Mercedes Mullins
Diesel fuel has long been associated with harmful emissions, contributing to air pollution and adverse health effects. The notion of clean diesel has been challenged by scandals and the inherent challenges of diesel engine technology. However, diesel engines remain prevalent, powering trucks, ships, machinery, and electricity generators worldwide. To address pollution, regulations like the Diesel Emissions Reduction Act (DERA) aim to reduce diesel emissions through fuel economy improvements and control strategies. Innovations in combustion science seek to fully burn diesel fuel to reduce soot and nitrogen oxide emissions, improving engine efficiency and environmental impact. Clean diesel fuel is also crucial for engine longevity, and proper storage and maintenance techniques are essential to prevent degradation and contamination. As a result of EPA regulations, modern diesel engines are cleaner, but older engines still contribute to pollution and health risks. The transition to cleaner diesel engines is gradual, highlighting the ongoing debate and efforts to make diesel a cleaner fuel.
| Characteristics | Values |
|---|---|
| Clean diesel fuel | Ultra-Low-Sulfur Diesel (ULSD) fuel |
| Clean diesel fuel characteristics | Clear with a distinctive amber or red coloration ("off-road" diesel); clear with a slight green color ("highway" diesel) |
| Clean diesel fuel benefits | More durable, last longer, fewer components, fewer repairs and maintenance services, more fuel-efficient, more energy per gallon, cost savings |
| Clean diesel fuel issues | Soot, nitrogen oxides (NOx), air pollution, acid rain, damage to soil, lakes, and streams, entry into the human food chain, property damage, reduced visibility, contribution to climate change, serious health issues (e.g., asthma, respiratory illnesses, exacerbation of heart and lung disease) |
| Clean diesel fuel regulations | EPA fuel standards for ULSD fuel, emissions standards for diesel engine highway vehicles (model year 2007 and later), Diesel Emissions Reduction Act (DERA) |
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What You'll Learn
Ultra-Low-Sulfur Diesel (ULSD) fuel
Diesel engines are robust, durable, fuel-efficient and can provide the torque needed to move large things. Hundreds of millions of medium and large long-haul trucks, the majority of the world's trains, ships, off-road vehicles, heavy machinery, electricity generators, domestic pickup trucks and European passenger cars run on diesel.
However, diesel engine exhaust contains harmful pollutants, such as nitrogen oxides (NOx) and soot particles. The soot-NOx trade-off has been a long-standing diesel engineering dilemma. To address this, engineers have been working on burning diesel fuel completely to avoid soot, while keeping temperatures low to avoid excess nitrogen oxide.
Since 2006, almost all petroleum-based diesel fuel available in Europe and North America has been ULSD. In the United States, EPA fuel standards require a major reduction in the sulfur content of diesel fuels, and as of December 1, 2010, all diesel fuel sold in the U.S. must be ULSD. Similarly, in 1993, the European Union began mandating the reduction of diesel sulfur content and implemented modern ULSD specifications in 1999. The United States started phasing in ULSD requirements for highway vehicles in 2006, with implementation for off-highway applications, such as locomotive and marine fuel, beginning in 2007.
ULSD fuel has also been adopted in other countries. For example, in 2020, Pakistan began importing Euro-V standard fuel, which includes ULSD diesel. In 2010, Delhi introduced 50 ppm sulfur diesel to curb vehicular pollution, and Bharat Stage VI with ultra-low sulfur was introduced in New Delhi in 2018. In Taiwan, diesel fuel has been limited to 10 ppm sulfur since 2007.
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Nitrogen oxides and soot particles
Diesel exhaust is a source of atmospheric soot and fine particles, which are components of air pollution that can cause human cancer, heart and lung damage, and mental functioning. It is also a Group 1 carcinogen, which causes lung cancer and is associated with bladder cancer. Diesel exhaust contains several harmful pollutants, including nitrogen oxides (NOx) and soot particles.
The soot-NOx trade-off, a long-standing diesel engineering dilemma, refers to the challenge of burning diesel fuel completely to avoid soot while keeping temperatures low to prevent excess nitrogen oxide formation. To overcome this, engineers must burn diesel fuel more efficiently while maintaining lower temperatures to minimize nitrogen oxide emissions.
Several methods exist to reduce nitrogen oxides (NOx) and particulate matter (PM) in diesel exhaust. One approach is Selective Catalytic Reduction (SCR), which involves injecting a reductant like ammonia or urea into the exhaust to convert nitrogen oxides into gaseous nitrogen and water. Another method is using Ultra-Low-Sulfur Diesel (ULSD) fuel, which helps reduce vehicle particulate emissions and nitrogen compound emissions.
Additionally, researchers have proposed placing a tiny version of a Bunsen burner in the diesel combustion chamber to promote better burning. The Bunsen burner design utilizes slots near the bottom of the tube to draw in air through the Venturi effect, allowing more oxygen to mix with the gas stream and ensuring more complete combustion. This innovation can potentially reduce soot emissions and enable cleaner burning.
Furthermore, advanced post-processing technologies, such as combining diesel particulate filters (DPF) with selective catalytic reduction (SCR), are employed to control soot particles and NOx emissions from diesel engines. DPFs capture particulate matter, but periodic regeneration is necessary to maintain efficiency due to soot accumulation. Regeneration methods include raising the exhaust temperature or coating catalysts on the DPF to promote soot oxidation.
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Fuel economy and idle reduction strategies
Diesel engines are robust, durable, fuel-efficient, and can provide the torque needed to move large things. Most of the hundreds of millions of medium and large long-haul trucks on highways today run on diesel, as do the majority of the world's trains, ships, off-road vehicles, and heavy machinery.
