The effects of exhaust gas recirculation on diesel combustion and emissions

Abstract

An investigation was conducted with the aim of identifying and quantifying the effects of exhaust gas recirculation (EGR) on diesel engine combustion and exhaust emissions. Five effects of EGR were identified and investigated experimentally: the reduction in oxygen supply to the engine, participation in the combustion process of carbon dioxide and water vapour present in the EGR, increase in the specific heat capacity of the engine inlet charge, increased inlet charge temperature and reduction in the inlet charge mass flowrate arising from the use of hot EGR. The experimental methodology developed allowed each one of these effects to be investigated and quantified separately. The investigation was carried out on a high-speed, direct injection diesel engine, running at an intermediate speed and load. A limited number of tests were also conducted in an optically accessible diesel engine, which established the effects of EGR on local flame temperature. Finally, tests were conducted with simulated EGR being used additionally to the engine air supply. This contrasts with the conventional use of EGR, whereby EGR replaces some of the air supplied to the engine. It was found that the first effect of EGR (reduction in the oxygen flowrate to the engine) was substantial and resulted in very large reductions in exhaust NOx at the expense of higher particulate emissions. The second and third effects (participation of carbon dioxide and water vapour in the combustion process and increase in the charge specific heat capacity) were almost insignificant. The fourth effect (higher inlet charge temperature) increased both exhaust NOx and particulate emissions. The fifth effect (reduction in the inlet charge due to thermal throttling) reduced NOx but raised particulate emission. Finally, when EGR was used additionally to the inlet air charge (rather than displacing air), substantial reductions in NOx were recorded with little increase in particulate emission.

Effects of High EGR Rate on Dimethyl Ether (DME) Combustion and Pollutant Emission Characteristics in a Direct Injection Diesel Engine

“…At an advanced injection timing (30° BTDC), ignition delays are considerably increased because of the dilution effects of EGR, and also because the low charge in-cylinder temperature creates a lean mixture, thus decreasing the peak release rate as shown in Figure 3. In general, the use of high EGR rate lowers cylinder temperatures during the expansion stroke due to its greater heat capacity, thermal and chemical effects, with the most important effect being the dilution effect, which increases the ignition delay period and thus allows more mixing of the charge [14]. Consequently, the heat-release rates will be dominated by the low-temperature premixed phase of combustion, thereby reducing the emissions of NO x and soot simultaneously.…”

Performance and Emission Analysis of Diesel Engine with Exhaust Gas Recirculation: A Review

“…Hence combination of minor oxygen quantity in the intake air and reduced flame temperature lowers rate of formation of NOx reactions. The EGR (%) is termed as the mass % of EGR (MEGR) in the total intake mixture (Mi) EGR (%) = (MEGR ÷ Mt) × 100 A pictorial representation of the EGR effect on diesel combustion is provided in fig.1 though EGR results in lower NOx emission and higher soot, CO, HC emission and inferior engine performance [38,26,21,22,19]. Fig.…”

“…Fig. 1 Effects of EGR on diesel combustion and pollutant formation [38,26,21,22,19 Landommatos et.al. [20] describes two more effects, viz.…”

“…increase in inlet charge temperature and thermal throttling hot EGR. According to his study he illustrates various diluents such as CO2 [21,22] water vapour and their combinations [24,25] were inducted along with the intake air to simulate EGR. For the diesel engine influence of dilution effect in the major factor responsible for reduction in NOx and soot emission.…”

Combustion and Exhaust Emissions in a Heavy-Duty Diesel Engine with Increased Premixed Combustion Phase by Means of Injection Retarding

“…Among the well-known effects of EGR on the combustion process [32][33][34], the thermal and dilution factors are expected to cause a slow down of the basic oxidation processes. This condition affects the ignition delay and the overall combustion duration, which become longer as it can be corroborated in the plots of Figure 12, where the rate of heat release and Figure 12 In-cylinder pressure evolution and normalized HRR.…”