Nitrogen oxides (NOx) is a family of air pollutants, namely, Nitric Oxide (NO) and Nitrogen Dioxide (NO2) emitted from on-road and off-road vehicles and power plants as a result of fuel combustion at high temperatures. These pollutants are responsible for smog, acid rain, ground-level ozone, secondary fine particulate matter (PM2.5), and respiratory diseases in humans.
A study conducted by the International Council on Clean Transportation (ICCT) in collaboration with the University of Colorado, Stockholm Environment Institute, and the International Institute for Applied Systems Analysis, found that air pollution from diesel NOx emissions caused 107,600 early deaths worldwide in 2015. Heavy-duty vehicles such as commercial trucks and buses were by far the largest contributor worldwide, accounting for 76% of the total excess NOx emissions. At a global level, the study projects that the impact of all real-world diesel NOx emissions will grow to 183,600 early deaths in 2040. The study recommends that the single most important action to reduce the health impacts of excess diesel NOx emissions is for countries to implement and properly enforce a Euro VI tailpipe emission standard for heavy-duty vehicles.
Does this mean that the diesel engine is the culprit and that the answer to an emissions-free future would be to replace diesel engines with petrol engines because diesel produces more NOx than gasoline? There is no easy solution to this problem because the challenge lies in reducing CO2 and pollutants at the same time, and such measures are limited by the fact that an attempt to decrease CO2 causes an increase in pollutant emissions. A car engine running at high temperature tends to emit less CO2 but more NOx, and a lower engine temperature will result in more CO2 but less NOx.
For decades, consumers in Europe favoured diesel vehicles over petrol because of their lower carbon emissions, but it came at the cost of early deaths due to higher NOx emissions, as mentioned above. When they decided to switch to petrol engines, CO2 emissions began to increase. The European Environment Agency (EEA) reported that CO2 emissions from new cars in the European Union (EU) have been increasing since 2017, after steadily declining during 2010–2016. For the first time, the average CO2 emissions from new vans also increased in 2018. Coincidentally, the number of new registrations of gasoline cars, SUVs and vans increased during these two years.
According to the European Automobile Manufacturers’ Association (ACEA), significant improvements in automotive technology have helped vehicle manufacturers cut both pollutant and CO2 emissions, significantly. Such technological advancements include improvements to engine combustion and fuel management such as variable valve timing, turbocharging, stop-start systems, direct engine injection technology and vehicle weight reduction to reduce CO2 emissions, and exhaust after-treatment systems such as lean-NOx catalysts and selective catalytic reduction (SCR) converter systems to reduce NOx emissions. Particle mass and particle number emissions have been reduced by high-efficiency diesel particle filters.
Currently, the most popular method of reducing NOx from the exhaust gases of diesel vehicles is after-treatment with SCR converter systems, which have been effective for almost a decade.
Selective catalytic reduction is a chemical reaction which converts NOx into water (H2O) and nitrogen (N2) by using a reducing agent such as urea (carbamide) and ammonia as the reductant. The reducing agent, also called diesel exhaust fluid (DEF), is a 32.5% solution of urea in deionised water. The DEF is not a fuel or fuel additive; it’s a clear, non-toxic liquid that is safe to handle and does not cause damage to the environment.
The DEF is filled in a dedicated tank installed beside the diesel tank. During the SCR process, DEF is injected into the exhaust gas stream, which converts urea into ammonia initially and then reduces 80–90% of the NOx.
The ISO 22241-1:2019 standard specifies the DEF as AUS 32, and the German Association of the Automotive Industry or Verband der Automobilindustrie (VDA) markets it worldwide as its registered trademark AdBlue.
After-treatment of NOx with SCR also complies with the Euro VI emission standards, which are being implemented globally. The VDA estimates that 20 million vehicles from German brands will be driving on Western Europe’s roads by 2020 equipped with SCR technology.
Diesel trucks imported in the UAE are required to comply with the Euro IV emission standards. Because, the diesel quality in the country is suitable for Euro V standards, manufacturers are offering Euro V models instead of Euro IV. The change in emission regulations has boosted the supply of AdBlue in the UAE. Trychem, a part of Germany-based Brenntag, supplies DEF in 10-litre cans for fuel service stations, 1,000-litre intermediate bulk containers (IBCs) with pumps, and bulk installations for both home depots and public filling stations which are monitored with telemetry solutions. Recently, the ENOC Group launched its own brand of DEF called ENOCBlue, which will soon be rolled out across it retail network in the UAE. However, there are barriers to the adoption of AdBlue, such as its relatively high price and lack of awareness among fleet operators about the necessity, application, availablity, and regulations related to the product.
