Effect of Hydrogen Addition on Natural Gas HCCI Combustion

Natural gas has a high auto-ignition temperature, requiring high compression ratios and/or intake charge heating to achieve HCCI (homogeneous charge compression ignition) engine operation. Previous work by the authors has shown that hydrogen addition improves combustion stability in various difficult combustion conditions. It is shown here that hydrogen, together with residual gas trapping, helps also in lowering the intake temperature required for HCCI. It has been argued in literature that the addition of hydrogen advances the start of combustion in the cylinder. This would translate into the lowering of the minimum intake temperature required for auto-ignition to occur during the compression stroke. The experimental results of this work show that, with hydrogen replacing part of the fuel, a decrease in intake air temperature requirement is observed for a range of engine loads, with larger reductions in temperature noted at lower loads. It is also shown that the low NOx emissions and high rates of heat release, typical for HCCI, are retained with hydrogen-assisted operation, especially at low engine loads. A practical possibility of producing the necessary hydrogen in a fuel reformer fitted in the exhaust gas recirculation system is illustrated for one engine condition.