Effects of Single versus Two-Stage Heat Release on the Load Limits of HCCI using Primary Reference Fuels
Homogeneous Charge Compression Ignition (HCCI) enables combustion with high efficiency and low emissions. Control over the combustion process and its narrow operating range are still the biggest challenges associated with HCCI. To expand the operable load ranges of HCCI, this paper explores the effects of single versus two-stage ignition fuels by studying the Primary Reference Fuels (PRF) in a variable compression ratio Cooperative Fuel Research (CFR) engine. The PRF fuels, iso-octane and n-heptane, are blended together at various concentrations to create fuel blends with different autoignition characteristics. Experiments were conducted using these PRF blends to explore the extent to which the load range can be extended with two-stage ignition fuels at various compression ratios and intake temperatures. The reactivity of the PRF blends increases with the fraction of n-heptane and so does the amount of low temperature heat release (LTHR). Since the low PRF number fuels have a higher reactivity, they can be autoignited at very low compression ratios while maintaining comparable combustion phasing and equivalence ratios. At the lower compression ratios, the low load limits were found to be extended while maintaining high combustion efficiencies. Additionally, lower peak pressures and pressure rise rates were achieved at low PRF number fuels as a result of its two-stage heat release, which can be used to reach higher loads. In addition, the energy released from the LTHR can be used to delay the CA50 combustion phasing (i.e., the crank angle timing where 50% of the energy has been released) beyond what is possible with a single-stage ignition fuel, which allows further high load extension. However, using lower compression ratios has a negative impact on the thermal efficiency. The effects of the extended load, single- and two-stage heat release, combustion phasing, and equivalence ratios on combustion efficiency, thermal efficiencies, and combustion durations were also explored.