The transportation sector accounts for approximately 30% of total US energy needs and 70% of US petroleum consumption. Idle reduction strategies to reduce petroleum consumption strengthen national energy security by increasing resilience to natural disasters and fuel supply disruptions, as well as reducing transportation energy costs for businesses and consumers.
Idle reduction strategies can reduce emissions of greenhouse gases and other harmful pollutants, such as nitrogen oxides (NOx) and soot particles. Argonne National Laboratory estimates that more than one million long-haul heavy-duty trucks idle during required rest stops, consuming more than one billion gallons of fuel per year. In addition, thousands of work trucks idle daily, and millions of passenger cars idle in traffic, drive-throughs, or drop-off/pickup situations. Accounting for all road vehicles, from passenger cars to heavy-duty trucks, more than 6 billion gallons of gasoline and diesel are lost to idling every year.
To achieve reductions in vehicle idling, strategies and actions must be taken to minimize the time spent by drivers idling their engines. Many light-duty vehicle manufacturers have implemented stop-start technologies on their vehicles, which automatically shut off the engine when the vehicle stops and quickly restart it when the gas pedal is pressed. Other idle reduction technologies include auxiliary power units (APUs), direct-fired heaters, fuel cells, thermal storage systems, truck stop electrification, battery-based systems, engine idle management (shutdown) systems, electrical (shore power) solutions, cab comfort systems, and hybridization. These technologies can reduce the amount of energy wasted by idling trucks, locomotives, and automobiles, saving fuel and maintenance costs and extending vehicle life.
Another potential strategy to reduce emissions from diesel engines is to improve the combustion process to reduce soot and nitrogen oxide emissions. One approach is to premix the fuel with air before ignition, allowing the charge to burn leaner at a lower temperature. This can be achieved by equipping diesel fuel injectors with tiny metal tubes installed a short distance from the injector nozzle hole and aligned with the fuel stream, enabling a more complete and even burn. This method has been compared to the Bunsen burner, which draws air into the fuel stream through the Venturi effect, resulting in a clean blue flame.
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The soot-NOx trade-off
Diesel engines are robust, durable, fuel-efficient, and can provide the torque needed to move large things. Hundreds of millions of medium and large long-haul trucks, the majority of the world's trains, ships, off-road vehicles, and heavy machinery run on diesel. Diesel engine exhaust contains harmful pollutants, such as nitrogen oxides (NOx) and soot particles. The soot-NOx trade-off, a long-standing diesel engineering dilemma, refers to the challenge of reducing both NOx and soot emissions simultaneously.
To overcome this issue, engineers must find a way to burn diesel fuel completely, eliminating soot, while maintaining low temperatures to prevent excess nitrogen oxide formation. Paul Miles, manager of Sandia's engine research program, emphasizes the critical nature of this research area. Experiments have been conducted to optimize emissions, focusing on strategies such as internal exhaust gas recirculation (EGR), external EGR, intake air heating, and diesel injection timing retardation.
One notable experiment investigated the impact of fuel oxygen content on emissions. It was found that increasing fuel oxygen to 15% resulted in very low soot levels at all test EGR dilution levels, achieving a breakthrough in the NOx-soot trade-off. Another study explored the use of a premixed low-temperature combustion (LTC) mode, which has the potential to overcome the trade-off by reducing both NOx and soot emissions simultaneously.
Additionally, combustion scientist Charles Mueller proposed a solution inspired by the Bunsen burner, suggesting the placement of a tiny version of the burner in the diesel combustion chamber to promote better burning. This approach aims to achieve a more complete combustion by enhancing the mixing of fuel and air, resulting in a cleaner blue flame burning. Despite these efforts, the "clean diesel" concept remains questionable due to scandals like "Dieselgate," where carmakers were accused of cheating on emissions tests.
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Fuel maintenance and storage
Diesel fuel storage tanks are an integral component of any firm or factory that relies on diesel generators, trucks, or machinery. As such, it is essential to carry out regular maintenance to ensure the tank is in proper working condition throughout the year.
One of the critical issues with diesel fuel storage is the buildup of sludge, biomass, and water, which can lead to microbial infestations that degrade fuel quality. To address this, regular cleaning of the tanks is necessary, and the use of commercial-grade treatments and biocides can help prevent and control the growth of microbes.
Oxidative fuel breakdown from exposure to air and light, and hydrolytic failure from exposure to certain metals, are also concerns that need to be managed. Ultra-low-sulfur diesel fuels, while helping to reduce emissions, are more susceptible to these issues due to the reaction between sulfur and the microbicide.
Fuel maintenance services can include fuel sampling, fuel polishing, fuel additive programs, and de-watering of storage tanks. These services can help maintain fuel quality and ensure efficient power output when it is needed.
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Frequently asked questions
Clean diesel is diesel fuel that is free of contaminants such as water, sediment, or microbial growth. It is important to use clean diesel fuel to maintain the efficiency and longevity of diesel engines.
Diesel engine exhaust contains harmful pollutants, such as nitrogen oxides (NOx) and soot particles, which contribute to air pollution and have negative impacts on human health and the environment. Older diesel engines are particularly polluting.
Fuel polishing can be used to restore aged diesel fuel by circulating it through filtration systems. Additives can also be used during storage to stabilise fuel and prevent microbial growth.
The US Environmental Protection Agency (EPA) has implemented fuel standards to reduce the sulfur content of diesel fuels, resulting in Ultra-Low-Sulfur Diesel (ULSD) fuel, which helps to reduce emissions. The EPA has also established emissions standards for diesel engine vehicles and provides funding for projects that aim to reduce diesel emissions.
