Stage V nonroad mobile machinery operating under EU Regulation 2016/1628 Annex II frequently uses close-coupled diesel oxidation catalysts and catalysed diesel particulate filters without aqueous urea injection when engine-out NOx is controlled by charge-air cooling, exhaust gas recirculation, or when the machine operates in a power band where selective catalytic reduction is not configured. In such non-urea systems, soot oxidation depends on two competing mechanisms: thermal oxidation by molecular oxygen at filter wall temperatures typically above 575–625 °C, and catalytic NO₂-assisted oxidation at lower wall temperatures between 250 °C and 450 °C. Fuel composition alters both pathways by changing soot nanostructure, ash residue, hydrocarbon slip, and the NO₂/NOx ratio entering the filter. EN 590:2022 reference diesel restricts sulfur to 10 mg/kg, fatty acid methyl ester content to 7.0% v/v, density to 820–845 kg/m³, and imposes a minimum cetane number of 51. The nonroad load profile defined by ISO 8178 C1 and the transient NRTC generates prolonged low-load operation in which diesel oxidation catalyst outlet temperature may remain below the catalytic soot oxidation window. Under those conditions, fuel-derived species that inhibit NO oxidation or add incombustible material to the filter become disproportionately important. The absence of urea eliminates the SCR consumption of NO₂, meaning the diesel oxidation catalyst outlet NO₂/NOx ratio is transferred directly to the catalysed diesel particulate filter. Fuel sulfur, aromatic content, FAME quality, and cetane number therefore regulate passive regeneration stability more strongly in a non-urea Stage V architecture than in an SCR-equipped system where part of the NO₂ stream is consumed by denitrification.

How Does Fuel Sulfur Govern NO₂-Assisted Soot Oxidation in Non-Reagent Stage V Systems?

Fuel sulfur is converted during combustion primarily to SO₂, which can be further oxidized to SO₃ on platinum-containing diesel oxidation catalyst surfaces. In the absence of urea injection, the SO₃ does not combine with ammonia to form ammonium sulfate; instead it remains as adsorbed sulfate species or sulfuric acid on the washcoat and filter cake. Sulfate accumulation masks platinum and palladium active sites, reduces the conversion of NO to NO₂, and fills micropores required for soot-catalyst contact. The direct consequence is a loss of passive regeneration capacity at low exhaust temperatures. A diesel oxidation catalyst operating on fuel containing sulfur below 10 mg/kg can typically sustain a NO₂/NOx fraction high enough to support soot oxidation at filter temperatures above approximately 300 °C. If sulfur concentration rises, the NO₂/NOx fraction declines progressively with operating hours, and the filter pressure differential measured across a wall-flow silicon carbide substrate increases at identical soot loading. The poisoning effect is temperature-dependent: sulfate decomposition requires temperatures above 600 °C for many washcoat formulations, while prolonged low-load operation below 250 °C can convert sulfates into stable deposits. Production-scale Stage V engines without urea often use platinum-rich oxidation catalyst formulations to improve low-temperature NO oxidation, but these same formulations can accelerate SO₂-to-SO₃ conversion when sulfur control is lost. Sulfur measurement by ISO 20846 with ultraviolet fluorescence is the established control method, and the EN 590:2022 sulfur limit of 10 mg/kg is a maximum, not a guarantee of long-term passive regeneration stability. Published quantitative data for Stage V nonroad DOC sulfation under variable fuel sulfur is limited; however, the relationship between fuel sulfur, NO₂ suppression, and DPF backpressure is widely documented in heavy-duty on-road studies. Field experience from nonroad machines indicates that even short periods of operation on off-spec fuel with elevated sulfur can require active regeneration intervals to be shortened, because passive soot oxidation is suppressed before the operator observes any change in engine power or fuel consumption.

