Helena Joyce
Diesel engines have been the subject of much debate in recent years, with some arguing that they are more polluting than petrol engines and others claiming the opposite. The answer to this question depends on various factors, including the type of pollution being considered and the age of the vehicle. While diesel engines generally emit less CO2 and greenhouse gases due to their higher compression ratio and better fuel performance, they can produce more harmful pollutants such as nitrogen oxides (NOx), ground-level ozone, and particulate matter, which have been linked to respiratory issues and cancer. On the other hand, petrol engines emit more fine particles and are better at regulating emissions without driver input. The discussion is further complicated by the fact that newer diesel engines with improved emissions technology may perform differently from older ones, and government standards and regulations also play a role in reducing diesel fuel pollution.
| Characteristics | Values |
|---|---|
| Diesel engines emit more... | Toxic emissions, nitrogen oxides, fine particles (including PM10, PM2.5, NO2, NOx, and N2O), soot, and fumes |
| Diesel engines are more common in... | Bigger, heavier, and more efficient vehicles that travel greater distances |
| Diesel engines are more polluting overall | Yes, especially older diesel engines |
| Diesel engines emit less... | CO2 and greenhouse gases |
| Diesel engines are more common for... | Trucks and heavy equipment for construction due to their efficiency |
| Diesel engines are more heavily regulated | Yes, the U.S. EPA has established stricter standards for diesel fuel and engine emissions |
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What You'll Learn
- Diesel engines emit more harmful pollutants, such as ground-level ozone and particulate matter
- Diesel vehicles cause more than four times the pollution of petrol cars
- Diesel engines require more air for combustion, causing more chemical reactions and air pollutants
- Newer diesel models emit higher levels of nitrogen oxides
- Diesel vehicles are bigger, heavier, and more efficient than petrol vehicles
Diesel engines emit more harmful pollutants, such as ground-level ozone and particulate matter
Diesel engines emit a complex mixture of gaseous and solid air pollutants. The burning of diesel fuel (refined from crude oil) produces many harmful emissions, and diesel-fueled vehicles are major sources of harmful pollutants, such as ground-level ozone and particulate matter.
Diesel engines emit nitrogen oxides (NOx), which are a significant pollutant and a precursor to the creation of ground-level ozone. NOx emissions from diesel engines can lead to the formation of particulate matter through chemical reactions in the atmosphere. NOx is linked to a range of respiratory health issues, including reduced lung function, inflammation, and the development of asthma.
Diesel exhaust also releases particulate matter, known as diesel particulate matter (DPM) or PM2.5. DPM is composed of carbon particles ("soot" or black carbon) and various organic compounds, including over 40 known carcinogens. The health effects of DPM exposure include cardiovascular and respiratory issues and even premature death. DPM also contributes to climate change by absorbing sunlight and converting it into heat radiation, warming the surrounding air.
To address the problem of diesel emissions, the U.S. Environmental Protection Agency (EPA) has established standards for the sulfur content of diesel fuel and emissions from new diesel engines. The petroleum industry has responded by producing Ultra-Low-Sulphur Diesel (ULSD) fuel, which helps reduce emissions in older and newer engines. Additionally, the EPA has set emissions standards for diesel engine highway vehicles for model year 2007 and later. Despite these efforts, diesel fuel use still contributes significantly to air pollution due to the large number of older diesel engine vehicles in use.
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Diesel vehicles cause more than four times the pollution of petrol cars
Diesel vehicles produce more than four times the pollution of petrol cars. Diesel fuel, which is refined from crude oil, produces a large number of harmful emissions when burned, and vehicles that run on diesel are a major source of pollutants such as ground-level ozone and particulate matter. These fine particles can cause cancer and acute respiratory issues. Diesel engines also produce more nitrogen dioxide, a toxic gas, than petrol engines.
In 2022, diesel fuel consumption accounted for about 25% of total US transportation sector CO2 emissions and about 10% of total US energy-related CO2 emissions. However, diesel engines are "lean-burn", meaning they use less fuel and more air than petrol engines to achieve the same performance. As a result, while diesel fuel contains slightly more carbon per litre than petrol, overall CO2 emissions from a diesel car tend to be lower.
Despite this, diesel cars have recently received negative press due to the amount of toxic emissions they produce. Some governments are even considering discouraging their use or banning them from urban areas. However, this has been met with backlash from diesel car owners, who argue that they bought their vehicles because they were supposed to be the environmentally friendly option. Indeed, diesel was promoted as a more environmentally friendly fuel as part of the EU's response to the 1997 Kyoto Protocol to reduce greenhouse gas emissions.
While new, well-maintained diesel cars built to the latest standards have similar emissions levels to new petrol vehicles, most vehicle fleets are dominated by older, more polluting models. It will take many years for these vehicles to be replaced, during which human health will continue to be impacted by their emissions. Overall, for most cars built in the past 20 years that are still in use, petrol is likely to be the less polluting option.
