Particle Emissions from Direct Injection Gasoline Engines
A team of SwRI engineers examines how emissions systems and engine technologies can affect the mass, numbers and average sizes of exhaust particles from gasoline engines
Dr. Imad A. Khalek is a program manager in the Engine, Emissions and Vehicle Research Division. He is an expert in particle emissions from internal combustion engines and has investigated the influence of engine stabilization, exhaust catalysts and filters (traps), dilution systems and dilution variables, and alternative fuels on particle size distribution and number emission measurements. He is the founder of the division’s nanoparticle laboratory.
In a gasoline direct-injection engine, an injector introduces fuel directly into the combustion chamber.
Vehicles with gasoline direct-injection engines are projected to grow to about 50 percent of the U.S. fleet by 2016.
The California Air Resources Board’s proposed new regulations target a reduction in particle emissions starting in 2017.
Comparative solid particle number profile for a 2009 model GDI vehicle using three types of fuel with varying volatility (ASTM 86) and double-bond species. A similar trend is observed for soot mass. Fuel A: E0 gasoline, more volatile than sample with 10 percent ethanol Fuel B: E0 gasoline, most volatile sample. Fuel C: E10 gasoline (10 percent ethanol), least volatile sample.
Adding EGR under stoichiometric and/or rich combustion greatly reduces the mass of particle emissions, but sometimes it leads to the presence of smaller particles in approximately the same numbers.
Projected solid particle number emissions show comparisons between conventional gasoline engines and GDI engines, and highway diesel engines with and without exhaust particle filters.
The search for lower fuel consumption and reduced exhaust emissions in gasoline engines has led to great successes through the application of technologies such as exhaust gas recirculation, turbocharging and sophisticated fuel injection systems. Gasoline engines, however, are the subject of new regulations that will lower the allowable limit of particle mass emissions in the United States, and particle mass and number in the European Union.
A team of engineers at Southwest Research Institute (SwRI), using internally and externally funded research, has examined how various emissions systems and engine technologies can affect the mass, numbers and average sizes of these particles.
Fuel injection strategies
Most gasoline engines in the world’s current vehicle fleet use multi-port fuel injection (MPI) systems, where fuel is injected into intake ports outside the combustion chamber during the intake stroke as the air moves into the combustion chamber. The quantity of injected fuel is calibrated toward complete, or stoichiometric, combustion so that the exhaust three-way-catalyst operates at optimum efficiency in reducing harmful gases in compliance with emissions regulations.
Better fuel economy and lower emissions of greenhouse gases such as carbon dioxide (CO2 ) have been achieved with the introduction of gasoline direct injection (GDI) fuel systems. GDI engines use a system similar to that of the diesel engine but at a much lower pressure, wherein fuel is injected directly into the combustion chamber during the intake stroke via a fuel injector tip inside the chamber near the spark plug. This injection strategy offers more flexibility and accuracy in fuel injection, leading to improved thermal efficiency and fuel economy as well as lower greenhouse gas emissions compared to MPI engines. With this improvement, GDI technology could be adopted for about half of new gasoline vehicles sold in the United States by 2016, according to some estimates.
However, GDI also results in higher particle emissions, mainly due to limited mixing of fuel and air within the combustion chamber. This limitation can lead to fuel-to-air enrichment near the spark plug, reduced droplet evaporation and wetting of the combustion chamber wall with injected fuel. By contrast, the older-style MPI engines pre-mix fuel with air to introduce a homogenous mixture into the combustion chamber before the combustion event is initiated by the spark plug. This leads to nearly soot-free combustion in most modern engines.
Recognizing the potential for increased particle emissions from gasoline engines, the California Air Resources Board (CARB) proposed new LEV III regulations targeting a reduction in particle emissions starting in 2014. The proposed regulations included an optional regulation of the numbers of solid particles in addition to particle-mass regulations that have been historically used in the regulatory environment. However, CARB recently changed the proposed implementation date for LEV III regulations to 2017. Furthermore, it dropped the optional solid particle number regulation as a topic for future discussion with the U.S. Environmental Protection Agency (EPA). CARB LEV III regulations progressively reduce the mass limit for particle emissions from the current level of 10 milligrams per mile (mg/mi) to a potential 1 mg/mi by 2025 (although such a limit may be below measurement capability).
Parallel to CARB, EPA is working on Tier 3 regulations that will include more stringent particle mass emissions, likely similar to that of LEV III. In the European Union, the Euro 6 regulations include a particle mass emission limit of 4.5 mg/km (~7.2 mg/mi), but will also include a more stringent solid-particle number emission limit that is still under consideration.
If GDI engines are treated like the diesel engines that they resemble, the limit would be 6 x 1011 particles per kilometer. This limit would make it difficult for the GDI to meet emission regulations without the use of a particle filter in the exhaust, as is the case with current-technology diesel vehicles. Such particle number enforcement would also slow the penetration of GDI engines into the vehicle fleet.
