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Diesel Tuning Fundamentals: Diesel Emissions, EGR and DPF

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Diesel Emissions, EGR and DPF

05.15

00:00 - One of the most important aspects of any engine's performance is its ability to meet emissions compliance.
00:06 If the engine can't be made to be emissions compliant, it simply isn't going to make it into production regardless how well it performs or how fuel efficient it is.
00:15 Diesel engines in particular generate relatively high levels of oxides of nitrogen, commonly referred to as NOx, and particulate matter in the exhaust.
00:26 Hydrocarbon production is also significant however typically at much lower levels than in a gasoline engine.
00:33 Oxides of nitrogen include nitric oxide, or NO, and nitrogen dioxide or NO2.
00:41 However nitric oxide is the predominant oxide formed during the combustion process.
00:47 It's not necessary to have a complete understanding of how and why diesel emissions form, in order to make tuning changes.
00:55 However some of the basic concepts are still valuable to understand.
00:59 NOx are produced during the combustion process as a result of the high local temperatures found in diesel engines, and there has been a large amount of development put into modern diesel engines in order to help reduce them.
01:12 Since combustion temperature is one of the factors that results in NOx formation, anything that can be done to reduce combustion temperature, is also effective at reducing NOx levels.
01:24 With this in mind, one of the most common methods to reduce NOx formation is exhaust gas recirculation or EGR for short.
01:32 As its name implies, EGR uses a computer controlled valve to introduce a stream of cooled exhaust gas into the intake system which has the effect of diluting the intake charge.
01:45 This deprives the combustion chamber of oxygen and reduces the heat released during combustion, and hence in cylinder temperatures.
01:53 This reduces NOx formation but it also has a negative impact on the engine's brake specific fuel consumption and hence fuel economy suffers.
02:03 In turbocharged engines, EGR also has the effect of reducing the exhaust energy provided to the turbo and hence boost and turbo response suffer.
02:13 Another technique used by OE manufacturers is to retard the injection timing which has the effect of reducing NOx formation but this is also detrimental to engine performance and hence involves some level of compromise.
02:27 Retarding the injection timing also has the negative effect of increasing particulate matter in the exhaust.
02:33 Pilot injection has also proved useful in helping to reduce NOx formation, however this can also cause an increase in exhaust particulate matter.
02:43 As you can see, the techniques employed for reducing oxides of nitrogen are often at odds with the considerations of power, torque, and fuel economy, so assuming that you're operating under conditions where emissions compliance is not required, significant improvements can be had at the expense of increased NOx output.
03:04 Another option that can be used to reduce NOx exhaust emissions is known as selective catalytic reduction or SCR for short which involves injecting a fluid into the exhaust stream which reacts with the oxides of nitrogen to produce nitrogen and water.
03:20 The fluid which is referred to as diesel exhaust fluid or DEF, and is sold under various brand names such as AdBlue is stored in a separate tank in the vehicle.
03:31 The fluid is injected at a relatively low rate of around 3% to 5% of the diesel fuel supply which provides long refill intervals.
03:40 Since some of the options manufacturers are using to reduce NOx levels, also result in an increase in soot or particulate matter in the exhaust, it's also become common for OE manufacturers to fit a diesel particulate filter or DPF which is fitted to the exhaust system and removes the soot from the exhaust gas.
04:00 DPFs however can become blocked with excessive soot, which must be burned off through a process known as regeneration.
04:08 This is handled automatically by the ECU, and can involved a late injection event that occurs well after the main injection event, in the engine cycle, with the intention of raising the exhaust gas temperature to a point where soot burn off can occur.
04:23 This can be problematic in some instances since it requires a reasonable distance of open road driving for the regeneration process to be completed.
04:33 It's unlikely to present an issue for owners of diesel cars that are regularly driven long distances, however for constant short run use, the DPF can prove problematic as there may not be sufficient opportunity for regeneration to occur.
04:48 Again, if you're operating in a situation where emissions compliance is not required removal of the DPF is a common modification, which also has the benefit of reducing the back pressure in the exhaust system, which can further aid performance.