Diesel Engine Emissions and Control Equipment
Lukas Willits
University of Idaho
ABSTRACT
This technical paper explores the major exhaust
pollutants of the 4-stroke compression ignition
(CI) diesel engine and how the diesel engine
compares to the spark ignition (SI) gasoline 4stroke engine.
Government regulations are becoming
increasingly strict on pollutant emissions of
diesel engines. The negative consequences of
these emissions to the environment and human
health will also be examined. Additionally, the
differences between these engine designs and
why they require different emissions control
equipment will be addressed. Finally, the
function of control equipment employed to
reduce these pollutant emissions in diesel
engines will be discussed.
EXHAUST POLLUTANTS
Here are the major exhaust pollutants we
generally focus on reducing and more
specifically why we want to reduce them.
Nitrogen oxides (NOx) contribute to the
formation of smog by reacting with volatile
organic compounds (VOCs), which includes
unburned hydrocarbons, to create tropospheric
or, “ground level” ozone. Nitric oxide also
depletes the ozone layer in the stratosphere.
Although ozone is harmful to our health in the
atmosphere closest to the Earth’s surface due
to smog formation, stratospheric ozone is what
protects us from excess UV radiation. [1]
Carbon monoxide is another ingredient in the
formation of tropospheric ozone and smog
formation.
Carbon dioxide is a greenhouse gas contributing
to man-made climate change. Also, it is
estimated that one-third of all man-made
carbon dioxide emissions have been absorbed
by our oceans, which increases their acidity [2].
This poses a threat to aquatic life that is
sensitive to pH balance.
Unburned hydrocarbons include volatile organic
compounds, as well as methane and ethane.
VOCs are highly reactive and a major ingredient
in tropospheric ozone and smog formation,
while methane and ethane are less reactive but
still contribute to smog. Methane is also a
greenhouse gas.
Particulate matter and soot cause a variety of
human health problems, such as asthma and
lung cancer. [3] Soot, or black carbon, also
causes positive radiative forcing like
greenhouse gases. Positive radiative forcing
means the earth receives more incoming energy
from sunlight than it radiates to space. The
radiative forcing of greenhouse gasses and
particulates are shown in Figure 1.
Figure 1. Radiative forcing in watts/meter2 of
greenhouse gasses and particulates. [4]
COMPARISON OF ENGINE DESIGNS
DIESEL ENGINE EMISSIONS
Figure 2 shows a comparison of emissions
between compression ignited and spark ignited
engines. Compression ignited engines have onefifth the unburned hydrocarbon emissions,
higher nitrogen oxide emissions, lower carbon
monoxide emissions, and comparable carbon
dioxide emissions. [5]
The lean combustion operation of compression
ignition engines adds complexity to emissions
reduction because the three-way oxidation
reduction catalysts used in spark ignition
gasoline engines aren’t effective at reducing
nitrogen oxides due to the excess oxygen in
compression ignition engine exhaust.
These differences are mainly due to the lean
combustion and higher compression of
compression ignition engines, resulting in
higher temperature and excess oxygen
combustion conditions.
Instead a diesel oxidation catalyst (DOC) is used
for oxidizing carbon monoxide and unburned
hydrocarbons, while other methods are used to
reduce nitrogen oxides. The main methods of
reduction are selective catalytic reduction (SCR)
and diesel exhaust fluid (DEF) injection, and
lean NOx traps (LNT).
The higher concentration of soot found in
compression ignition engines’ exhaust, which is
due to direct injection of diesel fuel into the
cylinder, is reduced by using a diesel particulate
filter (DPF).
DIESEL ENGINE EMISSIONS CONTROL
EQUIPMENT
DIESEL PARTICULATE FILTER
Figure 2. Comparison of the composition of
exhaust emissions between CI and SI engines.
[6]
Diesel particulate filters collect and remove
around 85% of soot from diesel exhaust,
however they will eventually clog or create
excessive backpressure if the soot is not burned
out of the filter with sustained high exhaust
temperatures. The process of burning the soot
is called regeneration and is achieved passively
and actively. Passive regeneration occurs during
extended highway driving where exhaust
temperatures reach the light-off temperature
required for regeneration, about 550 oC or
higher. Active regeneration can occur by the
engine control unit signaling an extra diesel fuel
injector upstream of the DPF to dose and ignite
fuel in the DPF, by using an electric heater, or
by using microwave heating. Roughly every
200,000 miles, the DPF must be removed and
professionally cleaned. [7]
SCR. Smaller passenger diesel powered vehicles
use LNTs as a cheaper and simpler method of
NOx reduction, removing the necessity of the
operator to refill the DEF storage tank. Another
benefit of LNT’s is in medium and heavy-duty
applications they can be calibrated to generate
ammonia for use in a downstream SCR system,
reducing the amount of DEF being consumed.
