Both petrol and diesel fuels are derived from crude oil. Petrol is composed of lighter hydrocarbon molecules with shorter carbon chains. Diesel is a middle distillate consisting of heavier, longer hydrocarbon chains. This fundamental difference in molecular structure results in varying combustion characteristics and a diverse range of environmental effects. Determining which fuel is “better” for the environment is not straightforward, as the answer depends on whether greenhouse gases or local air contaminants are being measured.
Comparing Greenhouse Gas Emissions
The primary environmental concern associated with fossil fuels is their contribution to climate change through carbon dioxide (\(\text{CO}_2\)) emissions. Diesel generally offers an advantage in \(\text{CO}_2\) output per distance traveled. This benefit stems from the inherent efficiency of the diesel engine’s combustion process, which uses a higher compression ratio than a petrol engine. The efficient conversion of fuel energy into mechanical power means a diesel vehicle consumes less fuel to cover the same distance.
Studies show that a diesel car typically produces 10 to 27 percent less \(\text{CO}_2\) per kilometer than its petrol counterpart. The lower consumption rate overrides the fact that diesel fuel contains slightly more carbon atoms by volume. Therefore, diesel maintains an advantage in reducing the largest contributor to global warming from a vehicle’s exhaust.
Impact on Local Air Quality
While diesel is superior in greenhouse gas emissions, it historically performed worse regarding pollutants affecting local air quality and human health. Diesel combustion naturally produces higher levels of two specific harmful pollutants: Nitrogen Oxides (\(\text{NOx}\)) and Particulate Matter (PM). \(\text{NOx}\) are a byproduct of high-temperature combustion that contribute significantly to the formation of ground-level ozone and acid rain.
The lean-burn characteristics of diesel engines lead to the formation of more \(\text{NOx}\) than in petrol engines without advanced after-treatment systems. Nitrogen dioxide is directly linked to respiratory problems. PM, commonly referred to as soot, consists of microscopic solid particles suspended in the exhaust. These particles pose serious public health risks, including cardiovascular disease, because they can penetrate deep into the lungs and enter the bloodstream.
Petrol engines use three-way catalytic converters to neutralize a significant portion of their \(\text{NOx}\). However, diesel catalytic technology is more complex due to the presence of excess oxygen in the exhaust. This difference in pollutant composition has been the primary reason for diesel’s negative reputation in urban environments.
Energy Density and Fuel Efficiency
The difference in environmental performance is rooted in the physical and chemical properties of the two fuels. Diesel fuel contains more energy packed into a given volume than petrol, a property known as higher energy density. A liter of diesel contains approximately 10 to 15 percent more energy than a liter of petrol, due to its longer and heavier hydrocarbon chains.
Because the fuel holds more potential energy, less volume is needed to produce the same amount of work or power. When combined with the higher thermal efficiency of the compression-ignition diesel engine design, this higher energy density results in a significantly lower fuel consumption rate.
The Role of Modern Engine Technology
Technological advancements and stricter emissions regulations have fundamentally changed the environmental profile of diesel engines, mitigating historical issues of local air pollution. The most significant change was the introduction of Ultra-Low Sulfur Diesel (ULSD), which dramatically reduced the sulfur content. This reduction was a prerequisite for the effective use of modern exhaust after-treatment systems, as sulfur poisons the catalysts used in these devices. Modern diesel vehicles are now equipped with sophisticated systems designed to neutralize the harmful byproducts of combustion.
The Diesel Particulate Filter (DPF) physically traps up to 90 percent of the hazardous Particulate Matter before it can be released into the atmosphere. To address \(\text{NOx}\) emissions, the Selective Catalytic Reduction (SCR) system is employed. SCR injects a liquid reductant, often a urea-based solution, into the exhaust stream. This chemical reaction converts the harmful \(\text{NOx}\) into harmless nitrogen gas and water vapor, achieving reductions of up to 80 percent. These combined technologies ensure that new diesel vehicles meet stringent emission standards, closing the gap with petrol vehicles on local air quality while retaining their advantage in \(\text{CO}_2\) emissions.