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Engine Building Fundamentals: The 4 Stroke Process

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The 4 Stroke Process


00:00 - An engine is a sophisticated piece of equipment that relies on a mind-boggling array of parts working in harmony in order to function.
00:09 When it comes to stripping and re-assembling any engine, it's going to be essential to have a solid understanding of what each part is, what it's called, and how it interacts with the other components.
00:22 Having this knowledge is essential to any engine builder, and it will also allow you to speak the same language as your engine machinist when it comes to specifying the work you need to have completed by them.
00:36 In this section, we're going to break the engine down and discuss each of the key components in turn.
00:42 Of course, there are a variety of engine designs and techniques used by different manufacturers, so there isn't one universal overview we can look at of how an engine is put together and what components must be included.
00:57 One engine, for example, may use push rod valve actuation while another may utilise direct valve actuation, where the cam lobe runs directly against a bucket.
01:08 And finally, another manufacturer may choose to use a rocker arm or finger follower arrangement for valve actuation.
01:16 With this in mind, we're not going to try and get too bogged down here with the specifics but rather gain a high-level understanding of each of the components we're likely to encounter.
01:28 It's also worth mentioning that there may be different names used to refer to various components depending on where in the world you're from and the origin of the engine you're working on.
01:39 Where possible, we'll discuss the various terminology relating to each component.
01:45 We're going to start right at the very beginning by re-visiting the four stroke principle.
01:51 This is the operating principle used by the vast majority of the engines we're likely to come across.
01:57 There are some exceptions, of course, such as two stroke engines and rotaries, but four stroke spark ignition piston engines are by far the most common engine type you'll be dealing with, and they are the focus of this engine building course.
02:12 Now, if you're a mechanic or you've been working with engines for any time, I'm sure you're thinking that the four stroke principle is child's play, and you're already going to know it all.
02:23 Well, bear with me, because there are a few subtle aspects that aren't immediately obvious.
02:29 And it's worth taking the time for a quick refresher, so we're all on the same page.
02:35 The four stroke process is 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.
02:48 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.
02:58 These are terms we'll be using repeatedly through this course.
03:03 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.
03:13 This is often referred to as TDC for short.
03:18 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.
03:31 When we discuss TDC, BDC, and the four strokes, we're going to look at one individual cylinder to understand what's happening.
03:42 This keeps everything nice and simple and easy to understand regardless how many cylinders the engine has.
03:50 We start with the intake stroke, where the engine is sucking in a fresh charge of air and fuel.
03:56 The intake stroke starts with the piston at the top of the cylinder, or TDC, with the intake valve open.
04:04 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.
04:15 As the piston moves down the cylinder, it creates a low-pressure area, or vacuum, in the cylinder.
04:22 This results in a difference in air pressure between the intake manifold and the cylinder.
04:28 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.
04:38 So this pressure differential is what causes the air to flow from the intake manifold into the cylinder.
04:46 Once the intake stroke is complete, we move on to the compression, or squeeze, stroke.
04:52 We start with the piston at BDC and the intake valve closing.
04:57 As the piston moves up the cylinder, the air can't escape because the valves are closed, so it's compressed tightly together.
05:05 Compressing the fuel and air molecules together is essential to extract the maximum amount of power from the engine.
05:14 We leave the compression stroke with the piston back at TDC and move on to the power stroke or the bang part of the analogy.
05:23 This starts with the spark plug igniting the tightly compressed fuel and air mixture.
05:28 This ignition of it typically happens a little before TDC, while the piston is still moving up towards the top of the stroke.
05:38 Once ignited, the fuel and air mixture begins to burn, which results in a rapid expansion of the gases in the cylinder.
05:46 These expanding gases create a huge amount of pressure, which acts on the top of the piston.
05:52 It's this pressure which generates the force that is transferred through the piston, into the connecting rod, and finally to the crankshaft.
06:01 This pressure then causes the crankshaft to rotate, creating torque and power.
06:08 The power stroke is finished when the piston reaches bottom dead centre, and we begin the exhaust stroke, which is the blow part of our analogy.
06:17 Here the exhaust valves open, and the exhaust gases are forced out of the cylinder as the piston rises towards TDC.
06:26 This part of the process gets rid of the exhaust gases and leaves the cylinder empty and ready to start the process all over again.
06:36 Now, this is all pretty basic stuff; however, it's often overlooked that it takes two full crankshaft revolutions, or 720 degrees, to complete an engine cycle.
06:49 This also means that the camshaft must operate at half the engine speed, which is achieved by the way the camshafts are driven from the crank.
07:00 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.
07:13 You'll hear these a lot through the course, so you need to understand what they mean.