Exhausting choice - Steve Skinner reports on meeting Stage IIIB compliance

Stage IIIB engine emissions legislation will see the use of two principal technologies to deal with exhaust gases. Steve Skinner reports on these systems and the implications for equipment owners and users.

Engine manufacturers will achieve Stage IIIB emission compliance through the use of either selective catalytic reduction (SCR) or exhaust gas recirculation (EGR) in association with a diesel particulate filter (DPF).

SCR uses a ceramic honeycomb catalyst contained in the exhaust line into which urea is injected. The resultant reaction between the urea and the nitrogen oxide (NOx) in the exhaust gasses creates non-polluting nitrogen and water. In all cases where SCR is used, the engine combustion is designed to create the minimum level of particulate matter (PM) - black soot - at the expense of creating NOx.

A trade off between NOx and PM is an inherent feature of diesel engines with the temperature of combustion tilting the balance in either direction. A cool combustion reduces NOx but increases PM while a high temperature reduces PM but increases NOx.

EGR sees some exhaust gases cooled and fed back into the engine to decrease the combustion temperature, which in turn reduces the production of NOx. The engine exhaust is then fed through a DPF which traps PM. Because the DPF is a trap, it requires regular cleaning, which is normally managed through a high temperature burn of the filter, known as regeneration, which incinerates the PM.

Now that all off-highway engine manufacturers have declared their choice of technology to reduce NOx and PM by -45% and -90% respectively, it is clear that Stage IIIB compliant construction equipment in Europe will feature either urea tanks or DPFs. This will apply to engines above 129 kW from January 2011 and for engines from 56 kW to 129 kW from January 2012.

Both technologies have already been tried and tested in trucks and busses, but the complexities of fitting either solution in off-highway equipment continues to challenge engine and equipment manufacturers alike.

Group engineering director at JCB, Tim Leverton told CE, "Previously, the fear was that DPFs would be of a similar size to the capacity of the engines they were being fitted to. In reality, they have turned out to be bigger, especially for the six litre engine. We have been able to compensate for this to a degree because the DPF acts as the muffler, but space remains an issue."

Chief operating officer at DPF manufacturer Baumot, Roger Kavena explained the significance of size when he told CE, "Dependent on the operation profile, larger sizes of DPF are necessary to offer a sufficient soot collection capacity to avoid high back pressure between regeneration, which has a negative impact on both engine durability and fuel consumption. Therefore, the size of the DPF is wholly dependent on engine performance and usage.

"At Baumot, we believe that for low back pressure, it is necessary to increase the volume of the DPF and we always recommend a round section as this is the most effective configuration," he said.

Mr Leverton confirmed that in some cases JCB has had to redesign the chassis in some of its models, while in others it has had to reposition key components to enable engineers to route the more complex exhaust system fitted with the DPF after treatment.

"The bonnet line on our Stage IIIB compliant backhoe loader will be slightly higher at the back edge because of the need to fit more into the space," he confirmed. "With our telehandlers however, we have managed to install the new engine without impacting on the operator's line of vision."

Like JCB, Caterpillar has opted to use EGR in combination with a DPF to attain Stage IIIB compliance. Caterpillar industrial engine spokesman Chuck Boysen told CE, "In many instances Caterpillar's ACERT Stage IIIB engines, in comparison to current Stage IIIA models, will offer up to +5% improvements in fuel consumption depending on application and operating conditions."

An important aspect of any Stage IIIB solution has been giving due consideration to Stage IV at the same time. Communications director at Cummins, Steve Nendick told CE, "With Stage IV running so close behind Stage IIIB, we looked at the two moves in one package. It is likely both EGR and SCR will be required for Stage IV compliance, so if you move to EGR and DPF now, then the changes for Stage IV will be minimal.

"We estimate that approximately 2% of SCR usage on top of the Cummins EGR system will attain Stage IV compliance in due course. For those choosing SCR for Stage IIIB, we believe they will have substantial installation work still to do in order to meet Stage IV compliance," he said.

Testing times

As part of its field test programme, Cummins has already conducted 20000 operating hours with its Stage IIIB engines. The tests cover a wide variety of equipment and the programme has been structured to enable field test customers to compare Stage IIIB powered equipment directly against Stage IIIA variants.

"The customer consensus from the programme is that Stage IIIB installations are exceeding the performance standards established by Stage IIIA powered equipment," Mr Nendick told CE. "We are being told that the next generation engines are offering better response, lower fuel consumption and cleaner operation."

Regarding regeneration of the Cummins self cleaning particulate filter, Stage IIIB field test programme leader, Maged Tadros said, "Field testing has demonstrated that most off-highway applications operate at a high enough engine load for the DPF to regenerate almost every time in passive mode. We typically logged the time spent in active regeneration at less than 1% of the equipment operating time, which proves we've managed to maximise the scope for passive regeneration."

Passive regeneration takes place within normal operating parameters and uses exhaust gas temperatures to incinerate PM. Active regeneration requires an additional heat source to sufficiently raise the temperature within the DPF to burn-off the PM.

Perkins too is already making progress with its machine installation and testing programme. Product marketing manager, Allister Dennis told CE, "Prototype six and four cylinder engines are now being validated in a number of machines, with over 50000 hours on the test bed and 4000 hours in machines already completed.

"Our test machines have been engaged in typical work patterns and a real highlight has been the discovery that both configurations are offering better fuel consumption and sound quality than the earlier Stage IIIA engines," said Mr Dennis.

