Optimization and Performance Evaluation of Diesel Oxidation Catalysts for Methane Removal in Dual-Fuel Diesel–CNG Engines

Abstract

Compressed natural gas (CNG) in dual-fuel diesel engines offers environmental benefits but significantly increases unburned methane (CH₄) emissions, especially at low engine loads. This study investigates the effectiveness of different catalytic converters in methane oxidation under transient test conditions (WHTC). Three types of catalysts (Pt-, Rh-, and Pd-based) were evaluated using a combined approach of empirical engine bench tests and mathematical modelling. The results showed that, under actual exhaust gas temperature conditions, the average methane conversion efficiencies were 3.7% for Pt, 17.7% for Rh, and 31.3% for Pd catalysts. Increasing the exhaust gas temperature by 50% improved the conversion efficiencies to 7.3%, 51.8%, and 69.2%, respectively. Despite this enhancement, none of the catalysts reached the 90% efficiency threshold required to increase the CNG content of the fuel beyond 6% without exceeding emission limits. The results highlight the need for high-activity Pd-based catalysts and optimised thermal management strategies to enable the broader adoption of dual-fuel engines, while complying with Euro VI standards.

1. Introduction

Internal combustion engines are recognised as a source of emissions of harmful chemical compounds that threaten human health and life, as well as the natural environment. For this reason, international environmental protection organisations are forcing engine manufacturers to meet increasingly stringent regulations regarding the emission of harmful exhaust gases. This is also the reason why governmental and non-governmental agencies exert pressure on diesel engine manufacturers to further improve this type of power units [1,2,3].

The European Green Deal has set a target of achieving climate neutrality by the 2050 and an ambitious interim target of reducing net greenhouse gas (GHG) emissions by at least 55% by the 2030 compared to 1990 levels. This is in line with the EU’s commitment to global climate action according to the Paris Agreement [4].

The desire to slow down climate changes caused by greenhouse gas emissions has led to increased interest in alternative fuels and powertrains with respect to diesel oil and gasoline. One such fuel is CNG. In their publications, authors [4,5,6,7] have proven that, with the current fuel mix (mainly gasoline and diesel), powering CNG internal combustion engines would reduce pollutant emissions. Natural gas is a good engine fuel, and chemically, it is a hydrocarbon of the paraffin series, the molecule of which is characterised by a significantly higher mass content of hydrogen compared to gasoline and diesel oil. Therefore, the use of methane as an engine fuel, and especially biomethane produced in biogas plants, can be one of the ways to reduce CO₂ emissions from vehicles equipped with internal combustion engines.

The research issue described in this article was to investigate the process occurring during the combustion of diesel oil with the addition of CNG in an internal combustion engine, where the main problem is the increase in hydrocarbon emissions, mainly CH₄. The problem here is the high methane emissions in exhaust gases from vehicles powered by diesel oil with the addition of CNG, which results in the failure to meet exhaust emission standards and hinders this fuel’s economic and environmental use. This article describes the causes of increased hydrocarbon emissions in the exhaust gases associated with dual-fuel power supply and presents innovative ways to solve this problem.

Recent research on the optimisation of exhaust aftertreatment systems highlights the necessity of integrating chemical simulations with engine models. Kakoee et al. [8] introduced a 1D simulation-based approach that accounts for processes occurring in methane oxidation catalysts (MOC) to optimise their efficiency. Their results confirm that accurate modelling improves catalyst parameter selection and increases methane reduction efficiency. Similarly, Hunicz and Medina [9] studied HCCI engines and demonstrated that proper three-way catalyst (TWC) configurations can significantly influence emission speciation and reduction under real-world operating conditions.

CNG is mainly obtained from gas wells or removed as a by-product during crude oil extraction. The gas typically contains 80–99% methane, as well as some higher hydrocarbons and impurities [1]. A product largely equivalent to natural gas can also be produced biogenically (called “biomethane”) [10].

Natural gas is a gaseous fossil fuel that is composed of different types of gases, mainly methane—92% and ethane—3%, as well as propane, butane, pentane, and carbon dioxide [11].

