State-of-the-Art of Establishing Test Procedures for Real Driving Gaseous Emissions from Light- and Heavy-Duty Vehicles

energies
Review
State-of-the-Art of Establishing Test Procedures for Real
Driving Gaseous Emissions from Light- and
Heavy-Duty Vehicles
S. M. Ashrafur Rahman 1 , I. M. Rizwanul Fattah 2, * , Hwai Chyuan Ong 2 , Fajle Rabbi Ashik 3 ,
Mohammad Mahmudul Hassan 4 , Md Tausif Murshed 5 , Md Ashraful Imran 5 , Md Hamidur Rahman 6 ,
Md Akibur Rahman 7 , Mohammad Al Mahdi Hasan 8 and T. M. Indra Mahlia 2

1 Biofuel Engine Research Facility, Queensland University of Technology (QUT), Brisbane, QLD 4000, Australia;

2 Centre for Green Technology, Faculty of Engineering and IT, University of Technology Sydney,
Sydney, NSW 2007, Australia; (H.C.O.);
(T.M.I.M.)
3 BUET—Japan Institute of Disaster Prevention and Urban Safety (BUET-JIDPUS),
Bangladesh University of Engineering and Technology, Dhaka 1000, Bangladesh;

4 International Training Network (ITN), Bangladesh University of Engineering and Technology,
Dhaka 1000, Bangladesh;
5 Department of Civil, Environmental, and Geomatics Engineering, Florida Atlantic University,



777 Glades Road Building 36, Boca Raton, FL 33431, USA; (M.T.M.);

 (M.A.I.)
6 Asian Disaster Preparedness Center (ADPC), Dhaka 1206, Bangladesh;
Citation: Rahman, S.M.A.; Fattah, 7 Petromax Refinery Ltd., Khulna 9351, Bangladesh;
I.M.R.; Ong, H.C.; Ashik, F.R.; Hassan, 8 School of Environment and Science, Griffith University, Nathan, QLD 4111, Australia;
M.M.; Murshed, M.T.; Imran, M.A.;
Rahman, M.H.; Rahman, M.A.; * Correspondence:
Hasan, M.A.M.; et al. State-of-the-Art
of Establishing Test Procedures for
Abstract: Air pollution caused by vehicle emissions has raised serious public health concerns.
Real Driving Gaseous Emissions from
Vehicle emissions generally depend on many factors, such as the nature of the vehicle, driving
Light- and Heavy-Duty Vehicles.
style, traffic conditions, emission control technologies, and operational conditions. Concerns about
Energies 2021, 14, 4195. https://
the certification cycles used by various regulatory authorities are growing due to the difference
doi.org/10.3390/en14144195
in emission during certification procedure and Real Driving Emissions (RDE). Under laboratory
Academic Editor: Attilio Converti conditions, certification tests are performed in a ‘chassis dynamometer’ for light-duty vehicles (LDVs)
and an ‘engine dynamometer’ for heavy-duty vehicles (HDVs). As a result, the test drive cycles used
Received: 17 June 2021 to measure the automotive emissions do not correctly reflect the vehicle’s real-world driving pattern.
Accepted: 7 July 2021 Consequently, the RDE regulation is being phased in to reduce the disparity between type approval
Published: 12 July 2021 and vehicle’s real-world emissions. According to this review, different variables such as traffic signals,
driving dynamics, congestions, altitude, ambient temperature, and so on have a major influence
Publisher’s Note: MDPI stays neutral on actual driving pollution. Aside from that, cold-start and hot-start have been shown to have an
with regard to jurisdictional claims in effect on on-road pollution. Contrary to common opinion, new technology such as start-stop systems
published maps and institutional affil-
boost automotive emissions rather than decreasing them owing to unfavourable conditions from
iations.
the point of view of exhaust emissions and exhaust after-treatment systems. In addition, the driving
dynamics are not represented in the current laboratory-based test procedures. As a result, it is critical
to establish an on-road testing protocol to obtain a true representation of vehicular emissions and
reduce emissions to a standard level. The incorporation of RDE clauses into certification procedures
Copyright: © 2021 by the authors.
would have a positive impact on global air quality.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
Keywords: air pollution; real driving emission; driving cycles; portable emission measuring systems;
distributed under the terms and
air quality
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).




