Improving the Efficiency of the Hydrogen Engine.

Abstract

Internal combustion engines fueled by hydrogen are among the most efficient means of converting chemical energy to mechanical work. The exhaust has near-zero carbon emissions and fuel efficiency exceeding fuel cells is achievable. Unfortunately, the fuel costs are high and efficient combustion of hydrogen in engines produces nitrogen oxides (NOx) that cannot be treated with conventional three-way catalysts.

This work presents the results of experiments which consider changes in hydrogen engine design and/or operating strategy to improve engine performance, consisting primarily of engine efficiency and NOx emissions. Various combustion chamber designs, compression ratios, operating parameters, and injector nozzles were tested, and the relative gains found ranged from 0 to 5%.

Three research areas were considered in greater detail to reduce NOx emissions and improve hydrogen engine efficiencies. The first effort focused on injecting liquid water into a cylinder filled with a premixed fuel-air charge. The results were compared against expectations for a conventionally operated hydrogen engine. Using this approach of direct injection of water into the cylinder, NOx emissions were reduced by up to 95%. At a threshold of 100 ppm of NOx, peak load possible increased by 17.3%.

The second research area considered injecting water into the intake air charge. With water injection into the intake air charge, the NOx emissions were reduced by 87%. At a threshold of 90 ppm of NOx, peak load possible increased by 23.9%.

Finally, experimental data were generated and analyzed for a combustion chamber with two spark plugs. An injector was designed to preferentially stratify the fuel towards the ignition sites. Results from a metal engine and an optically accessible engine are presented. Based on the metal engine data, the new cylinder head design produced a remarkable 47.7% net indicated thermal efficiency (ITE) while producing only 51 ppm of NOx. Laser induced fluorescence was used in the optically accessible engine to visualize the fuel distribution during non-firing operation. The most optimal injection conditions (based on the metal engine results) showed a fuel distribution where the equivalence ratio is approximately 0.65 near the ignition locations.