Exhaust gas recirculation
In a spark-ignition engine, an ancillary benefit of recirculating exhaust gases via an external EGR valve is an increase in efficiency, as charge dilution allows a larger throttle position and reduces associated pumping losses.
Gases re-introduced from EGR systems will also contain near equilibrium concentrations of NOx and CO; the small fraction initially within the combustion chamber inhibits the total net production of these and other pollutants when sampled on a time average.
[5] By 1973, an EGR valve controlled by manifold vacuum opened or closed to admit exhaust to the intake tract only under certain conditions.
Control systems grew more sophisticated as automakers gained experience; Volkswagen's "Coolant Controlled Exhaust Gas Recirculation" system of 1973 exemplified this evolution: a coolant temperature sensor blocked vacuum to the EGR valve until the engine reached normal operating temperature.
[5] This prevented driveability problems due to unnecessary exhaust induction; NOx forms under elevated temperature conditions generally not present with a cold engine.
In a typical automotive spark-ignited (SI) engine, 5% to 15% of the exhaust gas is routed back to the intake as EGR.
The maximum quantity is limited by the need of the mixture to sustain a continuous flame front during the combustion event; excessive EGR in poorly set up applications can cause misfires and partial burns.
Since the EGR system recirculates a portion of exhaust gases, over time the valve can become clogged with carbon deposits, which will prevent it from operating properly.
Cooled EGR components are exposed to repeated, rapid changes in temperatures, which can cause coolant leak and catastrophic engine failure.
Furthermore, since diesels always operate with excess air, they benefit (in terms of reduced NOx output) from EGR rates as high as 50%.
[11] 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.
The most common soot-control device is a diesel particulate filter (DPF) installed downstream of the engine in the exhaust system.
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.
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.
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.
