Introduction

To comply with the US Interim Tier 4 or EU Stage III B emission standards, implementation of either a particulate matter (PM) or a nitrogen oxides (NOx) aftertreatment is recommended. Deployment of aftertreatment enables the engine to extend its power range, altitude capabilities and transient performance characteristics. Both exhaust gas recirculation (EGR) and selective catalytic reduction (SCR) technologies are available for the Interim Tier 4 market. The choice between EGR and SCR for NOx reduction depends on the original equipment manufacturer's (OEM's) production history, technology experience, customer requirements and long term product strategy. Engines with EGR consume only diesel fuel but typically require the fitment of a DPF. DPF active regenerations mean that the maximum skin temperatures of the aftertreatment devices and tailpipe exhaust gases must be limited by the exhaust system design.

US Interim Tier 4 or EU Stage III B standards, shown in Table I, have been in effect since 1st January 2011. Besides significant tightening of criteria pollutant limits, a new Non Road Transient Cycle (NRTC) was introduced. Emission compliance on the NRTC is required on top of the existing eight mode steady state test. Speed and torque definitions of NRTC and eight mode tests are displayed in Figure 1. Emissions from cold and hot NRTC tests are weighted in a similar way to those for on-highway regulations. Not-to-exceed (NTE) rules equivalent to those for on-highway applications also apply to non-road engines.

NOx and PM criteria pollutant limits for engines above 130 kW are higher than those for 2007 on-highway trucks. However, the NRTC has a higher average load factor and is significantly more transient than the Federal Test Protocol (FTP) heavy-duty cycle. The NOx limit for engines below 130 kW is 3.4 g kW⁻¹. Engines with a power output below 56 kW must comply with a combined NOx + non-methane hydrocarbons (NMHCs) limit of 4.7 g kWh⁻¹. The required emission useful life is 8000 h for all diesel engines above 37 kW. This requirement differs signficantly from those for on-highway engines (1).

Different factors must be taken into account when designing exhaust aftertreatment systems. Non-road applications are very diversified with a wide range of engine configurations, power bands and machine forms. Some examples are shown in Figure 2. Agricultural applications, particularly row-crop tractors (Figure 2(a)) and harvesters (Figure 2(b)), have high load factors with signficant portions of operating time at full loads and rated speeds. Construction machines (Figures 2(c) and 2(d)) demand highly transient engine performance, altitude capabilities and longevity. Utility tractors (Figure 2(e)) and small construction machines (Figure 2(f)) can operate persistently at light loads with extended idle time. Their usage profiles are sporadic. This market segment is very sensitive to cost, especially the initial machine purchase price.

Most non-road machines are used for commercial purposes, therefore reliability and uptime are premium for the equipment owners and operators. Modern large scale agriculture and construction operations require a fleet of machines to work together. If one piece of equipment goes down, the whole operation may be jeopardised. Further, production volumes for the equipment vary drastically. High volume equipment is manufactured in tens of thousands of units annually, while specialty machines may be made at a rate of a handful a year. And non-road machines often have to perform at extreme ambient temperatures, high altitudes and off-level positions. Some machines operate in remote areas, in harsh terrains and under unique environmental conditions.

Engine and DOC-DPF Designs

Five engine families are offered from John Deere for the Tier 4 market. The engine line up is summarised in Table II.

Interim Tier 4 engines were based on Tier 3 engines using cEGR technology, re-optimised to meet the new NOx standards and to facilitate active DPF regenerations. Precise control of EGR rates and combustion events under transient operations was achieved by a redesigned engine control unit (ECU) with a new software package. A single ECU manages both engine operations and aftertreatment performance.

Engine out PM and NOx predictive models were used to calculate DPF soot loadings. New engine combustion modes enabled the engine to raise exhaust gas temperatures when DPF active regenerations were required. This was accomplished by increasing engine fuelling and reducing exhaust flow through an air intake throttle or an exhaust brake. Exhaust temperature management is critical to ensure the completion of an active DPF regeneration event when the exhaust temperature can fall below the pgm catalyst light-off temperature in a normal combustion mode. Capable engine hardware and calibration strategies eliminated the need for an exhaust diesel burner.