Most hydrocarbon fuels (with varying compositions of carbon and hydrogen, C_aH_b) burn completely in air according to the standard stoichiometric equation, producing CO₂, water (H₂O) and nitrogen (N₂) according to Equation (1).

$$C_a{H}_b+\left(a+{\displaystyle \frac{b}{4}}\right)\times \left(O_2+3.773N_2\right)\to aC{O}_2+{\displaystyle \frac{b}{2}}{H}_{2}O+(a+{\displaystyle \frac{b}{4}})N_2$$

(1)

During the combustion process under real conditions, chemicals dissociate at high combustion temperatures and pressures, so combustion is not stoichiometric. As a result, products of absolute and complete combustion, as well as not absolute and incomplete combustion, are produced, such as nitrogen oxide (NO) and nitrogen dioxide (NO₂) (commonly referred to as NOx), CO, and CO₂, as well as HC, and excess O₂ in both combustion processes with excess fuel as well as excess air.

As an automotive fuel, natural gas has properties that make it highly suitable for use in piston engines [12]. In this case, the octane number is a measure of the fuel’s resistance to knocking combustion [1]. A higher octane number indicates a greater resistance to auto-ignition, which in turn enables the utilisation of higher compression ratios [1,13].

CNG fuel has the highest hydrogen-to-carbon ratio, which results in lower CO₂ emissions than petrol or diesel. Natural gas engines emit less CO₂, NOx, PM, and NMHC, providing environmental and health benefits [14].

One way to use methane as an engine fuel is to convert used vehicles equipped with compression-ignition engines to a dual-fuel version, in which an additional gaseous fuel in the form of methane is injected into the engine’s intake manifold. This solution is particularly attractive for fleets of older vehicles that do not meet modern emission standards, and where such systems are used, national regulations allow them to benefit from advantages only available to engines that meet the highest emission standards, such as access to ‘green’ city zones, lower taxes, or road tolls [5,15].

It is well-known that a promising approach to achieving this objective is to switch to alternative energy sources, such as compressed natural gas (CNG). The low carbon-to-hydrogen ratio of methane, which is the main component of natural gas, allows for a significant reduction in CO₂, NOx, PM, and NMHC emissions. A dual-fuel vehicle provides 30–40% greater engine efficiency, which consequently reduces fuel consumption by 25% [14].

One of the main problems with CNG engines is the reduction in unburned methane in the exhaust gases. However, the system of additional gas fuel injection into the intake manifold of a compression-ignition engine has its technical limitations related to the fact that it increases hydrocarbon emissions, in particular CH₄ in the exhaust gases, which is a critical factor for selecting the dose of injected gas. Material imperfections and pitting in the combustion chamber are an inevitable source of unburned methane. It is necessary to additionally oxidize such methane emissions using an exhaust gas aftertreatment system, because methane is a potent greenhouse gas, and the raw emissions are usually too high under the latest pollution regulations [12].

In order to better flush the combustion chamber in a compression-ignition engine at the end of the exhaust stroke and at the beginning of the intake phase, it is common practice to open the exhaust and intake valves together to allow the fresh charge to better purge the cylinder of exhaust residues. In a classic SI engine, the flushing agent is a fresh charge in the form of induction air in a naturally aspirated engine or forced in by a turbocharger in a supercharged engine. In a dual-fuel engine, where additional gaseous fuel is injected into the engine intake manifold, the factor flowing into the cylinder is no longer pure air, but air mixed with a dose of injected gas (methane). As a result, part of the charge enters the exhaust system at the point where the intake and exhaust valves open together, increasing the emission of unburnt hydrocarbons, mainly CH₄.

The mass of methane entering the exhaust system depends on a number of engine operating parameters. The most important of them are the time the intake and exhaust valves open together, the pressure in the intake manifold, and the temperature in the combustion chamber. In the dynamic WHTC test, in which pollutant emissions are measured and where we are dealing with rapid changes in the above engine operating parameters caused by continuous changes in engine speed and load, the impact of these parameters on CH₄ emissions changes rapidly. The authors’ empirical research shows that methane leakage to the exhaust system in the WHTC test is of the order of 3% of the mass of methane injected into the intake system. The more methane is injected into the intake system, the more methane will flow to the engine exhaust system.

The exhaust gas aftertreatment system of a modern compression-ignition engine is usually equipped with several independent systems designed to selectively reduce nitrogen oxides, particulate matter, ammonia, carbon monoxide, and hydrocarbons. The latter is usually the so-called DOC (diesel oxidation catalyst), which is designed for the afterburning of hydrocarbons typical of diesel fuel. According to literature data [6,16], DOC is reliable in operation, which is advantageous from an economic and environmental point of view. Investigating the possibility of modifying the characteristics of DOCs to enable them to be used for exhaust gas purification by adding methane will have a positive impact on their further use as exhaust gas purification systems in dual-fuel engines.

