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Diesel Tuning Fundamentals: Comparison of Diesel vs Petrol Engines

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Comparison of Diesel vs Petrol Engines


00:00 - Before we get too deep into diesel engine operation, it's going to be useful to focus on the major difference between the way a diesel and gasoline engine operate, and this is to do with the way the fuel and air is ignited within the cylinder.
00:13 Gasoline engines work on a spark ignition process where the combustion is initiated by a spark plug located in the combustion chamber.
00:22 A diesel engine on the other hand relies on the 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 as it's running.
00:41 While two stroke diesel engines do exist, when it comes to automotive use, diesel engines predominantly operate on the four stroke principal and that's what this course will focus on.
00:52 In this respect, the basic operation of the diesel engine is actually very similar to a gasoline engine.
00:59 We have four strokes of the engine cycle which each take up 180 degrees of crankshaft rotation.
01:06 These strokes include the inlet stroke, the compression stroke, the power stroke, and the exhaust stroke.
01:12 While these should be familiar to anyone coming from a gasoline engine background, there are some subtle differences that I want to focus on.
01:21 First let's consider the intake stroke where the cylinder is filled with a fresh charge of air.
01:27 In a port injected gasoline engine we would be introducing a fuel and air mixture during the intake stroke.
01:34 However in a diesel engine, only air is introduced at this time.
01:39 Following the intake stroke we move into the compression stroke where the fresh air charge that entered the cylinder is compressed.
01:47 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 when it is injected and hence this is an essential aspect of the diesel engine's operating principal.
02:02 For this reason it's common for diesel engines to have compression ratios much greater than those of gasoline engines.
02:08 And for example compression ratios of 15:1 through to 20:1 or even higher, are not uncommon, even on turbocharged diesel engines.
02:18 The higher compression ratios are essential to produce the high temperatures required to ensure that the fuel is ignited.
02:26 Near the top of the compression stroke, the fuel is injected via an injector that is fitted directly into the cylinder head.
02:34 We'll discuss the actual combustion process in much more detail shortly so for now we're not going to get too much deeper into it other than to say that once the fuel is injected, the combustion process will begin, and the fuel in the cylinder will be consumed.
02:50 Following the compression stroke we move into 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 towards the bottom of the cylinder and generating torque at the crankshaft.
03:04 This process is essentially identical to what happens in a gasoline engine, as is the exhaust stroke where the combustion gases are evacuated into the exhaust system.
03:15 So on face value there are a lot of similarities between a diesel and gasoline engine, but also a few key differences.
03:23 The main difference of course being the compression ignition aspect of its operation.
03:28 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.
03:36 It's true that we will 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 commonly used on a gasoline engine.
03:50 In order to control the torque output from a gasoline engine, we need to control the air flow into the engine, since we can only operate a gasoline engine across a relatively narrow range of air fuel ratios, between perhaps 0.6 and 1.3 lambda.
04:06 In a diesel engine on the other hand, we don't need to modulate air flow, and instead the torque can be controlled solely by modulating the amount of fuel being delivered.
04:16 To reduce engine torque we simply inject less diesel fuel.
04:20 Diesel engines are able to run quite happily with exceptionally lean air fuel ratios, anywhere from lambda 1.0 through to lambda 10 or even leaner.
04:30 This is probably one of the biggest differences between diesel and gasoline engine operation from a tuning perspective and it's important to understand.
04:41 I mentioned that that diesel engine doesn't require a throttle body for torque modulation but we are still likely to see a throttle body fitted to these engines.
04:51 Now that might sound confusing but the throttle body is there for two very different reasons.
04:55 Firstly it can aid engine shutdown and secondly it can be used to aid exhaust gas recirculation.
05:02 We won't discuss these topics in more detail here as they are covered in separate modules a little further into the course.
05:09 So the key points that I want you 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.
05:23 On this basis we also need to remember that the fuel is only introduced into the cylinder near the top of the compression stroke.
05:30 And as such it's injected directly into the cylinder.
05:34 Also remember that a diesel engine doesn't require a throttle body in order to control engine torque, but instead torque and power are controlled by the amount of fuel being injected.
05:45 The last point that I want you to take away from this module is that diesel engines operated at much leaner air fuel ratios than gasoline engines.
05:53 And even at high load, we'll be operating at lambda values leaner than 1.0 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.
06:10 Conversely, with diesel engines, almost the exact opposite is true.
06:15 Under high loads, richer mixtures create more torque and also produce more heat which can be dangerous.
06:21 While exceptionally lean mixtures can still be reliably ignited by the high air temperatures inside the cylinder.

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