Challenges and strategies of clean combustion in automotive natural gas engines
Natural gas deserves to be a primary fuel for automotive engines because of the abundant supply in the world. As a gaseous fuel, natural gas differs from liquid petroleum and biofuels fundamentally in combustion. The primary constituent of natural gas, methane (CH4), is a very stable species that is difficult to burn completely. The activation energy for methane oxidation is much higher than most other hydrocarbon fuels. Therefore, a high flame temperature is generally required to oxidize methane completely.
Under a lower flame temperature, a longer residence time is required, unless in the presence of an oxidation promoting catalyst. The flame velocity of natural gas combustion is often slower than that of gasoline; there is a higher chance of extended combustion when the exhaust valve opens. By large, natural gas has a stronger ability of anti-knocking during combustion than gasoline or ethanol, which must be accommodated in combustion system designs. However, the knocking resistance is inconsistent, because of composition variations in supply and storage. Further, natural gas is often not sulfur free; the sulfur content does not only contribute to the particulate matter (PM) production, but also deteriorates the performance of catalytic converters.
Conventionally, the compression ignition (CI) and spark ignition (SI) engines are optimized with diesel and gasoline fuels respectively. The substitution of natural gas to the conventional liquid fuels inevitably causes compliance issuers, such as in power output, fuel efficiency, exhaust emissions, operating robustness and cost effectiveness. More importantly, when low temperature combustion is deployed to suppress the production of nitrogen oxides (NOx), the fuel and air must be tightly managed in accordance to the features of the combustion hardware, especially under transient operations; thereby any fuel property departures, e.g. from the designated conventions, inevitably affect the fuel efficiency and exhaust emissions significantly. It is crucial to develop new control strategies with improved sensors and actuators, including deploying adaptive control, enhanced ignition and flame acceleration.