Engines: The final stage?

Many of the machines launched at international exhibitions like ConExpo, Samoter and Smopyc at the start of this year featured new EU Stage IIIB/US Interim Tier 4 engines. The new regulations were introduced at the start of this year for 130 kW to 560 kW engines, and over the next two years the requirements will be rolled out to smaller diesel power units.

But engine manufacturers themselves already have their sights set on the Stage IV/Tier 4 Final requirements that will come into force from 2014.

The 2011 legislation called for a -90% reduction in particulate matter (PM) along with a -50% drop in nitrogen oxides (NOx), while 2014's Stage IV/Tier 4 Final emissions regulations take PM and NOx emissions to near-zero levels. In fact, some manufacturers say that in certain areas and inner cities, their Tier 4 Final engines will act as air cleaners, so strict are the final requirements.

Engines rated between 130 kW and 560 kW will be affected in 2014, and the 75 kW and 130 kW power band will have to comply from 2015.

Technology combination

To achieve this, engine manufacturers will have to use all the tools in their emissions reduction armoury. Depending on the engine size, this could mean a combination of cooled exhaust gas recirculation (EGR), selective catalytic reduction (SCR) and diesel particulate filters (DPFs).

EGR is designed to reduce NOx emissions. As the name suggests, it cools a portion of the exhaust gases and directs them back into the engine's combustion chamber. This reduces the combustion temperature, which reduces the formation of NOx - essentially the higher the temperature, the more nitrogen is fixed from the atmosphere to reduce NOx.

The downside to EGR is that the relatively low combustion temperatures make for greater PM formation. At higher temperatures, these solid pollutants are burnt off in the combustion chamber.

If manufacturers have gone down the EGR route, they will probably have added a DPF to their exhaust systems to meet Stage IIIB/Interim Tier 4 requirements. This physically traps the PM in a ceramic honeycomb where it is either burnt-off in everyday use (passive regeneration) or is incinerated by adding a squirt of fuel (active regeneration).

However, those that have gone for high combustion temperatures will not have a PM problem, but will have lots of NOx to deal with in the exhaust stream. This is where SCR comes in. This technology injects a urea solution into the exhaust stream, which reacts in another catalyst-impregnated honeycomb material to reduce the NOx back to nitrogen and oxygen.

Both the DPF and SCR route add bulk to the engine package, and some perceive a further disadvantage with SCR because the urea tank has to be periodically refilled. This is another expense for owners to manage and it also requires the availability of the fluid. Engine manufacturers have said this is not a major problem in Europe, but it can be an issue in the US.

EGR has certainly proved the most popular approach for construction machinery in terms of meeting this year's EU Stage IIIB limits. In the EGR camp are major engine manufacturers including John Deere, Volvo, Caterpillar, Perkins and Deutz, whose 2011 offerings each include DPFs.

Combustion management

Rivals JCB and Cummins have opted for EGR designs that meet the 2011/2012 requirements without using DPF or regeneration burners for engines below 129 kW. Instead they have managed to control emissions with careful management of the combustion process. Cummins' larger engines, however, use a DPF.

Meanwhile, Tognum subsidiary MTU Friedrichshafen decided to use SCR technology for Stage IIIB/Tier 4 Interim on engines below 560 kW, and EGR for engines from 560 to 730 kW. Agco Sisu and Scania have both pursued the SCR approach to comply with Stage IIIB/Tier 4 Interim regulations.

But as far as the final Stage IV/Tier 4 Final engines are concerned, it looks like manufacturers are going to have to use all the available technologies - EGR, SCR and DPFs.

MTU, for example, has said that for Stage IV/Tier 4 Final engines up to 560 kW, it will use EGR, SCR and DPF, although the company has stated that its ultimate goal is to fulfil the 2014 requirements without DPFs.

Engines above 560 kW, however, will be fitted with a technology package incorporating EGR, two-stage controlled turbo charging and high-pressure common rail fuel injection. The system involves piping fuel pressurised to 2500 bar through to injectors where it is precisely metered and injected into the combustion chambers as a fine vapour, resulting in efficient low-particulate fuel combustion in the cylinder.

And because the EGR simultaneously reduces the temperature in the combustion chamber, less NOx is produced during combustion, making exhaust aftertreatment systems largely unnecessary for this size engine, according to the company.

Cummins, meanwhile, premiered its new 503 kW QSX15 diesel engine at ConExpo in March. It is a power system that augments the company's existing EGR and Cummins particulate filter (CPF) technologies with SCR to achieve the final reduction in NOx emissions required to meet the 2014 standards.

Cummins said its new SCR system had moved beyond any of the technologies currently in use in the market to achieve Stage IIIB/Tier 4 Interim compliance. The system incorporates a copper zeolite-based catalyst capable of converting 95% of the NOx passing over it to non-polluting gasses. The CPF is then used to reduce black soot emissions.

The modifications to the engine have also seen power output increased by 56 kW compared to the Tier 4 Interim QSX15. Similarly, Cummins said its Stage IV/Tier 4 Final QSX11.9's top rating of 392 kW is higher than its Stage IIIB/Tier 4 Interim predecessor.

Jennifer Rumsey, Cummins' executive director for heavy duty engineering, said, "The QSX15 and QSX11.9 not only meet the challenge of near-zero emissions, but they are able to achieve this with higher power output and lower fuel consumption." She added that the CPF-SCR combination "raises the threshold" of what can be achieved in the trade-off between lower emissions and higher performance.

