WAYS TO IMPROVE THE ENVIRONMENTAL PERFORMANCE OF DIESEL ENGINES

ABSTRACT

As the diesel exhaust gases contain small amounts of carbon monoxide and unburned hydrocarbons in their normal technical condition, the main focus is to reduce emissions of nitrogen oxide and particulate matter - soot. Excessive presence of oxygen in exhaust gases does not allow neutralization of NOx as it is done in gasoline engines.

АННОТАЦИЯ

Поскольку в нормальном техническом состоянии выхлопные газы дизеля содержат небольшое количество окиси углерода и несгоревших углеводородов, основное внимание уделяется снижению выбросов оксида азота и твердых частиц - сажи. Чрезмерное присутствие кислорода в выхлопных газах не позволяет нейтрализовать NOx, как это происходит в бензиновых двигателях.

Keywords: diesel, exhaust gases, nitrogen oxides, combustion, excess air ratio.

Ключевые слова: дизельное топливо, выхлопные газы, оксиды азота, сгорание, коэффициент избытка воздуха.

As the diesel exhaust gases contain small amounts of carbon monoxide and unburned hydrocarbons in their normal technical condition, the main focus is to reduce emissions of nitrogen oxide and particulate matter - soot. Excessive presence of oxygen in exhaust gases does not allow neutralization of NOx as it is done in gasoline engines. In this case it is required to introduce additional reducing agents, for example, ammonia (NH₃) which complicates the engine significantly. Therefore NOx reduction directly at fuel combustion is predominant. To this end, the workflow organization of diesel engines is adjusted by:

- optimization of the timing and energy characteristics of fuel injection, ensuring the best possible charge micro- and macrostructure, as well as the lowest possible ignition delay;

- optimizing the vortex motion of the air charge;

- improving the design of diesel engines for water injection into the intake system;

- use of water-fuel emulsions;

- application of exhaust gas recirculation.

In general, the techniques listed aim to start the combustion process as close to the upper dead center as possible with the shortest possible ignition delay period.

A complex programme of changing the fuel injection timing is recommended, depending on the crankshaft speed, load and engine thermal condition. When running a cold engine at idle, it is advisable to increase the fuel injection advance angle with increasing crankshaft speed in order to avoid "white" smoke with high hydrocarbon content. However, after the engine warms up, the value of θinj should decrease by about 10 degrees of crankshaft rotation. When the "hot" engine is working under loading, the program of changing the moment of fuel injection has a completely different nature: θinj must decrease with increasing speed and load modes of the diesel engine from idle to 800-1200 rpm and remain at the level of 5-6 degrees of crankshaft rotation after the upper dead center at high rpm. Such programmes are implemented with the aid of electronic fuel management systems.

In addition to the starting point of fuel injection, the formation of harmful substances is influenced by the duration of injection. If the injection is too long, the last fuel portions are injected directly into the "hot" combustion products and are heated with a lack of oxygen. The result is an increase in incomplete combustion products and an increase in soot particle emissions. Therefore, in modern diesel engines the injection duration is reduced to the technically possible 20-30⁰ rotation of the crankshaft.

A reduction in injection duration is achieved by significantly increasing the injection pressure (p_inj). Increasing p_inj has a positive effect on atomization fineness, allowing optimization of the fuel jet microstructure and the charge macrostructure and thereby reducing exhaust gas smokiness (figure 1.1).

Figure 1.1. Dependence of particulate emission on fuel injection pressure

In connection with this, in modern diesel engineering there is a very clear tendency to increase p_inj up to 100-120 MPa. There are variants of fuel equipment, having injection pressure up to 180 MPa and more. Optimization of charge vortex motion. The charge vortex motion, together with the fuel jet parameters, significantly affects the macro-structure of the fuel-air mixture. As shown by experiments (Figure 1.2) [1], there is a well-defined vortex intensity at which both emissions of incomplete combustion products (CO, CH, C) and specific fuel consumption are minimized. However, in this case there is an increase of nitrogen oxides emission.

Figure 1.2. Pollutant content in diesel exhaust gases as a function of swirl ratio π_v/n

Improving the design of diesel engines. The main design methods for improving the environmental safety of diesel engines are:

- increase in the ratio of piston stroke to cylinder diameter;

- reduction of the piston volume above the piston;

- filling the gap between the cylinder liner and the head with a gasket made of heat-resistant synthetic materials;

- reduction in the piston flameband; conversion to four-valve timing systems.

Injection of water into the intake pipe. Water injection into the intake system also has a positive effect on the environmental performance of diesel engines. Under these conditions, the water vapour acts as inert ballast, having little effect on the ignition delay. Reduced charge temperature and a decrease in free oxygen concentration cause a decrease in NO_X emission. It was found [2], that addition of 6% (by weight) of water to the air entering the cylinder allows to reduce concentration of NOx in exhaust gases by 50% (figure 1.3).

Figure 1.3. Effect of water injection into the diesel inlet on nitrogen oxide emissions:

1 - without water injection; 2 - with 1% water injection; 3 - with 2% water injection; 4 - with 6% water injection

Application of water-fuel emulsions. An emulsion is a system consisting of two liquid phases, one of which is dispersed as 0.1-100 µm droplets (dispersed phase). The liquid in which the droplets reside is called the dispersed medium. Water-fuel emulsions can be direct (fuel droplets in water) and inverse (water droplets in fuel).

However, most studies [3 etc.] show that application of water-fuel emulsions with high water content decreases the concentration of nitrogen oxides in exhaust gases. Thus, with 20 % water in emulsion the content of nitrogen oxides decreases by 30-40 %, and with 40 % water - by 100 %, making 6-8 thousand ppm. In addition, with 20 % water in the emulsion, the concentration of CO decreased by 33 %, and with 40 % water by 66 % and is at 0.1 %. It was also found that the presence of water in the fuel reduces soot formation, preventing the coagulation of its molecules into large agglomerates.

Nevertheless, the prospects for water injection and the use of water-fuel emulsions are not uncontroversial, as they entail many problems. They require a water reserve (up to 20-30 % of the fuel stock). There are difficulties in preparing water-fuel emulsions and ensuring their stability over time, as water is released from the fuel over time and settles to the bottom of the fuel tank.

Figure 1.4. Schematic diagram of an exhaust gas recirculation system

References:

1. Lazarev E.A. Basic principles, methods and effectiveness of means to improve the combustion process to increase the technical level of tractor diesel engines. Textbook / E.A. Lazarev. - Chelyabinsk: CHSTU, 1995. - 215 p.
2. Marchenko A.V., Parsadanov I.V. Ecologization problems of internal combustion engines / A.V. Marchenko, I.V. Parsadanov // Internal combustion engines. - 2009. - № 2. - p. 3-8.
3. Goryachkin. A.V. Goryachkin Influence of moisture content in combustion zone on emission of nitrogen and sulfur oxides / A.V. Goryachkin // Science and Practice of Technogenic Bezpeka. - 2004. - Vin. 18. - Т. 31. - p. 27-37.