Thermally Stratified Compression Ignition: A new advanced low temperature combustion mode with load flexibility

Abstract

A new advanced combustion mode is introduced, called Thermally Stratified Compression Ignition (TSCI), which uses direct water injection to control both the average temperature and the temperature distribution prior to ignition, thereby providing cycle-to-cycle control over the start and rate of heat release in Low Temperature Combustion (LTC). Experiments were conducted to fundamentally understand the effects of water injection on heat release in LTC. The results show that water injection retards the start of combustion due to the latent heat of vaporization of the injected water. Furthermore, for start of water injection timings between 20 and 70 degrees before top dead center, combustion is significantly elongated compared to without water injection. The 10% to 90% burn duration with 6.6 and 9.0 mg of water per cycle was 77% and 146% longer than without water injection, respectively. Direct water injection reduces the heat release rate by local evaporative cooling that results in a forced thermal stratification. Finally, the load limits with and without water injection were determined experimentally. Without water injection, the load range was 2.3 to 3.6 bar gross IMEP. By using water injection to control heat release, the load range in TSCI was 2.3 to 8.4 bar gross IMEP, which is a range expansion of over 350%. These results demonstrate that direct water injection can provide significant improvements to both controllability and the range of operability of LTC, thereby resolving the major challenges associated with HCCI.

Implementation of thermal and fuel stratification strategies to extend the load limit of HCCI engine

“…The naturally occurring thermal stratification can be enhanced more commonly with increasing wall heat transfer by means of reducing the coolant temperature and increasing the intake air swirl Sjöberg and Dec 2005a). Additional combustion phase retard strategy is suggested by Lawler et al (Lawler et al 2017), is the direct injection of a small quantity of water into the cylinder prior to the start of combustion. In addition, the negative valve overlap (NVO) where the air-fuel mixture was diluted with hot in-cylinder residual helps for active combustion and provide much-extended load limit due to more combustion retard.…”

“…Early-direct injection with NVO provides better combustion while more NVO increases the pumping loss (Yun et al 2011). Negative valve overlapping interval (NVO) can be reduced to a certain amount by enhancing the effect of fuel stratification in HCCI operation for each set of load/speed and compression ratio conditions (Lawler et al 2017;Koopmans et al 2003). Enhanced thermal stratification by injecting the small quantity of water directly into the cylinder during the cycle can also help in reduction of the losses associated with NVO to increase the load over great extent (Lawler et al 2017).…”

Computational Fluid Dynamics Simulations of the Effect of Water Injection Characteristics on TSCI: A New, Load-Flexible, Advanced Combustion Concept

“…The simulations were performed of a single cylinder of a production 4-cylinder 2.0L engine manufactured by General Motors (type LNF). This was the same engine that was used in the experimental work of Lawler et al [21]. The production engine needed some modifications to enable HCCI combustion and conduct the experiments including NVO camshafts and a higher compression ratio piston.…”

“…Recently Lawler et al [21] proposed a new approach, called Thermally Stratified Compression Ignition (TSCI), which introduces a forced thermal stratification in HCCI by using direct injection of water into the combustion chamber. Injecting water directly into the combustion chamber affects the level of thermal stratification, and therefore allows control over the LTC heat release process.…”

“…In the present work, a 3D CFD model of an HCCI engine is utilized to investigate the underlying phenomena that occur due to water injection. The model is first validated against experimental data from Lawler et al [21] for both pure HCCI without water injection (WI) and for TSCI with WI. Second, the effect of injection pressure on combustion is investigated.…”