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Diesel Tuning Fundamentals: Advantages of Diesel vs Petrol

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Advantages of Diesel vs Petrol


00:00 - Diesel engines have gained in popularity over the last couple of decades with one of their main attractions being their fuel economy advantages when compared to traditional gasoline engines.
00:10 And in this module we'll discuss why the diesel engine does a better job of harnessing the energy contained in the fuel.
00:17 One of the key advantages is the fact that diesel engines can utilise a much higher compression ratio than a gasoline engine so we'll start by discussing why this is the case.
00:28 Since injection in a diesel engine begins just prior to the combustion starting, the diesel engine is not knock limited unlike a spark ignition gasoline engine.
00:38 This allows a higher compression ratio to be used in the diesel engine, and this is actually important in order to generate the heat required to achieve ignition of the diesel fuel when it's injected.
00:50 This higher compression ratio also improves its fuel conversion efficiency however when compared to a gasoline engine.
00:58 The reason a higher compression ratio increases efficiency is because it also offers a higher expansion ratio.
01:05 This is important because it allows the combustion gases to expand further and more energy can be harnessed from the expanding gases prior to the exhaust valve opening.
01:15 Another important advantage of the diesel engine is that the engine torque is controlled via fuel delivery rather than via throttling the air supply.
01:24 In particular at cruise speeds this means that pumping loses are much lower than a gasoline engine where the throttle is mostly closed and the piston must work against a partial vacuum during the inlet stroke.
01:37 Since the diesel engine always operates leaner than stoichiometric, and at cruise significantly leaner, the engine's efficiency increases because the combustion temperature is decreased which allows the burned gases to expand through a larger temperature ratio during the expansion stroke.
01:55 In simple english this means that more of the heat energy released from the fuel is harnessed and converted into power compared to the richer mixtures used in a gasoline engine under similar operating conditions.
02:07 Lastly we need to consider the amount of energy contained in the fuel being combusted.
02:13 In this case diesel offers a slight advantage with an energy density approximately 10% to 11% higher than gasoline.
02:21 All of these factors combined mean that a diesel engine can more efficiently convert the energy contained in the fuel into work and it's typical to see an improvement in fuel economy in the range of 20% to 25% between a modern common rail turbo diesel engine and a comparable sized gasoline engine.

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