Pre-treatment of NOx with diesel additives
An alternative solution to after-treatment of NOx is pre-treatment of NOx with diesel additives. About two years ago, UAE-based Emirates Leisure Retail (ELR), a subsidiary of the Emirates Group, began exploring environmentally friendly transportation solutions as part of the organization’s broader sustainability strategy.
ELR started looking for ways to reduce emissions from its fleet of over 50 trucks that deliver beverages to retail outlets in the UAE every day. In this case, the top-down approach of the management set long-term goals to mitigate both carbon and NOx emissions and formulated a plan for emission trials.
For over 13 months, ELR has been conducting emission trials on its light-duty delivery trucks using various blends of diesel and additives, under hot and cold weather conditions. The trucks are well maintained as per their service schedules. They operate for at least 10 hours a day and are at 80–90% of their payloads while leaving for deliveries.
The diesel additives are supplied by UAE-based Zaeto which provides eco-friendly lubricants and greases specifically designed for the climate in the Middle East. Zaeto’s diesel additive, called D-Zel Aid, can be added to diesel or biodiesel and helps the fuel burn cleaner and creates less soot and thus reduces NOx. D-Zel Aid also reduces the amount of fuel consumed. The dozing of the diesel additive in the fuel tanks is done manually by the drivers.
Nicholas Brooks, managing director, Zaeto, says: “D-Zel Aid is concentrated diesel additive which can be added directly into the fuel tank without the need for retrofitting or additional parts. We’ve been dozing 0.5 millilitres of D-Zel Aid per litre of diesel, which amounts to 50 millilitres for a 100-litre fuel tank. During these trials, we’ve seen reductions in CO2 by 50–70%, NOx by 35%, and CO by 8%. We’ve also achieved improvement in fuel economy by at least 10%.”
To validate the trials, ELR involved the Automobile and Touring Club of UAE (ATCUAE) as an independent testing authority. Since then ATCUAE has been conducting regular emission tests on ELR trucks running only on diesel and those running on diesel mixed with D-Zel Aid.
The initial trials involved 10 trucks ranging from 4.9 to 10 tonnes. This was later reduced to four 4.9 tonne trucks, all Mitsubishi Fuso Canter models with identical specifications, so that all parameters could be monitored thoroughly and compared it with the historical data of the trucks, particularly their fuel consumption during the last seven years.
Jason O’Keefe, distribution and warehouse manager, ELR, says: “Reducing the sample size enabled us to achieve consistency in the testing process; as a result, we saw the fuel economy double compared to the initial results.
The average fuel economy for the initial sample size of 10 trucks was 8–9%; this increased to 15–20% when we reduced the sample size to four trucks. We incentivised the four drivers to maintain consistency in dozing the fuel tanks by using syringes to measure the volume of additives.”
“Although this initiative was not prioritised on fuel economy, that’s exactly what we’ve achieved while also reducing emissions. These findings are crucial for us because it will generate more savings with increase in the price of diesel in the UAE,” adds Jason.
Table 1 shows the results of the idle and stall tests (60% load) for the four trucks tested in April 2019 by ATCUAE. Trucks 1 and 2 were running on diesel alone and Trucks 3 and 4 were running on a mixture of diesel and D-Zel Aid. The NOx emissions are expressed in parts per million (ppm).
In the idle test, the NOx values of Trucks 1 and 2 at 652 rpm and 654 rpm were 142 ppm and 170 ppm, respectively; those of Trucks 3 and 4 at 641 rpm and 844 rpm were 24 ppm and 57 ppm, respectively.
In the stall test, the NOx values of Trucks 1 and 2 at 3094 rpm and 2930 rpm were 407 ppm and 443 ppm, respectively; those of Trucks 3 and 4 at 2938 rpm and 2906 rpm were 223 ppm and 327 ppm, respectively. The increase in O2 emissions for Trucks 3 and 4 in both the idle and stall tests indicates that a lower amount of oxygen and nitrogen was burnt and thus less NOx was produced.
The significant NOx reduction in the idle tests is crucial for ELR because all its trucks idle for several hours during deliveries to keep the air conditioning and refrigeration systems running.
“All our trucks are reefer vehicles and we rarely turn the engines off during deliveries. During the 10 hours and more of operation of each of our trucks, the engine is switched on for 80–85% of the time. The results under load are fantastic, but those under the idle state are even better for us because of the day-to-day realities of our operations,” says Jason.
