Effects of Injection Timings and Energy Ratios on the Combustion and Emission Characteristics of n-Heptane/Ammonia RCCI Mode

Abstract

Blending with a high-reactivity fuel is recognized as an effective way to overcome poor combustion and heavy emissions of ammonia (NH 3 ). In this study, we investigated the combustion performance and nitrogen-based emissions of an nheptane/ammonia blend in an optical engine operating under reactivity-controlled compression ignition (RCCI) mode, addressing the n-heptane energy ratio and injection timing. Considering the long ignition delay time of pure ammonia, the n-heptane energy ratio was selected as 20−40%, and the injection timings were selected as −70 to −35 °CA aTDC. The results demonstrate the substantial impact of both variables on engine performance. Notably, a critical injection timing (SOI = −50 °CA aTDC) was identified, exhibiting optimal combustion stabilities, while an increased n-heptane energy ratio facilitated the expansion of the stable operating conditions and increased the indicated mean effective pressure by up to 6.4%. Combustion visualization showed a transition in combustion mode from flame propagation to sequential autoignition with the advancement of injection timing, resulting in a more homogeneous combustion. Meanwhile, optimal flame development was observed at the critical injection timing. As the heptane fraction decreased, autoignition timing was obviously retarded and flame development depended on the local temperature field, suggesting the potential for the modulation of combustion mode through controlling the injection timing and energy ratio. Regarding nitrogen-based emissions, an advanced injection timing correlated with more homogeneous combustion, leading to a decrease of NH 3 by up to 47.7% and an increase of NO and NO 2 by approximately up to 65.9 and 22.4%, respectively. As the n-heptane energy ratio decreased, unburned NH 3 emissions increased by up to 40.4%, while NO 2 emissions decreased by up to 11.2%. NO emissions exhibited an initial increase followed by a subsequent decrease, and the maximum value reached 2698 ppm at the n-heptane energy ratio of 30%. N 2 O emissions displayed negative correlations with the autoignition timing and flame propagation speed, consistent with the observed variations in combustion stabilities.

Combustion and Emission Characteristics of n-Heptane/NH3 at RCCI Mode: Emphasis on an Injection Strategy

“…This reduction is facilitated by an extended ignition delay that boosts OH and H radical production, further aiding in N 2 O reduction through reactions like N 2 O + H = N 2 + OH and N 2 O + OH = N 2 + H 2 O. Compared to the findings of Zhang et al, our study achieved a 32.32% reduction in N 2 O emissions. This notable improvement underscores the effectiveness of the pilot injection strategy in minimizing N 2 O formation.…”

“…Under the single injection strategy, as the injection timing advances, LTHR occurs earlier, although it is barely noticeable at an SOI of −40° ATDC. This effect is due to the dynamics of spray mixing and the complex interaction between NTC (negative temperature coefficient) chemistry and thermodynamic conditions . Moreover, when employing a pilot injection strategy, the intensity of the LTHR decreases and its occurrence advances.…”

Advancements in In-Cylinder and After-Treatment Strategies for Mitigating Pollutants from Diesel Engines: A Critical Review

“…RCCI is a dual fuel LTC strategy in which fuel with a lower reactivity is premixed by port fuel injection, and a higher reactivity fuel is directly injected into the cylinder during the compression stroke . The reactivity stratification attained using fuels of opposite reactivity helps achieve lower PPRR and broader combustion duration, thus, higher engine loads Figures and show the cylinder pressure histories and heat release rate characteristics for diesel in CDC, HCCI, PCCI, and RCCI modes, wherein RCCI exhibits a slower pressure rise rate and broader combustion .…”