Diesel Exhaust Emissions and Mitigations

*Nehemiah Sabinus Alozie and Lionel Christopher Ganippa*

#### **Abstract**

This chapter presents a concise treatment of diesel engine exhaust emissions and its mitigations. The working principle of the diesel engine is first given to establish the background and further to describe the influence of various parameters that affect the formation of engine exhaust emissions. The factors that influence exhaust emissions are linked to the engine design and the operating factors that promote good fuel-air mixing and combustion. These factors are air induction, fuel injection equipment, fuel injection schemes, in-cylinder gas exchange process and heat transfer. Thermochemistry essentially gives insight to the global reaction kinetics and how this is applied in practical engine combustion determinations in terms of equivalence ratios. Based on these, the fuel spray structure, atomization, penetration and the spray combustion model are described. The formation of exhaust emissions such as carbon monoxide, unburnt hydrocarbon and its intermediates, oxides of nitrogen and soot in diesel engines has been discussed. The techniques of their mitigation from the view of internal factors that deals with the optimization of engine design and it performance, as well as various exhaust after-treatment techniques used for NOx and soot reduction have been briefly discussed.

**Keywords:** diesel engine, fuel-air mixing, combustion, exhaust emissions, mitigations

#### **1. Introduction**

The diesel engine is a type of internal combustion engine invented in 1892 by Rudolf Diesel. It is so named in recognition of his patent which described a highly efficient, slow burning, compression ignition engine, where liquid fuel sprays were ignited solely by the heat of compression as piston reciprocates inside a cylindrical barrel. Compared to contemporary engines of that time especially the rival gasoline engine, it has superior efficiency in converting energy stored in liquid fuel to mechanical power. This superiority in fuel economy which has remained till date, is because diesel engine admits and compresses air only, (and can indeed be made to admit more externally compressed air into the cylinder) which is compressed to very high pressures and temperatures, before fuel is introduced into the combustion chamber as highly atomized sprays. Fuel-air mixing is achieved through combined factors of air swirl introduced through purposeful designed flow path, high fuel injection pressures, fine fuel atomization, and high temperature rapid fuel evaporation [1]. The high temperatures of the compressed air, reaches the auto-ignition temperature of the air-fuel mixture which burns and releases chemical energy in the fuel. Therefore, the way diesel fuel burns in the engine and the ultimate exhaust

emissions are related to the way it is introduced, mixed and ignited in the chamber. This is also dependent on the properties of diesel fuel and the geometric profiles of the chamber itself. These factors determine the index of air utilization in the combustion process and define the nature of exhaust emissions. Issues of diesel engine exhaust emissions have been of concern for diesel engine researchers and developers such that, although the principles remain the same; substantial improvements have been made to the original engine described by Rudolf. These have ensured better engine combustion and emissions; however stringent regulations on modern diesel engine exhausts, have further led to incorporation of after-treatment devices to attain compliant levels.

### **2. The working principles of diesel engine**

The diesel engine converts chemical energy stored in the fuel into work at the output shaft. This involves two steps; first is conversion of fuel chemical energy into thermal energy through combustion, followed by consequent conversion of thermal energy into mechanical energy by expansion of burning fuel in high pressure air. In effect, the following events take place: air induction, air compression, air and fuel mixing, burning of air-fuel mixture, expansion of the burning gases and their exhaust. These events or processes occur in four piston strokes during two revolutions of crankshaft. This gives the popular four-stroke cycle of induction stroke, compression stroke, power stroke during gas expansion and exhaust stroke, **Figure 1**. These events can also be arranged in a technology of two stroke operation using different design possibilities. Details of these are available in many standard diesel engine text books like [1–4] and only short summaries of these events are given with reference to **Figure 1** as follows.

#### **2.1 Intake stroke**

The intake stroke is the period when piston moves down from the top dead center (TDC) to the bottom dead center (BDC). During this time, the intake valve is open while the exhaust valve is closed. In operation, the intake valve is timed to open shortly before the TDC and closes after the BDC. This enables

**Figure 1.**

*The four-stroke cycle of typical diesel engine.*

fresh mass of filtered air to be drawn into the cylinder. The higher density fresh charge displaces burnt gases, thus enhances scavenging of the cylinder of burnt gases from previous cycle.
