At low load or temperature conditions, methane combustion is delayed because 1) at lower temperatures, it is difficult to break C-H bonds, as this reaction requires high energy (40 kJ), 2) slower attack by oxygen atoms, 3) the reactivity of methane decreases with temperature, and 4) the availability of free radicals is reduced at lower temperatures (Glassman et al., 2014). Gunea et al. (1998) suggested that the performance of M-DDF engines can be increased at lower load conditions by 1) using pilot fuel with a higher cetane number, 2) lowering the nozzle opening, 3) reducing the air intake by bypassing the excess air to provide a richer fuel-air mixture (Karim, 2003b), 4) skipping firing in multi-cylinder engines, 5) increasing the intake air temperature by preheating or by increasing the water jacket temperature, and 5) adding some other fuel during light load conditions (Khalil and Karim, 2002). At part load, only a fraction of the methane participates in combustion; the remaining methane either impinges into the cylinder wall or piston head or moves out with exhaust gases. Papagiannakis et al. reported that higher engine loads resulted in higher combustion pressure and heat release rate in M-DDF engines (Papagiannakis and Hountalas, 2003). In dual-fuel mode, the pressure and heat release rate increase with increasing load. These effects occur due to 1) an increased combustion temperature and 2) an increased mass flow rate of charge. The same trends in pressure, heat release rate, and noise (rate of pressure rise) with changes in the load in CNG-methane dual-fuel engines were reported by Abdelaal and Hegab (2012). Figure 3 shows that as engine load increases in methane-diesel dual-fuel engines, the rate of heat release increases dramatically (Tripathi et al., 2020a). This occurs due to the 1) more active participation of methane in combustion at higher loads, 2) increased oxidation rate and more rapid chemical reactions at higher loads, 3) enhanced combustion as the air-fuel ratio approaches a stoichiometric level at higher loads, and 4) increased numbers of free radicals at higher loads.

The supplementation of methane fuel in diesel engines increases the cycle-to-cycle variation in measured combustion characteristics. This problem can be easily solved using the exhaust gas recirculation technique. In this technique, some part of the exhaust gas is re-circulated with the intake air, which reduces the specific heat capacity of the intake mixture and results in a lower combustion pressure, heat release rate, and NO_x emissions. Abdelaal and Hegab reported reduced combustion pressure, heat release rate, and noise level (pressure rise rate) and an increased ignition delay period with increasing EGR (Figure 4) (Abdelaal and Hegab, 2012). These effects occurred because of 1) the replacement of oxygen with EGR gases (carbon dioxide and water) in the combustion chamber, which adversely affected the combustion process, 2) increased ignition delay by EGR gases (Karim et al., 1989), 3) a decreased charge temperature due to the mixing of air with EGR gases, 4) a reduction of the partial pressure of oxygen by EGR gases, 5) an increase in the specific heat capacity of the cylinder charges by EGR. Since EGR reduces the combustion pressure, the rate of pressure rise also decreases with increased EGR levels. Thus, combustion noise, which is highly dependent on the rate of pressure rise with respect to the crank angle, is also reduced. Higher EGR levels cause unstable combustion in natural gas-diesel dual-fuel engines (Chen et al., 2020). Tomita et al. focused on moderate utilization of EGR levels for effective combustion (Tomita et al., 2002).

Papagiannakis and Hountalas (2003) investigate the effects of varying natural gas ratios on combustion pressure, heat release rate, and ignition delay in natural gas-diesel dual-fuel engines. They reported an increased natural gas supplementation ratio decreased the peak pressure and heat release rate. An increase in natural gas ratio from 0 to 85% resulted in reductions in the peak pressure of 10 and 17.93% at loads of 40 and 80%, respectively. Natural gas burns slowly and most of its fraction remains unburned during the early part of combustion, resulting in reduced combustion pressure and heat release rate. The ignition delay period and combustion duration also increase with increased natural gas share. Figure 3 demonstrated the reduced rate of heat released with increasing methane energy share in diesel engines (Tripathi et al., 2020a). This occurs due to the replacement of oxygen with methane, which causes the incomplete combustion of alkanes. As the methane share increases, the combustion pressure of methane-diesel dual-fuel engines decreases (Figure 5) (Tripathi et al., 2020a). With increased methane share, more oxygen content is replaced in the intake manifold, resulting in the incomplete oxidation of methane and other hydrocarbons in the engine cylinders. As the methane content increases in the engine cylinders, a lower temperature is achieved after the compression stroke due to the higher specific heat capacity of methane, which makes it difficult for methane to achieve high auto-ignition temperatures compared to those in diesel fuel, which negatively affects combustion. Higher methane content in the engine cylinders quenches the flame during combustion as methane has a lower flame propagating speed, which results in incomplete combustion. Higher methane energy shares reduce the energy share of diesel, which acts as multiple ignition sources for methane in the engine cylinders. The difficulty in the ignition of methane reduces the combustion pressure and heat release rate. At higher loads, the MDDF engine characteristics are more sensitive to changes in methane energy share as methane rapidly participates in combustion at higher temperatures. Hence, at higher loads, methane supplementation results in higher combustion pressures and heat release rates compared to those at lower loads.

