Editing Exhaust gas recirculation (section)

== Diesel engines ==
[[File:Skoda BNM EGR.jpg|thumb|Electronically actuated EGR Valve for [[List of Volkswagen Group diesel engines#2.0 R4 16v TDI PD 103 125&nbsp;kW|VW BMN engine]]]]

Because diesel engines depend on the heat of compression to ignite their fuel, they are fundamentally different from spark-ignited engines.  The physical process of diesel-fuel combustion is such that the most complete combustion occurs at the highest temperatures.  Unfortunately, the production of nitrogen oxides ({{NOx}}) increases at high temperatures.  The goal of EGR is thus to reduce {{NOx}} production by reducing the combustion temperatures.

In modern [[diesel engine]]s, the EGR gas is usually cooled with a [[heat exchanger]] to allow the introduction of a greater mass of recirculated gas. However, uncooled EGR designs do exist; these are often referred to as hot-gas recirculation (HGR). Cooled EGR components are exposed to repeated, rapid changes in temperatures, which can cause coolant leak and catastrophic engine failure.<ref>{{Cite web |last=Editor |first=Digital360 |date=2024-04-02 |title=How do I know if my EGR cooler is failing? {{!}} Natrad |url=https://natrad.com.au/info-advice/how-do-i-know-if-my-egr-cooler-is-failing/ |access-date=2024-04-11 |website=Natrad Radiators & Auto Air |language=en-US}}</ref><ref>{{Cite web |date=2021-12-10 |title=Can a defective EGR valve cause coolant loss? |url=https://autoexpert.com.au/posts/can-a-defective-egr-valve-cause-coolant-loss |access-date=2024-04-11 |website=Auto Expert John Cadogan |language=en-AU}}</ref>

Unlike [[spark-ignition engine]]s, diesel engines are not limited by the need for a contiguous flamefront.  Furthermore, since diesels always operate with excess air, they benefit (in terms of reduced {{NOx}} output) from EGR rates as high as 50%.  However, a 50% EGR rate is only suitable when the diesel engine is at idle, since this is when there is otherwise a large excess of air.

Because modern diesel engines often have a throttle, EGR can reduce the need for throttling, thereby eliminating this type of loss in the same way that it does for spark-ignited engines. In a naturally aspirated (i.e. nonturbocharged) engine, such a reduction in throttling also reduces the problem of engine oil being sucked past the piston rings into the cylinder and causing oil-derived carbon deposits there.  (This benefit only applies to nonturbocharged engines.)

In diesel engines in particular, EGR systems come with serious drawbacks, one of which is a reduction in engine longevity.  For example, because the EGR system routes exhaust gas directly back into the cylinder intake without any form of filtration, this exhaust gas contains carbon [[particulates]].  And, because these tiny particles are abrasive, the recirculation of this material back into the cylinder increases engine wear.  This is so because these carbon particles will blow by the [[piston ring]]s (causing piston-cylinder-interface wear in the process) and then end up in the crankcase oil, where they will cause further wear throughout the engine simply because their tiny size passes through typical oil filters.  This enables them to be recirculated indefinitely (until the next oil change takes place).<ref>Dennis A., Garner C., Taylor D. (1999). ''The Effect of EGR on Diesel Engine Wear'', SAE 1999-01-0839, In-Cylinder Diesel Particulate and {{NOx}} Control 1999</ref>
   
Exhaust gas—which consists largely of nitrogen, [[carbon dioxide]], and water vapor—has a higher [[specific heat]] than air, so it still serves to lower peak combustion temperatures. However, adding EGR to a diesel reduces the specific heat ratio of the combustion gases in the [[Stroke (engine)|power stroke]]. This reduces the amount of power that can be extracted by the piston, thereby reducing the thermodynamic efficiency.

EGR also tends to reduce the completeness of fuel combustion during the power stroke.  This is plainly evident by the increase in particulate emissions that corresponds to an increase in EGR.<ref>Nagel, John (2002). ''Diesel Engine and Fuel System Repair'', {{ISBN|0130929816}}.</ref><ref>Bennett, Sean (2004). ''Medium/Heavy Duty Truck Engines, Fuel & Computerized Management Systems 2nd Edition'', {{ISBN|1401814999}}.</ref>

Particulate matter (mainly carbon and also known as soot) that is not burned in the power stroke represents wasted energy.  Because of stricter regulations on particulate matter (PM), the soot-increasing effect of EGR required the introduction of further emission controls in order to compensate for the resulting PM emission increases.  The most common soot-control device is a [[diesel particulate filter]] (DPF) installed downstream of the engine in the exhaust system.  This captures soot but causes a reduction in [[fuel efficiency]] due to the back pressure created.

