Homogeneous charge compression ignition (HCCI) is a promising low-temperature combustion strategy for reducing NOx emissions and increasing efficiency in internal combustion engines. However, HCCI has no direct combustion initiator and, when achieved by reinducting or trapping residual exhaust gas with a variable valve actuation (VVA) system, becomes a dynamic process as the temperature of the residual gas couples one cycle to the next. These characteristics of residual-affected HCCI present a challenge for control engineers and a barrier to implementing HCCI in a production engine. In order to address these challenges, this paper outlines physics-based control strategies for both the VVA system and the HCCI combustion process. The results show that VVA system control can provide arbitrary valve timings on a cycle-to-cycle basis, enabling tight control of HCCI. By abstracting these valve timings further into an inducted gas composition and an effective compression ratio, model-based controllers can be developed to control simultaneously load and combustion timing in an HCCI engine.

Experiments have been reported in the literature in which the low-load limit of a retention-mode HCCI engine operating on gasoline has been significantly extended by preprocessing of the fuel during negative valve overlap. This paper presents experimental studies in which this low-load-limit extension is demonstrated and characterized using simple, singlecomponent hydrocarbon fuels with relatively well-known chemical kinetics. The model fuels were n-heptane and i-octane and this choice was made both because of the extensive work that has been undertaken to develop their chemical kinetic mechanisms and because these fuels span the range of ignitability that is likely to be of interest for HCCI engines.The experimental results reported here show that both fuels exhibit load extension to as low as 1 bar net indicated mean effective pressure when operated at high residual mass fractions, low equivalence ratios, and an appropriate choice of compression ratio (13 for n-heptane, 18 for i-octane). Near the low-load limit, combustion is stable, exhibits slightly advanced timing, has relatively low unburned hydrocarbon emissions, negligible (, 5 ppm) NO emissions, and slightly increased CO emissions (compared to higher load conditions). The indicated efficiency of low-load operation with recompression reaction is somewhat reduced, mainly due to increased heat transfer and decreased combustion efficiency.

No abstract

Residual-effected homogeneous charge compression ignition (HCCI) was investigated using a single-cylinder research engine equipped with fully-flexible variable valve actuation. Operation at elevated compression ratios was explored to determine its effect on efficiency. Results showed that efficiency is decreased significantly by advanced phasing owing to increased thermal losses. However, if combustion phasing is held fixed, elevated compression ratio operation showed little effect on efficiency. Further experiments explored the use of IVC time as a control parameter at the elevated compression ratio. Tests were conducted to determine if an efficiency benefit could be realized from increasing the compression ratio and delaying IVC to increase the extent of expansion relative to compression. No significant change in efficiency was observed, although variation in IVC timing offered significant control authority. Delayed IVC was used in conjunction with variable IVO to control independently load and phasing of HCCI at the elevated compression ratio. Tests were also conducted to assess the significance of the measured exhaust temperature on HCCI phasing. EVO was introduced as a third control parameter, along with IVO and IVC, to control independently initial mixture composition, compression work, and exhaust temperature. Results indicated that the measured exhaust temperature was not a good indicator of HCCI phasing and suggested the stabilizing role of heat transfer to the reinducted exhaust gases. Results also suggested a set of control parameters that achieve the full range of HCCI operation with minimal degrees of freedom.

No abstract