Because ignition delay controls the premixed combustion fraction, fuel cetane number directly influences soot aggregate size, particle number, and the volatile organic fraction that accompanies dry soot. Low cetane fuels ignite later, increase the fraction of diffusion-controlled combustion, and produce soot with higher volatile hydrocarbon content. That volatile fraction can temporarily block filter pores and reduce the effective contact between NO₂ and carbon. High cetane fuels, such as paraffinic diesel and certain hydrotreated vegetable oil blends, shorten ignition delay and reduce soot formation under nonroad transient conditions. Soot from high cetane paraffinic fuels also tends to exhibit smaller primary particle size and a less graphitic carbon structure, which lowers the apparent activation energy for oxidation. Laboratory thermogravimetric analysis under 10% O₂ in nitrogen according to ASTM E1131 has been used to compare soot oxidation kinetics; published data indicate that non-catalysed soot oxidation exhibits apparent activation energy in the range of 140–170 kJ/mol, whereas catalytic NO₂-assisted oxidation can reduce that range to 70–110 kJ/mol. Those values are not fuel-specific constants but reflect the distribution of carbon bonding and surface oxygen groups. Fuels with high polycyclic aromatic content tend to produce soot with more ordered graphitic layers, which are less reactive toward both O₂ and NO₂. The EN 590:2022 polycyclic aromatic limit of 8.0% m/m constrains this effect for standard diesel, but off-spec or imported fuel can exceed that value. For non-urea Stage V equipment, a fuel with borderline cetane number may still meet the certification cycle particulate limit while increasing the rate of DPF loading during real low-temperature operation. The result is a shift from passive regeneration to active regeneration, where post-injection fuel is used to raise filter temperature above 550 °C. Fuel cetane quality is therefore not only an ignition parameter but also an indirect control variable for soot oxidation rate and DPF service interval.

Thermal and Catalytic Soot Oxidation Pathways When Urea Injection Is Absent

When urea injection is absent, the exhaust aftertreatment system has no downstream NO₂ sink other than the soot cake itself. The oxidation chemistry can be represented by the two stoichiometric pathways C + O₂ → CO₂ and C + 2NO₂ → CO₂ + 2NO. The second pathway is critical because it proceeds at diesel particulate filter temperatures that are attainable during normal nonroad duty cycles without active regeneration. NO₂ is generated upstream by the diesel oxidation catalyst from engine-out NO. The oxidation of NO to NO₂ is equilibrium-limited and kinetically controlled by precious metal dispersion, washcoat composition, space velocity, and catalyst inlet temperature. At temperatures below approximately 200 °C, NO oxidation is limited by light-off. At temperatures above approximately 450 °C, thermodynamic equilibrium restricts further NO₂ formation. The effective window for NO₂-assisted soot oxidation therefore lies between 250 °C and 450 °C. The absence of urea means that no ammonia-induced NO₂ consumption occurs, so the entire NO₂ stream remains available for carbon oxidation. This condition arguably improves passive regeneration compared with SCR-equipped systems, provided the fuel maintains high DOC NO oxidation activity. In production nonroad engines in the 56–130 kW power band, diesel oxidation catalyst inlet temperatures during ISO 8178 C1 modes can remain between 180 °C and 300 °C for significant periods. Under these conditions, the DOC formulation and fuel quality determine whether sufficient NO₂ is produced to offset the soot accumulation rate. Catalysed diesel particulate filters washcoated with platinum and palladium also promote direct soot oxidation at the filter wall, but the contact between carbon and catalytic sites is limited by the filter geometry. Wall-flow filters constructed from silicon carbide or cordierite provide an asymmetric cell structure that accumulates soot at the inlet channel surface; if that accumulation becomes excessive, the NO₂ cannot penetrate the soot cake, and passive oxidation slows. Active regeneration then becomes necessary. The active regeneration event, triggered by in-cylinder post-injection, raises DOC outlet temperature above 550 °C and initiates exothermic soot combustion. Fuel properties influence this process because post-injection fuel must vaporize and oxidize without excessive dilution of the lubricant sump. Low volatility fuel components, particularly a high final boiling point above 360 °C, can survive into the cylinder liner boundary layer and increase oil dilution. Fuel chemistry therefore affects not only passive regeneration but also the reliability of active regeneration under non-urea Stage V operation.

