How we're reducing diesel truck emissions

The truck manufacturing industry has been battling over the past 35 years or more to work out how to control exhaust emissions. The process has resulted in a dramatic reduction in the emission of both nitrogen oxide (NOx) and particulate matter (PM) by truck engines.

It was a lot different when the first rules were developed to reduce the allowed levels of NOx and PM in truck exhaust emissions.

Then, emissions associated with diesel engines in the road freight industry were causing major issues, with high smog levels around our cities and related health issues, especially respiratory problems, in populations near the road network.

Australia, as usual, came on board a little late with our equivalent to Euro 3 (ADR 80/00) -- the first real exhaust emission regulation for truck makers here – arriving in 2003.

Essentially, we jumped straight to Euro 3 (unlike Europe which had enforced Euro 1 from 1992, Euro 2 from 1996 and Euro 3 in 2000).

After that we continued to lag behind Europe; Euro 4 and 5 were two years after the EU, but Euro 6 was a full 12 years late!

Increasingly sophisticated technical solutions were found, and the truck industry learnt to talk about exhaust gas recirculation (EGR), diesel particulate filters (DPF) and selective catalytic reduction (SCR); all of which added weight and complexity to ensure compliant trucks.

NOx in the exhaust stream is increased by higher combustion temperatures in the combustion chamber.

EGR reduces NOx production by recirculating and cooling some of the exhaust gas back into the air as it enters the engine. This reduces the oxygen concentration, lowering peak combustion temperatures and reducing the production of NOx.

As the exhaust stream leaves the combustion chamber, the SCR system uses AdBlue which is injected into the flow of the exhaust stream.

The heat in the exhaust causes the Adblue to break down into ammonia and carbon dioxide.

As this exhaust stream continues through the system, to the SCR catalyst, the newly generated ammonia reacts with the NOx and transforms it into harmless nitrogen and water vapour.

“Tightening NOx control typically requires higher SCR dosing (increased AdBlue use), and more aggressive EGR strategies at lower loads,” explains Shayne Commons, Director of Product Strategy and Driver Development at Volvo Group Australia.

“These requirements do generally impose a fuel penalty, but Volvo Group products compensate with improved turbo technology, thermal management and after-treatment optimisation limiting the impact on fuel efficiency.

“Through these methods our engines are able to achieve increasing horsepower and torque levels, and combined with optimised rear axle ratios, it’s possible to have comparable if not better fuel efficiency from Euro 5 to Euro 6 step E.”

The trucking world has been working through the phased introduction of the Euro 6 and its A, B, C, D and E steps.

The first three, A, B and C, all came in at the same time in Australia as part of ADR 80/04.

They were all aimed at further reducing NOx and PM, plus a new PN metric (particulate number), or limiting the overall number of particles occurring in the exhaust stream to below 6x1011 per kilometre.

The journey from step A to step D was able to see NOx reduced by 77 per cent overall and PM by 66 per cent, when compared to the levels specified for Euro 5.

High PN can come from cold starts and high load acceleration, and will increase when the truck is travelling at less than 60km/h. The reduction required is achieved by higher functioning, and heavier DPFs, that are able to trap more of the soot and ash in the exhaust stream.

Another part of the Euro 6 problem for engine makers was the upgraded testing regime, which the new systems would need to undergo in order to achieve compliance with the new rules. The World Harmonised Stationary Cycle (WHSC) and World Harmonised Transient Cycle (WHTC) are upgrades on the regimes previously used to test for emission levels.

The WHSC is described as a ramped steady-state test cycle, using 13 steady-state engine test modes with the engine meeting defined speed and torque criteria at each mode. There are also defined ramps between these modes.

The WHTC requires two test cycles: the first from a cold start and the second from a hot start. The testing recreates typical driving conditions in the EU, USA, Japan and Australia, with a test run of 30 minutes.

The on-board diagnostics requirements, monitoring the effectiveness of the emission control equipment, were also upgraded for the introduction of Euro 6.

The progression from step A to step C simply ramped up the emission level requirements, over time, as Euro 6 progressed.

However, when it came to step D, there was an added focus on “in service conformity” or real-world requirements, to ensure exhaust emission levels remain compliant when trucks are performing their normal daily duties.

This change resulted in the introduction of Portable Emissions Measurement Systems (PEMS) fitted to working trucks running through their daily duty cycles. NOx and PM are measured this way, with periodic testing to ensure the emission control system remains effective over the vehicle’s life.

“Higher emission standards come with complex after-treatment systems and higher OBD monitoring,” Commons explained.

“However, this is overcome with designs for durability, preventative sensor monitoring, DPF regen strategy, catalyst coatings, software control etc., and coupled with Volvo real-time monitoring where unplanned stops can be predicted and planned.”

The latest move, to step E, takes the whole task up to another level, and is regarded as the most significant of all the steps.

At this level, the emphasis is on the cold start emissions, or the dirtiest phase of a diesel engine’s duty cycle. Step E also includes a requirement for a reduced level of PN during operation on the road.

Each move from step to step means changes to the operation of the emission control systems and the need to reduce combustion temperatures, in order to reduce NOx production.

This is normally controlled by the EGR and SCR systems and must be tweaked to take on the next Euro 6 step.

At this level of emission control, SCR catalysts have switched from using Vanadium to Zeolite, which is capable of handling higher temperatures. Its use also ensures more efficient filtering of the exhaust particles.

“Calibration workload, of course, expands with higher demands on the operating and monitoring systems,” explains Commons.

“Some examples of this are the need to optimise thermal management to keep SCR active under varied duty cycles, balance engine output emissions, maintain drivability while meeting stringent sensor monitoring requirements.

“With after-treatment size and real-estate to accommodate, increasing emission reductions requires complex muffler installations. More muffler space means less space for fuel or Adblue that can be carried onboard.

“High temperature component heat shields and muffler outlet directions also limit space. Muffler direction also becomes an important consideration.

“Potentially higher level electrical architecture sensors and ECUs to manage the emission level requirements, all have to be managed through either Volvo group core designs or by local Australian engineering,” he concluded.

These ever-increasing emission level and compliance controls have created a series of increasingly complex problems for diesel engine makers in the trucking industry. The solutions introduce more complexity, aided by more sophisticated and more expensive control systems.

For the truck operator, this has meant more costs to outsource or recruit technicians with higher skill levels using higher tech equipment.

On the flip side, engines have become more efficient, running lean to reduce emissions at the combustion phase.

Combustion control is handled with increasing precision, which has resulted in the side-effect of improving fuel efficiency across the board, albeit for some more than others.

While the fuel savings don’t compensate for the increased initial outlay, when purchasing a vehicle, they do relieve some of the pain.

Related: Zero emission trucking: what’s the current state of play?