The first Diesel engines were started 130 years ago, but they have moved a long way since then. Like their gasoline counterparts, the compression ignited engines have evolved from the rough units of the late 19th Century to reliable and refined units of today. However, the Dieselgate scandal showed us that getting them to match modern emission regulations is not easy. At first, Diesel engines started life in a similar form to industrial steam engines, but eventually found their way to passenger cars. From the first moment, they impressed engineers with the efficiency provided. Compared to a steam engine with a similar configuration, a compression ignition unit would provide a remarkable level of thermodynamic efficiency (18% compared to 32-35%). Compression ignition engines are more efficient than their spark-ignition counterparts because diesel fuel contains more energy by volume than gasoline. They also have a higher compression ratio, and that reduces consumption at idle. Modern diesel engines achieve a peak efficiency rating of 45% in passenger vehicles, and scientists expect them to reach a level of 55%. In theory, the diesel cycle has a maximum efficiency of 75%, but that has not been achieved in practice yet. Fans of diesel engines appreciate the high level of torque provided by these powertrains, as well as the fact that they tend to deliver peak torque earlier and for a generous rev range. However, diesel engines do not achieve the same power levels as equivalent capacity gasoline engines, because they must operate at lower engine speeds. What's the difficulty with diesel engine emissions?
Diesel Engines Then and Now – How the Emission Reduction Devices Were Developed
From the beginning, diesel engines produce less carbon dioxide than equivalent gasoline engines. However, they make more particulates and other toxic air contaminants. The most harmful of them are fine particles, which present health concerns, and some of them have been listed as carcinogenic (a cause of cancer) by the World Health Organization.
Therefore, diesel engines start out as more dangerous to the planet and living organisms. This health risk is already known as an occupational hazard for truckers and railroad workers, but the general population exposed to this type of engine is under threat.
Because of this, emission standards for diesel engines have become more and more strict, but not all manufacturers have managed to comply, as the Dieselgate scandal proved. The problem
The fuel/air mixture of diesel engines often leads to incomplete combustion, and the particles made by these power plants vary from one application to the other. We will focus on modern, four-stroke diesel engines for passenger vehicles. They can generate tiny nanoparticles, and those are dangerous to human health as they can infiltrate lungs and eventually cause respiratory illness.
The incomplete combustion of diesel engines also causes other particles and soot. Depending on fuel quality, the formation of the particles can vary. Modern compression-ignition engines require low-sulfur diesel fuel to operate properly. The first solutions
The catalytic converter was one of the first solutions the automobile industry provided to reduce emissions of diesel-engined vehicles. Initially, the technology had two-way catalytic converters that catalyzed a redox reaction. They eventually evolved to three-way converters. The latter had the ability to reduce nitrogen oxides, a dangerous gas for human health and also the element that Dieselgate-affected engines produced in excess.
Catalytic converters combined oxygen and carbon monoxide with unburned hydrocarbons to generate carbon dioxide and water. They do this by using rare metals such as rhodium, palladium, platinum, along with ceramic parts. While diesel and gasoline engines require different catalytic converters, they work on the same principle.Diesel Oxidation Catalysts
Eventually, stricter emission standards obliged carmakers to develop specific catalytic converters for compression ignition engines. These units use platinum, aluminum oxide, and palladium to act as catalysts that oxidize the hydrocarbons and carbon monoxide generated by the combustion process.
The process requires oxygen and forms carbon dioxide and water. Most converters end up operating at 90% efficiency and significantly reduced soot and diesel odor. But you are still left with some particles to eliminate.
However, the catalytic converters do not manage to reduce the harmful nitrogen oxide (NOx) gas, as attempting this would start a reaction with the high oxygen content in the converter. Therefore, a different method was necessary for reducing NOx emissions from diesel engines.
One of the first solutions to achieve this was exhaust gas recirculation. This works for gasoline engines as well.Exhaust Gas Recirculation
The emissions reduction technique of recirculating the exhaust gasses uses a special valve, called an EGR valve, that brings exhaust gasses back into engine cylinders through the intake port. The idea behind this is to provide gasses that are inert to combustion to absorb the heat generated by the ignition process.
