Diesel emissions

Fig. 1

Part two

Fig. 2

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.

“Not popular but…a priority” – MOT consultation emissions consideration welcomed
Ways to progress the monitoring of emissions are among the areas under consideration in the government MOT consultation that opened yesterday (Wednesday 18 January).
Diesel emissions
The technical specifics of diesel, its role and its future are considered by Frank Massey in the first of a two-part series
BorgWarner: More than 80 New EGR valves launched

BorgWarner increased its range of EGR valves in 2021, adding over 80 new part numbers to its product portfolio. In total, BorgWarner says its 80 new EGR part numbers cover 50% of the cars seen in the EU BorgWarner's solutions for exhaust gas recirculation help to reduce emissions for most engine types, thereby supporting cleaner and more energy-efficient mobility.
Nissens to acquire part of AVA Cooling
Nissens is set to buy-up a portion of AVA Cooling from Enterex International Limited Group as part of an overall ongoing plan to grow its engine cooling, A/C and emissions product coverage.
EU Emissions tighten on UK market
With or without Brexit, EU regulations surrounding emissions are playing an important part in pushing aftermarket requirements for
the correct emissions parts being fitted.

Mark Blinston, Commercial Director at BM Catalysts commented: “Emissions are a hot topic for discussion at the moment for many media outlets and manufacturers. All efforts are geared towards reducing emissions, as well as preventing the sale and fitting of catalytic converters and DPFs to vehicles that haven’t been approved to meet the relevant Euro Levels.

“Vehicles and replacement emission control devices must meet specific standards for exhaust emissions before they can be offered for sale in the European Union. Vehicle emissions are one of a series of performance standards that must be met to achieve Whole Vehicle Type Approval in accordance with EC Directives. In order to achieve type approval on the emissions levels for replacement emission control devices, such as catalytic converters and diesel particulate filters (DPFs), performance and noise levels must be within calculated limits as prescribed by legislation. Replacement catalytic converters and DPFs cannot be approved to a lower Euro level than that of the original vehicle. If the vehicle is Euro 5 then the replacement must be approved to Euro 5 levels. Testing and approving this part to Euro 4 would mean that it cannot be proven to meet emission standards and therefore cannot legally be fitted to any Euro 5 vehicle.”

Mark continued: “Helping to identify the correct products for stockists and garage professionals, the MAM software solution program is set to revolutionise the aftermarket. Used by the majority of distributors, the supply chain software is due to receive a V9 update, which will map the Euro level that a supplier’s part is approved for sale on against a vehicle registration number. Once the change is in place, we’ll start to see the correct catalytic converters and DPFs available matched against the correct vehicles for the first time. Additionally, the Department for Transport (DfT) is also helping to clarify the rules for the UK market, with recent guidelines reiterating the strict requirements for selling catalytic converters and DPFs to the UK market.”

But while the correct cataloguing of aftermarket parts is complex and challenging, perhaps the biggest shake up to the emissions market is the recent MOT changes. Mark continued: “The result of an EU directive, new regulations were introduced to the MOT test in May 2018, placing further emphasis on diesel emissions. The regulations require checks to the exhaust for visible smoke production and tampering or removal of the DPF. Any visible sign of alteration to the DPF or smoke of any colour will result in a fail for the vehicle. Prior to these tighter regulations, experts warned that the true figure of vehicles driving without DPFs were in the thousands, leaving many car owners at risk of hefty fines and penalty points.

“BM Catalysts acknowledges the need for up to date information on EU levels. As a leading manufacturer in Europe, we recognise we are a big part in meeting these standards, producing superior aftermarket products with the least harm to the environment. Additionally, we have recently produced free educational material on DPFs and the MOT changes to help our customers make informed motoring decisions.”

Mark added: “BM Catalysts produce catalytic converters and DPFs to high standards, yet to create a lasting change to the market, it is important that all hands along the supply chain, even the consumer, does their part to make sure they’re complying to regulations. If one link is in this chain proves weak, we all fail.”