[[Diesel particulate filter]]s come with their own set of very specific operational and maintenance requirements.  Firstly, as the DPF captures the soot particles (which are made far more numerous due to the use of EGR), the DPF itself progressively becomes loaded with soot.  This soot must then be burned off, either actively or passively.
    
At sufficiently high temperatures, the nitrogen dioxide component of {{NOx}} emissions is the primary oxidizer of the soot caught in the DPF at normal operating temperatures.  This process is known as passive regeneration, and it is only partially effective at burning off the captured soot.   And, especially at high EGR rates, the effectiveness of passive regeneration is further reduced.  This, in turn, necessitates periodic active regeneration of the DPF by burning diesel fuel directly in the oxidation catalyst in order to significantly increase exhaust-gas temperatures through the DPF to the point where PM is incinerated by the residual oxygen in the exhaust.

Because diesel fuel and engine oil both contain nonburnable (i.e. metallic and mineral) impurities, the incineration of soot (PM) in the DPF leaves behind a residue known as ash.  For this reason, after repeated regeneration events, eventually the DPF must either be physically removed and cleaned in a special external process, or it must be replaced.

As noted earlier, the feeding of the low-oxygen exhaust gas into the diesel engine's air intake engenders lower combustion temperatures, thereby reducing emissions of {{NOx}}. By replacing some of the fresh air intake with inert gases EGR also allows the engine to reduce the amount of injected fuel without compromising ideal air-fuel mixture ratio, therefore reducing fuel consumption in low engine load situation (for ex. while the vehicle is coasting or cruising). Power is not reduced by EGR at any times, as EGR is not employed in high load engine situations. This allows engines to still deliver maximum power when needed, but lower fuel consumption despite large cylinder volume when partial load is sufficient to meet the power needs of the car and the driver.

EGR has nothing to do with oil vapor re-routing from a positive [[crankcase ventilation system]] (PCV) system, as the latter is only there to reduce oil vapor emissions, and can be present on engines with or without any EGR system. However, the tripartite mixture resulting from employing both EGR and PCV in an engine (i.e. exhaust gas, fresh air, and oil vapour) can cause the buildup of sticky tar in the intake manifold and valves.  This mixture can also cause problems with components such as [[swirl flap]]s, where fitted.  (These problems, which effectively take the form of an undesirable positive-feedback loop, will worsen as the engine ages.  For example, as the piston rings progressively wear out, more crankcase oil will get into the exhaust stream.  Simultaneously, more fuel and soot and combustion byproducts will gain access to the engine oil.)

The end result of this recirculation of both exhaust gas and crankcase oil vapour is again an increase in soot production, which however is effectively countered by the DPF, which collects these and in the end will burn those unburnt particles during regeneration, converting them into CO2 and water vapour emissions, that - unlike NOx gases - have no negative health effects.<ref>[http://fleetowner.com/management/feature/scr_egr_0701/ SCR or EGR?] - FleetOwner magazine.</ref>

Modern cooled EGR systems help reduce engine wear by using the waste heat recouped from the recirculated gases to help warm the coolant and hence the engine block faster to operating temperature. This also helps lower fuel consumption through reducing the time after cold starts during which the engine controller has to inject somewhat larger amounts of fuel into the cylinders to counter the effects of fuel vapor condensation on cylinder walls and lowered combustion effectiveness because of the engine block still being below ideal operating temperature. Lowering combustion temperatures also helps reducing the oxidization of engine oil, as the most significant factor affecting that is exposure of the oil to high temperatures.<ref>{{citation |url=https://blog.amsoil.com/what-causes-engine-oil-oxidation/ |title=What Causes Engine Oil Oxidation? |date=18 March 2021 }}</ref>

Although engine manufacturers have refused to release details of the effect of EGR on fuel economy, the EPA regulations of 2002 that led to the introduction of cooled EGR were associated with a 3% drop in engine efficiency, thus bucking the trend of a 0.5% annual increase.<ref>{{citation |url=https://books.google.com/books?id=gcw2a3GTVxsC&pg=PT98 |title=Review of the 21st Century Truck Partnership |publisher=National Academies Press |page=98 |year=2008|isbn=9780309178266 }}</ref>