Reducing the temperature in the combustion chamber brings the reduction of NOx gas, and improves reliability through less stress on internal engine components. The reduction of NOx gas through this method is achieved because nitrogen oxide gas is only produced at very high pressures and temperatures.
Exhaust gasses are not permanently recirculated in modern engines, as this is not efficient in all operating conditions. In diesel engines, EGR rates go up to 50% and have a positive effect on reducing NOx emissions. Applications vary from one company to the next, but they usually use a heat exchanger to decrease the temperature of the recirculated exhaust gas before injecting it into the intake. Modern systems are operated electronically using multiple sensors to ensure maximum efficiency.
Mazda is the only carmaker that achieved the Euro 6 standard with EGR NOx control technology. They can do this because Mazda's SkyActiv diesel power plants feature the lowest compression ratio of any production compression-ignited engine.Diesel Particulate Filter
Since catalytic converters have no shot against carbon particles, the diesel-engined vehicle started to require DPFs. These also used several rare metals, and they channel exhaust gasses to their walls, covered in cordierite or silicon carbide, where soot particles get trapped. The engine’s ECU monitors the particle level of the DPF and occasionally trigger regeneration.
The DPF regeneration cycle temporarily uses more fuel so it can attain high temperatures that are required to clean the diesel particulate filter. Diesel-engined cars that are often driven only in the urban environment need this more often.
If the driver does not know that the DPF of their vehicle is going through a regeneration phase and turns the engine off, the filter could be clogged. If no action is undertaken, the DPF could eventually be damaged and require replacement.
A diesel-engined vehicle with a malfunctioning emission control device will not operate properly and will require expensive repairs. The same car will not pass emission testing if it is checked with the malfunctioning DPF.When the diesel particulate filter was not enough
In the case of most diesel engines for vehicles, the diesel particulate filter was proving insufficient to achieve new regulations. So manufacturers came up with several solutions: selective catalytic reduction and lean NOx trap.
Most carmakers use the Selective Catalytic Reduction, a technical solution that involves a particular reagent to turn NOx gas into Nitrogen. This works with an additive called Diesel Exhaust Fluid, commonly known as AdBlue. We have a separate story on AdBlue here.Lean NOx Trap
The Lean NOx trap emissions reducing system works with a catalyst support coated with a special wash-coat. The latter uses zeolites or alkaline oxide to absorb the particles, thus reducing NOx emissions. Like the DPF, this requires high-temperature regeneration periodically, also done by pumping more fuel into the cylinders.
Unfortunately, this process reduces the absorber’s operating life, so these systems must be designed in a way to ensure long-term reliability. This technology is widespread in European diesel-engined vehicles, but it is not used on the entire range of most carmakers.
The Dieselgate-affected vehicles from Volkswagen used Lean NOx Trap solutions to reduce their NOx emissions. As you know, they did not work as advertised, but not necessarily because of the LNT technology. The German company saved enormous amounts of money by not introducing SCR solutions for emission reduction in all of their cars and resorting to a "defeat device."The Ultimate Solution
Some diesel-engined cars come with combined LNT and SCR emission reduction technologies. This type is more expensive than both solutions took separately, but will eventually provide compliance with stricter norms like US Tier 2 regulations and the future Euro 6c standard.
Furthermore, carmakers will be able to replicate lab results in the future real-world driving cycle thanks to the combined effort of Lean NOx Traps and Selective Catalytic Reduction on Diesel engines.
A study made by the International Council On Clean Transportation revealed that EGR vehicles appear to have a stable NOx emission behavior, and that is less affected by the driving profile. SCR vehicles were better than the rest during high-load and extra-high-speed (over 120 km/h) sub-cycle test of the Worldwide harmonized Light vehicles Test Cycle. Meanwhile, the LNT technology did provide the best values in current NEDC testing, but had the worst ones in the WLTC.
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