Page 46 - Energize July 2021

TECHNICAL

The technologies
Now that we have an understanding of the legal requirements, let’s look
at the different technologies used to attain these emission standards and
air quality requirements.
Diesel emission control systems can be broadly broken down into
two categories: (1) in-cylinder strategies and (2) aftertreatment systems,
and the selection and configuration of technologies utilised is dependent
on the engine manufacturer and machine application.

In-cylinder
As emissions standards tightened, more advanced in-cylinder control
strategies were applied, which included energy-efficient cylinder heads
and valve train systems, closer piston-to-bore clearances and modified
ring positioning to assist in lower emissions output.
In the last two decades, the design of diesel engines has progressed
rapidly, most significantly in the areas of fuel injection systems,
Figure 2: Exhaust gas recirculation
electronic controls and air handling through the use of variable-geometry
turbochargers.
Many of the latest generation engines have common-rail or unit- Aftertreatment systems
injector designs, a common feature which produces far higher injection An aftertreatment system treats post-combustion exhaust gases prior to
pressure than the old mechanical systems, coupled with precise tailpipe emission. In other words, it is a device that cleans exhaust gases
electronic control of injection timing. Other in-cylinder techniques to ensure the engines meet emission regulations.
also included the adoption of the Miller cycle, diesel water injection Within the aftertreatment category there are a further two classes –
and homogenous charge compression ignition (HCCI). These various filters and catalysts.
techniques help achieve a more complete combustion and reduce
particulate formation and fuel consumption. Catalysts
Air handling strategies have been focused on the use of variable In chemistry, a catalyst is a substance that causes or accelerates a
geometry turbochargers to provide the right amount of air under specific chemical reaction without itself being affected. Catalysts participate
engine operational conditions. Increasing the pressure of the air entering in the reactions but are neither reactants nor products of the reaction
the chamber increases the air density and allows for better combustion they catalyse.
in the brief time available. Tuning these parameters minimises production A catalytic convertor in the broad sense of the term is a device
of both PM and NOx. that uses a catalyst to reduce the toxicity of emissions from an internal
Another popular in-cylinder technology for NOx control is exhaust combustion engine either through the process of oxidation or reduction.
gas recirculation (EGR). An EGR system recirculates a portion of cooled The first diesel emission catalysts, introduced in the 1970s for
exhaust gas back to the engine’s cylinder, which reduces both peak underground mining applications, were simple oxidation catalysts
combustion temperature and temperature-dependent NOx formation. designed for the conversion of CO and HC but as the years rolled on and
EGR is the most effective and commonly-used technology for in- requirements intensified, more specialised catalysts were developed as
cylinder NOx reduction in diesel engines. The EGR fraction, the share can be seen in the table below.
of recirculated exhaust gas in the total intake charge, is tailored to
each engine operating condition and can vary from 0% to 40% of the Filters
incoming air. A filter does exactly as its name implies - that is, it physically filters out
Since EGR reduces the available oxygen in the cylinder, the something. To be more specific, a filter is a porous device for removing
production of PM increases when EGR is applied. NOx and PM are impurities or solid particles from a liquid or - more relevantly - gas,
traded against each other in many aspects of diesel engine design. Very passed through it.
high temperatures in the combustion chamber help reduce the emission Ultimately, using a combination of physical mechanisms and
of soot but produce higher levels of NOx gases. Lowering the peak chemical reactions, these systems can, under the right conditions,
temperature in the combustion chamber reduces the amount of NOx achieve near complete removal of particulates and harmful gases. Let’s
produced but increases the likelihood of soot formation. take a closer look at how these devices work.

Table 2: Examples of diesel engine catalysts

Catalyst technology Reaction type Target emissions
Diesel oxidation catalyst Oxidation CO, HC, PM-(SOF)

Urea-SCR catalyst Selective catalytic reduction by ammonia NOx
Ammonia slip catalyst Selective oxidation of the ammonia slip NH₃

Hydrocarbon-SCR or Lean NOx catalyst Selective catalytic reduction by hydrocarbons (HC-SCR) NOx, CO, HC
NOx adsorber catalyst or Lean NOx Trap Adsorption (trapping) of NOx from lean exhaust, followed by NOx, CO, HC
release and catalytic reduction under rich conditions

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