PSI - Issue 16

Jacek Eliasz et al. / Procedia Structural Integrity 16 (2019) 273–280 Jacek Eliasz et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Combustion in a compression ignition engine

Chemical properties of fuel affect the course of the combustion process and the emission of toxic substances into the atmosphere. Combustion of the combustible mixture in the CI engine takes place in the form of a chain reaction, proceeding in several stages. The process is best presented and analyzed by Saltas et al. (2017) using the indicator diagram (Fig. 1).

Fig. 1. Combustion process presented on the indicator diagram.

Combustion of a combustible mixture in a self-ignition engine consists of several stages shown in Fig. 1: time of self-ignition delay (1 – 2), kinetic combustion phases (2 – 3), diffusion combustion (3 – 4) and afterburning (4 – 5) (Ambrozik et al. (2015), Balyts'kyi et al. (2017)). The first and most important phase is the time of self-ignition delay (I), which is the time from the moment of fuel injection to the combustion chamber until the first self-ignition centres appear. The first self-ignition centres appear in point 2 on the presented Fig. 1. As the chart shows, the pressure in the cylinder begins to be higher due to the release of heat from the compressed air pressure without fuel injection (Balyts'kyi et al. (2017)). During this stage, there is decomposition of streams into drops, partial evaporation and mixing of fuel vapours with air. If the self-ignition delay time lasts for too long, then a large amount of unburned fuel is supplied to the combustion chamber, which after evaporation is mixed with air. As a result of volumetric auto-ignition, high pressure and temperature will occur in the cylinder, which will adversely affect its operation and the emission of toxic substances into the atmosphere, in particular, nitrogen oxides. A self-ignition delay can be determined by the dependence (1) (Balyts'kyi et al. (2017), Osipowicz et al. (2014)):

a m E RT

n T e p

τ const 

.

(1)

s

Analysing the dependence (1) it can be stated that the self-ignition delay time depends on the amount of activation energy E a . Self-ignition delay is influenced by physical and chemical factors of the fuel (2). Therefore, conventionally, the self-ignition delay time can be described as the sum of two periods of physical and chemical self ignition delay (Balyts'kyi et al. (2017), Kurczynski et al. (2018)): τ τ τ s f ch     s τ f ch   . (2)