As mentioned earlier, natural gas is a mixture of hydrocarbon gases composed mainly of methane, which is becoming increasingly popular as an alternative fuel for vehicles and as a source of energy. Natural gas for automotive applications has a number of environmental advantages, including lower PM and NOx emissions compared to diesel oil and petrol. However, natural gas vehicles can emit unburned methane, which is a potent greenhouse gas with 21 times the global warming potential of CO₂. Therefore, catalytic methane oxidation has received much attention for the effective removal of methane from vehicle exhaust gases [17].

The DOC (diesel oxidation catalyst) oxidation converter is commonly used in exhaust gas aftertreatment systems of diesel engines as a basic solution for reducing HC and CO emissions. This converter can achieve conversion efficiencies in excess of 90% at exhaust temperatures above 190 °C. An additional advantage of DOC is its passive, continuous operation, which does not require interference in the operation of the combustion engine. The DOC converter operates in a large range of excess air in the combustion mixture, so its operating range covers all diesel engine operating points. The active layer of the converter contains platinum (Pt) and palladium (Pd) as catalysts for the oxidation reactions that occur according to Equations (2)–(4) [13,18,19]:

$$HC+O_2\to C{O}_2+H_{2}O$$

(2)

$$2CO+O_2\to 2C{O}_2$$

(3)

$$2NO+O_2\to 2N{O}_2$$

(4)

In the case of methane, the effectiveness of DOC turns out to be close to zero. This means that the original exhaust gas treatment system of a diesel engine practically does not work when methane appears in the exhaust gases. Therefore, we are dealing here with a case where the concentration of CH₄ in untreated and purified exhaust gases is practically the same. This condition means that the maximum share of the injected methane dose in relation to the diesel fuel dose may be around 2–3%. When this value is exceeded, the permissible value of CH₄ emissions in exhaust gases is exceeded [20,21,22].

The possibility of using an addition of 2–3% methane in the adapted dual-fuel engine defeats the economic sense of the entire project and casts doubt on its environmental sense. To ensure the possibility of using a higher proportion of methane, consideration should be given to measures that effectively reduce methane emissions. In engine practice, this requires at least the use of additional catalytic converters designed for this type of fuel and possible other technical measures and an appropriate gas fuel dosing strategy.

Methane, due to its high activation energy, is a difficult gas to utilize in the process of catalysis. It requires the use of special catalytic converters with a higher content of precious metals than is used in SI engines powered by diesel oil or in spark ignition engines running on petrol. Additionally, this type of converter may have different performance characteristics as a function of temperature and may achieve its highest efficiency at a higher temperature than in vehicles powered by diesel oil or gasoline. All this means that to combust methane, it is necessary to use catalysts of appropriate quality or ensure a high temperature of the oxidation process [23].

Recent studies confirm that Pd-based catalysts, while effective at higher temperatures, still face significant limitations in methane conversion below 400 °C, which remains a challenge for low-load engine operation in urban cycles [13,17,21].

The study uses a research methodology that involves modelling the process of methane exhaust gas purification based on data obtained from engine tests.

To ensure both practical relevance and scientific rigour, the study combines empirical engine test data with a polynomial approximation model of catalyst efficiency, enabling the evaluation of real-world emission behaviour under transient conditions.

This approach quantifies catalytic converter effectiveness under varying thermal and load conditions and provides insight into the feasibility of dual-fuel operation with CNG.

A methodology described below can be used to identify issues relating to the appropriate selection of an additional methane exhaust gas treatment system and to formulate guidelines for ensuring that the adapted engine meets the requirements of the Euro VI emission standard.

2. Materials and Methods

Catalytic converters containing precious metals, such as Pt, Rh, and Pd are covered with ceramic or metal substrates. There are many studies in the literature that show research on various applications of different noble metals in the exhaust gas treatment systems of vehicles powered by natural gas [24,25]. Generally, Rh reduces NOx, while Pd or Pt oxidises CO and CHx emissions. The interest in Pd in TWC converters has increased due to the increased use of CNG as a fuel. In the exhaust gas aftertreatment systems of stoichiometric CNG engines, Pd is the most commonly used noble metal, because it has the best ability to purify CH₄ emissions [25].

