Diesel Vs. Petrol: Which Cars Pollute More?

Diesel cars have been the subject of negative publicity due to the amount of toxic emissions they produce. Some governments are even planning to ban them from urban areas. However, some diesel car owners are upset by this, as diesel was promoted as the environmentally friendly option when they purchased their vehicles. So, are diesel cars really more polluting than petrol cars? Well, it's complicated. While diesel engines emit less CO2 and greenhouse gases than petrol engines, they require large amounts of air for fuel combustion, which causes more chemical reactions and releases more air pollutants, such as nitrogen oxides and fine particles. Modern diesel engines have particulate filters that can trap and burn soot, but these tend to clog up in urban areas. Petrol engines, on the other hand, self-regulate and need less driver input. However, it's important to note that the type of engine isn't the only factor in determining pollution levels; the injection system, vehicle type, usage frequency, and purpose also play a role. Ultimately, the best way to reduce our environmental footprint is to minimise our use of pollutant transport.

Diesel engines emit more fine particles, such as soot and NOx

To comply with new, stricter standards on diesel particulate matter pollution, manufacturers have installed particulate filters in diesel engines. These filters capture and burn off the soot, reducing the amount of soot emitted by diesel engines. Similarly, SCR systems have been implemented to address NOx emissions. Thanks to these advancements, between 90 and 99% of diesel polluting mass particles are now filtered.

However, it is important to note that these filters tend to clog up when used mainly for urban driving. This can lead to higher emissions if not properly maintained. Additionally, the latest emissions technology requires the regular addition of a urea mixture, such as AdBlue, which can be inconvenient for some owners.

While new and well-maintained diesel vehicles have similar levels of particulate emissions to petrol cars, older diesel vehicles are considerably more polluting. The higher pollution levels of older diesel engines contribute to poor heart health, leading to increased hospital admissions and deaths from heart attacks. Thus, it is crucial to phase out these older diesel vehicles and encourage the adoption of newer, less polluting models.

In summary, while diesel engines emit more fine particles, such as soot and NOx, than petrol engines, the implementation of particulate filters and SCR systems has helped reduce these emissions significantly. However, the maintenance requirements of these filters and the continued presence of older, more polluting diesel vehicles on the road remain challenges in the effort to reduce diesel particulate emissions.

Diesel engines require more air for combustion, leading to more chemical reactions and air pollutants

Diesel engines are unique in that they require a large amount of air for fuel combustion. This combustion process involves mixing fuel with excess air, which creates local hotspots that produce oxides of nitrogen and local fuel-rich locations that produce carbon monoxide and soot particles. These soot particles, or black soot, are a type of particulate matter known as PM10 or PM2.5.

The issue with requiring more air for combustion is that it leads to more chemical reactions, which in turn release a significant number of air pollutants. These air pollutants include dioxides, nitrogen oxides, gases, and fine particles such as polycyclic aromatic hydrocarbons, ethane, and ethylene.

Diesel engines were initially less efficient than petrol engines in terms of fine particles, emitting much more of these particles, especially NOx. However, manufacturers have since installed particulate filters to meet new, more restrictive standards on diesel particulate matter pollution. These filters, along with SCR systems, have helped reduce the environmental impact of new diesel engines, bringing their emissions closer to those of petrol engines.

Despite these improvements, diesel engines still emit more air pollutants due to the nature of their combustion process. The excess air used in diesel engines also means that they cannot utilize catalytic converters in the same way that gasoline engines do to reduce emissions. Overall, while new diesel engines have made strides in reducing emissions, the inherent need for more air during combustion continues to make them more polluting than petrol engines.

Petrol engines emit less CO2 due to smaller vehicles and shorter travel distances

The debate around diesel and petrol engines and their environmental impact has been a long-standing one. While diesel engines were initially touted as a more environmentally friendly alternative, with lower carbon dioxide emissions, the argument has shifted in recent years. It is now understood that diesel engines require large amounts of air for fuel combustion, which results in more chemical reactions and the release of various air pollutants. These pollutants include harmful gases and fine particles such as polycyclic aromatic hydrocarbons, ethane, and ethylene.