Diesel injection plays a crucial role in the combustion of diesel fuel. Yousefi et al. investigated the effects of diesel injection strategies in NG-DDF engines (Yousefi et al., 2019). They reported that advancing the diesel injection timing increased the maximum combustion pressure. Advanced diesel injection timing enhances the premixing process, resulting in an earlier start of combustion and a reduced ignition delay period. The earlier combustion start increases the combustion pressure and heat release rate for MDDF combustion. Figure 6 shows the effects of diesel injection strategies on the combustion characteristics in natural gas-diesel dual-fuel engines (Yousefi et al., 2018). As the first start of injection timing (SOI1) is advanced from 28° BTDC, the initial start of combustion (SOC) is also advanced. The SOC is advanced due to the enhanced premixing of natural gas and air. Further increases in SOI1 timing retarded the SOC due to the lower temperature inside the engine cylinder. The variation in maximum pressure rise rate (MPRR) showed a trend like that for the SOC variation curve.

In M-DDF engines, methane is sucked and compressed with air. The temperature achieved at the end of the compression of air-methane mixtures is less than that achieved after the compression of pure air. This is because of the high specific heat capacity of methane compared to that of air. Thus, methane enrichment in diesel engines results in slower combustion and lower in-cylinder pressure and HRR compared to those for pure diesel operation. Generally, HRR vs. the crank angle curve of diesel fuel shows four combustion phases; namely, the ignition delay period followed by the premix combustion and then controlled combustion periods, and finally the combustion phase. In the case of diesel fuel, the mixing-controlled combustion phase has a long combustion duration, during which diesel fuel is mainly burned. In the case of dual-fuel combustion, methane enrichment increases the premixed fuel mixture, which burns suddenly and fully or partially skips the mixing-controlled combustion phase depending on the enrichment (Wei and Geng, 2016). The peak of the HRR curve of MDDF engines was lower than that for pure diesel operation, as methane burns and releases heat slowly. Some studies have also reported the opposite trend for pressure and HRR with methane enrichment, as discussed earlier. These studies reported that methane enrichment in diesel fuel resulted in higher heat release rates due to increased OH radical formation, as well as in increased ignition delay period (Zang et al., 2016; Wei et al., 2018). In the case of dual-fuel combustion, combustion mainly occurred in the post-combustion phase, which explained the higher exhaust temperature. MDDF engines have an extended ignition delay period as methane supplementation alters the amount of free radicals and the mixing process, which increases the physical delay and suppresses the auto-ignition temperature of diesel (Karim et al., 1989). Methane supplementation in diesel engines has the start of combustion and extended the ignition delay, which shifted combustion overall towards the expansion stroke that resulted in reduced brake power output. Magno et al. (2015) reported that cases with combustion and ignition delay are mainly controlled by diesel injection strategies and are independent of the injected amounts of methane. Combustion noise [pressure rise rate] increases with load due to the increase in ignition delay, which accumulates more fuel that burns suddenly and increases pressure for increased rates as well. Lower engine speed results in lower cylinder pressures and HRR due to lower in-cylinder temperature and more time for the combustion process. Table 5 summarizes combustion characteristics, as well as the results of the comparisons between the combustion characteristics of diesel engines and the methane-diesel dual-fuel engine. The results studies regarding the addition of methane in diesel engines are controversial. While most studies reported decreased peak pressure and HRR with methane addition, while a few studies reported the opposite. However, there is no disagreement in the literature regarding the increase in ignition delay with methane supplementation. Table 5 describes the effects of the variation of parameters on the combustion characteristics of MDDF engines. Most studies showed that an increased load results in increased peak pressure, HRR, CD, and combustion noise. In contrast, increased speed, EGR level, and methane energy share decrease the peak pressure, HRR, CD, and noise.

