Emission Formation in Diesel Engines

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Abstract: Emissions formed during burning of the heterogeneous diesel air/fuel mixture depend on the conditions during combustion, during the expansion stroke, and especially prior to the exhaust valve opening. NOx emissions can be formed through a number of mechanisms during both the premixed and diffusion burning. PM is generated in diesels primarily during the diffusion flame. The visible smoke emission can be classified into black smoke, also known as hot or solid smoke, and white smoke also referred to as liquid smoke or fog.

Introduction

Unlike spark-ignited engines where the combustible mixture is predominantly homogeneous, diesel combustion is heterogeneous in nature. Diesel fuel is injected into a cylinder filled with high temperature compressed air. Emissions formed as a result of burning this heterogenous air/fuel mixture depend on the prevailing conditions not only during combustion, but also during the expansion and especially prior to the exhaust valve opening. Mixture preparation during the ignition delay, fuel ignition quality, residence time at different combustion temperatures, expansion duration, and general engine design features play a very important role in emission formation. In essence, the concentration of the different emission species in the exhaust is the result of their formation, and their reduction in the exhaust system. Incomplete combustion products formed in the early stages of combustion may be oxidized later during the expansion stroke. Mixing of unburned hydrocarbons with oxidizing gases, high combustion chamber temperature, and adequate residence time for the oxidation process permit more complete combustion. In most cases, once nitric oxide (NO) is formed it is not decomposed, but may increase in concentration during the rest of the combustion process if the temperature remains high [531].

Figure 1 summarizes the sources of unburned hydrocarbons (HC) and NO in direct-injected diesel engines. Species formed in both the premixed and diffusion (mixing controlled) combustion phases are shown [391].