Gas dosing system sets new standards

With both commercial vehicle and car-makers facing tough Euro 5 targets on emissions of nitrous oxides (NOx), research and development engineers are trying to drive up the efficiency of SCR while reducing both system and operating costs. A remarkable advance in the technology appears to meet both these objectives. Prototypes have been built and tested and the company behind it - IMI Vision -is just starting the process of approaching major truck and SUV manufacturers, as well as Tier One suppliers that specialise in exhaust systems. A family of patents that cover the basic design principles and engineering design have been filed and IMI Vision is now hoping to attract development partners (an engine mapping algorithm specialist ideally) to help take the product to market. The new SCR system, known as 'Sigma', injects gaseous ammonia - continuously produced on board from a standard urea (AdBlue) solution - rather than injecting an atomised liquid into the exhaust. It is far smaller and lighter than systems using liquid urea, more controllable, does not create hard chemical deposits and needs virtually no maintenance. The design has evolved through several prototypes and thousands of man hours. The latest version is being developed for the truck market but the technology can be readily scaled down for SUVs and heavy, or high powered, diesel passenger cars. These are sectors for which SCR has been identified as inevitable if proposed Euro 5 NOx levels (the same as for gasoline powered cars) are to be met over the whole life of such vehicles. The Sigma truck prototype, shown here, is positioned within the exhaust system just downstream from the turbocharger and close to the catalytic converter. Within it is a transient liquid urea storage vessel (fed from the main tank), a reactor, a flap valve with its actuator that manages the heat, and a heat shield. The construction is stainless steel throughout. The reactor is an anaerobic chamber in which liquid urea solution undergoes controlled hydrolysis at around 180ºC, generating ammonia, water and CO2. This gaseous mixture is buffered and available at a self-generated pressure of around 10 bar for dosing. In the catalytic converter, the NH3 reacts with the NOx to form harmless nitrogen gas (N2) and water. For this to happen efficiently, dosing needs to be precisely controlled as a function of engine NOx output and catalyst operating conditions. In the words of Patrick Ward, "Our Eureka moment came when the chemists working on the project discovered how to control the chemical reaction [hydrolysis] of the liquid urea solution." IMI Vision says that its research engineers are considering a range of dosing strategies. But the major benefits of the system are its simplicity - there is no requirement for a compressor or air reservoir - along with the dosing accuracy and high speed of response achievable. The latter are said to allow diesel engines to be mapped more aggressively, leading to fuel savings of between 1% and 3% over a standard SCR solution and up to 8% over a non-SCR engine. In reality, the technology should also lead to less de-tuning of the engine in order to keep it below the required NOx generation levels. So how does Sigma stack up alongside 'wet' SCR systems? Most current systems spray urea into the exhaust system almost like an aerosol. They are effective but have some drawbacks. They are bulky, generally designed to sit alongside the exhaust system and, in a truck application, the urea has to be sprayed into the exhaust at least one metre ahead of the catalytic converter. With Sigma, this distance from the catalytic converter can be halved, resulting in a more compact system. Some of the urea is converted into cyanuric acid, rather than ammonia gas, requiring up to 30% of the SCR's catalytic converter capacity to be used to re-convert this into ammonia gas. This, of course, reduces the converter's capacity to deal with NOx or requires it to be up-sized accordingly. Worse still, under some conditions urea will not decompose into ammonia and, instead, recombines into triuates, biurates or crystals. These hard deposits can accumulate like lime scale and stalactites in the exhaust system. IMI Vision says accumulations can be acute around the fine nozzles that inject liquid urea. To clear deposits, compressed air has to be periodically blasted through the system, leaving vehicles susceptible to shutdowns or low quality air supply. Sigma's high accuracy dosing, on the other hand, has been shown under test conditions to raise NOx reduction efficiency above 90% (70-80% for 'wet' systems) allowing more adventurous tuning and leading to improved fuel efficiencies. Overall, urea consumption is reduced. And, packaging is improved since the system is built into the exhaust pipe and doses the gas very close the catalytic converter. The converter can be made 25% smaller, because it doesn't have to deal with deposits resulting from isocyanic acid. It also has fewer moving parts than wet systems, so it is easier and more cost-effective to manufacture, and does not require complex maintenance. These are key issues for SUV and passenger car makers who are seeking compact, mechanically simple, solutions. Patrick Ward, business development manager at IMI Vision commented: "In an SUV or large passenger car, the whole system could be contained within a package having a volume no larger than two CocaCola cans. In a truck, on the other hand, it would probably be contained within the muffler box so that exhaust systems containing the technology would have a familiar design and appearance." He continued: "We have had audits carried out by several outside research and manufacturing organisations to validate different parts of the system and substantiate our claims. We believe gas-based systems could become the industry norm not only for trucks but also for diesel-powered vans, SUVs and cars." As a result of these tests, IMI Vision is claiming that, compared with wet systems, truck fleet operators using ammonia gas injection could save up to €6,000 per vehicle, over a five-year period. This would comprise a reduction in fuel bills, less maintenance and lower liquid urea consumption. Savings in the SUV and passenger car sectors would, of course, be smaller but still sufficient to excite vehicle platform managers. Pointers * In tests, the working prototype SCR system increased NOx reduction efficiency above 90% (compared to 70-80% for 'wet' systems) allowing more adventurous engine tuning and leading to improved fuel efficiencies. Overall, urea consumption was also reduced * IMI Vision claims that, compared with wet systems, truck fleet operators using ammonia gas injection could save up to €6,000 per vehicle, over a five-year period, due to a reduction in fuel bills, less maintenance and lower liquid urea consumption * Set up four years ago, IMI Vision is the innovation development centre of the global engineering group, IMI plc. It has a team of 40 scientists and engineers drawn from a variety of backgrounds and cultures and operates from locations in Alcester (UK), Osseo (Minnesota, USA) and Bangalore (India)