Abdelaal and Hegab investigated the effects of load on the performance of CNG-diesel dual-fuel engine. They reported an increase in engine load due to the BTE of CNG-diesel dual-fuel engines (Abdelaal and Hegab, 2012). The increase in load results in a higher combustion temperature, which stimulates methane to more actively participate in combustion and contribute to brake power, causing higher BTE at higher loads. Methane requires a higher compression ratio for auto-ignition (29:1) compared to that of diesel (15:1). Thus, it can be more highly compressed without ignition, resulting in increased thermal efficiency (Fraser et al., 1991). Papagiannakis et al. experimentally assessed the effect of load on BSFC at 40, 60, and 80% of the engine load for natural gas-diesel dual-fuel engines. They reported that at part load, the utilization of gaseous fuel was poor due to a lower combustion temperature, which also contributed to a lower brake power (BP) and higher BSFC (Papagiannakis and Hountalas, 2003). For MDDF engines, as engine load increases, the BTE is increased for each level of methane supplementation (Figure 7). At higher loads, combustion temperatures increased, which helped methane to achieve its high auto-ignition temperature. High combustion temperatures at higher loads also help in the oxidation of methane and other hydrocarbons (Tripathi et al., 2020b).

It is important to study the effect of EGR on the performance characteristics of M-DDF engines as it directly affects engine BTE. Abdelaal and Hegab examined the effect of EGR on BTE and the equivalence ratio of methane-diesel dual-fuel engines (Figure 8) (Abdelaal and Hegab, 2012). They reported an increase in EGR level up to 20% increased the BTE and reduced the equivalence ratio at loads <70%. In contrast, increased EGR levels up to 20% decreased the BTE and increased the equivalence ratio at loads >80%. At lower loads, the BTE increased with increased EGR due to the re-combustion of EGR gases, which contribute to higher power output and increased BTE. These partially hot EGR gases act as a preheater for intake air that enhances the BTE. The number of active radicals increased in the combustion chamber due to increased EGR level, which enhanced the BTE at lower and medium loads. At higher loads, BTE decreased with increased EGR due to the expansion of EGR gases at high temperatures, which replaced the oxygen content and reduced the volumetric efficiency, resulting in a reduced BTE. At higher loads, more carbon dioxide was present in the combustion chamber, which increased the specific heat capacity for intake charge to suppress combustion and reduce the BTE. At lower loads, increased EGR level reduced fuel consumption due to the presence of highly active hydrocarbons and radicals, which decreased the equivalence ratio. EGR replaced a significant amount of air at higher loads, resulting in a higher equivalence ratio.

Belgiorno et al. (2018) examined the effect of different methane energy shares on the thermal efficiency of MDDF engines at different pilot fuel quantities. They reported that increased methane injection resulted in increased and decreased thermal efficiency at higher and lower loads, respectively. due to the more active participation of methane combustion at higher load conditions. Tripathi et al. studied the effect of methane energy share on heat balance sheets in MDDF engines (Tripathi et al., 2020b), reporting reduced energy equivalent to brake power output, heat absorbed by exhaust gases, and heat absorbed by cooling water with increases in MES from 0 to 75%. These findings occurred due to the incomplete combustion of methane, which does not contribute to BP and the heat absorbed by exhaust gas and cooling water. In contrast, the unaccounted heat loss increased at different engine loads with increased methane energy share. The unaccounted loss increased because most of the methane burns after the combustion stage due to extended SOC timing, prolonged ignition delay, and slow burning speed of methane. Moreover, increased methane energy share resulted in increased heat removed by the exhaust gases. As the methane gas supplementation ratio increased in diesel engines, the brake-specific energy consumption (BSEC) increased for each load condition (Figure 9) (Tripathi et al., 2020b). The BSEC is reciprocal of the BTE. As the BTE decreases with methane energy share (Figure 7), the BSEC increased with MES.

Yousefi et al. (2019) investigated the effects of diesel injection strategies on the performance characteristics of NG-DDF engines, reporting that advancing the diesel injection timing enhanced the premixing of air-NG, which increased the thermal efficiency. Cameretti et al. explored the effect of diesel injection timing on the global efficiency of a natural gas-diesel dual-fuel engine (Cameretti et al., 2016). They observed that the global efficiency first increased and then decreased as the diesel injection timing was advanced. The global efficiency of natural gas-diesel engines peaked at around 35.5% at the start of the pilot timing, corresponding to 32.5° before the top dead center. Furthermore, advancing the injection timing decreased the global efficiency of natural gas-diesel dual-fuel engines.

