SwRI IRD 2003--In-Cylinder Emissions Reduction Technologies in a Light-Duty Diesel Engine, 03-9281

In-Cylinder Emissions Reduction Technologies in a
Light-Duty Diesel Engine, 03-9281

Background - Light-duty vehicles (e.g., passenger cars) face stringent emission legislation in the near future. Diesel powered light-duty vehicles enjoy much popularity in many European countries due to the superior fuel economy, durability and driveability characteristics. Diesel vehicles, without any exhaust treatment, emit higher levels of NO_x and particulate matter (PM) emissions than gasoline vehicles. The upcoming Euro V and US Tier II emission regulations will require the diesel vehicle to be as clean as a gasoline vehicle that uses a three-way catalytic converter and closed-loop control.

Today's low emission levels from diesel engines have principally been met by controlling the formation of the pollutants inside the engine. Injection timing retard, injection pressure and number of injections, and intake charge swirl ratio are just a few of the technology areas that have been successfully employed. Future emission requirements will necessitate the use of exhaust treatment devices such as catalysts and particulate filters. Though very promising results have been demonstrated using these exhaust treatment devices, many issues remain before cost-effective and durable emissions control can be expected from post-engine approaches.

One area that still holds potential for lowering emissions within the engine is to modify the combustion process so that more of the combustion occurs under lean conditions to reduce the combustion temperatures and, hence, NO_x. Further reductions of engine-out emissions can directly translate to smaller or cheaper or more durable exhaust treatment devices. The purpose of this internal research project was to explore the effectiveness of modifying the combustion process in a light-duty, passenger car, diesel engine.

Approach - The objective of this project was to explore the reduction potential of NO_x emissions in a light-duty diesel engine using premixed charge compression ignition combined with regular direct injection combustion (PCCI-DI). The PCCI mode of combustion was obtained by injecting part of the fuel early in the cycle (pilot injection) to allow a premixed charge to form prior to the main fuel injection and diffusion-controlled combustion. The test engine used on this project was a Peugeot 2.0 liter equipped with a common rail fuel injection system. The fuel system was controlled with a SwRI developed Rapid Prototyping Electronic Control System (RPECS), giving flexible pilot and main injection timing and quantity control. Engine tests consisted of sweeps of pilot injection timing and quantity, and number of pilot injections. The study focused on two operating conditions, a light load/low speed condition (17% load, 1500 rpm) and a medium load/medium speed condition (54% load, 2600 rpm). These conditions were representative of light-duty vehicle operation.

Accomplishments - At 1500 rpm and 17% load, moderate NO_x reductions of up to 15% were achieved, while a 9% NO_x reduction was achieved at 2600 rpm and 54% load. The reductions are relative to the stock engine. For the 1500 rpm case, the effect of pilot timing and quantity on BSNO_x emissions using a single pilot injection is shown in Figure 1. Depending on the pilot timing, the BSNO_x increased or decreased with increasing pilot quantity. To help understand the observed NO_x trends, heat release rate and bulk gas temperature (BGT) in the cylinder were computed from the measured cylinder pressure. Heat release rates and BGT are shown in Figure 2 for three pilot timings, all with 35% pilot quantity. Note that 0° is Top-Dead-Center (TDC). Two heat release events were observed for all three cases, however the shapes of the first heat release events differed in magnitude and shape. The early heat release for the 22° pilot timing case was indicative of normal diesel combustion, which increased cylinder temperature and NO_x formation. The 30° and 38° cases gave a different early heat release shape that was indicative of premixed combustion. This premixed combustion resulted in lower BGT and NO_x formation.


Figure 1. Effect of Single Pilot Injection Timing and Quantity on BSNO_x at 1500 rpm	Figure 2. Heat Release and Bulk Gas Temperature for 35% Pilot Quantity

The results of this study suggest the greatest potential for reducing NO_x emissions with PCCI-DI combustion exists at light loads (below 30%) and medium to low speeds. Optimization of the combustion chamber (piston bowl shape) and injection nozzle characteristics (spray angle, hole size, etc.) must be done to explore the full NO_x reduction potential of PCCI-DI combustion.