Development of high BMEP natural gas engine with knock mitigation

The stringent emission norms over the past few years have driven the need to use low carbon fuels and after treatment technology. Natural gas is a suitable alternative to diesel for heavy duty engines with applications in power generation and transportation sectors. Stoichiometric combustion mode offer the advantages of complete combustion and low carbon dioxide emissions. The combination of turbocharging and cooled exhaust gas recirculation (EGR) technology enhances the power density along with reduced exhaust emissions simultaneously. However, there are several constraints in the operation of natural gas spark ignition engine such as exhaust gas temperature (EGT) limit of 750 °C, sufficient before turbine (BT) pressure for EGR drivability, boost pressure (2.5 bar), peak cylinder pressure limit and knocking. These limits may restrict the brake mean effective pressure (BMEP) of the engine. In the present study, tests were conducted on a V12, 24 litre, dedicated heavy duty natural gas fuelled spark ignition engine (~ 600 HP rated power) with different turbocharger configurations to achieve the target BMEP (~ 16 bar) without abnormal combustion. It was found that the turbine and compressor trim significantly affect BT pressure and boost pressure, respectively. Higher turbine trim increases BT pressure which restricts exhaust gases to flow out of combustion chamber leading to reduced scavenging and excess hot residual gases in the cylinder. Although higher BT pressure increases cooled EGR rate, the influence of high hot residual gas is more than EGR which results in knock. The increase in compressor trim reduces the boost pressure which decreases the pressure and temperature at intake valve closing (IVC). This reduces the possibility of engine knock. Thus, decrease of turbine trim and increase of compressor trim eliminates knock. This study would be useful in the development of heavy duty natural gas spark ignition engines having high BMEP.