1. Introduction

The development of a country greatly depends upon its transportation, mining, and power generation sectors. Diesel engines, also known as compression ignition (CI) engines, have become a source of power for these sectors due to their having high innate efficiency, durability, and better power output. Hence, it shares a large portion of the passenger car and heavy machinery market ^[1]. However, with rapid growth in demand, the diesel engine has also become one of the key sources of pollutants. Both the environment and human health are adversely affected by the pollutants resulting from petroleum-derived fuel combustion. United Nations Intergovernmental Panel on Climate Change has reported a rapid surge of global warming due to increased greenhouse gas emissions, including methane, nitrogen oxides, and carbon dioxide. It is predicted that more than a hundred million lives will be in danger if the average global temperature increases by more than 2 °C ^[2].

The main constituents of emission resulted from the combustion of diesel fuel are carbon monoxide (CO), nitrogen oxides (NO_X), hydrocarbons (HC), and particulate matter (PM)—these are considered as regulated emission and Polycyclic aromatic hydrocarbon, benzene, toluene, xylene, soot—these are considered as unregulated emission. Diesel engines emit hydrocarbon as the by-product of incomplete or partial combustion ^[3]. Sneezing, coughing, drowsiness, eye irritation, symptoms akin to drunkenness, several lung diseases- these are the problems that can be caused by HC emission ^[4]. The incomplete oxidation product of hydrocarbon fuel results in CO emission ^[5]. Excessive inhaling of CO disrupts the proper functionality of several vital organs such as the brain, nervous tissue, and heart by reducing the oxygen-carrying capacity of blood ^[6][7]. CO emission also can cause morbidity in people who have respiratory or circulatory complications ^[4]. NO_X formation is a complex mechanism, which can be divided into three parts, thermal, prompt, and fuel. At first, the high combustion temperature breaks the triple bonds of “Nitrogen molecules.” Then, these nitrogen molecules dissociate into their atomic states and produce NO_X while reacting with oxygen. The development of free radicals in the flame front of hydrocarbon flames leads to rapid production of NO_X ^[8]. During combustion of fuel, oxygen reacts with nitrogen bound in the fuel and forms NO_X. Irritation of the lungs, lowering respiratory infection resistance, edema, bronchitis, and pneumonia—these are the problems caused by NO_X emission ^[4]. Exposure to heavy metals causes adverse health effects, including toxicity, severe respiratory, and cardiovascular problems, and shorten life expectancy ^[9][10].

The acute effect of polycyclic aromatic hydrocarbon (PAH) on human health depends upon several factors, such as concentration, extent, the process of exposure, etc. ^[11]. Exposure to PAH may result in nausea, diarrhea, vomiting, skin irritation, etc. ^[12]. Long-time exposure increases the chances of lung, skin, bladder, and gastrointestinal cancers ^[13][14]. The most prominent aldehydes have carcinogenic effects, which are harmful to human health. Aldehyde over-exposure results in sore throat, nausea, headache, irritation of eyes, nose, skin, and throat, and difficulty in breathing and can cause chronic diseases at higher concentrations. Studies have shown that hematological, immune, neurological, and reproductive systems are each affected by N₂O emissions ^[15]. Furthermore, human exposure to benzene may cause adverse health effects and diseases, including cancer and aplastic anemia ^[16]. Toluene is a respiratory irritant that can affect the central nervous system. Inhalation of high levels of toluene vapors for a short period may cause headache, drowsiness, visual changes, nausea, dizziness, muscle spasm, and loss of coordination. Long time exposure to high-level toluene may result in attention and concentration and motor performance deficits ^[17]. To reduce the health effects of the combustion of diesel in internal combustion engines, regulators have imposed more and more stringent regulations on manufacturers.

The increased effect of global warming, limited efficiency of diesel engines, and stringent anti-pollution laws (especially NO_X and PM emissions) enforced by the governments have generated a spur to develop efficient engines with the acceptable emission level ^[18][19]. This development can be divided into three categories: