Homogeneous Charge Compression Ignition (HCCI)

Homogeneous charge compression ignition (HCCI) has the advantage of low NOx and soot emission, because he global equivalence ratio is limited to values below about 0.5, which inevitably results in relatively small operating load, with similar efficiency to diesel combustion in part load because of low pumping loss. HCCI operates by having a more or less homogenous mix of fuel and air within the combustion chamber and the whole charge auto ignites when the pressure and temperature

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necessary for autoignition is reached. One challenge for HCCI combustion is controlling the autoignition timing. The small operating load limit restricts the application of HCCI but some of the principles of HCCI can be applied to partially premixed diesel and low temperature combustion.

Controlling methods for HCCI combustion include altering inlet temperature and/or pressure, fuel injection strategies, EGR rate, compression ratio, timing for valves closing and opening and equivalence ratio. Autoignition quality of the fuel, EGR quality, EGR unmixedness and pressure rise rate affect the high-load limit of HCCI combustion (Sjoberg and Dec (2009)). Study of the effect of boost pressure in HCCI condition has been conducted from a chemical kinetics point of view (Silke, Pitz et al.

(2008)). Modelling with CFD on a range of equivalence ratios was conducted to compare with research engine data (Hessel, Foster et al. (2008)).

Thermal stratification affects combustion in HCCI operation which is caused by wall heat transfer and large scale turbulence transport. Simulations have been used to investigate the thermal stratification effect on HCCI combustion (Aceves, Flowers et al. (2002), Aceves, Flowers et al. (2004)) and compared with experiments in engines (Sjoberg, Dec et al. (2005)). The main objective in studying thermal stratification is to understand how thermal stratification controls ignition and combustion phasing. It is also used for increasing high and low load limits and decreasing pressure rise rate.

Development of thermal stratification during the compression and expansion stroke was investigated and the thickness of the thermal boundary layer was identified (Dec and Hwang (2009)). Effects of thermal and compositional stratification on HCCI combustion have been investigated in a test engine and found competing stratification effects makes technological application difficult (Herold, Krasselt et al.

(2009)).

Fuel stratification controlled by fuel injection strategy can be used to control combustion phasing and reduce pressure rise rate. It can also extend the limit of high and low load HCCI operation. Dual injection consisting of one injection in the intake stroke and one in the compression stroke was found to reduce pressure rise rate and

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heat release rate as well as increase IMEP (Sjoberg and Dec (2006)). However, it was noted that injection penetration may have to be altered as early injection can lead to wall wetting and increased NOx emission (Sjoberg and Dec (2006)). Retarded injection timing changes stratification and improves combustion efficiency. It also reduces penetration of fuel towards the wall and crevice while creating a rich fuel zone for complete combustion (Dec, Davisson et al. (2008)). Leaner mixture increases hydrocarbons and oxygenated hydrocarbon emissions (Dec, Davisson et al.

(2008)) and injection timing also affects unburned hydrocarbons emissions (Dec, Davisson et al. (2008)).

The effect of the autoignition quality of a fuel and load limit in HCCI has been explored and fuel with medium reactivity was found to produce the highest IMEP (Sjoberg and Dec (2008)). Comparison of gasoline and diesel fuel has been done for HCCI and MK system and higher resistance to autoignition is more advantageous by encouraging premixing (Kalghatgi, Risberg et al. (2007), Kalghatgi, Risberg et al.

(2007)). The effect of fuel volatility on HCCI combustion has been investigated (Collin, Nygren et al. (2004)) and effects of oxygenates on soot formation processes has been researched (Mueller, Pitz et al. (2003)). Spectroscopy and chemical kinetics of HCCI combustion of single and two stage ignition fuels has been conducted and stages of ignition and combustion have been identified (Hwang, Dec et al. (2008)).

The combustion stability of single and two stage ignition fuel in HCCI combustion has also been studied (Sjöberg and Dec (2007)). In addition, the use of blends of Diesel with biofuels may also modify the ignition delay in an advantageous way.

The limitation of HCCI has been discussed with its limited practical application.

However, insofar as PCCI and LTC have aspects of combustion which are similar to HCCI, understanding the effects of changing operation strategy and of variables leads to useful guidelines, at least in the initial stages of research, for the operation of HCCI and LTC regimes.

56 1.3.5 Gasoline Compression Ignition

Another way to reduce emission of the diesel engine combustion is to utilise fuel chemistry and properties to overcome the problems of LTC and HCCI. The concept of using gasoline like fuel in a compression ignition has been tested to provide diesel like efficiency with reduced NOx and PM emission (Kalghatgi, Risberg et al. (2006), Kalghatgi, Risberg et al. (2007), Sellnau, Sinnamon et al. (2011)). There has been report of peak gross indicated efficiency of 57% (48.47% brake efficiency) in heavy duty engines showing the promising potential of this combustion system (Manente, Johansson et al. (2009), Splitter, Hanson et al. (2011)). The mechanism behind this concept is the auto ignition resistance quality of gasoline which means that the fuel injection can take place earlier than would have been the case with diesel fuel allowing for more premixing and hence producing less NOx and soot. The longer mixing time also means fuel can be injected at lower injection pressure to prevent wall impingement/wetting. This also brings the possibility of lower injection equipment cost and parasitic losses because of the lower injection pressure required. There is yet further reduction in cost because there is no need of after treatment for NOx and soot.

However, conventional gasoline fuel is not the ideal fuel for this mode of combustion as its high resistance to auto ignition means there is the possibility of misfiring and lack of ignition reliability. In contrast to conventional diesel fuel and its propensity for auto ignition, early injection and long mixing time creates early combustion phasing and limited load capacity. Fuels with different octane number (Hildingsson, Kalghatgi et al. (2009)) and fuel type such as naphtha (Viollet, Chang et al. (2014)) have been tested. There is a need for lower octane gasoline for this type of combustion system which will not only benefit the vehicle emission, it will result in lower energy cost in producing a less volatile fuel than current gasoline from the refinery perspective.

Various aspects of gasoline compression ignition have been investigated such as the lower load limit (Weall and Collings (2009), Borgqvist, Tunestal et al. (2012),

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Kolodziej, Ciatti et al. (2014)), various injector designs and compression ratios (Won, Peters et al. (2013)) and injection pressures (Kolodziej, Ciatti et al. (2014)) to compensate for the lower load.

In addition to challenges at low and high load, other obstacles such as cold start and idle, hardware optimisation and fuel quality will need to be resolved. For this study, the investigation of the viability of using gasoline in a compression ignition engine begins in Chapter 4 of this thesis. The composition, properties and blending of gasoline will be described in order to understand the fuel chemistry which may assist in designing a better fuel blend for gasoline compression ignition. This is a starting point in the application of gasoline compression ignition engine.