Energies 2021, 14, 4195. https://doi.org/10.3390/en14144195 https://www.mdpi.com/journal/energies

,Energies 2021, 14, 4195 2 of 32




1. Introduction
As the world progresses, technological development has resulted in the availability of
machinery, which translated into the development of a machine-based industry, resulting in
increased emissions. The imposed travel restrictions worldwide in 2020 due to the COVID-
19 pandemic has resulted in a 6% reduction in energy demand compared to 2019 [1].
Despite the fact that the global energy-related CO2 emissions declined by 5.8% in 2020,
they still remained at 31.5 Gt, which contributed to CO2 reaching its highest-ever average
annual concentration in the atmosphere of 412.5 parts per million in 2020—around 50%
higher than when the industrial revolution began [2]. While it would be ideal to have the
latest data, at the time of publishing the report ‘Emissions by sector’ [3] (September 2020),
the most recent comprehensive data set were available for 2016. The authors reported
that the global greenhouse gas (GHG) emissions were 49.4 billion tonnes (Gtoe) of carbon
dioxide equivalents (CO2 eq) at that time [3,4] (CO2 eq sums all of the warming impacts of
the different greenhouse gases together. To calculate CO2 eq of non-CO2 gases, their mass
was multiplied by their ‘global warming potential’ (GWP). GWP measures the warming
impacts of a gas compared to CO2 ). They reported that the energy sector contributed almost
3/4th of the total GHG emissions (73.2%) [3]. Of this 73.2% emitted by the energy sector, the
majority of contributors were the industry, transport and building sectors which consisted
of 24.2, 16.2 and 17.5%, respectively (Figure 1). About 73.5% of transport sector emission is
contributed by ‘road transport’, followed by ‘aviation’ and ‘’shipping’ at 11.7 and 10.5%,
respectively, as shown in Figure 1. A report claims that to achieve net-zero-emission
by 2050, the world needs to maintain a drop in emissions of around 5% each year from
now forth [5].




Figure 1. Global greenhouse gas emission by sector for 2016. Total greenhouse gas emissions were 49.4 billion tonnes
CO2 equivalent [3].

As of 16 March 2021, the total number of confirmed COVID-19 cases had surpassed
183 million, with the pandemic affecting more than 192 countries/regions [6]. COVID-19

,Energies 2021, 14, 4195 3 of 32




has now resulted in the death of over 3,962,550 people [6]. More than a year into the
pandemic, the countries worldwide are still struggling to combat the spread of COVID-19,
which is hurting the economy. Thus, when the world returns to a new normal state, it will
be hard to keep the energy demand low and reduce GHG emissions due to the sudden
increase in social and economic activities. Decisive actions are needed to reduce transport
emissions to achieve the zero-emission target. Thus, the vehicles that are used for everyday
transport must follow the emission standards set by the regulatory body. Figure 2 shows
the timeline of emission standards for passenger cars for the United States, Europe, China
and Japan [7].




Figure 2. Timeline of emissions standards for passenger cars in different parts of the world [8].

There is ongoing concern about public health owing to air pollution caused by vehi-
cle emissions. It is well-recognised that air pollution is a primary risk factor for chronic
non-communicable diseases [9]. Pollutants carry microorganisms that are highly inva-
sive to humans, affecting the immune system and making people more susceptible to
pathogens [10]. The COVID-19 pandemic has also manifested the importance of a healthy
environment. Vehicle emissions depend on the vehicle nature and mode of operation
factors such as driving style [11], traffic conditions [11], fuel quality and specifications [12],
the technology behind the vehicle design, such as emission control technology [13,14], and
ambient conditions [14,15]. These factors determine the number and amount of pollutants
emitted during the driving interval and cannot be replicated through engine test cycles.
Hence, these pave the way for vehicle development and the consequent recent advance of
vehicle technology and emission control strategies. This also depends on driving behaviour
and traffic conditions—several factors such as changing lanes, overtaking or merging
result in increased engine loads [16]. As a result, the engine operates in a rich fuel-air
ratio, and thus emissions are increased [17]. Furthermore, vehicle acceleration and speed
also affect emissions. Vehicle acceleration significantly impacts CO and HC emission,

