Ignition delay and CCAI - A physical meaning.

Until the oil crisis at 70/80, the bunker or maritime fuel, was elaborated by the blend of residual oils and diesel as cutter stock adjusting the viscosity required. The viscosity was the parameter used for commercialization once till then there were no standards yet.

The International Organization for Standardization (ISO) published at 1987 and based at marine fuel specification BSMA100:1982 of British Standards Institution, was the first edition of a standard for residual and distillates marine fuels.

The main engines (ME) were stopped after the changeover to diesel oil and there was no trunk engines running with bunker, only diesel fuel.

Thus, as smaller was the bunker viscosity better was its quality.

The table 1 shows the trend of refineries output by the increase of refining degree of severity as installation of process as fluid catalytic cracking (*) units compatible with the distillates demand and by 1990 there were about 350 commercial fluid crackers in operation. Growth of FCC as the major petroleum refining process continues at a rate of about 1.7% per year (1989-92) [1].

The higher added value of on distillates market allowed investments at refineries and the progressive reduction of residues production.

(*) Catalytic cracking processes also alter the chemical composition of residual fuel. Is used a catalyst rather than high pressure to break down complex hydrocarbons into simpler molecules. The catalyst is a substance that assists the process of the chemical reaction but does not change its own properties. The most common process is fluid catalytic cracking (FCC), which can convert gas oil and residual oil into high-octane gasoline and diesel fuel.

Table 1 – Identifies the trend in product output (%) in response to demand for middle distillates, average worldwide

The cutter stock was changed from diesel (density of 845 kg/m3 and 5 cst) to cycle oils after the oil crisis (density>900 kg/m3 and 2~6 cst) and soon in the began 1980s the engine makers and users started to see a non-expect specific fuel consumption increase in main engines. In the early of 1990s the trunk engines become to use bunker as ME and followed damages in piston rings package as shown at above figure.

The damage of compression piston rings and the misfiring of the engines (even with the mechanical timing correct) indicated ignition problems in the fuels [2], thus based on the observations and a understanding of combustion process was developed at laboratories equipped with research engines a relationship between viscosity and density denominated CCAI (Calculated Carbon Aromacity Index) [3] [4]. For easy readout one can get the CCAI value thru the nomogram as shown below in Figure 1. The non-dimensional number, CCAI, means the ignition quality of the fuel being the higher number worse ignition performance, considering 860 as the limit number for a good ignition.

In the nomogram there are two lines that depicts a 180 cst bunker (blue) and a 380 cst bunker (green). It can be noted that 180 cst bunker has a worse ignition quality (CCAI of 860) than the 380 cst (CCAI of 850), so the idea of "minor viscosity-better the bunker" was changed.

Figure 1 - Nomogram for CCAI determination. The most left scale is the fuel oil viscosity (cst) in the middle the density scale (kg/m^3@15 deg.C) and most right the CCAI number, the higher, worse is the effect of ignition delay .

The task of the fuel-injection system is to meter the appropriate quantity of fuel for the given engine speed and load to each cylinder, each cycle, and inject that fuel at the appropriate time in the cycle at the desired rate with the spray configuration required for the particular combustion chamber employed.

Allow me to do a fast review of the engine combustion process divided by “phases”: In the first phase, the fuel is injected into the engine under high pressure (around 1000 bar). These large pressure differences across the injector nozzle are required to builds a spray formed by fine liquid droplets (not always spherical) increasing the area to heat and mass exchange. The break-up of a single droplet of 3 mm of diameter in smaller droplets of diameter of 30 μm will increase the area to exchange mass and heat in 10000 times, thus the nozzle holes state is one most important issue concerning spray formation once the time of droplet evaporation is function of the droplet initial diameter.

The droplet initial diameter is driven by injection pressure and the shape and size of atomizer holes (nozzle), being every droplet formed by residual and the diluent portion, what means different molecular weight, surface tension, boiling points and dynamic viscosity in the inner of the droplet (diluent) and the surface(residual).

As these droplets are formed, the friction and convection of the turbulent hot air in the cylinder it begins to vaporize and mix with the surrounding. Several physico-chemical process take place inside a burning droplet of heavy fuel oil. The fuel in the droplet is pyrolysed and changes chemically, becoming highly viscous and forming a shell that can trap gases.

There is a great entrance of hot air inside of spray spreading the fuel envelop exchanging heat and thus reducing the air temperature. At two-strokes engines the air turbulence is caused by the swirl, a directional flow taken by the air driven by the angle of inlet ports while at four-strokes engines the air turbulence is induced by the fuel spray itself. 

Secondly, after a delay the heat of compression and the localized fuel/air ratio provokes spontaneous ignition at spray borders, and a period of rapid uncontrolled combustion follows as the accumulated vapor formed during the initial injection phase is burned. This delay between the fuel entrance in cylinder and the moment of spontaneous ignition is named the ignition delay. Thereafter the combustion occurs at the fuel spray entrance in combustion chamber.

In the next phase occur the controlled combustion, or combustion by diffusion which keep a constant pressure over all combustion chamber surfaces while the fuel is injected into the cylinder and finish with the complete burn out of the fuel after injection has ended.

During phases two and three, the pressure in the cylinder rises rapidly and considerable forces are imposed over behind of piston rings. It is desirable to keep the rate of pressure rise of design (4.4~5 bar/°crank-angle), and this is reach by ensuring that the minimum quantity of fuel is present in the cylinder prior to ignition.

This means that the physical/chemical ignition delay period should be as short as possible.

Power output of the engine is optimized if ignition takes place at piston top dead center (TDC) and is followed by diffusion combustion. To satisfy these requirements it is necessary in practice to begin the injection of fuel just before TDC to allow time for the chemical-physical process occurs. In low NOx engines (TIER I, II) the ignition delay timing is zero, i.e., the injection occurs at TDC.

