Boosted HCCI - Experimental Observations in a Single Cylinder Engine 2014-01-1277
Naturally aspirated Homogeneous Charge Compression Ignition (HCCI) operational window is very limited due to inherent issues with combustion harshness. Load range can be extended for HCCI operation using a combination of intake boosting and cooled EGR. Significant range extension, up to 8bar NMEP at 1000RPM, was shown to be possible using these approaches in a single cylinder engine running residual trapping HCCI with 91RON fuel with a 12:1 compression ratio.
Experimental results over the feasible speed / load range are presented in this paper for a negative valve overlap HCCI engine. Fuel efficiency advantage of HCCI was found to be around 15% at 2.62bar / 1500RPM over a comparable SI engine operating at the same compression ratio, and the benefit was reduced to about 5% (best scenario) as the load increased to 5bar at the same speed. The primary intention of this paper is to evaluate the compatibility of the presented HCCI concept in a future downsized and boosted engine for improving fuel efficiency over typical drive cycles. Since the fuel efficiency benefit of this concept was noted to be primarily limited to lower loads, it is presumed to have limited potential to significantly impact fuel economy in such a scenario.
Current testing was limited to steady state engine operation. It is anticipated that a significant amount of work is required for developing control strategies to mitigate NVH issues associated with high MRPR (Maximum Rate of Pressure Rise), as well as, to resolve potential issues with torque oscillations/emissions associated with transitions between HCCI and SI modes.