How do aftertreatment systems differ between hydrogen and diesel engines?

As fleets work towards reducing emissions, understanding the differences in aftertreatment systems for various fuel types is crucial. The adoption and growth of hydrogen internal combustion engines (ICE) is a significant step as hydrogen ICE provides a low-emission, low-carbon option for a more sustainable transportation sector. The mechanisms that clean exhaust gases in these engines are fundamentally the same as diesel ICE, with the key difference being what is in the exhaust gas to start with.

This article delves into the key differences between aftertreatment systems for diesel and hydrogen engines, providing essential insights for fleets navigating this transition.

How does the aftertreatment system work in diesel engines?

Diesel engine aftertreatment systems play a crucial role in reducing emissions from diesel engines. These systems act as a filter for the exhaust gases, ensuring harmful pollutants are converted into harmless substances before they are released into the air. You will find aftertreatment systems in every diesel-powered application.

Diesel aftertreatment uses technologies like diesel particulate filters (DPFs) and selective catalytic reduction (SCR) units to filter exhaust gases. DPFs capture soot particles directly from the exhaust, while SCR units utilize a urea-based solution to transform nitrogen oxides into nitrogen and water.

Interestingly, not all applications require aftertreatment systems. For example, emergency power generators in North America are exempt due to their specific use cases.
This is true, however, in industrialized countries, but there are many regions globally that are still lagging on emissions requirements that would drive the need for aftertreatment systems.

By integrating advanced aftertreatment technologies, diesel engines can continue to offer the efficiency and power they are known for, while significantly reducing their environmental footprint. It is a balance of performance and responsibility, ensuring that diesel engines can meet today's emission standards and contribute to a cleaner environment.

Developments in aftertreatment system for diesel engines

Diesel engine aftertreatment systems have evolved significantly over the years, adapting to stringent emissions standards across various regions. In North America, diesel oxidation catalysts (DOC) have been utilized since the 1990s, depending on the engine manufacturer. The role of the DOC has been to initiate the chemical process that breaks down pollutants in the exhaust stream, preparing them for further treatment.

In 2007, diesel particulate filters (DPF) were introduced, bringing a more complex system that shifted from a passive to an active state, with the latter requiring periodic intervention to burn off accumulated particulates. Further advancements came in 2016 with the introduction of single-module products that housed these components in a more compact, lightweight design. These systems have continued to develop to meet even stricter emissions standards, paving the way for ultra-low NOx (Nitrogen Oxide) systems featuring dual dosing and heaters to enhance performance and control.

The diesel aftertreatment system has matured across global markets, unifying around core technologies that cater to environmental regulations and market needs. As a result, the once-divergent paths of diesel emissions technology in different parts of the world are converging, creating a more standardized approach to reducing the environmental impact of diesel engines.

How does the aftertreatment system work in hydrogen ICE?

Here is the thing with hydrogen engines: ideally, when you combust hydrogen (H2) and oxygen (O2), you expect to get only water vapor and no NOx emissions – a harmful pollutant. But that is in an ideal world. In real-world conditions, however, a lean-burn H2 ICE does produce NOx along with water vapor. This happens because air, which contains nitrogen, is used for combustion. This is where the Selective Catalytic Reduction (SCR) system comes into play to address the NOx in the tailpipe emissions.

Small amounts of hydrocarbons in the exhaust, due to engine oil consumption, may necessitate an oxidation catalyst in hydrogen internal combustion engines (ICE). However, in some hydrogen applications, hydrocarbon levels are so low that the ammonia slip catalyst at the end of the SCR system may be sufficient to convert harmful emissions to harmless ones, eliminating the need for an additional oxidation catalyst.

Although hydrogen combustion does not directly produce CO2, the operation of the SCR system can result in minor CO2 emissions. Interestingly, some regions, such as Europe, may still classify hydrogen internal combustion engines (ICEs) as zero-emission vehicles despite this. In North America, regulators are still debating this classification. Nonetheless, even with these minimal emissions, hydrogen ICEs achieve a significant reduction in CO2 compared to diesel engines—around 90 percent.

That is why hydrogen is often referred to as “bridge” technology. It provides an opportunity to get remarkably close to zero CO2 emissions, serving as a stepping-stone, or bridge, depending on how regulators view it. It must be noted that hydrogen fuel cell electric vehicles (FCEVs) are typically seen as the long-term, truly zero-emission goal, but their adoption comes with higher investment costs.

Aftertreatment systems become a key component for H2 ICEs in this transitional phase. They ensure that hydrogen engines can significantly reduce emissions, making them a viable and cleaner alternative in the market. H2ICE technology is poised to evolve with aftertreatment systems in the mix, hydrogen engines can indeed serve as a robust bridge to a greener automotive future.

Difference between hydrogen and diesel aftertreatment systems

Diesel engines produce CO2, NOx, and particulate matter (PM). Aftertreatment methods like Exhaust Gas Recirculation (EGR) lower NOx emissions by reducing combustion temperatures. Additional methods, such as Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR), also remove NOx from diesel exhaust.

While NOx emissions in hydrogen engines are like diesel, there are key aftertreatment differences. Hydrogen fuel does not require a particulate filter to capture soot, which is large, expensive and needs service. Instead, a simpler oxidation catalyst can be used. The aftertreatment systems for both diesel and hydrogen ICE are fundamentally similar, with SCR for hydrogen ICE using the same diesel exhaust fluid as diesel engines.

Hydrogen, however, poses unique challenges in the aftertreatment process. The water content in the exhaust and hydrogen's reaction with certain metals and welds necessitate the use of materials compatible with hydrogen to maintain system integrity. Hydrogen engines have a cleaner combustion process, eliminating the need for Diesel Particulate Filters (DPFs) and simplifying the aftertreatment system.

By focusing on lean-burn technology and the SCR system, we can effectively control and reduce NOx emissions in hydrogen ICE without the added complexity of diesel aftertreatment systems.

Advancing emissions control

As we approach the implementation of the United States Environment Protection Agency’s 2027 emissions standards, additional tools for thermal management are anticipated to become more widespread, potentially in 2024. These advancements aim to further reduce NOx emissions, ensuring diesel engines can meet the forthcoming stringent standards.

The approach to emissions control varies slightly between Europe and North America due to different market drivers. In Europe, the primary focus has traditionally been on controlling NOx emissions, driven by a strong emphasis on fuel economy. Some regions may require particle number control using filters, while others may necessitate basic filtration to meet particle size limits, as particulate filters are not essential for capturing soot like they are in diesel engines. Consequently, hydrogen aftertreatment systems will differ across markets, with North America employing more advanced technology to meet ultra-low NOx regulations. However, with the introduction of Euro 6 regulations, emissions control technology has become more aligned between North America and Europe.

Cummins is leading the charge to develop technologies that cater to both diesel and hydrogen engines. As hydrogen technology continues to mature, the need for complex aftertreatment systems will decrease, offering a simpler, more environmentally friendly alternative to traditional diesel engines. Cummins remains committed to leading this transition, developing engine technologies that meet the demands of the future while minimizing environmental impact.