When FAME Content Exceeds 7% v/v in Stage V Duty Cycles

FAME is not a single chemical species; it is a mixture of methyl esters whose carbon chain length and unsaturation vary with feedstock. EN 590:2022 permits up to 7.0% v/v FAME in automotive diesel, and this limit is generally carried into nonroad fuel supply contracts where Stage V engines are fuelled with standard automotive diesel. The oxygen atoms in FAME can reduce soot formation during combustion, because oxygenated fuel species reduce the local equivalence ratio in fuel-rich zones. Soot produced from oxygenated fuel may also contain surface carbonyl, carboxyl, and hydroxyl groups that enhance reactivity toward NO₂. However, FAME also introduces higher distillation end points, higher viscosity, and greater deposit-forming potential than paraffinic diesel. The most significant non-urea soot oxidation risk is not the ester itself but the impurities and degradation products. FAME with high unsaturation measured by iodine value according to EN 14111 and poor oxidation stability measured by EN 15751 can polymerize during storage or in the fuel injection system. The resulting gum and insoluble oxidation products increase injector fouling, alter spray atomization, and produce incombustible organic films that contribute to DPF pressure drop. Fuel ash measured by ISO 6245 remains low for specification FAME, but off-spec or poorly processed biodiesel can contain sodium, potassium, and phosphorus that are not removed by regeneration. Alkali metal compounds accumulate in the filter as ash and can mask catalytic sites. The experience from production-scale Stage V engines without urea is that FAME-containing diesel can reduce soot mass emissions while simultaneously increasing the frequency of injector maintenance and raising DPF backpressure if fuel oxidation stability is inadequate. The thermal stability of FAME also influences post-injection behavior: methyl esters with high boiling range components can delay vaporization, leading to incomplete post-injection combustion and diesel fuel accumulation in the engine oil. Oil dilution reduces effective lubricant viscosity and can lead to increased oil consumption, which then introduces lubricant-derived ash into the DPF. Since the filter cannot convert ash, the net effect is a reduction in available soot oxidation surface. The use of FAME above 7.0% v/v in Stage V nonroad machinery without urea requires fuel system validation because the combination of high FAME and low-load duty can produce accelerated DPF loading that is not detected during certification tests.

Paraffinic Drop-In Fuels and Passive Regeneration Balance

Paraffinic diesel fuels conforming to EN 15940:2023 represent a distinct alternative to conventional B7 diesel in non-urea Stage V applications. These fuels include hydrotreated vegetable oil and gas-to-liquid cuts with sulfur typically below 5.0 mg/kg, total aromatics below 0.1% m/m, FAME content below 0.1% v/v, and cetane number above 70. The combination of low aromatic content and high cetane number reduces soot formation at the source, which lowers the specific soot loading per operating hour. At the same time, low sulfur preserves DOC NO oxidation activity by reducing sulfate accumulation. The net effect is a higher NO₂/soot ratio at the diesel particulate filter and a longer passive regeneration interval under comparable duty cycles. However, paraffinic diesel has a lower density, typically 765–800 kg/m³, and a lower volumetric energy content than EN 590:2022 fuel. Electronically controlled common-rail injection systems compensate to some extent, but the lower density can alter injection timing and spray penetration if the calibration is not updated. The lubricity of paraffinic fuel is controlled by the HFRR wear scar limit in ISO 12156-1, but in practice paraffinic fuels may exhibit higher wear sensitivity when stored for long periods without additives. Any increase in injector wear introduces metal and hard particle debris into the filter as ash. Since ash cannot be oxidized, the filter’s effective volume for soot storage is reduced. The thermal behavior of paraffinic soot is also relevant. Soot from paraffinic combustion tends to be less graphitic and may oxidize at lower temperatures than soot from aromatic-containing diesel. This property is beneficial for passive regeneration but can lead to a lower exotherm during regeneration and a reduced tendency for uncontrolled soot combustion. In a non-urea Stage V system, the threshold at which passive regeneration balances soot accumulation depends on engine-out soot mass, NO₂ availability, and filter temperature. Paraffinic fuels simultaneously reduce soot mass and protect NO₂ availability, so they are generally compatible with non-urea DPF strategies. Published data for long-term Stage V nonroad operation on EN 15940:2023 paraffinic diesel is limited, but the fuel’s regulatory specification directly addresses the two parameters most responsible for DPF passive regeneration failure: sulfur and aromatic soot precursors. The comparative fuel parameters are summarized in the table below.