The DOC and DPF were sized according to exhaust flow rates, which correlated well with engine power outputs if EGR rates and air to fuel ratios were similar. A total of seven DOC-DPF sizes were designed and released for the five engine families ranging from 37 kW to 460 kW. The DPF dimensions are summarised in Table III. Each design also features ash serviceability, inlet/outlet rotatability, three temperature sensors and one delta pressure sensor. Round filters with a larger diameter and shorter length were preferred for vehicle installations as they provide lower DPF pressure drop, higher volumetric soot loading and active regeneration robustness. Filters made of 200 cells per squre inch (cpsi) cordierite and 300 cpsi silicon carbide (SiC) materials were applied to engines above 130 kW and under 130 kW, respectively. DOC substrates were sized to have the same diameters and approximately half the volume of the filters.

DPF designs must consider the worst case pressure drop when loaded with ash and soot. The ash cleaning service requirement is 4500 h for engines above 130 kW and 3000 h for engines under 130 kW. A 200 cpsi cell structure is more tolerant towards ash accumulations, and is therefore preferred for applications above 130 kW. For engines under 130 kW, a higher volumetric soot limit and smaller volume filters favour 300 cpsi cell structures. A 300 cpsi filter offers lower pressure drop in a soot loaded state due to its higher geometric surface area and a thinner soot layer. In addition, smaller SiC filters fit better into compact vehicles.

A DOC was designed to convert nitrogen monoxide (NO) to nitrogen dioxide (NO₂) for passive regenerations and to provide high hydrocarbon (HC) oxidation activity for active DPF regenerations. PGM loading was selected to provide adequate residual conversion efficiencies of NO to NO₂ as well as sufficient HC light-off performance beyond 8000 h. A catalysed DPF with a low pgm loading was highly effective to prevent HC emissions during active DPF regenerations. The catalyst on the DPF was highly active for HC oxidations during active regeneration because of high reaction temperatures and an abundance of oxygen. NO₂ generated by the DPF catalyst also promoted more passive regenerations. Design and function relationships are summarised in Table IV.

The DOC, DPF and exhaust gas sensors were packaged and integrated into a converter assembly, shown in Figure 3. An integrated DOC-DPF design was preferred over separated DOC and DPF converters as it required less space and had higher efficiency. This system design eliminated the need for two additional end cones and reduced heat loss and pressure drop. To accommodate diversified vehicle installations, the inlet and outlet cones were made fully rotatable. Two serviceable flanges, one on each side of the DPF, allowed the DPF to be removed for ash cleaning. Cylindrical converters with two service flanges provided flexibility in the installation of aftertreatment sensors and the positioning of wire routings. Each DOC-DPF converter contained three temperature sensors and one delta pressure sensor across the DPF. The DOC inlet temperature sensor (T1) was used to initiate HC dosing for active regenerations; the DOC outlet sensor (T2) was used primarily for temperature control; and the DPF outlet sensor (T3) was used for temperature diagnostics.

The DOC-DPF converters were heavily insulated, including areas around the sensor ports, to keep converter skin temperatures below the required limits even during active DPF regenerations. The design assumed no air flow around the converters. Under normal engine operating conditions, which accounted for over 97% of the total time, the DOC-DPF converter skin temperature was lower than that typically found for a traditional muffler. Since the DOC-DPF converters were internally insulated with a stainless steel sheet metal surface, any external air flow would effectively reduce skin temperatures further.

For engines above 130 kW, HC was delivered to the DOC through an airless exhaust fuel doser, as shown in Figure 4. A tip coking resistant design was selected. No air purging or tip cleaning service was necessary. Late post injections were used for engines under 130 kW.

The exhaust fuel doser was mounted next to the engine turbocharger to maximise fuel evaporation and mixing. For engines with two-stage turbochargers, a fuel doser was placed between the two turbines. The second stage turbine served as an active mixer. Uniform HC distributions maximised DOC catalytic efficiencies and exhaust temperature homogeneities for DPF regenerations. Perfect HC mixing avoided hot spots on the DOC and reduced its degradation rate.

