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Launch Control: Launch Control Methods

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Launch Control Methods

06.39

00:00 - As I've discussed, there are a few different techniques for implementing launch control.
00:04 The correct option for you will depend on your ECU as well as your car and your application.
00:10 In this section, we'll cover the common techniques and talk about how they function, as well as any potential pitfalls.
00:17 We'll start with one of the most basic systems, which consists of a simple launch control rev limit.
00:24 This type of launch control is often referred to as two step or two step launch control in drag racing circles.
00:31 It consists of a secondary rev limiter that is active when the car is stationary.
00:37 This rev limiter is set at the RPM you want the car to launch at, and you can simply apply full throttle and then release the clutch.
00:45 This works well to achieve a consistent amount of engine power during launch, and it's also very quick and easy to set up.
00:53 It's a system that can't do anything to help you once you've released the clutch and started moving, though.
00:59 So if the car immediately breaks into wheelspin, two step launch control won't reduce the engine power.
01:06 With any type of launch control system, we need some way of signaling to the ECU that we want the launch control to become active.
01:15 The two simplest ways of doing this are by using a digital switch such as a clutch switch or by basing the launch control off wheel speed so it only becomes active when the car's stationary.
01:28 While a clutch switch is a common way of triggering launch control, there is no reason why this couldn't be triggered via a button on the steering wheel, or perhaps even a hand brake switch.
01:39 This does require the driver to synchronize releasing the clutch and the launch control switch at the same time, though, and this can be more difficult than it sounds.
01:49 While a launch control RPM limit is the simplest form of launch control, it's often coupled with ignition retard, too.
01:57 This isn't necessary or even useful with a naturally aspirated engine, but on a turbocharged engine, and particularly a small displacement engine with a large turbo, ignition retard can be very effective in helping to spool the turbo prior to launching.
02:14 This works in conjunction with an ignition cut rev limiter, where the spark is randomly cut to individual cylinders to control engine speed.
02:23 This naturally results in a lot of unburnt fuel and air moving through the engine and entering the exhaust.
02:30 If, at the same time, the ignition timer is dramatically retarded back to TDC or even later, the cylinders that are still firing will tend to continue combustion into the exhaust manifold.
02:43 This, in turn, will ignite the unburnt fuel and air in the exhaust manifold, creating small explosions that provide a high level of energy to drive the turbine wheel and hence create boost pressure.
02:55 We'll talk more about setting this type of launch control up in the next module.
03:01 While a basic two step launch control system is effective in achieving consistent engine RPM and power on the start line, it has little effect once the clutch is released and the car actually begins moving.
03:15 Particularly with a high powered two wheel drive car or a two wheel drive car on a slippery surface, some further assistance can be beneficial as the car begins to move.
03:26 There are a couple of ways of dealing with this, which could be likened to a passive form of traction control.
03:33 These techniques include controlling the engine RPM limit or adjusting the opening of an electronic throttle body relative to the wheel speed from an undriven wheel.
03:45 Again, we'll look at this in more detail, but before we move on, I'll explain how this works.
03:51 If you have a speed sensor input from a non-driven wheel and you put the car in first gear and accelerate smoothly with no wheel spin from a standing start, you'd find that the engine RPM is directly relative to ground speed.
04:06 What I mean by this is that each increment of ground speed from, say, zero to 40 kilometers per hour, there will be a specific engine RPM required to achieve that speed.
04:17 Let's call this the "zero slip RPM." This will depend on your gear ratio, final drive ratio, as well as the rolling diameter of your wheel and tire combination.
04:30 If we know what the zero slip engine RPM is at a certain ground speed, we then know that if the RPM is higher than this, the car must be wheelspinning.
04:40 On this basis, we can apply an engine RPM limit slightly higher than the zero slip RPM at each ground speed increment and the limiter will help control wheelspin.
04:52 Now let's consider how we would control the throttle opening in a powerful car if we wanted to launch it from a standing start.
04:59 Initially, we might want to use a large throttle opening to ensure the car doesn't bog or stall when the clutch is engaged.
05:07 Once moving, though, the same large throttle opening may cause excessive wheelspin, so naturally we'd back off the throttle to reduce wheelspin and achieve good traction.
05:18 The effect of this is that we would adjust the throttle position relative to ground speed in order to achieve maximum acceleration without excessive wheelspin.
05:29 If we have a car equipped with an electronic throttle body in a capable ECU, we can electronically control the maximum allowable throttle opening during launch to achieve the same aim.
05:42 The problem with both of these techniques is that we need a ground speed or undriven wheel speed input.
05:49 This type of control can't be achieved using a driven wheel speed sensor, because when the car is wheelspinning, the wheel speed simply increases, giving nothing useful to base a control strategy off.
06:01 This means we can't apply this sort of technique to a four wheel drive vehicle.
06:07 In theory, a GPS speed sensor could be used.
06:10 However, GPS speed input still can't be considered 100% reliable and consistent.
06:17 The update speed of GPS speed inputs also doesn't lend itself well to this sort of control, as most sensors operate at either 5 Hz or 10 Hz.