Although polyoxymethylene dimethyl ethers are not currently included in EN 590:2022 and their use in Stage V nonroad machinery remains subject to equipment-specific approval, published combustion studies indicate that oxygenated methyl ethers with high cetane number can reduce soot formation dramatically while lowering the carbon reactivity threshold for oxidation. Polyoxymethylene dimethyl ethers, particularly oligomers in the PODE₃–PODE₅ range, contain no carbon-carbon bonds, which suppresses soot precursor formation in fuel-rich zones. The resulting particulate matter consists of smaller aggregates with higher surface oxygen content. Such soot oxidizes more readily by NO₂-assisted pathways at filter temperatures below 350 °C. The absence of sulfur and aromatic species in these fuels would also preserve DOC NO oxidation activity. However, the physical properties of polyoxymethylene dimethyl ethers introduce compatibility risks. Their density, viscosity, and boiling range fall outside EN 590:2022 limits, and their lower lubricity may increase high-pressure pump wear. Sealing materials in common-rail systems may also require fluorocarbon or other resistant compounds because of the solvent character of oxygenated ethers. In non-urea Stage V operation, any fuel that reduces soot mass while maintaining NO₂ availability can extend passive regeneration intervals, but the equipment must be recalibrated for the lower volumetric energy content and the changed injection timing. Published data for polyoxymethylene dimethyl ether use in production Stage V nonroad machinery is limited; most available results derive from single-cylinder research engines and heavy-duty bench tests. The applicability of those results to transient nonroad load cycles should be treated with caution, particularly where fuel system warranty and elastomer compatibility are concerned.

Alongside fuel chemistry, lubricant-derived ash must be considered in any non-urea Stage V soot oxidation evaluation because the diesel particulate filter cannot distinguish between fuel-borne ash and lubricant-borne ash. Engine oil consumption is a normal operating variable, and the metallic compounds in the oil accumulate in the filter as sulfated ash, phosphorus, and sulfur. Even when fuel ash measured by ISO 6245 remains below the specification limit, the combined ash load from lubricant and fuel can occupy a significant fraction of the filter volume after several thousand operating hours. This ash layer reduces the effective filtration area and restricts access of NO₂ to the soot cake. In Stage V engines without urea, passive soot oxidation is more sensitive to this ash accumulation because there is no downstream SCR to absorb NO₂ variability. The use of low-SAPS engine oils meeting ACEA E9, API CK-4, or manufacturer-specific low-ash approvals is therefore inseparable from fuel selection when the objective is stable passive regeneration. At the same operational boundary, fuels with high sulfur or high FAME degradation products can accelerate lubricant degradation and increase oil consumption, creating a feedback loop that further raises ash loading. The filter differential pressure transducer provides the primary production-scale signal of this degradation, but it cannot differentiate between soot, sulfate, and ash. Field experience from nonroad deployments indicates that backpressure-based regeneration triggers may occur more frequently when the fuel has borderline sulfur or the lubricant ash exceeds the engine oil specification. This operational boundary is particularly important for low-load machines that never reach sufficient exhaust temperature for thermal soot oxidation. Without urea, the only available low-temperature oxidation path is NO₂-assisted carbon removal; any fuel property or ash accumulation that suppresses NO₂ availability or masks the catalytic surface will reduce that path until active regeneration or mechanical cleaning becomes necessary.