Same components

Perkins, too, will add SCR to its Stage IIIB/Tier 4 Interim solution of reducing NOx, using EGR and a DPF to meet Stage IV/Tier 4 Final standards for its 400 series engines above 56 kW.
And for engines below 56 kW, Perkins is using advanced EGR technology to meet the 2014 regulations. The new 46 kW 400F three and four cylinder engine models feature 97% of the same components from Tier 4 Interim/Stage IIIB, but include a new passive regeneration aftertreatment system to burn away the soot in the DPF.

This automatic regeneration technology does not impact the machine's duty cycle and has a minimum of service requirement of 3000 hours. Perkins also designed the system to be as compact as possible, with flexible mounting arrangements to ease installation.

Indeed, for those manufacturers which have already gone down the aftertreatment route to meet the Stage IIIB/Tier 4 Interim emissions standards, one key concern has been how to package the components in the machines without jeopardising safety by limiting the view of the operator.

It is an issue to which Volvo has given a lot of consideration. Arvid Rinaldo, Volvo Construction Equipment's global market communications manager, said that the company completely redesigned the DPF filter for its new Stage IIIB/Tier 4 Interim V-ACT (Volvo-Advanced Combustion Technology) D13 (13 l), D11 (11 l) and D16 (16 l) engines.

"We used cooled EGR technology for our new engines and developed a regeneration burner with its own pumped air supply. This regeneration burner burns away the soot which collects in the filter when it reaches set limits, and is separate from the engine. This way we avoid disturbing the operation of the machine, and full power is always available to the operator during regeneration," Mr Rinaldo said.

Regeneration is automatic and the filter is changed as part of a normal service every 4500 hours. As well as EGR, the engines are also equipped with XPI variable fuel injection systems - a combination that has produced fuel savings of up to -15% on some machines.

But Volvo was staying tight lipped on its plans for meeting the 2014 regulations. Mr Rinaldo said future technologies would be based on knowledge and technology throughout the Volvo group, which includes on-road trucks and power generation equipment.

Volvo Penta, for example has used SCR to comply with this year's emissions limits on its new 685 kW, 16 l TWD1663GE engine, designed for power generation applications.

Fuel injection

Meanwhile, two features that are likely to appear on many Stage IV/Tier IV Final engines are advanced combustion and fuel injection technologies - key elements in JCB's Stage IIIB/Tier IV Interim offering, the Ecomax T4 4.4 l engine.

JCB invested around £ 80 million (US$ 125 million) developing its latest generation JCB Dieselmax engine, and came up with an EGR solution that eliminated the need for a DPF filter or any exhaust after-treatment.

Alan Tolley, director of JCB Engine Programmes, said meeting the legislation had been a challenge and an opportunity for innovation.

He said, "We focused our research and development efforts on a high efficiency combustion system - in other words we have made sure we don't create the pollutants to start with rather than try to deal with them later. This approach also gives us very low fuel consumption levels."

Tolley explained that JCB's second generation common rail fuel injection technology had raised injection pressures to 2000 bar, while nozzle hole geometry had been refined to atomise and distribute the fuel within the cylinder. JCB has incorporated variable geometry turbochargers on all but the lowest powered 55 kW Dieselmax engine, while cooled EGR is used to clean up the exhaust gases before they are passed from the engine.

"The nozzle itself is critical to how the fuel is atomised and we spent a lot of time designing this," Mr Tolley said. "The fuel injection system has produced fuel consumption improvements of between +5% and +10% - figures that are clearly of value to our customers."

But while JCB has said it will also use the same components and systems to achieve Stage IV/Tier IV Final compliance for its 55 kW to 129 kW engines, it has conceded that exhaust after-treatment for NOx reduction may be "unavoidable" for 2014.

Scania, meanwhile, used SCR in conjunction with XPI common rail fuel injection, turbo charging and increased combustion pressure to meet Stage IIIB limits, and has said that it has now completed its engineering for Stage IV and Tier 4 Final. But details were scarce - the company said, "For Scania, this step will only involve minor changes to the installation. Equipment manufacturers can thus continue with their current designs, without any need for re-engineering to accommodate Scania's power units for Tier 4 Final or Stage IV."

So far, major engine manufacturers including Deutz, Cummins, Perkins and MTU have said that a combination of SCR and EGR/DPF technologies will be present on their 2014 engine ranges, and the industry is waiting with bated breath to find out what solution rival manufacturers have developed for Stage IV/Tier IV Final.

Next stage?

The current round of legislation on diesel engine emissions for the off-highway sector began in 1999 with the introduction of Stage I/Tier 1 requirements. The amount of technology applied to diesel engines in the intervening 15 years to reach Stage IV/Tier 4 Final has been extraordinary, but now the end is in sight, should the industry be asking, "What next?"

Emissions levels may be close to zero with the final round of laws, but there could still be more to come. One area of future legislation is perhaps to focus on particulate size. It is believed that smaller soot particles are the most damaging to human health, so there is a possibility of legislation in this area.

There is also the wider question of CO2 emissions. While there are no plans iC is aware of for a direct carbon tax or legislation on fuel efficiency in the off-highway diesel sector, this is sure to be an issue. If governments want to tax CO2, they may do it indirectly by increasing fuel duties. In any case, with oil prices likely to go on rising over the long term, better fuel efficiency (and therefore lower CO2 emissions) are likely to continue to be an issue.

There may also be a swing to other technologies. The use of hybrids may grow as a way of improving fuel efficiency, and there is also a move towards diesel-electric type machines like Caterpillar's D7E dozer. This technology uses the diesel engine as an on-board generator, with electric motors providing the drive. This eradicates the substantial friction losses that are a feature of mechanical drives.

And there may be moves to other fuels in time. Fully electric machines are already a feature of some industries - mining shovels for example are often 'plugged in' because they have a fairly stationary application. For mobile machines, perhaps hydrogen or natural gas will replace diesel.