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Diesel engines require more air for combustion, causing more chemical reactions and air pollutants
Diesel engines are a major source of harmful pollutants, such as ground-level ozone and particulate matter. Diesel exhaust contains gaseous pollutants, including volatile organic compounds and oxides of nitrogen (NOx). NOx emissions from diesel engines are particularly important because they can undergo further chemical reactions in the atmosphere, leading to the formation of PM2.5 and ozone.
PM2.5 is the size of ambient particulate matter most associated with adverse health effects, and it has ambient air quality standards. These health effects include cardiovascular and respiratory hospitalizations and even premature death. As a California statewide average, DPM comprises about 8% of PM2.5 in outdoor air, although levels vary across the state.
Diesel engines are lean combustion engines with a consistently high air-fuel ratio. This means that diesel engines require more air for combustion than other engines. The air required for combustion is highly compressed inside the combustion chamber, generating high temperatures. These temperatures are sufficient for the diesel fuel to ignite spontaneously when injected into the cylinder. This process releases the chemical energy contained in the diesel fuel and converts it into mechanical force.
Because diesel engines use more air, they can also produce more chemical reactions and air pollutants than other engines. The formation of carbon monoxide (CO), for example, is dependent on the air-fuel mixture. In engines with a rich mixture (where the excess-air factor is less than 1.0), not all carbon can be converted to CO2, and CO is formed. Although CO is produced during operation in rich mixtures, a small portion of CO is also emitted under lean conditions due to chemical kinetic effects. However, because diesel engines have a high air-fuel ratio, the formation of CO is minimal.
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Newer diesel models emit higher levels of nitrogen oxides
Diesel fuel, which is refined from crude oil, produces many harmful emissions when burned, and diesel-fueled vehicles are a major source of pollutants. While diesel engines have been improved over the years, they still contribute to air pollution. This is because it will take a long time for newer and cleaner diesel engine vehicles to fully replace older diesel engine vehicles.
Diesel engines produce more nitrogen oxides (NOx) than petrol engines. This is because diesel engines operate at a higher temperature and pressure than petrol engines, and these conditions favour the production of NOx gases. The amount of NOx formed during combustion varies with peak combustion temperature: as the temperature rises, so does the rate of NOx formation.
NOx gases include nitrous oxide (N2O), nitric oxide (NO), and nitrogen dioxide (NO2). Nitrous oxide is a serious greenhouse gas, defined as being 298 times worse than CO2 due to its radiative effect and the time taken to break it down. Nitric oxide is readily oxidised in the atmosphere to nitrogen dioxide, which is a major pollutant and a component of smog. Nitrogen oxides have harmful direct effects on human health and indirect effects through damage to agricultural crops and ecosystems.
There are techniques to remove NOx from exhaust gases, such as Selective Catalytic Reduction (SCR), which is the most common method in diesel vehicle exhausts. However, it is expensive and therefore not used in small, cheap vehicles.
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Diesel vehicles are bigger, heavier, and more efficient than petrol vehicles
Diesel vehicles are generally bigger and heavier than their petrol counterparts, and they are also more fuel-efficient. This is due to the fact that diesel fuel is thicker and burns more slowly than gasoline, resulting in a more efficient combustion process with less energy loss. Diesel engines have a higher compression ratio than petrol engines, typically ranging from 14:1 to 25:1 compared to 8:1 to 12:1 for petrol engines. This higher compression leads to more efficient combustion, with the fuel burning more completely, resulting in increased thermal efficiency.
The greater efficiency of diesel engines is particularly notable in heavy vehicles, where diesel engines are often preferred due to their ability to produce higher torque. Diesel engines are significantly more expensive than gasoline engines, but they have much longer lifetimes, making them a cost-effective choice for commercial vehicles that are on the road constantly. The longer lifespan also results in less downtime for repairs, further reducing costs for commercial users.
While diesel engines are more efficient in terms of fuel consumption, they have historically been associated with higher levels of harmful emissions, such as ground-level ozone, particulate matter, and nitrogen oxides. However, advancements in emission control technologies have significantly reduced diesel emissions, and the development of Ultra-Low-Sulfur Diesel (ULSD) fuel has further contributed to cleaner-burning diesel engines. Despite these improvements, the older diesel engines still contribute to air pollution, and it will take time for them to be fully replaced by newer, cleaner models.
In summary, diesel vehicles tend to be bigger, heavier, and more fuel-efficient than petrol vehicles, especially for heavy-duty applications. While emissions have been a concern with diesel engines, ongoing technological advancements are helping to address this issue and make diesel engines cleaner and more environmentally friendly.
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Frequently asked questions
Overall, diesel vehicles produce more toxic emissions than petrol cars and are more harmful to human health. However, diesel engines emit less CO2 and greenhouse gases than petrol engines.
Diesel engines require large amounts of air for fuel combustion. This additional air causes more chemical reactions, which release air pollutants such as nitrogen oxides and fine particles.
Long-term exposure to nitric oxide can increase the risk of respiratory problems. Fine particulate matter emitted from diesel engines is associated with poor heart health and an increased risk of hospital admissions and deaths from heart attacks.