Diesel Oxidation Catalyst-Diesel Particulate Filter Performance

Wall flow DPFs were selected due to their high PM trapping efficiencies and their robustness towards diversified applications and engine operating conditions. Measured PM trapping efficiencies on a 9 l engine are summarised in Table V. The results demonstrated a filtration efficency of over 95% with a brand new DPF. The PM trapping efficiency well exceeded 99% when the ramped modal tests were repeated and a soot layer had been established on the DPF. Similar performance data for a DPF after 5000 h field usage showed efficiencies greater than 99% both before and after an ash cleaning service.

The full benefits of passive regenerations were achieved with a DOC and a catalysed DPF (4). The DOC oxidised NO to NO₂ under normal engine operating conditions. A production design DOC, after accelerated ageing to simulate 8000 h field usage, was capable of providing NO to NO₂ conversion efficiencies of over 50%, as shown in Figure 5. Each data point in Figure 5 represents an engine operating condition. The bubble size signifies the actual engine out NOx ppm level. To fully benefit from passive soot oxidations, the DPF soot predictive models must account for the soot and NO₂ reaction rates, and adjust for catalyst degradation over time.

Due to a higher NOx limit of 3.4 g kWh⁻¹, engines for applications under 130 kW produce less PM. Table VI compares the NOx:PM ratios of an engine running at 2 g kWh⁻¹ vs. 3.4 g kWh⁻¹ permitted NOx output levels. A higher NOx:PM ratio provided a greater opportunity for passive regenerations. In principle, active regeneration is only required when an engine operates persistently at low loads with low exhaust temperatures. Under low load conditions, the engines produced little soot and the DPF soot loading rates were low. Infrequent active regenerations for engines under 130 kW enabled HC to be delivered by late post injections without oil dilutions or compromises in engine durability.

At 2 g kWh⁻¹ NOx for engines above 130 kW, active regenerations were more frequent. But significant passive regenerations were also observed. Engines for applications above 130 kW tended to operate at higher average loads with temperatures over 250°C, which are more favourable for passive regenerations. High speed and low load engine operations tended to produce more PM and lower exhaust temperatures, and therefore required more frequent active regenerations than other operating conditions.

Steady State Tests

Steady state eight mode emission results of a fully aged engine aftertreatment system for a 9 l engine are shown in Figure 12. The blue bars represent the US Interim Tier 4 or EU Stage III B standards for CO, HC, NOx and PM. The engine out NOx emission is under the limit with a reasonable engineering margin. HC, CO and PM criteria pollutants are far below the regulatory limits.

Conclusion

Cooled EGR, DOC and DPF are proven technologies for meeting the US Interim Tier 4 and EU Stage III B emission control standards for non-road diesel applications. High trapping efficiency wall flow filters enable flexibility in engine design, broad engine applications and wide operating windows. The platinum-palladium based DOC is cost effective and robust and provides the benefit of passive regenerations through NO₂ and soot reactions. The DOC oxidises HC and the soluble organic fraction of PM and heats the exhaust gas for active DPF regenerations under a wide range of exhaust flow, O₂ level and inlet temperature conditions. The robust HC performance and thermal inertia of a DOC are beneficial for precise control of the DPF inlet temperature for active regenerations.

For non-road applications, passive soot regeneration occurred extensively in the DOC-DPF system. The aftertreament control alogrithm within the engine management system was designed to take advantage of this. The DOC-DPF system is less complex than the burner-DPF alternative. A key enabler was a new engine exhaust temperature management mode to ensure exhaust gas temperatures are above the DOC light-off temperature. Active regenerations are recommended for wall flow DPF applications to provide a reliable and robust system for diversified non-road applications. Uniform HC distributions and precise DPF inlet temperature controls are critical for reliable active DPF regenerations. Additional vehicle level integrations are required to effectively manage the DOC-DPF converter skin and exit gas temperatures. Overall, the EGR and DOC-DPF solution offers the best in class engine and emission performance as well as being cost effective.