In general, drive cycle development phases include three main components: test route discovery, data collection, and cycle design methods
. Route selection is an important aspect of the cycle development process where considerations are given on travel activity patterns as well as traffic flow characteristics. The data collection should consider driving both weekends and weekdays so that collected data represent different driving conditions based on the selected routes. Cycle construction is done by feeding driving data records into the representative cycle and then modifying and smoothing the cycle based on statistical analysis.
5. Real Drive Emission Cycle Tests
Yang et al. 
evaluated diesel and gasoline vehicle real drive emissions. The authors developed an RDE route of 60 km consisting of urban, rural and motorways. The speed limit for the respective parts was 50, 90 and 130 km/h, which falls within the design specification. The vehicles were equipped with different emission reduction techniques. For emission measurement, a PEMS consisting of a Horiba OBS 2200 was used to collect unit to collect instantaneous and cumulative data of gaseous emissions (CO2
, CO, THC, and NOx). The authors reported a higher gaseous emission for urban roads due to poor driving conditions and motorways due to excessive energy demand. CO and THC emission for all the vehicles were below the specific limit (for gasoline CO: 1.00 g/km and THC: 0.10 g/km, for diesel CO: 0.50 g/km and THC: 0.09 g/km). However, NOX
emission was above the vehicle types’ limits (gasoline: 0.06 g/km, diesel: 0.08 g/km). Diesel vehicles emitted lower average trip CO2
compared to gasoline vehicles which might be associated with better thermal efficiency.
Thomas et al. 
tested EURO V compliant gasoline vehicles fitted with a 3-way catalyst for emission reduction. The RDE tests were performed using PEMS devices: the Horiba OBS-ONE-GS PEMS equipment and the AVL MOVE Gas PEMSiS equipment. The trials started with urban driving, followed by rural and motorway driving sections. Urban, rural and motorway roads were approximately 34, 33 and 33% of the total route. RDE CO2
emissions were higher than the EU passenger car CO2
goal for 2015 (130 g/km). However, RDE CO emissions were 60% lower than the WLTC (well below the Euro 5 limit of 1 g/km), and RDE NOX
emission was 34% lower compared to the WLTC emissions.
Wang et al. 
evaluated volatile organic compounds emission from 4 different vehicles, including gasoline cars, light-duty diesel trucks, heavy-duty diesel trucks, and LPG-electric hybrid buses. The authors used two routes. Route A was approximately 68 km, including urban, suburban and highway roads, while route B was 18 km long route of No. B12 bus in Zhengzhou, China, which includes 29 stations. To simulate passengers dropping off and getting on, the bus was stopped at each station for 10 s. A SEMTECH ECOSTAR PLUS with an extra VOC sampling unit comprising three units: micro proportional sampling system (MPS), gas analyser, particulate matter (PM) analyser was used to measure emissions. The VOC emissions were different for different vehicles. The major VOC species for vehicles were as following:
Gasoline cars: i-pentane, acetone, propane, and toluene.
Light-duty diesel truck: mainly long-chain alkanes- dodecane, n-undecane, naphthalene and n-decane, which in total contributes to 70.4% of total species
Heavy-duty diesel truck: naphthalene contributed 31.8% of total VOC, which might be due to the engine operating conditions and the pyrolysation from incomplete combustion (Lin et al., 2019a, 2019b).
LPG bus: short-chain hydrocarbons, acetone, i-pentane, i-butane, n-butane and propane (46.7% of the total VOCs).
For the gasoline cars, aromatics take up 39.4, 35 and 41.7% of the tailpipe VOCs under the urban, suburban and highway conditions, respectively. The higher aromatic emission at the highway may be caused by the lower air/oil ratio of port fuel injection (PFI) under ultra-high-speed working conditions. Heavy-duty diesel trucks emitted 8.6 times higher VOC than light-duty diesel trucks, which can be attributed to poor vehicle maintenance as exhibited by the inefficient Selective Catalytic Reduction (SCR) device. Gasoline and LPG vehicles produced relatively lower VOC emissions compared to diesel vehicles. This could be attributed to the TWC of gasoline vehicles which might have reduced the VOC emissions. Highway operating conditions emitted much lower VOC emissions compared to urban road conditions.