He also told CE that that there had been no real issues with engine installation in the test machines thanks to the flexibility of the after treatment components that Perkins had developed.

Like Perkins, John Deere has found fuel economy benefits and sound reductions from its Stage IIIB engines. Product marketing manager, Joe Mastanduno said, "John Deere is ready for 2011 legislation. Our machines are already out in field tests with customers and they are telling us that our Stage IIIB compliant equipment is performing as well, if not better than Stage IIIA models."

For its part, Volvo is to launch a new generation of four and six cylinder engines developed from its proven Volvo-advanced combustion technology (V-ACT) range. The new engines will feature ultra high pressure variable fuel injection systems, cooled EGR, variable geometry turbochargers, powerful new electronic engine management systems (EMS) and a DPF with an active regeneration capacity.

The cooled EGR will allow for more exhaust recirculation than found with the Stage IIIA version, which in turn will help to reduce NOx levels at the exhaust manifold. For rapid engine response across the range, Volvo will use a variable geometry turbo designed to continually vary the airflow into the engine. A smart remote actuator will regulate the turbo based on engine load to vary the flow of exhaust gases into the turbine wheel, providing a rapid boost at low engine speeds while maintaining boost at higher engine speeds.

A statement from Volvo said its DPF would reduce PM by -99% over its existing engines and that the 20 minute thermal regeneration of its DPF will not affect the machine's operation, performance or productivity. Volvo envisages that regeneration, which will take place at temperatures exceeding 600⁰C, will only be required once during a normal working day.

Regeneration is critical to the ongoing healthy operation of a DPF and managing the regeneration strategy is an area that has been at the forefront of research and development at DPF manufacturer, Huss Group.

OEM product manager, Konrad Sörgel told CE, "We see two main directions in demand. The first is wall-flow DPFs using a silicon carbide (SiC) base to collect up to 99,9% of the PM while also offering minimal back pressure.

"The second is active regeneration starting from a low base exhaust temperature. Using our modular diesel post injection (MD) system we are able to start regeneration at exhaust temperatures as low as 220⁰C, which enables the filter to be cleaned of PM even when the engine's not under load."

Filter free

MTU believes in some cases it will be able to meet Stage IIIB compliance without the need for exhaust after treatment. Project manager for development of the company's 4000 series engine, Ingo Wintruff said, "MTU has pulled off a master stroke with some of our Stage IIIB engines. We have kept within emissions limits without the need for after treatment on engines that suit equipment such as mobile cranes and wheeled loaders."

MTU attributes its ability to avoid after treatment to extremely high fuel injection pressures of 2200 bar, up from 1800 bar. "The high injection pressures mean the fuel burns more cleanly and less PM is produced in the first place," said Mr Wintruff. "To keep within NOx limits, while maintaining low fuel consumption, we have adopted EGR in tandem with our 'Miller process' which reduces NOx by closing the inlet valves earlier. This enables the air charge to cool and therefore lowers the combustion temperature."

Volvo Penta's Stage IIIB solution centres on SCR technology following the success of the system in on-highway applications. Business development manager, Darren Tasker told CE, "At Volvo Penta we had both EGR and SCR technologies available to us from the Volvo Group. With over 500000 SCR units operating in on-highway applications in Europe we considered this to be a mature technology that would suit off-highway equipment and so we have chosen this as our route to compliance."

Mr Tasker believes the high volume of SCR systems produced for on-highway trucks means SCR is cost competitive, and with no need for cleaning or replacement during its lifetime, it will lead to lower operating costs. This is in addition to the savings in fuel consumption, which Mr Tasker believes are up to +8% better than with an equivalent EGR with DPF solution.

"Installation is also much easier as the engine block and components on the engine are largely unchanged from the Stage II variant," said Mr Tasker. "Compliance is basically through the addition of the SCR muffler - which in itself replaces the current silencer - and the urea tank.

"Because there is no added heat rejection from SCR, there is no need to alter the cooling system either, which again aids installation," he said.

Agco Sisu Power has also adopted SCR for Stage IIIB compliance and the company expects the first serial production engines to be delivered in the final quarter of this year.

Marketing director, Juha Tervala told CE, "Several prototype engines have been built for various applications and field tests are on-going, the results of which are matching our expectations.

"Fuel consumption is approximately -10% lower than found with Stage IIIA engines and urea consumption is working out at no more than 4% of the fuel consumption. We therefore believe that total savings for end users will be significant."

As well as fuel savings, Mr Tervala believes a significant advantage of Ago Sisu Power's SCR system is that the cooling packages will be similar to those found in Stage IIIA compliant equipment. "This is one of the main advantages compared with cooled EGR. Our customers will face no space problems with their cooling packages and through avoiding the use of larger fans, there will be no associated noise or power loss issues," he said.

Warning

The new generation of engines will offer more power and fuel efficiency along with reduced engine noise due to the muffling effect of DPFs, but it will be at an increased capital cost for users. For engines with SCR, the availability of urea is critical while for engines with EGR and DPFs, the regeneration may also affect day-to-day operation, although manufacturers are agreed that fitting units that avoid the need for manual intervention is paramount.

As Mr Leverton told CE, "The worst case scenario for us is operators having to 'stand down' their equipment so that a regeneration cycle can take place. We have to provide a solution that avoids intervention for regeneration!"