Euro 6 SCR

Frank looks at some of the challenges faced when dealing with the latest diesel engines

Published:  18 September, 2019

With a focus on technical challenges and potential cost with diagnostic equipment and servicing, I think we should explore the technology that drives the need for specialist tools in both service and repair. I’m going to look at Euro 6 generation 2 diesel emission systems.

I’m convinced that the more technology manufacturers throw at improving diesel combustion, the more problems they introduce. As usual, my knowledge is based on Volkswagen-Audi Group design. Engine design innovation is now closely following that of gasoline direct injection, alike to that of the EN888.


MDB concept
The VAG MDB concept engine design is a world based modular system. This allows for a more flexible production with regional variation based on local emission standards. The three basic modules are the intake systems, a central engine core based on the EN288, and the exhaust or emission module.

The EN288 engine has 3-cylinder and 4-cylinder options with EU4/EU5/EU6 compliance. It is a cast iron block, alloy with the 3-cylinder variant, with and without balance shafts, crossflow alloy cylinder head with variable valve timing. A fully mapped and integrated coolant pump ensures maximum thermal efficiency.

It important to understand that there are significant differences between the 2.0/1.6/1.4 3-cylinder and 4-cylinder design concepts, so various comments across the range of options will not reflect every variant.

The 4-cylinder head has an offset valve layout. This introduces turbulence within the combustion chamber. The 3-cylinder valve layout is a conventional layout with swirl flaps in the intake module. Intake valve variation allows for a delay of intake valve closure (IVC) with a reduction of cylinder pressure during compression, reducing temperature and NOx. The control variator utilises oil pressure, with a backup accumulator to adjust IVO/IVC.


Emission control module
The emission control module is without doubt the most radical evolution. High pressure EGR is introduced via a valve directly from the exhaust manifold to the inlet, with the single aim of heating the 3way Euro 5 catalyst, or 4way Euro 6 catalyst when the engine is cold (see fig 1.)
Low pressure exhaust gas passes via the EGR cooler, catalyst and particulate filter into the exhaust system. During NOx reduction strategies, exhaust gas is re-circulated aided by the EGR control valve and exhaust venturi or brake as it is referred to. This device partially closes the downstream exhaust circuit increasing upstream exhaust gas pressure by 30-40mb. This helps self-cleaning of the cooler and allows for AdBlue to be injected post cat pre DPF. Mixing is aided by the turbo. This also provides for the wideband NOx sensor to monitor NOx content before it enters the catalyst and particulate filter.

The exhaust brake also increases the upstream exhaust gas volume through the cooler, aiding self-cleaning. In addition, the emission control module has the task of reducing ammonia NH3.

Fuel delivery pressures have increased to 2000bar with delivery phases from 3/5/6 events depending on the operating profile. Additional combustion monitoring is achieved via a pressure sensor built in a heater plug. The sensor data helps the PCM calculate fuel quantity, timing and EGR values.

There is also a feature I have supported for some time, relating to how the DPF is subject to regeneration or replacement based on saturation levels.

Catalytic reduction
4-way catalytic reduction, co, hc, NOx, nh3. is based on principles of absorption followed by reduction (see fig 2). This is assisted with noble metals; platinum, palladium, and rhodium. An additional ingredient, namely barium, is used to assist in NOx reduction. Barium also helps absorb sulphur requiring periodic de-sulphation. The PCM performs this process every 600mls by ensuring exhaust gas temperature around 600-650°C. This should take 15-20 minutes.

The location of the cat and SCR has required copper zeolite to assist with higher operating temperatures. The additive injector is water-cooled to help protest both the nozzle and electrical circuit. The exact control of injector timing and additive quantity is a precise value based on the specific vehicle ID. To test the 5bar delivery pressure and two-way control valve in the additive tank module requires OEM software. Additive delivered into a calibration flask must meet exacting min-max values.

We have also conducted tests on the variation in quality of adblue. I recommend either a SG test or refractometer ensuring 32.5% ratio of active agent and de-ionised water. We have seen large variations in agent quality. It should have little or no odour. Please note; a strong smell of ammonia should not be present.

Performance
I’m not insensitive to the improvements that diesel vehicles have attained. It’s just that they don’t perform as intended under actual road conditions. We find SCR additive consumption is often excessive requiring premature refill. Additive injector crystallisation and EGR cooler blockages are commonplace as well.

Be careful when interpreting DTCs suggesting a blocked DPF. It can often be the cooler that is blocked,  restricting gas flow and affecting the algorithms for AMM, gas temperature, and DPF pressure. This will of course directly affect regeneration strategies.
Returning to my initial opening thoughts, is it clear that the fiscal life of a vehicle, especially diesels, could be ended by the cost of a single repair. The future will I believe move very quickly within certain demographics to PCPs and rental rather than ownership. This is just what the manufacturers want.

This means that in a shrinking market is even more vital to understand and invest in the latest evolutions.

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