Du et al. 
tested the effect of cold-start on primary emission parameters. For the experiment, light-duty gasoline vehicles were selected, which were equipped with a TWC converter, closed-loop control of fuel injection and gasoline particle filter (GPF). The PEMS used in the tests was a HORIBA OBS-ONE. A route of 75.4 km was selected, which consisted of an urban section (32.6%), a rural one (32.4%) and a motorway section (35.0%). The altitude of the route ranged from 192.7–313.2 m, which satisfies the RDE test regulations requirements. The authors reported a significant increase in CO, CO2
and PN emission at the cold-start compared to a hot start.
studied three vehicles of varying age groups (BS-II/post- 2000; BS-III/post-2005, and BS-IV/post-2010). The fuels used in the vehicles were also different: gasoline, compressed natural gas (CNG) and diesel. The author selected a route of 10 km consisted of heavy traffic, lean traffic and traffic signals. On-road emissions were measured using Aerosol Emission Measurement System (AEMS), which consists of heated duct, dilution tunnel, zero air assembly, heated particle sampling probe and power supply unit. The author reported that diesel emitted the lowest CO, where gasoline emitted the highest. CO2
of gasoline vehicles were dependent upon acceleration and deceleration. Furthermore, the gasoline vehicle emitted a higher amount NOX
of compared to the other two vehicles.
Cao et al. 
also evaluated on-road VOC emission; however, they opted for a car chase method. The emission measurement system included an exhaust flow meter tube, mobile emission analyser, micro proportional sample system and VOC sampling unit. The test route included arterial roads (19.5 km) and highway roads (14.2 km). The route was divided into three defined driving cycles: arterial road hot start (ARHS), highway hot running (HWHR) and arterial road hot running (ARHR). The test vehicles were light-duty gasoline conformed to diverse emissions standards (Pre-China I to China IV). The authors reported that, for most of the vehicles, hot start resulted in the highest VOC emission, and highway hot running produced the lowest emission, which can be attributed to the low average speed and the effect of the hot start.
Daham et al. 
investigated the impact of micro-scale driving actions (left and right turns, stops and start at traffic lights, etc.) on on-road vehicle emissions. The authors selected a route of 0.6 km, which consisted of four left-hand turns. They used several manual vehicles which conformed to Euro I to IV. They evaluated several driving variations, left turn (anticlockwise) with and without a stop at intersections; right turn (clockwise) with and without a stop at crossroads. The HC emission for the tests was well below the corresponding legislation. However, the NOx emission for all the vehicles were all above the legislative limits. The authors reported that aggressive driving behaviour heavily affects NOX
and CO emission; a three times increase was observed. Aggressive driving also increased CO2
Roso and Martins 
compared two different methods to develop an RDE cycle: average method and cumulative method. The emissions were evaluated using computational models developed through GT-Suite software. The authors used two vehicles to develop the cycle: bus and passenger vehicle and reported a significant increase of emission for cumulative methods compared to the FTP-75 cycle.
Zhou et al. 
evaluated the effect of various environmental constraints on real drive NOX
emission. If there is a difference in altitude or temperature, it will have an impact on parameters such as oxygen content, air intake and air-fuel ratio of the engine and which will vary in-cylinder combustion, amount of pollutants and fuel consumption of vehicles. Emission varies with temperature and speed. For example, with vehicle speed <20 km/h, low altitude NOX
emission is higher than that of moderate altitude. High temperature and oxygen enrichment result in reduced NOX
emission, and thus high altitude produces a lower emission. Akard et al. 
also reported the influence of altitude on CO2
emission. The authors reported that a change in altitude of 90 m might result in a change in CO2
emission by 10 gm/mile.
Ren et al. 
evaluated the effect of various alternative fuels on the emission of buses on their usual daily route. The fuels considered were diesel, 20% Waste cooking biodiesel+ 80% diesel (B20) and liquid natural gas (LNG). The PEMS was composed of a gas test unit: an onboard emissions measurement system and a particle test unit: engine exhaust particle sizer. Compared to diesel-operated buses, B20 and LNG buses had a reduction in PM emissions; however, NOx and PN emissions were higher.
Braisher et al. 
evaluated on-road emission against legislative cycles such as NEDC, WLTC. The test cell is equipped with the Horiba MEXA-2000SPCS, and PN was measured using the pre-series instrument Nanomet3-PS where particle detection was based on aerosol particles corona charging. The selected driving route consists of an urban route (max speed of 50 km/h) and short segments (speed 70 km/h). The second route includes a long autobahn (high-speed road network) route with a max speed of 100 km/h. the complete PEMS evaluation run consists of two urban routes and one autobahn route. The total run time was approximately 90 min. Cold-start tests were conducted at different ambient temperatures (8 to 28 °C). The on-road measurements were performed using gasoline operated Euro-5 compliant passenger vehicle. The obtained results show that 60% of the trip PN emission is generated while the vehicle operates during the cold-start period. Braisher et al. 
reported that during the NEDC cycle, 77% of the total PN emission is generated during the cold-start period. Furthermore, the tests also reveal that driving style is an essential parameter with a substantial impact on the PN emission. The autobahn trips show a significant change in PN emission (10× increase) when switched from normal to severe driving. This can be attributed to larger accelerations, higher velocities, and higher power demand.
