration enrichment", but equally, it could be referred to as transient enrichment or tip-in enrichment.
01:34 These terms are all interchangeable and mean the same thing.
01:37 Essentially, when the throttle pedal is opened very quickly, then engine can, momentarily, run lean, requiring some amount of additional fuel, over and above what is being provided by the main fuel map.
01:49 The ECU can monitor the rate of change of the throttle position sensor, and this signal can be used to add acceleration enrichment fuel.
01:57 Similarly, we'll see that when the throttle is rapidly closed, the engine will momentarily run rich.
02:03 While this situation isn't ideal, it's also less likely to cause drive-ability issues than throttle tip in.
02:10 Some ECUs may provide an enleanment function to deal with this, though, and allow stable control of the air/fuel ratio, regardless of throttle movement.
02:19 Next, we have the throttle close signal, which is used to tell the ECU that the driver's foot is completely off the throttle pedal.
02:27 This can signal the ECU to employ idle speed control, and perhaps use idle ignition control to achieve a stable idle speed.
02:35 We only want this happening when the throttle is truly closed, otherwise the idle functions will be fighting against the driver opening the throttle and attempting to increase the engine RPM.
02:46 This is actually quite a common scenario when the idle close signal isn't correctly adjusted, and it results in some frustrating drive-ability problems at very slight throttle openings.
02:56 Aside from the idle speed functionality, most ECUs will also employ what's known as overrun fuel cut, where the fuel injection is completely disabled when the throttle is closed at high RPM.
03:08 This is done to reduce fuel consumption and minimise emissions in a condition when engine power is not required.
03:15 This is the sort of situation you can expect when you're coasting down a steep hill in gear, with the throttle closed.
03:21 As the RPM begins to drop close to the normal idle speed, or the throttle is opened by the driver, normal fuel injection is resumed.
03:29 In some situations, we may choose to utilise the throttle position signal as the main load input, instead of the MAP or MAF sensor.
03:37 This is often referred to as alpha-n, and while it isn't common, it's worth understanding the scenarios when this technique may be used.
03:46 The first situation would be any engine which uses individual throttle bodies instead of a plenum and single throttle.
03:52 In this case, the manifold pressure signal generated between the throttle plates and the intake valves isn't a good representation of the engine load, and instead, we use the throttle position sensor for the load source.
04:03 This technique is also used in conjunction with a manifold pressure compensation table to deal with turbo-charged engines using multi throttle bodies, like Nissan's RB-26.
04:15 We may also need to use the throttle position sensor as a load input in a naturally aspirated engine running a very large cam profile with a lot of overlap.
04:23 In this sort of engine, we may not generate sufficient manifold vacuum at idle in order to give us sufficient resolution for the fuel and the ignition maps, and alpha-n can offer better results, better tunability and better drive-ability.
04:37 In this module we've learned that the throttle position sensor is fairly straightforward as far as sensors go.
04:43 Its purpose is to tell the ECU where the throttle plate is as well as how fast it's moving so that the appropriate amount of fuel can be supplied.