Petrol engines, on the other hand, emit less CO2 due to several factors, including the type of vehicle and usage patterns. Firstly, petrol engines are often found in smaller vehicles that are used for shorter travel distances. This means that even though petrol engines may have lower fuel efficiency than diesel engines, the overall CO2 emissions are reduced because of the shorter distances travelled.

Additionally, petrol engines have a different combustion process. Unlike diesel engines, which use excess air, petrol engines mix fuel and air before combustion, resulting in a well-mixed air-fuel combination. This leads to a more complete combustion process, reducing the amount of unburned fuel and other pollutants in the exhaust gases. By passing these gases through a catalytic converter, harmful substances can be effectively converted into harmless products.

The type of vehicle and its usage patterns play a significant role in the environmental impact of petrol engines. Smaller vehicles with petrol engines are often used for shorter commutes or urban driving, which further contributes to lower CO2 emissions. This is in contrast to diesel engines, which are commonly found in larger vehicles or those used for longer journeys, resulting in higher overall CO2 output.

While the discussion around diesel and petrol engines is complex, it is clear that petrol engines have certain advantages when it comes to CO2 emissions. The smaller vehicles and shorter travel distances associated with petrol engines contribute to lower overall CO2 output, making them a more environmentally friendly choice in certain contexts. However, it is worth noting that the best way to reduce our environmental footprint is to transition to electric motors and reduce our reliance on pollutant means of transport.

Diesel engines are lean-burn, using less fuel and more air than petrol engines

Diesel engines are unique in that they require large amounts of air for fuel combustion. This means that diesel engines are lean-burn engines, using less fuel and more air than petrol engines.

In a petrol engine, the amount of fuel injected is calculated according to the amount of air entering through the throttle valve. This is known as maintaining the stoichiometric ratio—the amount of oxygen required to burn all the gasoline completely. In contrast, a diesel engine does not have a carburetor to mix air and fuel at the right ratio. Instead, it controls the volume of fuel injected into the cylinder each cycle, and the timing of that injection.

The lack of air control in diesel engines means that they can burn lean without damaging the engine. This is because diesel engines compress the fuel-air mixture to the point of self-ignition. The speed of a diesel engine is controlled by managing the amount of fuel injected into the cylinders.

Diesel engines are generally more fuel-efficient than petrol engines due to their lean-burn capabilities and higher compression ratios. However, one drawback is that diesel engines emit more fine particles, such as NOx, than petrol engines. Manufacturers have addressed this issue by installing particulate filters, which now filter out between 90 and 99% of diesel polluting mass particles.

Modern diesel engines have similar emissions to petrol engines, but older diesel engines are more polluting

The debate around diesel and petrol engines and their relative pollution levels is a complex one. While diesel engines emit less CO2 and greenhouse gases than petrol engines due to higher compression ratios and better performance, they emit more fine particles, such as PM10, PM2.5, NO2, NOx, polycyclic aromatic hydrocarbons, ethane, and ethylene. Initially, diesel engines were significantly less efficient than petrol engines in terms of fine particles, but manufacturers have since installed particulate filters that can trap and burn soot, reducing diesel particulate matter pollution by up to 99%.

Modern diesel engines, when well-maintained and built to the latest standards, now have similar emissions levels to petrol engines. However, it is important to note that most vehicle fleets still consist of older diesel engines, which are considerably more polluting. These older diesel engines emit more toxic emissions and are subject to bans in some urban areas. The negative publicity around diesel engines has led to a backlash, as some owners argue that they purchased diesel vehicles believing them to be more environmentally friendly.

The higher pollution levels of older diesel engines are due to the unique requirements of diesel fuel combustion, which needs large amounts of air. This additional air results in more chemical reactions and the release of more air pollutants. While gasoline engines mix fuel and air before combustion, achieving a close-to-ideal ratio, diesel engines mix fuel with excess air, resulting in local hotspots that produce oxides of nitrogen and fuel-rich areas that produce carbon monoxide and soot particles.

To address the higher emissions of older diesel engines, manufacturers have adopted new, more restrictive standards, and technologies such as SCR systems and urea mixtures like AdBlue have been introduced to reduce emissions. However, these technologies can be less effective in urban driving conditions and may require regular input from the vehicle owner. Overall, while modern diesel engines have made significant improvements in reducing pollution levels, the presence of older diesel engines on the roads continues to impact air quality and human health.

Frequently asked questions