Emission control

25 April 2008

As successive new engine emission regulations come into force manufacturers launch waves of new machines that are reconfigured to comply. This year's adoption of the US Tier 3 and European Stage IIIA laws on 1 January for the 130 to 560 kW powerband has been no exception.

April's Intermat exhibition in France was an opportunity for equipment manufacturers to debut their new Tier 3/Stage IIIA powered machines. For engine manufacturers, however, Tier 3/Stage IIIA is old news and most are already focusing on the US's Interim Tier 4 and European Stage IIIB.

From 1 January 2011, engines in the 130 to 560 kW class will have to meet these new regulations. As with Tier 3/Stage IIIA the laws will be phased in for other engine sizes over several years (see table).

“The challenge of Tier 3 was to reduce the combined level of oxides of nitrogen (NOx) and hydrocarbons (HC) by 40%,” explains Howard Gerwin, external product development and applICation manager at John Deere Power Systems. “The move to Interim Tier 4 varies slightly from one powerband to another but the main change is the reduction in partICulate matter (PM),” Gerwin continues.

For most manufacturers the key to reducing HC and NOx emissions to meet the Tier 3 limits has been to lower the combustion temperature and improve the combustion process. Most manufacturers have been able to meet Tier 3 without resorting to electronIC engine management systems on all engines.

One of the simplest ways to reduce the combustion temperature is by charge air cooling, whICh involves lowering the intake air temperature. “The lower air temperature not only reduces the formation of NOx, it also improves engine durability and increases low speed torque and power density,” Gerwin explains.

Changes to cylinder and piston design and the way fuel is introduced can also help lower the combustion temperature. The shape of the piston crown and combustion chamber influences the 'swirl' or turbulence of the intake air in the cylinder and can be used to optimise the completeness of combustion. Premixed compression ignition, multiple fuel injection and common rail systems are also used to increase fuel pressure for more effICient combustion.

Many Tier 3 engines use exhaust gas recirculation (EGR) where a measured amount of exhaust gas is fed back into the intake manifold and mixed with the incoming fresh air. This lowers the proportion of oxygen in the intake to reduce the formation of NOx.

“The new Tier 3 compliant engines are often more fuel effICient than their Tier 2 counterparts,” Howard Gerwin says. “But for many manufacturers, meeting Interim Tier 4 is going to be more of a challenge than Tier 3 and is going to call for more cooperation between engine producers, OEMs and construction equipment manufacturers.”

Engines in the 130 to 560 kW powerband will be the first to be affected by the Interim Tier 4/Stage IIIB regulations, whICh call for NOx to be cut to 3.5 g/kWh, HC to 0.19 g/kWh and partICulate matter (PM) to be cut from the Tier 3 level of 0.2 to 0.025 g/kWh. “PartICulate traps are likely to be the most common way to meet the reduction in PM needed to meet both Interim and Final Tier 4,” Gerwin says. “But at present traps to suit construction equipment are not readily available, so it is very uncertain at the moment about how big they will be, how much they will cost and how often they will need replacing. Resolving issues like this will call for more liaison through the supply chain.”

PartICulate traps may not resolve the problem completely, so electronIC control may become more widespread as engine manufacturers attempt to further raise fuel pressure and improve mixing. EGR and turbo charging may also become more common. “Achieving Final Tier 4 and Stage IV calls for a further 80% reduction in NOx, whICh will bring emissions in line with those produced by on-road trucks,” Gerwin says. “Reducing NOx to these levels will probably call for some form of after treatment. There are several catalyst options but these have a signifICant impact on fuel economy.

“The alternative is a urea-based additive, whICh, when mixed with exhaust gases, converts the NOx to oxygen, nitrogen and water. This technique would not impact on fuel effICiency but needs a urea tank, pumps and a control system. It also relies on the operator to re-fill the tank on a regular basis.”

Engine manufacturers are either looking at the options for developing Tier 4 engines, or already testing them. Fitting additional filters, tanks and catalytIC converters into the often already confined engine compartment will be a challenge met by construction equipment manufacturers over the next few years.

Can engine emissions be reduced any further? Howard Gerwin says, “Between introduction of Tier 1 in 1996 and Stage 1 in 1999 and Final Tier 4 and Stage IV, NOx, HC and PM will have been reduced by 99%.” Gerwin declines to comment on whether the technology exists to meet a Tier 5/Stage V but says Tier 4 engines will likely be further refined to compensate for the loss in fuel economy inevitable in meeting the next set of regulations. •

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