The addition of methane in diesel engines through the air intake port replaces the oxygen content that reduces the volumetric efficiency and power output. Diesel injection techniques are effective in enhancing engine performance. The BTE of MDDF engines is reduced in part-load conditions. The BTE decreases because the lean mixture of methane-air at low loads prevents the diesel fuel from combusting; thus, a major portion of fuel remains unburned and is emitted as exhaust. In contrast, at higher engine loads, the BTE of MDDF engines was enhanced. At higher loads, the flame becomes more stable due to higher combustion and boundary temperatures that result in increased brake power output and brake thermal efficiency. In the case of methane-enriched diesel combustion, the coefficient of variance (COV) for the indicated mean effective pressure (IMEP) increases. Since existing CI engines were manufactured for single-fuel rather than dual-fuel combustion, MDDF engines have higher COV values for maximum pressure and indicated mean effective pressures. In the case of natural aspiration, the methane induction is not the same in each cycle because different atmospheric and operating conditions and resistance offered by frictional pathways and valve hindrance lead to higher COV values for IMEP. EGR helps address cycle-to-cycle variations in MDDF engines. The COV for IMEP is higher at part-load conditions compared to those at higher loads. At higher loads, higher combustion temperature and higher intake of premixed charge result in a compact combustion duration that reduces the COV for IMEP. The BSFC is reduced at higher loads due to better fuel utilization at high temperatures. Table 6 describes the effects of methane addition on the performance characteristics of methane-diesel dual-fuel engines. This table also shows the results of the comparisons of performance characteristics between diesel and MDDF engines, demonstrating the disagreement in the literature regarding the effect of methane addition in diesel engines on engine BTE and BSFC. Most of the literature suggests that methane addition in diesel engines results in reduced BTE and increased BSFC. However, some studies reported the opposite trends. Methane supplementation reduces volumetric efficiency. Increased loads result in increased BTE and reduced BSFC, while increased engine speed and MES result in increased BSFC.

The addition of methane to diesel fuel in dual-fuel mode reduces the in-cylinder temperature (Hountalas and Papagiannakis, 2000), which directly reduces NO_x formation (Mustafi and Raine, 2008). Methane has slow flame development and flame propagation speed, which result in higher HC and CO emissions (Mustafi and Raine, 2008). The addition of methane at different loads and speeds also differently affects engine emissions (Iorio et al., 2017). Cozzolini et al. (2013) analyzed the emissions from M-DDF engines at different speeds and loads in European stationary cycle (ESC) modes. They observed that at low speed and load (torque), the M-DDF engine showed lower NO_x emissions and particulate matter concentrations and higher CO and non-methane HC emissions compared to those of conventional diesel fuel operation. In contrast, at high speed and load, the comparative emission trends reverted. Moreover, the CO₂ emission was lower in the methane-diesel dual-fuel engine in ESC modes, which will help reduce global warming. The unburned methane from diesel-methane dual-fuel engines was higher at low load and speed conditions compared to those at higher load and speed conditions.

The main problem of compression ignition engines is particulate matter emission. Higher particulate formation occurs due to high carbon-to-hydrogen ratios in conventional CI engine fuels and fuel-rich packet formation inside the engine combustion chamber. The addition of methane to CI engines reduces the PM emission as it has lower amounts of carbon and hydrogen atoms compared to those in conventional fuel. PM emission is also reduced with methane supplementation owning to the physical state of gaseous methane fuel, as it improves the in-cylinder fuel-air mixing compared to conventional liquid diesel fuel. Methane expands widely in the engine cylinders. This enhances the mixing and reduces the chances of rich fuel packet formation, as well as reducing PM emissions. Boretti (2019) reported that quick vaporization of CNG resulted in drastic PM emissions. Lounici et al. (2014b) investigated biogas-diesel dual-fuel engines, reporting decreased PM emission compared to pure diesel operation. The authors suggested that the lower combustion efficiency and gaseous nature of methane gas were the main causes of the reduced particulate emission from biogas-DDF engines. As the methane supplementation ratio increases in diesel engines, the particulate concentration decreases (Figure 12) (Tripathi et al., 2020b). PM emission is reduced with methane energy supplementation due to reduced carbon-to-carbon and carbon-to-hydrogen bonds. Moreover, increased engine load increases particulate matter emission (Tripathi et al., 2020b). PM formation is reduced at increased loads as higher temperatures oxidize more hydrocarbon species and reduce PM emissions.

Table 8 summarizes the particulate matter emissions of methane diesel dual-fuel engines and the comparisons of particulate matter emissions between diesel and MDDF engines. The effects of methane supplementation on the PM emissions from the engines remain controversial. While most studies reported reduced PM emissions in MDDF engines compared to diesel engines (Gunea et al., 1998; Liu et al., 2003a; Wannatong et al., 2007; Zang et al., 2016), several studies reported increased PM emissions from MDDF engines compared to those in diesel engines (Belgiorno et al., 2018). Generally, the addition of fuel-borne catalysts to CI engines is the preferred choice for reducing PM emission; however, early injection in methane-diesel engines is also an effective technique for reducing PM emissions (Shukla et al., 2021; Tripathi et al., 2022). This technique is shown in Table 8, which also describes the effect of parameter variation on PM emission in MDDF engines. Increased load or speed in MDDF engines results in increased PM emissions, while increased MES reduces PM emissions.