Williams, D., Koci, C., and Fiveland, S., "Compression Ignition 6-Stroke Cycle Investigations," SAE Int. J. Engines 7(2):656-672, 2014, doi:10.4271/2014-01-1246.
Driven by the desire to implement low-cost, high-efficiency NOx aftertreatment systems, such as Three Way Catalysts (TWC) or Lean NOx Traps (LNT), a novel 6-Stroke engine cycle was explored to determine the feasibility of implementing such a cycle on a compression ignition engine while continuing to deliver fuel efficiency. Fundamental questions regarding the abilities and trade-offs of a 6-stroke engine cycle were investigated for near-stoichiometric and lean operation. Experiments were performed on a single-cylinder 15-liter (equivalent) research engine equipped with flexible valvetrain and fuel injection systems to allow direct comparison between 4-stroke and 6-stroke performance across multiple hardware configurations. 1-D engine simulations with predictive combustion models were used to support, iterate on, and explore the 6-stroke operation in conjunction with the experiments. Output from the experiments and simulations were then used to perform Availability and Energy balances for a thermodynamic comparison of the two cycles.Compared to 4-stroke cycle operation, the 6-stroke cycle exhibited lower PM emissions at stoichiometric operation, while higher NOx/PM emissions were observed under some lean conditions. The ratio of fuel burned in the first combustion event to the second event had a strong impact on performance, heat loss, and emissions. For some 6-stroke strategies engine breathing and airflow management were found to be challenging, and beneficial for other strategies. The thermodynamic analysis showed that under similar boundary conditions, the 6-stroke and 4-stroke engine cycles could attain very similar brake efficiencies, though the detailed availability balance breakdown could differ substantially.