Effects of Single versus Two-Stage Heat Release on the Load Limits of HCCI using Primary Reference Fuels 2019-01-0950
Homogeneous Charge Compression Ignition (HCCI) enables combustion with high efficiency and low emissions. Challenges in the control of combustion and its narrow operating range are still the biggest hindrances of 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 CFR engine. The PRF fuels, Iso-octane and n-heptane are blended together at various concentrations to create different fuels. Experiments were conducted using these PRF blends to explore the limits upto which the standard load ranges can be extended at various compression ratios and intake temperatures. The reactivity of the PRF blends increases with the fraction of n-heptane and so does the 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, which allows further high load extension. The effects of the extended load, single and two-stage heat release, combustion phasing and equivalence ratios on combustion efficiency, thermal efficiencies, combustion durations and exhaust emissions were also explored.
Deivanayagam Hariharan, Ruinan Yang, Sotirios Mamalis, Benjamin Lawler