Fuel property effects
An important issue is the effect of fuel properties on particle emissions from GDI engines. Recent work by the team of SwRI researchers showed that fuel properties of commercially available gasolines can have a great impact on particle emissions of GDI engines. For example, high-volatility fuel (based on ASTM D86 method) having carbon numbers of 10 or less can result in reduction of particle emissions by more than 75 percent. By comparison, the work showed that oxygenated fuel, such as an E10 ethanol blend, may not result in particle reduction, mainly due to baseline fuel properies that exhibit low volatility with an increased level of double-bond compounds. This indicates that the baseline fuel property used in ethanol blends can influence the reduction of particle emissions. The SwRI team’s work also identified three potential key areas – cold-start operations, hard acceleration and hard acceleration to high speed – where GDI engines may need to be improved to reduce particle emissions. Precise fuel control and good fuel-mixing strategies could reduce the impact of these events on particle emissions.
The particles emitted from GDI engines are mainly comprised of soot or elemental carbon. Their size distribution is very similar to that of old-technology on-highway diesel engines; typically with a mean diameter distribution between 50 nm and 100 nm, with sizes ranging between 5 nm and 300 nm. A comparison of particle size distribution between 2009 and 2010 GDI vehicle technology under cold-start operation, using the same fuel, indicated that vehicle technologies can have a wide range of particle emissions even though they can meet the particle emission standard.
Exhaust gas recirculation effect
SwRI manages an active High Efficiency Dilute Gasoline Engine (HEDGE®) consortium, where exhaust gas recirculation (EGR), high turbo-boost and long-duration ignition systems are used to improve gasoline engine efficiency and fuel economy, as well as providing a substantial reduction in oxides of nitrogen (NOX) emissions. Parallel to this work, the SwRI team’s internally funded research examined the effect of EGR on exhaust particle emissions from a GDI engine that had been modified with a multi-stage boosting system and a long-duration ignition system to accommodate ignition under a high level of EGR.
EGR resulted in a substantial reduction in soot mass emissions, and to a lesser extent in solid particle number emissions, at stoichiometric operation. EGR also reduced soot mass emissions even under fuel enrichment, and to a lesser extent, solid particle numbers. While this showed the benefit of EGR relative to particle reduction, it also highlighted a paradoxical implication in soot emission control from modern GDI engines: While soot formation can be suppressed with EGR due to low-temperature combustion, there may not be a similar reduction in solid particle numbers. Also, the trend in particle emissions as a function of engine speed or fuel enrichment can be opposite between mass and number. In some cases, the volume or massweighted size distribution of particles was reduced with EGR while the number- weighted size distribution simply shifted to a smaller size range. In other words, the more substantial the mass reduction, the smaller the particles became. At high fuel enrichment, EGR achieved a 90 percent reduction in soot mass, but the number of particles changed only slightly as their mean size shifted from 40 nm to about 15 nm. While these small particles are below current EU solid particle number regulations that focus on particles larger than 23 nm, this highlights the importance of the sub-23 nm size range in future consideration. It also highlights some of the challenges that can face the industry in meeting both soot mass and number regulations with engine control strategies.
One way to control particle emissions from engines is to use a particle filter in the exhaust. High-efficiency filters have been used in all post-2007 highway heavyduty and light-duty diesel engines. They provide a substantial reduction in soot mass and number, whereby they meet a very stringent Euro 6/VI emission limit on solid particle number, which is much more stringent than the standard for particle mass. They are also very effective in removing and retaining lube-oil derived ash present in engine exhaust. As part of an internally funded research activity, the SwRI team used an exhaust particle filter to investigate its effectiveness in removing particles from the exhaust of a GDI engine. Substantial reduction in soot was observed. This reduction would put the GDI engine technology on an equal footing to that of a highway diesel engine, relative to particle emissions. However, putting a filter in the exhaust of a GDI production vehicle will require additional research to address issues related to filter cleaning and regeneration, ash accumulation and durability. SwRI has very active research work in this area, where solutions can be provided to engine original equipment manufacturers if they elect to pursue this route.
While current exhaust filtration technology is highly efficient, the requirement for GDI engines to meet future particle emissions standards may not be as stringent as that for diesels. The SwRI researchers projected that a filtration efficiency of better than 92 percent will be needed for diesel vehicles to meet Euro 6, and 98 percent will be needed to have a 50 percent safety margin by being at 50 percent below the number standard. For GDI vehicles, there is a wide range of efficiency requirements, depending on vehicle technology, but the efficiency required is less than that for diesel engines. Thus, this can provide an opportunity for designing filters in such a way as to reduce pressure drop and decrease the number of regeneration (cleaning) events needed during the vehicle’s useful life. SwRI’s aftertreatment and particle science and technology sections can help clients develop filters from small-scale prototype size to production size.
The application of exhaust filters to diesel or gasoline vehicles would require a particle sensor in engine exhaust for onboard diagnostic (OBD) purposes. The sensor would trigger a fault when particle emissions exceed a certain threshold. Having an accurate and durable particle sensor is critical to the success of OBD. SwRI recently launched a Particle Sensor Performance and Durability consortium to investigate the potential accuracy and durability of commercially available particle sensors. The consortium will help the industry understand what the sensors are capable of measuring and how accurately they can measure it. Sensors, if well investigated, can be used for OBD as well as for online particle emission control from vehicles and engines.
Questions about this article? Contact Khalek at (210) 522-2536 or imad.abdulkhalek@ swri.org.