In figure 2 is represented the hypothetical physical meaning of the CCAI in an open diagram of a two-stroke engine. Above certain value, the rate of pressure destroys the lubricant oil film stability built by hydrodynamic effect between ring and cylinder liner surface.

 Figure 2 - CCAI hypothetical interpretation in an open cycle diagram of a two-stroke engine.CCAI is the rate of pressure inside of cylinder by the crank angle. The blue array show the TDC.

In October 2008, the 58th IMO MEPC session adopted significant changes to Annex VI under Resolution MEPC 176(58). This introduced a reduction in the global sulfur fuel limit to 3.5% from January 2012 with a further global reduction to 0.5% from January 2020.

Inside of Environment Control Areas (ECA) the vessel must lead to an emissions equivalents to the consumption of a fuel with 0.1% of sulfur, and outside of these areas (open waters), emissions equivalents to a fuel of 0.5% of sulfur content. So, ultra-low sulfur fuels (ULSF) must be used on engines. The CCAI number is a fast way to evaluate the fuel ignition proprieties. Thus, is not clear if this method can be useful for "future fuels distillates" elaborated by new blends types pools.

At the CCAI nomogram (Fig.1) if one plot a fuel with 4 cst of viscosity and 991 kg/m^3 of density the resultant CCAI number will be 930 (the worse ignition quality of the scale), thus a 4 cst fuel shall be a fuel with a maximum grade of density about 900~910 kg/m^3 to result in a CCAI acceptable number of 850.

Until now, distillates are being used only in ECA, from 2020 the demand of low sulfur fuels will increase globally .

Some considerations regarding distillates at trunk engines.

The conventional trunk engines were designed for running with bunker, thus fuel pump plunger and barrel were designed for operate with 12 cst fuel, for a continuous disttilate operation consider consult the engine maker (original engine maker) regarding the fuel pumps renew.

The evaluation of fuel valves nozzle holes is very difficult even on shore, renew the parts as maker recommendations or even before conform engine performance figures.

The key of engine performance is the systematic monitoring of lubricant in use associate to the running parameters of the engine, such as combustion pressure x load, scavenger air pressure x load and fuel pump index x load. Plote these curves from the test bed data and compare the performance on test with the engine performance on board.

Delayed ignition due mechanical degradation or bad ignition quality fuels means higher fuel consumption.

When bunkering a new fuel, keep one of the auxiliary engines set up with the previous fuel on board which was consumed whitout any troubles until do a test of the new fuel at another engine.

REFERENCES

1.      Geldart, G., "Challenges in Fluidized Bed Technology" AIChE Symposium Series No. 270, 85, p. 111, 1989.

2. Steernberg, K., Forget, S., "The effects of a changing oil industry on marine fuel quality and how new and old analytical techniques can be used to ensure predictable performance in marine diesel engines" CIMAC Congress, Paper 198, Viena 2007.

3.      A.P.Zeelenberg, H.J. Fijn van Draat, H.L.Barker, “The ignition performance of fuel oils in marine diesel engines”, 15th CIMAC conference, Paris 1983, Paper D13.2.

4.      A.P.Zeelenberg, H.L.Barker, “The ignition performance of fuel oils in marine diesel engines”, 10th Dutch National CIMAC Committee, Amsterdam, November 1988.

ADITIONAL READINGS

AGLAVE, R., REIDEL, U., WARNATZ, J. – Turbulence-chemistry interactions in CFD modeling of diesel engines – Combustion Theory and Modeling, 12:2,305-325, 2008.

ANDREASEN A., MAYER, S. - Modelling of the oxidation of fuel sulfur in low speed two stroke Diesel Engines - INTERNATIONAL CONGRESS ON COMBUSTION ENGINES, 39, 2010, Bergen. Proceedings: Conseil International des Machines a Combustion – CIMAC, 2010.

ARCOUMANIS, C., GAVAISES, M., FRENCH, B. - Effects of Fuel Injection Process on the Structure of Diesel Sprays - – SAE PAPER 970799 – SAE International – 1997.

BAUMGARTEN, C., LETTMAN, H., MERKER, G., P., - Modelling of Primary and Secondary Break Up Process in High Pressures Diesel Spays - In: INTERNATIONAL CONGRESS ON COMBUSTION ENGINES, 7, 2004, CIMAC. Proceedings: Conseil International des Machines a Combustion – CIMAC, 2004.

KIM, C., S., LEE, D., H., CHO, Y., S. - The comparison about CFD simulation and measurement results of a two strokes diesel engine. In: INTERNATIONAL CONGRESS ON COMBUSTION ENGINES, 23, 2001, Hamburg. Proceedings: Conseil International des Machines a Combustion – CIMAC, 2001.

CHRYSSAKIS, C., PANTANZIS, K., KAIKTSIS, L., - Development and validation of a CFD Heavy Fuel Oil Model – 20th Int. Multidimensional Engine Modeling User′s Group Meeting at the SAE Congress, April 12, 2010, Detroit USA.

GOLDSWORTHY, L., TAJIMA, H., - A Model for Ignition and Combustion Quality of Heavy Fuel Oil - INTERNATIONAL CONGRESS ON COMBUSTION ENGINES, 107, 2007, Vienna. Proceedings: Conseil International des Machines a Combustion – CIMAC, 2007.

GOLDSWORTHY, L., - Computational Fluid Dynamics Modelling of Residual Fuel Oil Combustion in the Context of Marine Diesel Engines, - Int. J. Engine Research, Vol. 7, pp. 181-199 – 2006.