Fig. 1

Diesel emissions

Part two

Fig. 2

By Frank Massey |

Published:  14 February, 2022

Here comes the science part; Concentrate as Frank pulls out the Periodic Table to delve deeper into the diesel conundrum 

Diesel 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:

  •  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 the specific gravity value of the SCR additive, which should be exactly 32.5%. At ADS, we have conducted tests on several budget suppliers of additive, or AdBlue, which is a brand trademark, with alarming results. Check all deliveries and reject all under or over 32.5% urea. If the additive in the tank does not meet the correct ratio, drain and discard.
    
How additives are stored is critical. Very low and very high temperatures will degrade the solution. As a guide, fresh solution will not have a pungent odour, but aged solution will smell of ammonia, i.e. like urine. Yuck!  


First reaction
The additive solution is injected into a traject or diffuser within the exhaust system post DPF where it is mixed with the exhaust gas flow. The first reaction is thermolysis, evaporation of the ionised transport fluid releasing ammonia (NH3) and isocyanic acid (NHCO) into the exhaust gasses. This is followed by hydrolysis where an additional ammonia molecule is produced, the effect of which separates NOx into N, N2, and O2. This occurs within the secondary NOx reduction catalyst which has an additional barium coating for NOx reduction.  It also acts as a secondary reduction for CO and HC.
    
It is essential that exhaust gas temperatures are well above 150°C. This value is taken from exhaust sensor 3. Refer back to Bosch’s 200°C developments.
    
The amount of injected urea additive is more critical than you may imagine. The PCM will send a PWM signal to the additive injector via a dedicated CAN line, using the following variables:

Fig. 3

  •  Air mass meter value
  •  EGR recirculation value (open ratio vis amm reduced mass value)
  •  Exhaust gas temperature


 The injector is also cooled by the cold side of engine temperature control system.
    
Do not trust this to be sufficient however. During the injector delivery period, it must deliver an exact amount of additive; This must be checked through the serial platform into a measuring beaker. A failure to confirm this will result in incorrect NOx reduction, often resulting with excessive additive consumption.


Good news
The good news is that injector blockage and atomisation problems are often cured with a hot water flush.
    
Generation two SCR implemented an integrated DPF with copper zeolite catalyst and the SCR reduction catalyst combined in a close coupled location. So, we now have two EGR flow control valves, a third exhaust flow valve, high and low-pressure cooling and two separate catalyst systems. You may also notice there are two exhaust pressure sensors. Please refer to Fig.2. The integrated additive tank incorporates several critical functions which are non-serviceable:

  •  Brushless additive A/C pump
  •  Level sensor
  •  Three heaters, tank, pump and delivery line
  •  Reversable motor or two-way valve
  •  Heated additive supply line


High pressure EGR diverts EGR gasses directly back into the inlet manifold, avoiding the intake cooler during warm-up to quickly heat the catalyst. This normally takes approximately two minutes. Low pressure EGR is taken from further down the exhaust system via the EGR cooler, back into the inlet system and is employed to reduce combustion temperatures.
    
The low temp EGR also employs an exhaust brake or valve. This partially closes the exhaust system on overrun to create a positive pressure differential across the SCR catalyst. This in turn causes the exhaust gasses a second pass through the SCR catalyst, where SCR additive is injected. it also passes through a cooler which often blocks with soot. This often leads to incorrect diagnosis of a DPF blockage. It may also become porous from the coolant system, so just fit a new one.
    
SCR also employs a dedicated CAN network between the additive control module and engine PCM. I have not mentioned cylinder combustion monitoring or several other sensors, all of which are dedicated to what I call a secondary combustion system.
    
Gasoline direct injection also has high particulates and NOx problems, therefore I expect these to be fitted with a gasoline SCR system (GPF). What fun.

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