Khalfan et al. 
evaluated the effect of traffic congestion on vehicle emissions. A portable Fourier Transform Infrared (FTIR) spectrometer was used to measure real-world on-road emissions. The total route distance was 5 km, and the speed limit was 48 km/h. Two different cycles were conducted with varying numbers of right and left turns, traffic lights and pedestrian crossings. These dynamic factors would result in numerous stop/start events. The authors reported that congestion affected the CO2
emission; most of the trips produced more than 180 g/km CO2,
which is above NEDC certification limits. THE CO and THC, and NOX
emissions were dependent upon the traffic speed. These emissions were higher than the legislative limits at lower speeds.
Rosenblatt et al. 
tested three variants of a motorbike on a 30 min test cycle, which consists of a cold-start phase, intermediate speed and high speed, with each stage being weighted equally at 0.33. The authors found that the PM emission was lower than the current Tier 3/LEV III light-duty highway vehicle limit of 3 mg/mile. A considerable fraction of CO and HC emissions were produced over the cold-start phase and/or high-speed phases, whereas NOX
emission was most influential over the high-speed stage.
Modern-day vehicles have new technologies that help the driver, such as adaptive cruise control (ACC) and start-stop systems. ACC assists the driver in maintaining longitudinal control of their car when travelling on the highway, automatically adapting to different traffic situations. The system regulates the accelerator, engine powertrain, and vehicle brakes to maintain the desired time gap to the vehicle ahead. The start-stop system is a simple and low-cost solution in which the internal combustion engine is automatically turned off when the automobile is stopped and re-started at the driver’s request or as necessary. As a result, it avoids idle fuel use, such as that caused by traffic lights or congestion, which may account for up to 10% of overall consumption 
. It is to be noted from the research of Warguła et al. 
that a system similar to the start and stop system is only used in a machine (wood chipper), where idling occurred more frequently. This did not turn off the engine rather reduced its rotational speed, reducing fuel consumption 
, CO, CO2
and NOx emissions with increasing HC emission 
. This confirms the fact that the internal combustion engine performs best under constant operating conditions. Limiting the idling fuel consumption as well as exhaust emission can be further achieved through cylinder deactivation 
. This is also known as variable displacement engine technology. The method entails deactivating one or more cylinders by inhibiting valve actuation, causing the engine to run at a greater specific load across the remaining cylinders to provide the required torque [108,109]
. Operating at such a position with a wider throttle opening decreases the engine’s pumping losses and, as a result, fuel consumption.
Dvorkin et al. 
evaluated the effect of ACC on vehicle emissions on-road. The performance of ACC depends upon two factors: platooning and controlled acceleration. When vehicles closely follow each other to reduce aerodynamic drag, it is known as platooning, which is most effective in highway conditions where aerodynamic drag dictates road load forces. The author reported that active ACC resulted in less CO2
g/miles, and optimum GHG reduction is achieved when the following distance is 20–40 m and vehicles operating at highway speeds. In another study 
, the authors concluded that if households switched from conventional vehicles to electric vehicles, they could reduce their GHG emissions significantly.
Andersson et al. 
evaluated two EURO 6 compliant light-duty diesel vehicles. These vehicles had two different technology installed to reduce NOX
emission. One vehicle had SCR while the other had exhaust gas recirculation. Both consisted of diesel particulate filters for PM reduction. Furthermore, the vehicles were tested in two different routes, one with 60% urban condition and another with 60% motorway. The authors reported that both vehicles produced significantly higher NOX
emissions compared to the EURO 6 legislation limit; however, THC and CO emissions were within the limit.
Wang et al. 
compared PEMS with a mobile monitoring system for on-road emissions. The authors selected heavy-duty diesel trucks that complied with China III to China V emission standards. The authors reported that the mobile monitoring system reported similar NOX
results compared to that of the PEMS.