A comprehensive overview of current aftertreatment technologies for lean-burn natural gas engines, including DOC, TWC, and methane oxidation catalysts, is provided by Lott et al. [3], highlighting the complexity of optimising such systems under transient conditions.

Noble metal catalysts, including Pd, Pt, Rh, and Ru, have been widely studied in the context of methane oxidation. Among them, Pd-based catalysts are the most active in methane oxidation due to their naturally high catalytic activity. The active phase of Pd forms for methane oxidation is largely dependent on the reaction temperature; PdO is the active phase for low-temperature reactions (<400 °C), and metallic Pd is active during high-temperature reactions [25].

In this study, the authors have applied a research method that involves modelling the process of methane exhaust gas purification using empirical data from an experiment in the form of tests on a real engine installed on an engine dynamometer, on which the WHTC (World Harmonised Transient Cycles) emission test was reproduced, which is used, among others, in the European regulations for the type approval of engines with regard to the emission of pollutants into the atmosphere. Mathematical modelling involved approximating with a polynomial the characteristics of three real catalytic converters made of the Al₂O₃ monolith, shown in Figure 1, and determining the efficiency factors for the methane purification of exhaust gases based on the exhaust gas temperature measured in the WHTC test. It is worth noting that only this type of system was studied, and it was the prototype for the mathematical analysis extended to include parameters of systems containing various precious metals.

Optimisation is key to improving the efficiency of internal combustion engines and their emission control systems. Kozłowski [26] defines optimisation as finding the best solution under given constraints, which is particularly relevant in the design of engine control systems. In the context of dual-fuel engines, optimisation involves balancing the fuel mixture, combustion conditions, and exhaust gas aftertreatment to achieve maximum power with minimum emissions. His work on the stabilisation of linear systems with a random horizon provides insights into dynamic system control, which can be applied to the real-time adjustment of engine parameters for optimal efficiency. Therefore, the work investigated the efficiency of the catalytic system depending on the degree of reactor heating and checked which part of the methane added with fuel escapes with the exhaust gases. This determined which operating parameters allow for the effective reduction in this gas in the exhaust gases.

The tests were carried out at a measuring station designed to measure pollutant emissions in accordance with UN Regulation No. 49/06. The engine tests were carried out at the Motor Transport Institute on an automated dynamometric measurement station

The course of characteristics presented in Figure 1 was selected based on the literature, as typical representatives of catalytic converters used in the automotive industry. The best of these catalysts using palladium (Pd) was characterised by a light-off temperature of approximately 270 °C.

The model was verified by empirical tests of the engine on an engine test bench and equipped with a catalytic converter designed for methane combustion.

3. The Course of Tests and Analysis of the Results

3.1. Modelling the Exhaust Gas Treatment System

To model the exhaust gas purification system to remove excess methane, the exhaust gas temperature curve of the tested engine in the WHTC test, shown in Figure 2, was used. Using the approximated catalyst effectiveness characteristics shown in Figure 1 and the exhaust gas temperatures shown in Figure 2, the course of the methane purification coefficient was determined, which is shown in Figure 3. The exhaust gas temperature in the WHTC test is subject to strong fluctuations. The WHTC cycle consists of three 600 s periods corresponding to urban, rural, and motorway driving, respectively.

Figure 2 clearly shows that, due to significant differences in engine loads, the exhaust gas temperature varies in each of these three phases. In the first phase (urban driving), the exhaust gas temperature oscillates below 250 °C, except for the initial part, where the impact of intensive engine heating before the start of the test is visible. During rural driving, the exhaust gas temperature increases by several tens of degrees compared to urban driving and only rises above 400 °C during motorway driving.

The described exhaust temperature course has an impact on the effectiveness of methane purification (Figure 3). The exhaust gas methane oxidation efficiency (η) follows the exhaust gas temperature. In the first 600 s, the simulated catalytic converters practically do not work. During urban driving, their efficiency increases, but the converters are still not operating for half of this part of the cycle. Only during the motorway part do the converters start to work, but the efficiency of the converter built, based on covering the monolith with a Pt layer, is unsatisfactory.

Figure 3 clearly shows that, from the characteristics presented, it can be concluded that from the characteristics presented in it, only the Pd catalyst works correctly in the area of motorway driving; in other areas, the methane oxidation efficiency is insufficient for this catalyst material and for other catalysts. The average exhaust gas purification rates determined in the entire WHTC test for individual catalytic converters are, respectively, η_Pd = 31.3%, η_Rh = 17.7%, η_Pt = 3.7%, which cannot guarantee an overly high increase in the share of injected methane in the fuel.

