February 2022

BY Frank Massey D iesel exhaust emission reduction systems do not have a good reputation, and with justification. Can we improve on their reliability? Will understanding how they work help in keeping them serviceable? With London recently going over to a super ultra-low emission vehicle (SULEV) zone, operating a diesel vehicle is going to become financially prohibitive for many owners. However it will lead to a much more proactive repair and service opportunity. Components Let us take a little step back to ULEV, to best understand the exhaust system components: High pressure EGR Low pressure EGR Cooled EGR valve SCR system EGR cooler Additional temperature sensors Cylinder pressure monitoring What and why? Soot particles originate from incomplete diesel fuel combustion (see Fig.1). Under ideal conditions, you get the combustion of diesel fuel, excluding the resultant emission issues. This requires high compression, perfect atomisation, and fuel delivery, including quantity and timing with high internal cylinder temperatures and an excess oxygen ratio. When this is presented into the DPF, passive generation will take place, organic conversion of soot to CO2, with no requirement to store unconverted soot particles. The DPF frontal area is coated with platinum and rhodium (loading), to promote efficient conversion of soot. Exhaust gas temperatures must exceed 150°C, and ideally much higher, all of which is difficult to maintain in urban environments. These conditions will adversely affect flame spread, usually due to injector problems and or poor EGR mixing and mechanical engine efficiency. Active reduction The next phase of soot treatment is active reduction. This process takes place when the pressure in the DPF reaches a pre-determined threshold. Nominal pressures; 25MB at idle, 150MB at full load. The PCM will adjust the fuelling to increase exhaust gas temperature. Soot deposited within the rear area of the DPF will burn off leaving ash as the residual deposit. Ash quantity is a theoretical value within the PCM based on data from active regeneration cycles, DPF back pressure, and a volumetric efficiency calculation. Several critical events must take place to achieve active regeneration: 28 AFTERMARKET FEBRUARY 2022 TECHNICAL www.aftermarketonline.net PART TWO DIESEL EMISSIONS Here comes the science part; Concentrate as Frank pulls out the Periodic Table to delve deeper into the diesel conundrum No DTCs in engine PCM Functional heater plug system Closed EGR with no leaks Post injection on exhaust cycle Turbo boost increase Accurate air mass value Exhaust gas temperature input DPF pressure sensor value Please refer to Fig.2. Additive SCR systems were introduced to further reduce emissions, especially NOx, which unfortunately increases with higher combustion temperatures and excess air ratios. My first advice is to purchase a refractometer and check Fig 1