Park et al. 
studied the on-road emission of 109 gasoline and diesel vehicles using PEMS on roads in Seoul (South Korea). The authors compared the emission results against laboratory experiment results and found that Euro 5 and Euro 6B standard vehicles emitted 5 times more on-road NOX
compared to laboratory-based tests. The authors also reported that the on-road NOX
emission is also dependent on factors such as driving dynamics and ambient temperature. One drawback of the study is that the stop ratio of more than 30%, which does not meet the EU RDE regulations. However, the average speed of the vehicles was within 15 to 40 km/h, which is within the regulations.
Bischoff et al. 
compared the on-road emission of motorcycles. The authors evaluated two driving styles, normal and dynamic driving, where the latter represents a sporty driving style with increased longitudinal dynamics. The authors reported that CO and NOX
emissions were 58 and 36% higher than that of WMTC certification limits. Furthermore, CO, CO2
emissions increased significantly when switched from normal driving to dynamic driving.
presents a summary of RDE route descriptions and emission results from various literature.
6. Comparison of RDE and laboratory testing
Degraeuwe and Weiss  compared NOX emissions from on-road testing and laboratory-based NEDC tests for diesel and gasoline vehicles. According to the authors, the NOX emissions of diesel engines in laboratory tests are below the emission level limit, but on-road emissions are far above the limit. The on-road emissions from gasoline vehicles are higher than the NEDC pollution values, but they are still within the emission limits. On-road NOX emissions for diesel cars are 181 percent higher than the NEDC average. Several national form approval authorities confirmed that tested vehicles emit emissions below the limit in a laboratory setting, but 4.5 to 4.7 times higher than the limit for EURO 5 and EURO 6 vehicles [119,120]. In contrast to laboratory-based experiments, on-road tests released 50% more NOX, according to Pirjola et al. . Valverde et al.  conducted on-road and in-laboratory NEDC studies on diesel and gasoline vehicles. On-road emissions for diesel vehicles were 14 times higher than NEDC tests and 6 times higher than type approval limits, according to the authors. The authors also stated that PN emissions for diesel cars were below the TA limit in both on-road and laboratory tests, and CO2 emissions for on-road were marginally higher than the TA limit. On-road and in-lab NOX emissions from gasoline vehicles were within the TA cap. On-road emissions and chassis dynamometer-based in-lab measurements were compared by Besch et al. . The authors used two different routes with a Jeep Grand Cherokee and found that NOX emissions were significantly higher on both routes as compared to chassis dynamometer cycles. Park et al.  also reported that on-road NOX emission was five times higher than laboratory-based emission tests. On the contrary, Thomas et al.  reported 34% lower NOX emission and 60% lower CO emission compared to laboratory cycle-based emission results. The major factors for such discrepancies between on-road and laboratory-based emission findings are that on-road emissions are affected by various factors that are not considered in laboratory-based driving cycles, such as driver aggression, congestion, road gradient, etc. Furthermore, most authors indicated that NOX and PN emissions are the ones that differentiate from laboratory studies.
In recent years researchers have found a significant difference in emissions reported using type emission tests and on-road emission tests. The test drive cycles employed to measure the emissions produced by vehicles are expected to adequately represent the vehicle's real-world driving pattern to provide the most realistic estimation of these levels. However, which is not the case. Furthermore, two recent scandals have identified that auto manufacturing companies use emission defeat devices that would reduce vehicle emission by tracking when used in chassis dynamometer and enabling emission reduction techniques. These have resulted in researchers prioritising on-road emission tests known as RDE testing. The paper discusses RDE development methods and reviews past works. The review shows that various factors such as traffic signal, driving dynamics, congestions, altitude, ambient temperature etc., have a significant impact on on-road emissions. In addition, driving behaviour, along with the driving route, significantly impacts vehicle emission, which is not represented in type emission tests. For example, aggressive driving behaviour increased NOx and CO emissions three times than normal driving behaviour. Route characteristics such as traffic lights, right/left turns, roundabouts, gradients were all found to have a significant impact on vehicle emissions. The literature review also indicated that modern vehicle technologies such as start-stop systems also affect vehicle emission. The start-stop system results in increased emission due to the engine not operating in conditions intended, i.e. stoichiometric operation and operating temperatures because of frequent stops and starts. This contributes to the disturbance of the mixture composition (enrichment during acceleration, idle when reducing the rotational speed). These factors cause the engine to work on mixtures other than stoichiometric, i.e. unfavourable from the point of view of exhaust emissions and exhaust after-treatment systems. This also corroborates the fact that an internal combustion engine performs best under constant operating conditions. The driving dynamics are not represented in the current laboratory-based test procedures. Thus, it is important to consider all these factors when developing a real drive emission cycle to evaluate the emission level of any vehicle. This, in turn, will significantly improve the air quality around the world.