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Practical Diesel Tuning: Diesel Operating Principles

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Diesel Operating Principles

04.54

00:00 - While our Diesel Tuning Fundamentals course provides a thorough understanding of the operation of a diesel engine, it's still worth starting this course with a brief refresher on the key concepts.
00:09 In particular here we'll reference the key differences between the operation of a diesel engine and a gasoline engine which comes down to the way the fuel and air is ignited within the cylinder.
00:18 Gasoline engines work on a spark ignition process where the combustion is initiated by a spark plug located in the combustion chamber.
00:24 A diesel engine on the other hand relies on a compression ignition process where the heat being produced during the compression stroke initiates combustion of the fuel when it's injected into the cylinder.
00:35 Let's jump back a little though and we'll discuss what happens inside a diesel engine when it's running.
00:40 While two stroke diesel engines do exist, when it comes to the automotive use, diesel engines predominantly operate on the four stroke principle and that is what we'll focus on.
00:49 In this respect, the basic operation of a diesel engine is actually very similar to the gasoline engine.
00:53 We have four strokes of the engine cycle which each take up 180 degrees of crankshaft rotation.
00:59 These strokes include the inlet stroke, the compression stroke, the power stroke and the exhaust stroke.
01:05 While these should be familiar to anyone coming from a gasoline engine background, there are some subtle differences that I want to focus on.
01:10 First, let's consider the intake stroke where the cylinder is filled with fresh air.
01:15 In a gasoline engine, we'd be introducing fuel and air during the intake stroke.
01:20 However in a diesel engine, only air is introduced at this time.
01:24 Following the intake stroke, we move onto the compression stroke where the fresh air charge that entered the cylinder is compressed.
01:30 As the air is compressed the pressure and heat in the cylinder both increase and it's the heat that is the key to igniting the fuel as it is injected and hence this is the essential aspect of a diesel engine's operating principle.
01:41 For this reason, it's common for diesel engines to have compression ratios much higher than those of gasoline engines.
01:46 Compression ratios of 15:1 up through 20:1 or higher are not uncommon.
01:51 Even on turbocharged engines.
01:54 The higher compression ratios are essential to produce high temperatures required to ensure that the fuel ignites.
01:59 Near the top of the compression stroke, the fuel is injected via an injector that's fitted directly into the cylinder head.
02:05 The complete combustion process is covered in detail in our diesel tuning fundamentals course so we won't repeat it here.
02:11 However in simple terms, once the fuel is injected into the cylinder, the combustion process is initiated and the fuel in the cylinder will begin to be consumed, creating pressure.
02:20 Following the compression stroke, we move on to the power stroke, and this is where the pressure in the cylinder caused by the expanding combustion charge acts on the top of the piston pushing it toward the bottom of the cylinder and generating torque at the crankshaft.
02:31 This process is essentially identical to what happens in a gasoline engine.
02:36 As is the exhaust stroke where the combustion gases are evacuated into the exhaust system.
02:41 So on face value, there are a lot of similarities between diesel and gasoline engines but also a few key differences.
02:47 The main difference of course being the compression ignition aspect of its operation.
02:52 The differences don't stop there though and another important difference we'll see on diesel engines is that a throttle body is not essential.
02:59 It's true that we may see a throttle body on many current generation diesel engines however they're not necessary in order to modulate or control engine torque which is how they're used on a gasoline engine.
03:09 In order to control the torque output from a gasoline engine, we need to control the airflow into the engine.
03:14 Since we can only operate a gasoline engine across a relatively narrow range of air/fuel ratios betwteen perhaps 0.6 and 1.3 lambda.
03:22 On a diesel engine on the other hand, we don't need to modulate airflow and instead the torque can be controlled solely by modulating the amount of fuel being delivered.
03:31 To reduce engine torque we simply inject less diesel fuel.
03:34 Diesel engines are able to run quite happily with exceptionally lean air/fuel ratios, anywhere from lambda 1.0 through lambda 10 or leaner.
03:43 The fact that we control engine torque via fuel delivery is probably one of the biggest differences between diesel and gasoline engine operation as far as a tuning perspective goes and it's very important you understand it.
03:53 So the key points you need to understand from this module are that a diesel engine is a compression ignition engine that relies on the heat produced during the compression stroke to ignite the fuel when it's injected.
04:03 On this basis, we also need to remember that the fuel is only injected into the cylinder near the top of the compression stroke.
04:09 Also remember that a diesel engine doesn't require a throttle body in order to control the engine torque.
04:15 Instead, torque and power are controlled by the amount of fuel that is injected.
04:18 The last point I want you to take away from this module is that diesel engines operate at much leaner air/fuel ratios than gasoline engines.
04:25 And even at high load we'll see operating lambda values leaner than 1.0.
04:28 In a gasoline engine, lean air/fuel ratios can be dangerous under high loads and if the air/fuel ratio becomes too lean, we may not be able to ignite the air/fuel charge.
04:38 Conversely with diesel engines, almost the opposite is true.
04:41 Under high loads, richer mixtures create more heat and can be dangerous.