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EFI Tuning Fundamentals: Ignition Drives

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Ignition Drives


00:00 It's now time to discuss the ECU outputs.
00:02 These are the functions on the engine that the ECU controls, and we have a lot of possibilities, here.
00:08 The ECU's main job, though, is to provide the spark and the fuel, so let's start with these basic tasks, dealing with the ignition system or ignition drives first.
00:18 The ECU will include ignition drives that provide an output to control the ignition system and produce the spark.
00:26 To understand how these outputs function, we need to also understand how the ignition coil works.
00:31 The ignition system uses an ignition coil to create the high voltages required to produce a spark from the relatively low, 12-volt automotive electrical system.
00:41 An ignition coil, for example, can increase the 12-volt input to 20,000 volts or more.
00:47 at the spark plug gap.
00:49 In order to create the spark, the coil must first be allowed to charge.
00:53 During this time, the ECU provides voltage to the primary side of the ignition coil, creating a magnetic field in the coil windings.
01:02 When the ECU then breaks the circuit, the energy stored in the coil induces a large voltage in the secondary side of the coil, and this in turn creates the spark.
01:12 A key part of this process is the charging time, which is also known as the dwell time.
01:18 It takes a finite amount of time for the energy to be built up in the coil, and if it's allowed to discharge too soon, the result will be a reduction in spark energy.
01:27 How long it takes to charge will vary from coil to coil and will also depend on the battery voltage.
01:33 For example, at lower voltages, there is less energy available to charge the coil, and hence, it will take longer to completely charge.
01:42 To ensure the ignition coil is fully charged, you'll usually find a dwell or charge time table in the ECU, which is a two-dimensional table of dwell time versus battery voltage.
01:52 It's important to ensure this is correctly configured, because a dwell time that's too short will reduce the spark energy available while a dwell time that's too long can potentially damage and overheat the coil.
02:04 Dwell times are typically defined in milliseconds and values in the order of two to four milliseconds are relatively normal at an operating voltage of 14 volts.
02:14 As tuners, we aren't expected to come up with these dwell time values ourselves.
02:19 And the coil manufacturer or ECU manufacturer will usually be able to advise on appropriate values.
02:26 The ECU will be equipped with a certain number of ignition drives, which will depend on its type and complexity.
02:32 The number of drives available will affect the type of ignition system that you can control, and this is one aspect that you need to consider when deciding on an ECU.
02:42 You're going to need sufficient ignition drives to control the number of coils fitted to your engine.
02:48 Controlling the switching of the voltage supply to the coil is done by an igniter module, rather than the actual ECU itself.
02:55 You can think of the igniter module as an electronic switch that's controlled by the ECU.
03:01 The igniter module is designed to handle the voltage and current requirements of the ignition system, which can be too much for the delicate ECU.
03:10 In some ignition systems, the igniter module will be an external module that's wired between the coils and the ECU.
03:16 While some ignition coils, referred to as smart coils, have an internal igniter module built into them.
03:23 A good guide as to what sort of coil you have is to check the number of pins.
03:28 A dumb coil, which requires an external igniter module, will usually have two pins, while a smart coil, with an internal igniter module, will typically have three or perhaps four pins.
03:39 There are a wide range of ignition systems fitted to different engines, and the type used may depend on the engine configuration, as well as the ECU.
03:49 Let's briefly discuss the most common types of ignition system you may come across now.
03:54 First up, we have the distributor ignition system, which the simplest ignition system from the ECU's perspective.
04:01 Here, the ECU controls a single ignition coil, and the spark is transferred to the distributor cap by an ignition lead.
04:09 Once the spark reaches the distributor cap, it travels down the rotor arm and is directed to the correct cylinder.
04:15 There are a few downsides to the distributor, though.
04:18 Since we only have one coil in a distributor ignition system, at very high RPM, it may not have enough time to completely charge between spark events.
04:27 This can result in a reduction in spark energy, and may result in a misfire.
04:32 With a variety of moving parts to wear out, distributor ignition systems can also become unreliable over time.
04:39 Finally, the long path that the spark needs to take can also reduce the amount of spark energy that actually reaches the spark plug.
04:47 Distributors are pretty uncommon on new engines these days, and waste spark or direct spark systems have become the norm.
04:56 A waste spark system gets rid of the distributor and uses a twin post coil that's fitted with two ignition leads.
05:03 When the coil produces a spark, it's sent to both leads at the same time.
05:08 The waste spark system supplies spark to one cylinder that's on the compression stroke, which begins the combustion process and the other side of the coil supplies spark to a cylinder which is on the opposing cycle, the exhaust stroke.
05:19 This second spark does nothing, because the cylinder is on the exhaust stroke, and hence the term waste spark.
05:26 Waste spark systems are common and quite effective in most applications.
05:31 Since each coil is creating a spark event once per engine revolution, instead of once per engine cycle, though, we may still run into problems at very high RPM with the coil not having sufficient time to charge fully.
05:44 Direct spark is where we have an individual coil on every cylinder.
05:48 This provides the best possible spark energy, and is ideal for high-RPM or high-power engines.
05:55 The advantage over waste spark is that each coil only needs to fire once per engine cycle.
06:00 This provides the coil with more time to charge, which will result in more spark energy at very high RPM.
06:07 So from this module, you need to understand how the ECU controls the ignition system, and the role of the igniter module in this process.
06:16 Remember that some coils will require an external igniter module, while others will have this integrated within the coil, and we can tell what type of coil we have by the number of terminals on it.