, Energies 2021, 14, 4195 4 of 32




especially at high speeds and low vehicle speed in congested traffic results in increased
emission. Auxiliary loads such as air conditioning system can increase CO and NOx
emission and sometimes result in double the emissions. Road type is another critical factor,
e.g., hill ascents result in high NOx emissions. Also, the horizontal curvature of roads and
roundabouts increases engine load and thus results in increased emissions. Thus, the gap
between regulated vehicle emissions from certification procedures and real-world driving
emissions has become increasingly wider [16,18]. The inclusion of real driving test proce-
dures can attenuate the discrepancies between emission values determined in laboratory
tests and emission values produced during on the road driving because they take greater
acceleration into account, along with gradients, stop-and-go, or higher speeds. Emission
measurements under real driving conditions can significantly contribute to improving
the air quality of the world. The development of an RDE test cycle has been thoroughly
discussed in the literature, with many methods suggested [19–21]. It is to be noted that this
review article does not focus on engines for non-road applications that constitute: handheld
portable devices (lawnmowers, grass mowers, chainsaws, hedge trimmers, twig choppers,
snow removal machines, and devices applied in forestry), power generators, and non-road
vehicles also referred to as Non-Road Mobile Machinery (NRMM) [22]. The NRMM vehicle
group includes construction machinery, farm tractors and machines, and special-purpose
machinery [23,24]. This group of vehicles is subject to separate regulations on exhaust
emissions which was detailed by Waluś et al. [25]. Emissions from such machinery are
also tested against standards during approval; however, these tests are conducted in real
operating conditions [24,26–28].
Previous reviews on emission test procedures generally focused on various aspects of
vehicle certification procedures. For example, Mahlia et al. [29] reviewed motor vehicle fuel
efficiency research procedures in order to establish a test protocol to endorse Malaysia’s fuel
economy standard, labelling, and other associated services. They came to the conclusion
that Malaysia should use the Japanese JC08 fuel economy evaluation protocol for motor
vehicles. Hooftman et al. [30] studied the history of European emission regulations and
offered a comparative overview of the European market’s approaches with the approaches
of other major automotive markets around the world. They concluded that a significant
revision of the European regulatory system governing automotive emissions is needed.
Agarwal and Mustafi [31] conducted a more recent study of real-world vehicle emissions,
focusing on the more recent methodologies for monitoring vehicular emissions under real-
world driving conditions. This report would concentrate on research that has contributed to
developing the RDE test protocol, which aims to minimise the gap between type-approval
and real-world driving emissions.

2. Vehicle and Engine Test Cycle Basics
Vehicle emissions are one of the main sources of greenhouse gas (GHG) emissions in
modern cities, leading to air pollution [32]. The rising number of passenger cars, especially
in the last decade, has resulted in a complicated traffic issue with significant implications
in terms of vehicular emissions [33,34]. Since the early 1960s, vehicles’ compliance with
emission regulations has been checked using standardised tests [35]. These have been
known as driving/drive cycles, test cycles, or transient cycles. Even though these three
terms are often used in the literature interchangeably, they might not mean the same
test procedures or parameters. Driving test cycles involve testing the whole vehicle and
typically comprise of a series of data points representing a speed-time profile that is
representative of urban driving [36–39]. In particular, the test cycle consists of a series of
test points, where the vehicle or engine in question has to follow a certain speed at each
point. Thus, in this regard, test cycles are primarily classified as (a) chassis dynamometer
cycles used for vehicle testing and (b) engine dynamometer cycles used for engine testing.
Engine tests cycles are carried out for exhaust emission certification procedures for heavy-
duty and off-road vehicles as it is often impractical to put those vehicles on a chassis
dynamometer. These tests are performed in an engine test-bed following a pre-determined