Internal Combustion Engines: non-engine solutions for emission reduction.

In recent years, internal combustion engines were expected to decline quickly as electric mobility gained momentum, driven by environmental concerns and technological advancements. However, today, manufacturers are significantly focusing on advancing internal combustion engine technology, with the future of these engines largely contingent on their ability to meet stringent low-emission standards.

From this article, you will learn:

• the importance of ICEs.
• how emission standards were created.
• briefly what the key internal engine technologies for emission reduction are.
• what the non-engine purification methods are.
• the importance of synergy between primary and secondary solutions.

Emission challenges and regulatory advances

The rise of internal combustion engines (ICEs) in the mid-20th century led to significant environmental pollution, especially in the U.S. In response, stricter emission standards, like CARB (1967) and EPA (1970), were introduced to reduce harmful pollutants. These early regulations have influenced global standards, which now target not just exhaust gases but also pollution from tires and brakes.

Despite the momentum toward electric vehicles, ICEs remain vital in industries like automotive, aerospace, and off-highway applications. Ongoing efforts to reduce toxic emissions from ICEs focus on advancing engine technologies and adopting cleaner fuel options.

Primary engine technologies for reducing emissions

Reducing emissions in modern combustion engines starts at the design stage, focusing on the interaction of various engine components. Key technologies include exhaust gas recirculation (EGR) to lower nitrogen oxides (NOx) and hydrocarbons, advanced fuel injection systems for optimal fuel-air mixing, and improved turbocharger efficiency for better combustion. Other methods like Closed Crankcase Ventilation (CCV) and valve timing optimization help reduce hydrocarbons and unburned fuel, while engine cooling system improvements and high-quality fuels contribute to overall emission reductions.

If you want to learn more about internal exhaust gas purification methods, read this article: Internal Combustion Engines: internal solutions for a greener future.

Integrating secondary purification methods

The modern engine design, despite numerous innovative solutions that were described in the previous article, is currently not sufficient to achieve the level of toxic emissions required by standards only when using primary treatments. Further development of engine methods for preventing pollution in exhaust gases is ongoing but requires high financial outlays and may lead to the complexity of the entire structure which may negatively affect its reliability, durability and cost. For this reason, every effort was made to ensure that the process of reducing exhaust emissions was transferred externally and became separate elements integrated in the exhaust system. These are the so-called non-engine (secondary) exhaust gas purification methods. Typically including the following components:

• Catalytic Converters (TWC / DOC) – catalytic converters are primarily used to reduce emissions of nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons (HC). The most common are three-way catalytic converters (TWC), used in gasoline engines, which simultaneously reduce NOx, CO, and HC. In diesel engines, two-way oxidation catalysts (DOC) are used which reduce CO and HC. Currently, used catalytic reactors are required to have very high efficiency achieved in the shortest possible time. Their design requires a large contact surface of the catalytic reactor material with the exhaust gas phase quickly achieving the optimal operating temperature, low flow resistance and high mechanical and thermal performance.
• Diesel / Gasoline Particulate Filters (DPF / GPF) – the filter traps Particulate Matters (PM) which are then burned off during the filter regeneration process. Solid particles formed in the process of incomplete combustion consist mainly of soot-carbon particles with an adsorbed hydrocarbon layer. In practice, there are two types of particulate filters – open and closed. Open filters allow free flow and have a structure similar to a catalytic oxidation reactor. Contact of solid particles with the catalytic material causes their oxidation. They only limit the PM value (particle size) and the PN (particle quantity) remains unchanged. Closed particulate filters are more popular. They are composed of a large number of filter channels into which exhaust gases flow. However, the channels are blocked at the end which allows solid particles to accumulate there. The horizontal walls of the channels are highly porous which allows exhaust gases cleaned of solid particles to escape to the further part of the exhaust system. The filter can be made of metal or ceramic with a catalytic coating to improve the oxidation process. Closed filters have the ability to reduce PM and PN emissions much above 95%. The collected solid particles may self-ignite if the temperature in the exhaust system is at the level of 500-550°C. The self-cleaning process of the filter is called regeneration and applies to reusable filters. In some cases of engine applications, especially less loaded ones where a high temperature of the exhaust system is not achieved, single-use filters are used. At lower temperatures the regeneration process takes place but with much less intensity and the process of increasing the amount of solid particles is much faster than their self-cleaning. It is also possible to reduce the auto-ignition temperature of particulate matter using catalytic additives to fuels – wet DPF filter. Particulate filter regeneration is divided into a passive and active process. During a passive process, the purification of particulate matter occurs automatically as a result of the high temperature that the exhaust system could reach naturally. At temperatures of 300-500°C particulate matter is oxidized using nitrogen dioxide formed on the catalytic layer of the particulate filter. Above this temperature solid particles are burned. The active process requires an artificial increase in the temperature of the particulate filter to approximately 550 °C. This can be done by several methods – changing the characteristics of fuel injection into the cylinder, fuel injection into the exhaust system or using an electric heater. The strategy of controlling the particulate filter regeneration processes is very important.
• Lean Nox Trap (LNT) – also known as a NOx storage-reduction catalyst, is used particularly in lean-burn engines like diesel engines. Its primary function is to reduce nitrogen oxide (NOx) emissions. During normal operation the LNT stores NOx on the catalyst surface. In order to convert them to nitrogen and water the engine switches mode to rich burn allowing stored NOx to be released and reduced.
• Selective Catalytic Reduction (SCR) – SCR systems are used to reduce NO_x emissions and are mostly used in diesel engines. This process involves injecting a urea solution (AdBlue) into the exhaust stream, which converts NOx into nitrogen and water. AdBlue fluid is colorless and odorless and consists of a 32.5% solution of urea in water. As a result of the thermolysis and hydrolysis reactions, ammonia is produced from urea during injection into the exhaust before the SCR catalytic reactor. The layer of catalysts that covers the reactor core causes the adsorption of ammonia produced from the AdBlue on the surface of the reactor. Exhaust gases flowing through the SCR catalytic converter enter into a chemical reaction with ammonia, resulting in the formation of nitrogen and water reducing toxic overall NO_x quantity. Ammonia presence in the exhaust gas environment is needed for the effectiveness of SCR system, but a high concentration of unreacted ammonia leaving the exhaust system is not allowed – ammonia slip. Excessive ammonia may cause system failure, such as AdBlue leaks from the injector, too low temperature or the system cleaning process when the engine is stopped. To limit the released ammonia ASC (Ammonia Slip Catalyst) may be used to oxidate ammonia to nitrogen and water.

Synergy of internal and non-engine solutions for cleaner emissions

Achieving cleaner emissions from internal combustion engines (ICEs) requires a multi-faceted approach, involving both primary and secondary methods for reducing harmful pollutants. While internal engine technologies, such as exhaust gas recirculation (EGR) and advanced fuel injection systems, contribute significantly to reducing emissions directly at the engine level, non-engine solutions integrated into the exhaust system are equally crucial. Technologies like catalytic converters, particulate filters, Lean Nox Traps (LNT), and Selective Catalytic Reduction (SCR) systems play vital roles in reducing nitrogen oxides (NOx), carbon monoxide (CO), particulate matter (PM), and other harmful emissions. These innovations help to ensure that engines meet increasingly stringent emission standards.

However, as emission regulations become more stringent, the integration of these systems becomes necessary to achieve the required emission reductions, especially in diesel engines. Despite the complexity and increased cost that these solutions introduce, their possible effectiveness in making exhaust gases cleaner than the air entering the engine highlights the potential for ICEs to contribute to future pollution reduction efforts, particularly in highly urbanized areas. The continued development of these technologies offers significant promise for improving air quality and advancing towards a more sustainable automotive future.

Jakub Szylar

Project Leader, Powertrain Systems

I have always been passionate about automotive. Some say I must have been born in the garage, but it was my father, a car mechanic, who introduced me to the world of automobiles. As a Project Leader at Endego I feel creative and responsible for work results that bring satisfaction to our clients. Performing both design and management activities I can be a part of any team on the way to success.