Using Hydrogen Injection to Improve Idle Combustion and Emissions Performance at Lean Conditions of Gasoline Engines
|Gasoline and hydrogen rails and injectors installed on the intake manifolds. Credit: ACS. Click to enlarge.|
Spark-ignited engines suffer high specific fuel consumption, emissions, and cyclic variation at idle and lean conditions. Increasing levels of hydrogen enrichment of a gasoline fuel charge in a spark-ignited (SI) engine can increase engine-indicated thermal efficiency and emissions at idle and extend the lean-burn limit, according to a study by researchers from the Beijing University of Technology. A paper on their work was published online 18 August in the ACS journal Energy & Fuels.
Changwei Ji and Shuofeng Wang modified a 4-cylinder (SI) engine to permit hydrogen and gasoline to be injected into the intake ports simultaneously to realize a hybrid hydrogen-gasoline engine (HHGE). The hydrogen and gasoline flow rates were governed by a hybrid electronic control unit (HECU).
Three hydrogen volumetric fractions of 0, 3, and 6% in the intake were applied to investigate the effects of hydrogen addition on engine thermal efficiency, combustion duration, cyclic variation, and emissions at idle conditions using a fixed spark advance.
Under each hydrogen enrichment level, the engine was gradually leaned by reducing the gasoline flow rate until the coefficient of cyclic variation in indicated mean effective pressure (imep) was above 10%.
The test results showed that, with the increase of hydrogen enrichment level, engine-indicated thermal efficiency was improved, and the lean burn limit was extended. The peak in-cylinder temperature and in-cylinder temperature at exhaust valve opening decrease with the increase of excess air ratio and hydrogen blending level.
For a specified excess air ratio, HHGEs tend to get a high residual gas fraction compared with a conventional engine. The combustion duration was gradually shortened with the increase of hydrogen addition level at diluent conditions. Moreover, CO and NOx emissions were reduced at lean conditions for the HHGE. HC emissions from the HHGE were also lower than the original one at all excess air ratios.
Specifically, the researchers found that:
Under the same excess air ratio, indicated thermal efficiency of the test engine improves with the increase of hydrogen fraction. The relative excess air ratio for the maximum indicated thermal efficiency increases with the addition of hydrogen.
At idle condition, due to the high flame speed, small ignition energy of hydrogen, and the increased OH radical after hydrogen enrichment, the flame development and propagation durations are shortened by the addition of hydrogen.
Because of the shortened CA0-10 and CA10-90, COVCA0-10 and COVimep of the HHGE are lower than the original engine at the same λ. The engine lean burn limit is also extended after hydrogen enrichment.
NOx emissions at idle condition are reduced by increasing hydrogen blending fraction. When the hydrogen-gasoline mixture combusts near stoichiometric excess air ratio, HC emissions from the 6% HHGE are slightly higher than those from the 3% HHGE but are still lower than those from the original engine.
At the lean burn limit, the engine-indicated thermal efficiency is improved, whereas CO2, CO, and NOx emissions are reduced with the increase of hydrogen enrichment level. However, HC emissions at the lean operating limit are raised with the increase of hydrogen addition fraction, due to the further increased λLL, which causes lower combustion temperature and partial burning of gasoline.
Changwei Ji and Shuofeng Wang (2009) Effect of Hydrogen Addition on Idle Performance of a Spark-Ignited Gasoline Engine at Lean Conditions with a Fixed Spark Advance. Energy Fuels doi: 10.1021/ef900517t