4.3.2. System Efficiency for Methane and Propane Combustion

Taking into account Equations (4)–(6), the efficiency of the engine was determined at each operating point when the prechamber was fed with methane and propane (Figure 14). It was found earlier that the operating conditions at λ = 1.8 were not fully acceptable. Due to stable operating conditions, the highest engine efficiency was observed at λ = 1.5 when the prechamber was fueled with methane. The values of ηi reach more than 40% at a low fuel initial dose. Slightly lower values were observed at this operating point when PC was fueled with propane (η<sup>i</sup> = 39.3%). The combustion of mixtures with λ = 1.3 results in an efficiency slightly lower than at λ = 1.5, but higher than at λ = 1.8, at low doses of fuel delivered to the prechamber.

**Figure 14.** Indicated engine efficiency related to the value of energy supplied to the prechamber during the combustion of methane and propane.

Prechamber methane combustion is more beneficial at small fuel doses to PC and at λ = 1.3 and 1.5 in the range up to 30 J of energy in PC. The areas of increased engine efficiency for propane combustion in a two-stage system cannot be clearly identified.

Based on the above relationships, interpolated maps of the indicated engine efficiency fueled by propane and methane to PC were determined (Figure 15). As can be seen from the data presented, there are areas of higher efficiency when burning methane in PC than when burning propane. The combustion of methane in the PC results in higher efficiency for both small and large doses of this fuel fed to the PC. The combustion of small doses of propane in the PC at λ = 1.8 results in combustion efficiency being rapidly reduced. Similar negative engine operating conditions were noted when burning methane (no measuring point).

λ λ λ

λ λ λ

**Figure 15.** Indicated engine efficiency maps related to the energy contained in the fuel dose to the prechamber: (**a**) propane; (**b**) methane.

## 4.3.3. Analysis of Emission Indexes

During engine operation, the concentrations of carbon monoxide CO, hydrocarbons HC, and nitrogen oxide NO were analyzed, which were then converted into specific emissions relative to the power generated by the engine, and the results are shown in Figures 16–18. The main factor affecting emissions is the lambda excess air factor, and the trends obtained are consistent with the results presented in another paper [29].

**Figure 16.** Carbon monoxide emissions at different values of energy supplied to the prechamber: (**a**) λ = 1.3; (**b**) λ = 1.5; and (**c**) λ = 1.8 (the arrows indicate the trend).

**Figure 17.** Hydrocarbon emissions at different values of energy supplied to the prechamber: (**a**) λ = 1.3; (**b**) λ = 1.5; and (**c**) λ = 1.8 (the arrows indicate the trend).

**Figure 18.** Nitrogen oxide emissions at different values of energy supplied to the prechamber: (**a**) λ = 1.3; (**b**) λ = 1.5; and (**c**) λ = 1.8 (the arrows indicate the trend).

The specific carbon monoxide CO emissions generated due to incomplete combustion, among other factors, are shown in Figure 16. For charges with an excess air ratio of 1.3, regardless of the amount of fuel delivered to the ignition chamber, lower emissions were generated by injecting propane into the PC. The largest differences of 1 g/kWh were achieved for the minimum energy in the PC. This may be due to more intensive ignition processes of the main charge determined by the flow of charge between chambers when combustion starts in the PC. Subsequently, increasing the charge dilution, i.e., λ = 1.5, reverses the trend of lower CO for propane as the fuel initiating combustion except for the two smallest values of the share of energy delivered to the PC. For ultra-lean charges of λ = 1.8, emissions for the single-fuel mode were more or less constant regardless of the amount of fuel delivered to the PC, which correlates with the stability of engine operation in this area. Using propane for lower fuel doses to the PC where the engine operated unstably resulted in higher emissions, which decreased sequentially as the proportion of the dose to the PC increased.

The next step analyses hydrocarbon emissions from unburned fuel and lubricating oil (Figure 17). Increasing the proportion of air in the mixture promotes increased HC emissions due to the deterioration of the combustion process and the incompletion of the flame in all areas of the combustion chamber. In the case of excess air ratio λ = 1.3 and λ = 1.5, emissions increase as the proportion of fuel to PC increases, i.e., the main charge becomes leaner. In this case, the primary factor determining combustion efficiency throughout the cylinder volume is the lower ignition energy requirements of the main charge rather than

the amount of energy supplied to the initial combustion chamber. Increasing the excess air ratio in the leanest area decreases the importance of the fuel dose size to PC, especially for single-fuel operation. Propane supply to PC is better for lean charges, while for ultra-lean charges, single-fuel supply is better.

Nitrogen oxide NO emissions depend mainly on peak temperatures in the cylinder during the combustion process [30]. Dilution of the charge causes a decrease in the mentioned temperature, which leads to a decrease in NO emissions with an increase in the lambda excess air ratio. In the cases analyzed, NO emissions decrease with an increase in the proportion of fuel supplied to the PC. This is due to the dilution of the main charge mostly responsible for the emission of toxic exhaust components. In most cases, better NO emission rates were obtained for the single-fuel mode (Figure 18), corresponding to higher thermal efficiency, i.e., improving the combustion process due to energy indicators.
