Alternate Modes Combustion Study: HCCI Fueled With Heptane and Spark Ignition Fueled With Reformer Gas

Homogenous Charge Compression Ignition (HCCI) combustion is an efficient operating mode for internal combustion engines operating at low specific power and has the further advantage of very low specific NOx emission rate. High Compression Lean Burn (HCLB) spark ignition (SI) provides a more conventional approach to achieving high engine efficiency. Specific NOx emission rates are low though not as low as with HCCI. Compared with HCCI engines, HCLB-SI engines have the advantage of direct combustion control, (through spark timing), and thus are able to start from cold and operate over a wider range of conditions including higher specific power than HCCI. On the other hand HCCI still has the advantage in efficiency and NOx emission rate. These trade-offs makes it desirable to develop an engine which can operate in either mode, hopefully without significantly adding to engine cost or compromising either mode. This paper presents a first exploration of a dual-mode engine which can use either HCCI or HCLB mode by operating on either a base, low-octane liquid fuel or on reformer gas (RG) produced by steam reforming or partial oxidation from the base fuel. Engine experiments with a CFR engine are used to demonstrate engine operation in both HCLB-SI and HCCI operating modes fueled with some combination of n-heptane and reformer gas. The combustion characteristics including combustion stability and cyclic variation are measured and compared for HCCI operation and RG-fueled HCLB-SI engine operation. CNG-fueled SI engine operation is also used as an additional basis for comparison.