The anatomy of a modern diesel engine

Lighter, cleaner, and more technologically advanced, modern diesels are completely different from their predecessors.

Europe is leading the way on diesel modernisation with a target of attaining the highest performance standards with the lowest environmental impact.

Through relentless investment in diesel engine technology, European vehicle manufacturers and suppliers have revolutionised diesel engines with innovations in diesel technology including the ground-breaking exhaust emission control device, the catalytic converter, and ‘AdBlue®’, an effective chemical that helps to reduce harmful emissions.


Aftertreatment technology

Diesel exhaust aftertreatment devices are critical, as they allow for stricter emissions control and improved air quality.

Initially introduced in the mid-1970s for petrol cars, the original catalyst was known as an ‘oxidation catalyst’ until it was superseded by the ‘three-way catalyst’. The Diesel Oxidation Catalyst (DOC) remains a key piece of technology for diesel engines, as it converts carbon monoxide (CO) and hydrocarbons (HC), into carbon dioxide (CO2) and water (H2O). The DOC also decreases the mass of diesel particulate emissions by oxidising some of the hydrocarbons that are adsorbed on the carbon particles.

Following the development of new technologies and systems that reduce emissions and improve fuel efficiency, the Diesel Particulate Filter (DPF) was created to further lower emissions by trapping the solid soot particles that the DOC was not able to oxidise.

DPFs were introduced on diesel cars in 2000 and since 2011, all new diesel cars in the EU have been fitted with this technology, which prevents the emission of harmful exhaust particles.

Compared to what they used to be 10 years ago, today’s diesel engines are cleaner and more efficient, being equipped with emission control systems to eliminate harmful emissions from the vehicle’s tailpipe.

The latest improvement to exhaust systems is Selective Catalytic Reduction (SCR). With the aid of a catalyst, the SCR system converts nitrogen oxides (NOx) into diatomic nitrogen (N2) and H2O. When conditions are not optimal for SCR catalysts, NOx adsorbers are useful because they collect NOx coming from the engine in order to store them, and treat them when conditions are suitable.

Ultimately, the combination of different emissions control technologies ensures greater control over harmful emissions.


Diesel particulate substrate filter - diesel engine anatomy

A Diesel Particulate Filter (DPF) traps over 99.9% of combustion particles


How diesel innovation is reducing air pollution

The new generation of diesel engines is made up of an integrated three-part system: a highly efficient engine, ultra-low sulphur diesel fuel and an advanced emissions control system.

The advanced electronic engine management system is the ‘brain’ of a modern engine which controls, among others, emission levels, as it collects and processes signals and data from on-board sensors and then coordinates the DPF and SCR exhaust aftertreatment systems.

The ultra-low level (less than ten parts per million) of sulphur in the fuel allows the use of improved emission control devices. This enables the engine to utilise the particulate filter to trap the soot particles, reduce its emissions and thus improve air quality.

Additionally, the common rail high pressure diesel injection systems contribute to the high efficiency of diesel engines. These systems increase the pressure in the injectors and provide better fuel atomisation, which in turn improves the ignition and combustion processes. This ensures that only the necessary amount of fuel required by the injectors is supplied to the common rail. It also enables the filter to be regularly ‘regenerated’ by burning off the collected soot.

The latest generation of diesel engines uses catalytic converters, adsorbers and particle filters that convert up to 99% of combustion engine exhaust pollutants (HC, CO, NOx and particulates). The catalytic converter plays an important role in controlling emissions: carbon monoxide (CO) and unburnt hydrocarbons (HC) are oxidised into CO2 and H2O while NOx, i.e. NO and NO2 is reduced into inert N2, thus virtually eliminating toxic gases and reducing air pollution.

The industry has demonstrated its commitment to improving air quality by developing and combining technologies that can directly address air pollution issues.

At a time when there was still a lack of epidemiological evidence regarding health effects of particles, the ‘precautionary principle’ was invoked, requiring the elimination of carbon particles through the use of DPFs on all new diesel cars sold in Europe from 2011 onwards. This was confirmed by the 2013 World Health Organization (WHO) Review of Evidence on Health Aspects of Air Pollution (REVIHAAP) which indicated that ultrafine particles have toxic effects on the human body.