One downside of LNT’s is they gradually
become poisoned by sulfur dioxide in the
exhaust and require high temperature
regeneration periods like the DPF to regain
effectiveness of reducing NOx. [8]
Figure 3. Diesel Particulate Filter [3]
SELECTIVE CATALYTIC REDUCTION
Selective catalytic reduction (SCR) converts NOx
into nitrogen and water by injecting diesel
exhaust fluid (DEF) into the exhaust. DEF is a
mixture of water and urea, and the urea
thermally decomposes into ammonia in the
exhaust and is absorbed by the SCR catalyst,
allowing the chemical reactions in figure 4 to
occur. Urea is used as opposed to ammonia as it
is safer to handle and store. SCR has a
conversion efficiency of about 90% for NOX. [7]
Figure 5. Diagram of standard emissions
control equipment employed in diesel engines.
[7]
SUMMARY
Diesel engines differ from 4-stroke gasoline
engines due to their high compression
combustion ignition. The high compression and
lean air/ fuel mixture inherent to the diesel 4stroke engine creates the need for alternative
exhaust pollution reduction equipment.
Figure 4. Chemical reactions occurring within
the SCR system. [7]
LEAN NOX TRAPS
Lean NOx Traps are another method used for
reduction of nitrogen oxides in diesel engines
and can either be a replacement for Selective
Catalytic Reduction or used in conjunction with
The pollution control equipment used are the
diesel particulate filter to remove and burn
soot, selective catalytic reduction with DEF
injection to reduce nitrogen oxides into
nitrogen, and a diesel oxidation catalyst to
oxidize carbon monoxide and unburned
hydrocarbons into carbon dioxide and water.
REFERENCES
1. “Tropospheric Ozone.” Wikipedia,
Wikimedia Foundation,en.wikipedia.org
/wiki/Tropospheric_ozone.
2. Mathez, Edmond A., and Jason E. Smerdon.
Climate Change: The Science of Global
Warming and Our Energy Future. Columbia
University Press, 2009.
3. Reşitoğlu, İ.A., Altinişik, K. & Keskin, A. The
pollutant emissions from diesel-engine
vehicles and exhaust aftertreatment
systems. Clean Techn Environ Policy 17, 15–
27 (2015). https://doi.org/10.1007/s10098014-0793-9
4.
“Causes of Climate Change – Online
Course.” FutureLearn, University of
Bergen,www.futurelearn.com/courses/caus
es-of-climate-change.
5. Heywood, John. Internal Combustion Engine
Fundamentals. McGraw Hill, 1988.
6. Motor Vehicle Exhaust Emissions.
Volkswagen AG,
www.volkspage.net/technik/ssp/ssp/SSP_2
30.pdf.
7. Alozie, N. S., & Ganippa, L. C. (May
29th,2019). Diesel Exhaust Emissions and
Mitigations. doi: 10.5772/intechopen.85248
8. Johnson, Timothy V. “Diesel Emissions in
Review.” SAE International Journal of
Engines, vol. 4, no. 1, 2011, pp. 143–157.
JSTOR, www.jstor.org/stable/26278142.
ME 433 Project Evaluation Rubric
Student: Lukas Willits
Title: Diesel Engine Emissions
Deliverable: Research Paper
Exemplary
DELIVERY
CONTENT
Date: 8/9/20
Style
Fully
Meets
Criteria
Background
Provides clear context with
compelling purpose and strong
inquiry questions.
Analysis
Appropriately describes relevant
engineering concepts and tools.
References
Cites robust set of credible
resources, appropriately cited and
formatted.
X
Organization
Logically ordered with effective
transitions between sections.
X
Visuals
Graphics, tables, and equations
used to support project narrative.
X
Language, sentence structure, and layout
conveys professional meaning.
X
Partially
Meets
Criteria
Minimally
Meets
Criteria
X
X
Comments: The background category is satisfied with a solid set of inquiry questions which determined
the structure of the paper. The analysis category is where I see the greatest
weakness in the content of this paper. I read about a lot of technical topics
along the lines of diesel engine emissions, such as engineering concepts and
tools/ equations. The problem was that I couldn’t decide how to incorporate
them into my research paper in a coherent manner. My set of references are
robust and include multiple credible sources like the Heywood text we used for
the class, as well as a few papers published by SAE. I believe I organized my
paper in such a way that the reader progresses through the inquiry questions I
set out to answer in a logical manner. I added more visuals and diagrams to aid
in understanding of the concepts and systems being discussed throughout the
paper. Style is not one of my strengths, but I feel I did an adequate job using
proper language for a technical research paper and avoided obvious errors by
rereading my writing