One of the challenges ELR aims to overcome with the diesel additive is its dozing consistency, which could be achieved if it is premixed with diesel by fuel suppliers.
“Dozing the fuel tank manually comes with the risk of human error, which will not give the same fuel economy across the entire fleet. The ideal solution is to have the diesel and additive premixed and delivered to our yard in fuel tankers,” says Jason.
This year, ELR will be replacing its current fleet of heavy-duty trucks with new Mercedes-Benz Actros Euro V models that will run on AdBlue. ELR is also conducting trials with biodiesel blends such as B5.
“We’re trying to find the ideal combination of diesel, additives, pre-treatment and after-treatment that works best for our fleet and contributes to our sustainability strategy. Currently, AdBlue is relatively expensive in this region; so, we’re keen to find whether or not the diesel additive can help reduce the volume of AdBlue required per truck,” he says.
NOx testing has a controversial past, but that’s changing with government-industry partnerships and new testing methods
Remember ‘Dieselgate’, the Volkswagen emissions scandal in 2015 when the US Environmental Protection Agency (EPA) and California Air Resources Board (CARB) found that Volkswagen, Audi, and Porsche installed a defeat device – a software designed to detect when the vehicles were undergoing emissions testing – on more than 600,000 2.0-litre and 3.0-litre diesel passenger cars in the US during 2009–2016. The software cheated emission tests by switching on full emissions controls only during laboratory tests and not during normal driving. As a result, the cars met emissions standards in a laboratory or testing station, but emitted up to 40 times the standard NOx levels during normal operation.
Volkswagen’s multi-billion-dollar settlement with the US government includes a series of three partial settlements to resolve the company’s violations of the Clean Air Act (CAA). Under the CAA 2.0 litre partial settlement, Volkswagen must recall or perform an approved emissions modification on at least 85% of the affected 2.0 litre vehicles by June 2019; Volkswagen estimates the total cost of achieving this target will go up to $10 billion. The CAA 2.0 litre partial settlement also requires Volkswagen to invest $2.7 billion in a NOx mitigation trust fund and $2 billion in zero emission vehicle (ZEV) charging infrastructure. Under the CAA 3.0 litre partial settlement, Volkswagen must contribute an additional $225 million in funding to the mitigation trust fund. This additional funding is intended to fully mitigate the total, lifetime excess NOx emissions from the 3.0 litre vehicles. Volkswagen must meet an 85% recall rate for the generation 1 affected 3.0 litre vehicles by November 30, 2019, and the generation 2 affected 3.0 litre vehicles by May 31, 2020. Volkswagen has also paid a $1.45 billion civil penalty under the third partial settlement.
The EPA is taking collaborative measures to prevent future violations by light-, medium-, and heavy-duty vehicle manufacturers. In July 2018, the agency demonstrated how the government and industry could work together to manage compliance failures.
When initial testing of certain trucks equipped with Cummins engines revealed excess NOx emissions, the EPA shared the results with Cummins, which prompted the engine manufacturer to recall roughly 500,000 medium- and heavy-duty trucks to replace a faulty component in the SCR system, making it the largest voluntary truck emissions recall to date.
The EPA estimates that although NOx emissions in the US have dropped by more than 40% over the last decade, heavy-duty trucks will be responsible for one-third of NOx emissions from transportation in 2025. In order manage this projected increase, the agency has launched the cleaner trucks initiative (CTI) to further decrease NOx emissions from on-highway heavy-duty trucks and engines.
The CTI aims to update the existing NOx standard, which was last set in 2001, and publish a proposed rule in early 2020. The EU introduced a new test in 2017 to reduce discrepancies in data from laboratory tests and real-world emissions. The worldwide harmonized light-duty vehicles test procedure (WLTP), which replaces the outdated European driving cycle (NEDC) test, applies to new passenger cars in the EU since September 1, 2017.
The EU is also the first in the world to introduce on-road testing, called the real driving emissions (RDE) test. The RDE test complements the WLTP lab test by ensuring that vehicles not only comply with NOx limits in lab tests but also under a wide range of real driving conditions. The RDE test requires vehicles to be equipped with portable emission measuring systems (PEMS) that monitor NOx and other pollutants emitted by the vehicles in real time. The RDE test has proved that the latest-generation diesel cars emit low NOx emissions on the road. In 2018, the ACEA published data that proves 270 new types of diesel cars introduced in the EU over the previous year performed well below the NOx threshold of the RDE test and below the stricter NOx threshold that will be mandatory from January 2020.