An analysis of Figure 3 leads to the conclusion that the low effectiveness of the catalytic converters considered is caused by the exhaust gas temperature being too low. Considering the operation principle of a spark ignition engine, in which the fuel–air mixture has a stoichiometric composition, it is characterised by a lower exhaust gas mass, which allows the energy contained in the fuel to heat the charge filling the cylinder to a higher temperature. This is particularly visible at low loads, where, for example, a spark ignition engine will have approximately five times less exhaust gas mass when idle compared to a CI engine. This observation suggests that, if a throttle was used in the air inlet of the engine tested, especially at low loads and low rotational speeds, the exhaust gas temperature would increase, which would result in an increase in the effectiveness of methane removal from the exhaust gases. Figure 4 is an attempt to estimate this phenomenon. It was prepared in a similar way to Figure 3, assuming that, as a result of certain actions (e.g., using an additional air throttle at the engine inlet), the exhaust gas temperature would increase by 50% throughout the WHTC cycle. As a result of the assumed increase in exhaust gas temperature, the efficiency of methane combustion during urban and suburban driving improved. In these fragments of the WHTC test, a clear increase in methane afterburning efficiency can be observed compared to Figure 3. The Pd and Rh converters begin to work noticeably, and in the area of motorway driving, their efficiency reaches 100%, which is the value that a catalytic converter in an engine should have. In this case, the average exhaust gas purification rates for the entire WHTC test were η_Pd = 69.2%, η_Rh = 51.8%, and η_Pt = 7.3%, i.e., as a result of increasing the exhaust gas temperature by 50%, the efficiency of catalytic reactors reached over 100%.

The influence of the exhaust gas temperature in the WHTC test on the efficiency of methane afterburning in the exhaust gas can be seen in Figure 5, which shows the distribution function of the ethane post-combustion efficiency index for the Pd catalyst. For example, it can be seen from this graph that the methane oxidation efficiency η_Pd < 50% is achieved for 71% of the WHTC test measurement points, i.e., 29% of the points have a methane oxidation efficiency greater than 50%. If the exhaust gas temperature was increased by 50%, this methane oxidation efficiency would be achieved by 30% of the test points, i.e., the rest, 70%, would have better efficiency, which is more than twice that amount.

The values of methane exhaust purification coefficients shown in

Table 1. Calculated average values of the purification coefficients of clearing exhaust gases from CH₄ in the WHTC test for individual catalytic converters.

	Measured Exhausts Temperature	Exhausts Temperature Increased by 50%
ηPt [%]	3.7	7.3
ηRh [%]	17.7	51.8
ηPd [%]	31.3	69.2

indicate the possibility of increasing the share of CNG in the fuel compared to an engine without a CH₄ purification system, i.e., equipped with the engine’s original exhaust gas purification system. The exhaust gas methane oxidation efficiency index obtained for the most effective of the analysed catalysts (η_Pd = 69.2%) is still too low and does not meet the challenges faced by engine designers. Considering that, in a dual-fuel engine, the CH₄ concentration limit in the WHTC test is 30 ppm, which occurs in an engine without a methane afterburning system and already with the share of approximately 2% CNG in the fuel, the catalytic converter with a methane after-combustion efficiency of 69.2% may allow for a safe increase in the share of methane in the fuel to approximately 3–4%, which is far from sufficient. For the addition of methane-to-diesel fuel to be economically and ecologically justified, a 10% share of CNG in the fuel should be considered as such. To accomplish such a task, one would need a system with methane afterburning efficiency of at least 90%.

Knowing the efficiency characteristics of the Pd converter (Figure 2 and Figure 3), it is possible to calculate what maximum CH₄ concentrations are permissible at each point of the WHTC test, so that the final concentration of this exhaust component in the purified exhaust gases does not exceed the permissible value, which, in this work, is assumed to be 30 ppm. In the test areas where the converter efficiency was equal to zero, the CH₄ concentration in the raw exhaust gases was assumed to be 30 ppm, while in the areas where the reactor efficiency was low, the CH₄ concentration in the exhaust gases was assumed to be 150 ppm, which would approximately correspond to the share of 10% CNG in the fuel. The results of these calculations are shown in Figure 4. The use of a Pd-type reactor, the characteristics of which are shown in Figure 1, allowed us to increase the average CH₄ concentration in the raw (unpurified) exhaust gases from 30 ppm to 52 ppm, i.e., by 70% compared to the initial concentration, which translates into the possibility of increasing the CNG share in the exhaust gases by approximately 1%. If the exhaust gas temperature was increased, as described in Figure 3, the CH₄ concentration in the raw exhaust gas could be increased to 92 ppm, which is more than three times the initial value. This would make it possible to increase the proportion of CNG in the fuel to over 6%.

