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EFI Tuning Fundamentals: Engine Principles

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Engine Principles

07.35

00:00 - Let's start right at the very beginning by revisiting the four stroke principle.
00:04 This is the operating principle used by the vast majority of engines you're likely to come across.
00:10 There are some exceptions like two stroke engines, diesel engines, and rotaries of course but four stroke piston engines are by far the most common engine type you'll come across.
00:20 Let's just talk for a moment about some of these exceptions so you'll know exactly what to expect from this course.
00:27 This particular course will focus solely on spark ignition engines, and we won't be covering any concepts relating to compression ignition, diesel engine tuning.
00:37 With that said, we do offer separate diesel tuning fundamentals and practical diesel tuning courses and these are perfect if you want to learn more about this particular style of engine.
00:47 You'll find a link below this module to those courses.
00:50 While we're not going to be covering two stroke applications in this course, if you're a rotary enthusiast, then this course is still going to be beneficial to you.
00:59 While obviously the rotary engine's operation is quite different to that of a piston engine, we still find that the way a rotary engine responds to both fuel and ignition timing are very similar to a piston engine, so the lessons are still very relevant.
01:14 The key with a rotary engine is understanding that they are a lot more fragile when compared to a piston engine, and a conservative approach to tuning is required.
01:24 In particular, a rotary engine typically likes to run with a richer air-fuel ratio and is very intolerant of detonation.
01:32 Before we move on, I'll also mention direct injected engines, as these are becoming more common in the tuning market.
01:39 While we won't be dealing specifically with direct injection technology here, the majority of this course is still relevant and applicable to DI engines.
01:49 Understandably, direct injection engines entail some specific idiosyncrasies when compared to a port injection engine, and these will be dealt with in detail in the future with a direct injection specific course.
02:02 Now that you have a better idea of what engines we'll be discussing, let's continue.
02:07 If you're a mechanic or you've been working with engines for any time, I'm sure you're thinking the four stroke principle is child's play for you and you already know it all.
02:16 Well, bear with me, because there are a few subtle aspects that aren't immediately obvious, and it's worth taking the time for a quick refresher so we're all on the same page.
02:27 The four stroke process is also often referred to as suck, squeeze, bang, blow, which is a nice easy analogy to help you remember what's going on inside the engine.
02:37 Before looking at the individual strokes, though, there are two terms we need to understand that refer to the most important points in the engine cycle.
02:45 When the piston is at the very top of the cylinder and the big end journal of the crankshaft is pointing directly up, the piston is said to be at top dead centre.
02:55 This is often referred to as TDC.
02:57 Likewise, when the piston reaches the bottom of the cylinder and the big end journal of the crankshaft is pointing straight down, the piston is said to be at bottom dead centre, or BDC for short.
03:09 When we discuss TDC, BDC, and the four strokes, we're going to look at one individual cylinder to understand what's happening.
03:17 This keeps everything nice and easy to understand regardless how many cylinders the engine has.
03:23 We start with the intake stroke where the engine is sucking in a fresh charge of air and fuel.
03:29 The intake stroke starts with the piston at the top of the cylinder, or TDC, with the intake valve open.
03:36 The intake stroke will take the crankshaft through 180 degrees of rotation, and the piston will finish up at the bottom of the cylinder, or BDC.
03:45 As the piston moves down the cylinder, it creates a low pressure area or vacuum inside the cylinder.
03:52 This results in a difference in air pressure between the intake manifold and the cylinder.
03:57 When we have a pressure differential like this, air will always move from the high pressure area to the low pressure area to create an equilibrium.
04:06 So this pressure differential is what causes the air to flow from the intake manifold into the cylinder.
04:12 Let's just discuss this for a moment because it's an important concept that's easy to overlook.
04:18 The larger this pressure difference is, the more air we'll end up with in the cylinder, and the more air we have in the cylinder, the more potential we have to make power.
04:27 When we have a naturally aspirated engine, we will only have atmospheric pressure in the intake manifold, and this limits the maximum pressure differential we can achieve.
04:37 Turbos and superchargers, on the other hand, artificially raise the air pressure in the intake manifold.
04:43 The result is a larger pressure differential between the intake manifold and the cylinder which causes more air to be packed into the cylinder during the intake stroke.
04:53 Once the intake stroke is complete, we move on to the compression or squeeze stroke.
04:58 We start with the piston at BDC and the intake valve now closed.
05:03 As the piston moves up the cylinder, the air can't escape, so it's compressed tightly.
05:08 Compressing the fuel and air molecules together like this is essential in order to extract the maximum amount of power from the engine, and we'll discuss the effect of compression ratio on the engine's performance a little bit later.
05:22 We leave the compression stroke with the piston back at TDC and move on to the power stroke, the bang part of our analogy.
05:29 This starts with the spark plug igniting the tightly compressed fuel and air mixture.
05:34 Once ignited, the mixture begins to burn, which results in a rapid expansion of the gases in the cylinder.
05:40 These expanding gases create a huge pressure which forces down on the top of the piston.
05:46 It's the pressure which generates the force that is transferred through the piston to the connecting rod and finally into the crankshaft.
05:54 This pressure then causes the crankshaft to rotate, creating torque and power.
05:59 The power stroke is finished when the piston reaches BDC, and we begin the exhaust stroke, which is the blow part of the analogy.
06:07 Here, the exhaust valves open and the exhaust gases are forced out of the cylinder as the piston rises up towards TDC.
06:15 This part of the process gets rid of the spent exhaust gases leaving the cylinder empty and ready to start the process again.
06:23 Now, this is all pretty basic stuff.
06:26 However, there are a couple of points I want to focus on.
06:29 Firstly, it's important to understand that the four stroke process takes up two complete engine revolutions or 720 degrees of crankshaft rotation.
06:39 This is known as an engine cycle.
06:42 This means that we only have one intake stroke for every engine cycle or every two engine revolutions.
06:49 This is going to be important to remember when we start calculating air flow later on in the course.
06:55 Secondly, we only have one power stroke per engine cycle, and this is the only time that the engine actually provides torque to the crankshaft.
07:04 To conclude this module, the key points that I want you to take away are the terms TDC, which stands for top dead centre, and BDC, which stands for bottom dead centre.
07:15 You'll hear these a lot through the course, so you need to understand what they mean.