Numerical investigation of recompression and fuel reforming in a SIDI-HCCI engine
SAE Technical Paper Series, (2007)Abstract
Homogeneous Charge Compression Ignition (HCCI) is a combustion concept which has the potential for efficiency comparable to a DI diesel engine with low NOx and soot emissions.
However, HCCI is difficult to control, especially at low speeds and loads. One way to assist with combustion control and to extend operation to low speed and loads is to close the exhaust valve before TDC of the exhaust stroke, trapping and recompressing some of the hot residual. Further advantages can be attained by injecting the fuel into this trapped, recompressed mixture, where chemical reactions occur that improve ignitability of the subsequent combustion cycle. Even further improvement in the subsequent combustion cycle can be achieved by applying a spark, leading to a spark-assisted HCCI combustion concept.
This study demonstrates some of the characteristics of a recompression-fuel injection strategy in a single-cylinder research engine with negative valve overlap, and is a first step in developing understanding of its performance.
Numerical simulations for two combustion modes (pure HCCI and spark-assisted HCCI) were carried out with a zero-dimensional, multi-zone chemistry model fully coupled with the zero-dimensional engine simulation code WAVE. Furthermore, recompression and fuel reforming effects for both operating modes are studied using the same zero-dimensional, multi-zone chemistry model partially coupled with the three-dimensional, CFD-code AC-FluX. The latter solves for detailed fluid flow mechanics in the engine and thus, provides insight into the effects of fuel distribution and mixing processes on the recompression and fuel reforming process.