An analysis of Figure 6 shows that, during the first 1200 s of the WHTC test, the selected catalytic converter does not operate effectively, which means that the concentration of CH₄ in the raw exhaust gases cannot be increased, and therefore, the share of CNG in the fuel cannot be increased.

3.2. Engine Bench Tests

The engine dynamometer bench used in the research is the AFA100 4Z4/4, manufactured by AVL List GmbH, Graz, Austria, designed to measure the performance and efficiency of engines under various conditions, along with exhaust gas analysers (Figure 7). The system has a nominal torque (Tnom) of 2801 Nm within the range of 0 to 1500 rpm and a nominal power (Pnom) of 440 kW at 1500 to 3800 rpm with a maximum speed (nmax) of 4200 rpm. The engine mode operates with a Tnom of 2521 Nm in the range of 0 to 1500 rpm and a Pnom of 360 kW between 1500 and 3800 rpm with a maximum speed of 4200 rpm. The mass moment of inertia is 4.64 kgm², and the system can handle an overload of up to 25% for one minute, with a maximum continuous duration of 15 min. Additionally, the AVL exhaust gas analysers are integrated to measure the emissions during the tests (Figure 8). The engine tested was an engine from a heavy transport vehicle—Volvo D13K460.

At the beginning of the engine bench tests, based on literature data [20] and the tests conducted by the authors, it was assumed that one of the main reasons for the low efficiency of devices burning methane in exhaust gases would be the low exhaust gas temperature. Figure 5 and Figure 6 show the distribution of the measured exhaust gas temperature in the engine tested during the reconstruction of the WHTC test cycle required by EURO VI regulations. Figure 2 shows that, during the reconstruction of the WHTC cycle, in approximately 50% of the measurement points, the exhaust gas temperature is lower than 250 °C, i.e., the value considered the temperature at the beginning of operation of an average three-function catalytic converter. It should be emphasized that the test was conducted in the standard 1800 s WHTC homologation cycle.

In connection with this, additional studies were conducted, consisting of recording measurement points at fixed values of engine operation and exhaust gas cleaning device temperature. This approach allowed us to determine at what point the excess of emitted methane occurs and helped to determine the efficiency of the system at specific temperatures.

This observation, made at the very beginning of the research, allows us to formulate the thesis that even if the tested engine was equipped with a Pd catalytic converter with the characteristics shown in Figure 1, 50% of the measurement points of the WHTC test would be outside the effective operating range of this converter, because the ignition temperature (light-off temperature), read from Figure 1, of this catalytic reactor is approximately 270 °C, which corresponds to its operation with a load of 40% in the conditions of stationary engine operation. It can, therefore, be concluded that, in order to minimise CH₄ emissions from the exhaust system of the engine tested, methane should not be injected into the engine when the load on the heated engine is less than 40%. Since the Pd catalytic converter in Figure 1 achieves full efficiency of purifying exhaust gases from methane when their temperature is higher than approximately 400 °C, this means that only when the engine load is higher than approximately 60% can a significant drop in CH₄ emissions be expected, as a result of the effective operation of the catalytic converter. A practical implementation of this should be to equip the engine CNG injection control system with an exhaust gas temperature measurement system and use this parameter in the CNG injection system algorithm to correct the doses of injected CNG, according to the exhaust gas temperature (Figure 9 and Figure 10).

In order to investigate the influence of engine load on hydrocarbon emissions (THC is the sum of NMHC and CH₄), a load characteristic was performed (Figure 11) with a constant share of methane (gas equivalence ratio—GER) in the fuel of 10%. The engine was equipped with a catalytic converter designed to reduce methane concentration, coming from a city bus powered by a spark ignition engine, fuelled by CNG, the characteristics of which were similar to the curve lying between the Rh and Pt curves in Figure 1. The concentration of hydrocarbons, especially methane, is strictly dependent on the engine load. As stated earlier, the average THC concentration corresponding to the Euro VI emission limit in the WHTC test for the tested engine is approximately 30 ppm. Figure 9 shows that, if the tested engine was not additionally fuelled with CNG, the concentration of emitted NMHC would be lower than this value. In order for the CH₄ concentration in exhaust gases to reach 30 ppm in the torque range T < 300 Nm, CNG should not be dosed at all, while a more effective catalytic converter should be used in the remaining area. The converter used gives satisfactory results only when the torque exceeds 600 Nm, when the exhaust gas temperature reaches the required value. It only starts to work effectively when the torque reaches T > 300 Nm. The catalytic converter should reduce CH₄ emissions by approximately five times.