The wall-flow DPF has the function of trapping and storing particulates over the whole particles’ size range. The exhaust gas passes through a porous ceramic wall honeycomb structure, where soot particles are deposited as a soot layer. This ‘soot cake’ not only stores soot but also filters out the ultrafine particles.

Consequently, particle mass and number emissions from diesel vehicles are efficiently controlled under real-world driving conditions, as the number of particles is being reduced by several orders of magnitude. High exhaust temperatures then periodically burn the soot collected in the DPF thus regenerating the filter and making it ready for the next round of soot collection.

The industry has demonstrated its commitment to improving air quality by developing and combining technologies that can directly address air pollution issues.


High-performing, efficient diesel engines

Improvements in diesel engine performance mean that, over the past 15 years, NOx and particulate matter (PM) emission limits have both been drastically reduced.

In practice, DPFs remove over 99.9% of particles, including ultrafine ones. The reduction of real-world NOx emissions did not always improve at the same pace as the Euro 3 to 5 standards, but an overall downwards trend has been observed from AECC’s vehicle measurements database.

AECC has been measuring vehicles’ emissions that occur outside of regulatory procedures for more than a decade. First it was on the Artemis test cycle, a lab-based test which is more representative of real-world driving than the former regulatory NEDC test cycle.

Since 2012 it has then been with on-road tests where the vehicle is fitted with a Portable Emissions Measurement System (PEMS). The picture below indicates the real-world NOx reduction from the diesel vehicles AECC has tested over the years.

From representative to actual real-world diesel NOx in AECC database

While engine improvements tend to decrease NOx emissions, they also tend to increase PM emissions. In other words, decreasing NOx through lowering the maximum combustion temperature increases the PM emissions from the engine, as it inhibits the complete oxidation of soot. This is called the ‘NOx-PM trade-off’.

The majority of manufacturers in Europe have chosen to use this trade-off to minimise particulate emissions at the engine-out, while using SCR aftertreatment to control emissions of NOx coming from the engine. This method allows the improvement of fuel economy compared with the previous generation of engines.

In practice, DPFs remove over 99.9% of particles, including ultrafine ones.

What innovations can we expect next?

Since the early 90s, the European Union has introduced increasingly strict emissions limits on vehicles known as the ‘Euro standards series’. The Euro 1 to 4 standards were not as stringent as the Euro 5 and 6, as they did not require particle or NOx aftertreatment devices to be fitted to diesel cars. These older, ‘dirtier’ diesel cars are contributing to the air quality challenges European cities are facing.

In order to follow Europe’s new and more stringent emissions standards, the diesel vehicle industry has consistently innovated and improved engine efficiency.

London skyline - Anatomy of a diesel engine

Upgraded in 2017, the Euro 6 standard now accounts for Real-Driving Emissions (RDE). These ensure NOx control aftertreatment technologies are used to their full potential, decreasing vehicle NOx emissions. Today, the Conformity Factor (CF) allows a margin for on-road NOx emissions.

Since 1 September 2017, a Not-To-Exceed (NTE) emissions limit has been set for the RDE of new cars, with an interim CF of 2.1 for NOx emissions. This means that real-world emissions can be 2.1 times higher than the regulation’s limit value. This new on-road and more stringent standard is called the Euro 6d-temp and has applied to all new cars registered from September 2019 onwards.

The European Commission and EU Member States considered the CF to be essential from a technical point of view, in order to reflect the RDE measurement uncertainty, as it is conducted on the road and not in a laboratory. Without the CF, compliant vehicles could fail a RDE test since it wouldn’t accurately represent how much a car emits.

However, the discrepancy will be further reduced through lowering the CF to 1.0 plus a measurement uncertainty starting from January 2020 for all new models of vehicles and from 2021 for all new vehicles.

The measurement uncertainty was originally set to 0.5 but an annual review of the CF will be conducted, in order to take into account technical improvements to the on-road test equipment. In March 2018, the European Commission proposed lowering the measurement uncertainty from 0.5 to 0.43. This more stringent CF forms the Euro 6d standard.

Diesel engine technology has come a long way in an incredibly short timeframe. Compared to what they used to be 10 years ago, today’s diesel engines are cleaner and more efficient, being equipped with emission control systems to eliminate harmful emissions from the vehicle’s tailpipe.

Ever more-stringent targets mean the diesel industry will continue to reach even higher efficiency standards.