Held et al (1990) conducted experiments with diesel engines, SCR technology using urea as a reducing agent. The remaining problems are still the low space velocity and the narrow temperature window of the catalyst. The production of reaction products and secondary reactions of urea with other components in the diesel exhaust gas are still unclarified in their study.

Konno et al (1992) presented results of experiments to reduce smoke emitted from direct injection diesel enters by strong turbulence generated during the combustion process. The turbulence was created by jets of burned gas from an auxiliary chamber installed in the cylinder head. Strong turbulence, which was induced rate in the combustion period, enhanced the mixing of air with unburned fuel and soot, resulting in a remarkable reduction of smoke and particulate; NOx did not now any increase with this system, and thermal efficiency was improved at high loads.

Herzong et al (1992) reported on research and development work conducted at AVL to determine the reduction potential of in-cylinder charge conditions, fuel injection system parameters, adjust gas recirculation, fuel formulation, and adjust gas after treatment by catalyst. Based on the findings, development options are derived the assigned to the various future emission standards in USA, Europe and Japan.

Boest et al (1993) developed a new analytical method for the automobile industry a pulsed tunable laser system and a Reflectron time-of-flight mass spectrometer. The goal was to achieve the following conditions. High time resolution (100ms), high sensitivity down to 1 ppm), high accuracy (10%) and applicability to most exhaust emission components. The main problem is the large number of components with very different and fast varying concentrations. For a preliminary list of 25 exhaust emission components, all necessary parameters have been determined.

Alatas et al (1993) conducted an experimental study to characterize NO evolution and soot evolution in an optically possible D.I. diesel engine with a square combustion chamber. Two- dimensional laser-induced fluorescence was to characterize NO evolution. Soot evolution was characterized by twodimensional laser-induced incandescence (LII) and Mie scattering techniques as well direct photography of the flame luminosity. The engine operating parameters were set to provide optimum conditions for NO imaging. The NO images showed that the NO formation almost immediately after ignition and ceased not than 40 degrees ATDC. No soot images could be by the laser-induced incandescence or Mie scattering methods before 20 degrees ATDC since the soot concentration was very low.

Nehmer and Reitz (1994) studies how this study was conducted to develop an understanding of rate-shaped and split injections can affect the soot and ox emissions of a heavy-duty diesel engine.

Tow et al (1994) conducted a experimental study which evaluated the veness of using double, triple and rate shaped injections simultaneously reduce particulate and NOX emissions. The equipments were done using a single cylinder version of a caterpillar 3406 heavy duty D.I. diesel engine. The fuel used was a common rail, electronically controlled that allowed flexibility in both the number and of injections per cycle. Injection timing was varied for injection scheme to evaluate the particulate vs. NOX and fuel consumption.

Chan and Chan (1995) proposed a phenomenological modeling approach which simulates the behaviour of gas transportation through an emission analyzer by a series of alternately arranged pipes and surge volumes such that the distortion of the emission signal can be physically explained and modelled. A computer program has been developed which provides a continuous signal inference fro a series of distorted emission pulses measured during transient engine operation.

Ford and Collings (1999) introduced tow techniques for measuring the residual gas fraction in internal combustion engines. Both techniques use a fast chemiluminescent detection (CLD) type NO sensor. The measurement is made in real-time and requires a single misfire of the engine. Development of the techniques revealed several unexpected, but interesting effects; the results obtained show good agreement with existing knowledge.

Inagaki et al (1999) applied to an optically accessible single-cylinder diesel engine to measure incylinder soot concentration quantitatively. The detailed investigations based on the sequences of spatial and temporal quantitative images reveal the following points. (1) a correction of the LII signal intensity profile distorted by the laser attenuation due to the soot clouds on the laser path, (2) a correction of the LII singal intensity attenuated by soot clouds between a camera and a measurement plane, (3) Soot particle sizing up using 2-colour LII signals and (4) conversion from a signal intensity to a soot concentration based on a calibration data.

Koebel et al (2000) discussed the fundamental problems and challenges if urea-SCR is extended to mobile applications. The major goal is the reduction of the required catalyst volume while still maintaining a high selectivity for the SCR reaction over a wide temperature range. The much shorter residence time of he exhaust gas in the catalyst will lead to higher secondary emissions of ammonia and isocyanic acid originating from the reducing agent. Additional problems include the control strategy for urea dosing, the high freezing point of urea, and the long term stability of the catalyst.

Schar et al (2003) presented an advanced controller for a urea SCR (Selective Catalylic Reduction) catalytic converter system for a mobile heavy-duly diesel engine. The after-treatment system is composed of the injecting device for urea solution and a single SCR catalytic converter.

Schar et al (2003) presented an advanced controller for a urea SCR (Selective Catalylic Reduction) catalytic converter system for a mobile heavy-duly diesel engine. The after-treatment system is composed of the injecting device for urea solution and a single SCR catalytic converter. The control strategy consists of three parts: A primary feedforward controller, a surface coverage observer, and a feedback controller. A nitrogen oxide (NOx) gas sensor with non-negligible cross-sensitivity to ammonia (NH3) is used for a good feedback control performance. The control strategy is validated with ESC and ETC cycles. While the average NH3 slip is kept below 10 ppm, the emission of NOx is reduced by 82%.

Helden et al (2004) presented optimal hydrolysis and mixing conditions are of major importance. Non reduction High speed photography and droplet size measurements are presented as methods for characterizing and modeling aqueous urea spray patterns. The information from these measurements is used as input parameters for simulation tools. These consist of a 2D/3D CFD program for urea mixing analysis and a 1D SCR system model for development of dosage control and optimization of catalyst dimension. Finally, simulations with the SCR system model are compared with measurements on the engine test bench.

Options for reducing nox emissions

In principle, there is a variety of options to reduce NOx emissions from energy combustion, i.e through:

1. changes in the energy system leading to lower consumption of fuels (by energy conservation or fuel substitution),
2. combustion modification and
3. treatment of the flue gases.

Further, the emission control options available for mobile sources can be divided into the following categories:

• Changes in engine design to better control the combustion processes in the engine.
• Changes in fuel quality. For instance, a changed sulfur content of the fuel has an impact on emissions of particulate matter. Lower sulfur contents enable the application of more advanced catalytic converters. Changes in the contents of aromatics and benzene impact emissions of NOx.
• After-treatment of the exhaust gas by various types of catalytic converters.
• Better inspection and maintenance, e.g., by in-use compliance testing, in-service inspection and maintenance, on-board diagnostic systems, etc.