knowledge of the composition of sulfur species in the EGR line will be helpful in the design and control of the EGR scrubber in order to control the level of sulfur species entering the cylinder through he scavenge ports potentially increasing the risk of cylinder liner corrosional wear if not neutralised.The basis of this study is real measured engine data from a large two-stroke diesel engine e.g.directly measured in-cylinder pressure as a function of degree crank angle as well as exhaust gas NO concentraion, derived fuel mass burned fraction and trapped air amount (air excess ratio). This is used as input for simulations invoking a detailed reaction mechanism describing the formation of oxidised sulfur species from fuel injection start to exhaust valve opening. 

METHODS  The engine used to acquire input data to the kinetic model is the MAN Diesel custom built test engine located in Copenhagen. A summary of engine characteristics is given in Table 1. The engine is equipped for full electronic-hydraulic control of the fuel injection and the exhaust valve timing. A total of four engine tests have been conducted at the following loads: 100, 75, 50, and 25% (at 123, 112,98 and 78 rpm, respectively) of the maximum continuous rating (MCR). The engine speed decrease as a function of load according to a simulated propeller curve [23]. The air excess ratio(trapped air only, scavenging air not included) for the four tests was 2.27, 2.43, 2.36, 2.74 for 100, 75,50, and 25% load, respectively. The used fuel was diesel oil with the following atomic composition of C and H atoms: 86.8 w/w % and 13.2 w/w %. The sulfur content of the fuel was 0.05 w/w % and heating value of the fuel is 42820 kJ/kg. All values determined from an analysis performed by an external laboratory. The exhaust gas concentration of NO has been measured in the exhaust duct between the turbine and the stack by the heated chemiluminescence technique (HCLD) according to ISO 8178 standards.

Table 1 Test engine specifications

Manufacturer                MAN B&W Diesel

Type                       4T50ME-X

Number of cylinders          4

Bore                       500 mm

Stroke                      2200 mm

Connecting rod length        2885 mm

MCR speed                 123 RPM

MCR power                 7050 kW

MEP @ MCR                20 bar

Turbocharger                MAN TCA55-VTA

Model and calculational setup 

The starting point in the kinetic model is measured cylinder pressure as a function of crank angle.From the measured in-cylinder pressure the fuel

mass burned fraction is derived from the integrated heat release rate as obtained from a single zone heat release analysis using an in-house code.The measured cylinder pressure as a function of degree crank angle for the four engine tests is depicted in Figure 1. The rate of heat release as determined from the in-cylinder measured pressure traces shown in Figure 1 is shown in Figure 2.

Figure 1 — Measured in-cylinder pressure

averaged over 50 revolutions as a function of degree crank angle after top dead center (ATDC). Figure 2 — Rate of heat release for the 4 load cases

In order to model the kinetics of the formation of oxidised sulfur species i.e. SO2, SO3, and H2SO4, a detailed reaction mechanism is called upon. For this purpose the Glarborg sulfur mechanism is used [24] which is a result of a continuous effort in literature data review, experimental laboratory reactor measurements as well as theoretical predictions [25, 26, 27, 28, 29]. Other sulfur oxidation mechanisms exists e.g. the Leeds sulfur mechanism [30], however a substantial part of the S subset is based on the work of Glarborg et al. anyway. The S subset in the Glarborg mechanism contains 96 elementary reactions. In addition to the S subset the mechanism contains an O/H subset describing the formation of O, OH, HO2, H2O2 . A complete listing of all reactions along with rate coefficient and thermodynamic parameters will not be given. In order to describe the formation of sulfuric acid a single reaction has been added in which H2SO4 is formed from SO3 and H2O. The