The course of CH₄ concentration changes in Figure 11 quite faithfully reproduces the courses presented in Figure 1, except that the efficiency of the converter used in the tests is 50% at a temperature of approximately 450 °C, which turns out to be clearly insufficient to meet the Euro VI emission standards.

4. Discussion

The studies carried out indicate that the main problem with dual-fuel engines fuelled with diesel oil and CNG is excessive methane (CH₄) emissions in the exhaust gases. The results showed that standard exhaust gas treatment systems, such as DOC (diesel oxidation catalyst), are ineffective in eliminating methane. As indicated in the literature, effective methane oxidation requires catalysts containing palladium (Pd) or platinum (Pt), whose flash point (light-off temperature) is in the range of 270–400 °C [18,23]. The presented results show that, in the dynamic WHTC test, in which significant changes in exhaust gas temperature occur, the efficiency of methane exhaust gas treatment strongly depends on the engine load. In urban driving conditions, the exhaust gas temperature often does not reach the level necessary for effective methane oxidation, which leads to low conversion efficiency in the catalysts used. It is only at motorway speeds that the efficiency of the Pd catalyst increases to acceptable levels, as confirmed by the work of Monai et al. [20] and Yan et al. [22].

An attempt to increase the exhaust gas temperature by using an additional air throttle in the engine intake showed an improvement in methane oxidation efficiency. The analysis indicates that, with a 50% increase in exhaust gas temperature, the efficiency of methane purification for the Pd catalyst increased from 31.3% to 69.2%. Despite this improvement, the required level of 90% was still not achieved, which suggests the need for the further optimisation of the catalyst composition and the gas fuel dosing strategy [3,24].

Alternative approaches to improve low-temperature activity include the use of modified PdO catalysts, zeolite supports, or dopants, such as phosphorus, which have shown promising performance in recent experimental studies [13,22].

The combined empirical and modelling approach of this work provides a versatile framework for evaluating and optimising methane aftertreatment systems in dual-fuel engines, bridging the gap between bench testing and predictive simulation.

An important area for further research is the application of catalyst preheating or activation techniques to overcome low efficiencies at sub-250 °C exhaust temperatures. Several strategies have been proposed in the recent literature, including electrically heated catalysts (EHCs), which can significantly reduce light-off time and improve methane conversion during cold-start and low-load conditions. For example, electrically assisted Pd catalysts have demonstrated enhanced CH₄ conversion in hybrid vehicles with frequent thermal cycling [27].

Another technique under consideration is chemical pre-treatment or surface doping, which can modify catalyst reducibility and increase low-temperature activity. Additionally, intake throttling or exhaust insulation can serve as passive thermal management strategies to increase the catalyst bed temperature during urban operation.

Although such methods were not implemented in this study, they present promising opportunities to address the observed thermal limitations and will be considered in future work.

Recent studies indicate that an important element influencing the efficiency of exhaust gas aftertreatment systems in CNG-fuelled engines is the precise modelling of engine operating parameters and combustion conditions. Tucki et al. [28] showed that advanced computer modelling can optimise the combustion process to reduce emissions of harmful compounds, including methane. The results of these studies can help to design more effective catalysts adapted to the specific operating conditions of dual-fuel engines. In turn, the research by Rybak et al. [29] on optimising energy consumption in biogas fermentation processes emphasises the importance of the appropriate selection of operating conditions in the context of methane emissions. Although their work focuses mainly on biogas production, a similar approach can be used to optimise methane combustion in combustion engines. In addition, issues related to vehicle dynamics and road surface classification, which Surblys et al. [30] analysed, may affect combustion efficiency in real road conditions. Vehicle operating conditions, such as road surface characteristics and engine load, can affect exhaust gas temperatures and, thus, the efficiency of methane catalysts. As shown in the work of Kozłowski et al. [31] on the readiness of technical systems, the implementation of new solutions in the field of exhaust gas purification requires the consideration of the reliability and durability of catalytic systems. The optimisation of catalyst operating parameters and their long-term stability should be considered when designing new methane reduction systems.

