THE MAIN FACTORS INFLUENCING THE FORMATION OF HARMFUL SUBSTANCES IN DIESEL ENGINES

ABSTRACT

Products of incomplete combustion and partial decomposition of fuel (carbon oxides, hydrocarbons and soot) are formed due to general or local oxygen deficiency. The determining factors for their formation are: fuel composition and quality; excess air ratio, uniformity of the macrostructure and optimum microstructure of the operating mixture.

АННОТАЦИЯ

Продукты неполного сгорания и частичного разложения топлива (оксиды углерода, углеводороды и сажа) образуются из-за общего или местного дефицита кислорода. Определяющими факторами их образования являются: состав и качество топлива; соотношение избыточного воздуха, равномерность макроструктуры и оптимальная микроструктура рабочей смеси.

Keywords: carbon monoxide, hydrocarbons, nitrogen oxides, combustion, excess air ratio.

Ключевые слова: окись углерода, углеводороды, оксиды азота, сгорание, коэффициент избытка воздуха.

The currently regulated pollutants produced during operation of reciprocating internal combustion engines include carbon monoxide, hydrocarbons, particulate matter and nitrogen oxides [4, 86, etc.]. Proceeding from the reversibility of chemical reactions, combustion cannot theoretically come to an end; the completeness of combustion is determined by the conditions of the process [2, 3], especially by the quality of mixture formation.

Lack of oxidant in the local flame zones in the combustion chamber, in particular diesel, reduced gas temperature, leads to incomplete fuel oxidation and increased concentration of hydrocarbons, carbon monoxide and soot particles in the exhaust gas. Excess oxidant at high temperatures and pressures leads to intensive formation of nitrogen oxides.

Let's consider briefly (see, for example, [5, 6] for details) the main factors influencing the formation of harmful substances in diesel engines.

Based on these determining causes, the formation of harmful substances can be analysed in relation to control and regime factors.

Fuel composition and quality. Fuel quality mainly has a direct influence on the content of both regulated and unregulated toxic components in the exhaust gases. The content of sulphur and its compounds in exhaust gases (mainly in the form of particulate matter) is proportional to the sulphur content of the fuel. Increasing the share of cyclic and polycyclic aromatic hydrocarbons in the fuel increases the smokiness of exhaust gases. The effect on NOx emission is directly manifested through organic nitrogen compounds in the fuel.

The effect on the reduction of all toxic emissions obtained by improving the fuel quality is quite obvious and is in the range of 10-20%.

Excess air ratio. In diesel engines, the overall excess air ratio determines the particulate (soot) and hydrocarbon content in the exhaust gases, but has little effect on the CO concentration. A clear increase in incomplete combustion products and soot formation is only observed when the excess air ratio is less than 1.35-1.40.

Increasing the air charge pressure (e.g. by increasing the supercharger degree, using a two-stage supercharger) entails a reduction in carbon monoxide and soot particles emissions. The effect of increasing pressure on nitrogen oxides is ambiguous, as on the one hand the excess air ratio increases, reducing NOx emissions, and on the other hand the temperature of the air at the cylinder inlet increases, increasing nitrogen oxide emissions.

Reducing the air charge temperature (e.g. by using a charge air cooler) by every 10 °C, can reduce the specific NOx emissions by about 10% [4].

The resistance of the intake and exhaust systems influences the air charge pressure and temperature. The valves are normally opened by a cam follower with a constant cam profile, and the valve lift law is not optimal at engine operating conditions other than nominal. The current trend is to replace conventional mechanical timing mechanisms with electromagnetic, hydraulic or electro-hydraulically actuated systems. Caterpillar, for example, uses Variable Valve Actuation (VVA) on the C13 and C15 diesel engines.

The aerodynamics of the air charge, i.e. the characteristics of the mixture formation process, depend on the geometry of the intake valve and duct.

The size of the over-piston gap determines the volume of the flame-extinguishing zone at the piston position near TDC, i.e. determines the emission of incomplete combustion products, mainly hydrocarbons.

The geometry of the piston side surface determines the amount of oil entering the combustion chamber from the cylinder walls. This parameter determines the emissions of hydrocarbons, carbon monoxide and soot particles.

Increasing the compression ratio results in higher combustion temperatures and combustion products, higher NOx emissions and lower CH emission.

Type of mixture formation. In case of film mixing, the smallest quantity of CO, CH and soot particles in exhaust gases is observed; in case of volumetric mixing, the smallest quantity of nitrogen oxides is observed.

Fuel injection torque. The formation of pollutants in diesel engines is significantly influenced by fuel injection timing, determined by the angle of crankshaft rotation to the upper dead center (θinj). Late injection shifts the end of the combustion process beyond the expansion phase to the beginning of exhaust, with the result that the amount of incomplete combustion products in the exhaust gas increases.

Increase in θinj has a favorable effect on unburned hydrocarbons content, but causes a sharp increase in nitrogen oxides emission. The latter is explained by the fact that as θinj increases the ignition delay period increases, which, in turn, leads to an increase in the share of cycle fuel, which has undergone pre-ignition preparation and burns at a high rate. This causes an increase in maximum cycle pressure and temperature, naturally leading to increased nitrogen oxide emissions.

Atomizer geometry determines the character of fuel jets development: their quantity, opening angle, dispersion of fuel droplets, range. The volume of the sub-air well determines the effect of fuel flow after the main injection, which leads to increased emission of hydrocarbons.

Engine operating mode. Increase in load on the diesel engine, compensated by increase in a cycle fuel supply, leads to growth of the maximum pressures and cycle temperatures, accordingly, to increase in NOx concentration in exhaust gases, increase in fuel evaporation time, that entails increase in concentration of products of incomplete combustion [5].

The general pattern of formation of toxic components in diesel exhaust gases is an increase in incomplete combustion products (CO, CH and soot) as the load increases (figure 1.1), when the excess air ratio decreases from 6-8 units at idle to 1.4-1.6 at nominal power mode.

Exhaust gases of diesel engines contain only about 0.1 % (by volume) of CO at idle, the concentration of which increases as the average effective pressure rises up to 0.2 % at full throttle.

Figure 1.1. Dependence of the exhaust emission content of a diesel engine on load [1]

Therefore, the highest amount of hydrocarbons (up to 0,8 g/m3) is registered at low loads and idle running of diesel engines. Minimum CH emissions occur at pe=0,4-0,6 MPa and at full fuel supply, due to the local oxygen deficit in the diffusion combustion zones, the CH concentration increases again.

The increase in soot emissions is even more significant with increasing load. A sharp increase in smokiness, starting from pe=0,4-0,5 MPa, is explained by deterioration of mixture formation processes, change in total and local excess air ratios and slowing down of diffusion stage of combustion against the background of increasing temperature in the flame zone.

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