The engine tests confirmed that, in order to ensure an adequate reduction in CH₄ emissions, CNG dosing should be limited at low engine loads, and the catalysts used in the exhaust system should have an optimised temperature characterisation. This is confirmed by Chen et al. [12] and Nau et al. [21], who indicate that the proper activation of the Pd catalyst is critical for effective methane elimination.

Additionally, recent studies have explored advanced catalyst formulations, such as Pd supported on CeO₂ and perovskite-based materials, due to their potential for lowering light-off temperatures and improved thermal stability. For example, Pd/CeO₂ catalysts exhibit high catalytic activity and durability, with strong metal–support interactions that stabilise active PdO species, even under fluctuating temperatures and wet conditions [13,19].

While these advanced materials show clear potential, the present study focused on the more commonly used Pt, Rh, and Pd catalysts, which remain the benchmark in commercial diesel aftertreatment systems. Integrating emerging catalyst technologies, such as Pd/CeO₂ or perovskites, into future work may offer significant improvements in methane conversion efficiency and system durability, particularly under low-load urban driving conditions.

5. Conclusions

Based on the research conducted, several key findings can be found:

• Standard DOC converters are ineffective in methane removal. Under real-world WHTC conditions, their efficiency was close to 0%, confirming literature observations and our empirical results (Section 3.1, Figure 3).
• Among the tested catalytic materials, Pd-based converters showed the highest performance. Their average CH₄ conversion efficiency reached 31.3% at actual exhaust temperatures, compared to 17.7% for Rh and only 3.7% for Pt (Table 1). However, even the best-performing catalyst did not reach the 90% threshold required for meaningful CNG integration.
• Exhaust gas temperature plays a critical role. Increasing temperature by 50% raised Pd converter efficiency to 69.2%, a more than twofold improvement (Figure 4). Nevertheless, this still falls short of enabling a >6% share of CNG in the fuel without exceeding emission limits.
• Our results show that a Pd-based catalytic converter with a minimum efficiency of 90% is required to meet Euro VI methane emission standards. However, under real-world conditions, even the best-performing catalyst (Pd) achieved only 69.2% efficiency, highlighting the need for the further optimisation of both catalyst design and engine thermal management.
• CNG injection must be dynamically controlled based on engine load and exhaust temperature. Engine loads below 40% result in exhaust temperatures too low for effective CH₄ oxidation, rendering methane dosing counterproductive. A Pd catalyst only achieves optimum performance at loads above ~60% (Section 3.2, Figure 9, Figure 10 and Figure 11).
• Under current technical constraints, the CNG share in dual-fuel engines is effectively capped at 3–6%. This range allows compliance with the Euro VI THC limit (30 ppm), assuming a Pd catalyst is used, and the engine operates under favourable conditions. Achieving a 10% CNG share would require at least 90% conversion efficiency, which is currently unachievable with existing hardware.

Without the further optimisation of catalyst formulations or the integration of thermal management strategies, dual-fuel diesel–CNG engines will remain limited in their ecological and economic potential due to excessive methane emissions. The presented study highlights the technical limitations of current systems and provides clear operational guidelines to mitigate CH₄ emissions.

Building on the findings of this study, future research should focus on several key areas that address the limitations identified in dual-fuel diesel–CNG engines. One priority is the development of palladium-based catalysts with improved low-temperature activity, which would enhance CH₄ conversion efficiency during urban driving conditions, where exhaust temperatures are typically low. Additionally, implementing advanced thermal management strategies, such as intake throttling or exhaust heat recovery, could effectively increase exhaust gas temperatures during low-load operation, thus improving catalyst performance.

Another important avenue is the design of adaptive CNG dosing algorithms that incorporate real-time exhaust temperature data to dynamically regulate methane injection dynamically, ensuring efficient combustion and minimal emissions under varying load conditions. In parallel, the integration of engine and aftertreatment system simulations—especially those based on transient test cycles like WHTC—would support optimising system performance under real-world driving scenarios. Finally, long-term durability studies are needed to assess catalyst degradation over time and evaluate the stability of methane oxidation efficiency in operational settings.

Together, these directions form a coherent and practical research agenda that may advance the development of cleaner, more efficient, and regulation-compliant dual-fuel powertrains.