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A medium size diesel engine converted to natural gas operation on the Otto principle has been studied under stoichiometric and lean burn operation in order to evaluate the potential to reduce the NOX exhaust gas emissions below the stringent limit prescribed by the Swiss Federal Clean Air Act - 250 mg/mN3, 5%O2 (at normal (N) conditions, 5 % residual oxygen and dry). While both operational modes fulfill the prescribed NOX limit, lean burn operation, combined with turbocharging, provides a higher brake power and a better fuel conversion efficiency.

The major advantages with Homogeneous Charge Compression Ignition, HCCI, is high efficiency in combination with low NOx-emissions. The major drawback with HCCI is the problem to control the ignition timing over a wide load and speed range. Other drawbacks are the limitation in attainable IMEP and relativly high emissions of unburned hydrocarbons. But the use of Exhaust Gas Recycling (EGR) instead of only air, slows down the rate of combustion and makes it possible to use lower air/fuel ratio, which increases the attainable upper load limit. The influence of mixture quality was therefore experimentally investigated. The effects of different EGR rates, air/fuel ratios and inlet mixture temperatures were studied. The compression ratio was set to 18:1. The fuels used were iso-octane, ethanol and commercially available natural gas. The engine was operated naturally aspirated mode for all tests.

Pilot fuel quantity and intake temperature are two important parameters controlling the combustion process in dual fuel engines. Experiments were conducted on a LPG diesel dual fuel engine at various intake temperatures and pilot quantities. Ignition delay, rate of pressure rise, combustion duration and heat release patterns have been presented at low and high loads. An increase in the concentration of the gaseous primary fuel significantly increased the ignition delay. At high outputs the combustion of the gas by flame propagation which follows the ignition process of the pilot and the entrained gas was the dominant feature. However, at low loads combustion of the pilot fuel and the gas entrained in it were only significant.. The rapid combustion of the gaseous fuel at high output conditions, particularly when the intake temperature was high, resulted in rough engine operation.

A two-zone model was developed to describe the NO and soot formation in diesel engines. Nitric oxide formation is computed with the extended Zeldovich mechanism. The kinetic rate constants referring to the soot formation process are transferred from known investigation results of stationary flames to the diesel engine combustion. Adjustment between calculation and measurement is achieved by preset profiles for the local air/fuel ratios. The results of computation are verified with experimental investigations using a fast gas-sampling valve and an optical measurement technique. The simulation model is able to approximate the measured NO and soot concentrations during combustion. Thus, the fundamental influences of different engine operating parameters on pollutant formation can be explained.

New York City Department of Sanitation has operated natural gas fueled refuse haulers in a pilot study: a major goal of this study was to compare the emissions from these natural gas vehicles with their diesel counterparts. The vehicles were tandem axle trucks with GVW (gross vehicle weight) rating of 69,897 pounds. The primary use of these vehicles was for street collection and transporting the collected refuse to a landfill. West Virginia University Transportable Heavy Duty Emissions Testing Laboratories have been engaged in monitoring the tailpipe emissions from these trucks for seven-years. In the later years of testing the hydrocarbons were speciated for non-methane and methane components. Six of these vehicles employed the older technology (mechanical mixer) Cummins L-10 lean burn natural gas engines.

Flamelet modeling allows the application of comprehensive chemical mechanisms, which. include all relevant chemical combustion processes that occur in a DI Diesel engine during autoignition, the burnout in the partially premixed phase, the transition to diffusive burning and formation of pollutants like NO, and soot. The highly nonlinear dependencies of the chemistry need not to be simplified, and the complete structure of the flame is preserved. Using the Representative Interactive Flamelet (RIF) model the one-dimensional unsteady set of partial differential equations is solved online with the 3-D CFD code. The flamelet solution is coupled to the flow and mixture field by the current boundary conditions (enthalpy, pressure, scalar dissipation rate). In return, the flamelet code yields the species concentrations, which are then used by the 3-D CFD code to compute the temperature field.

Design of experiments (DoE) has been used to optimise the accuracy of CFD predictions for diesel combustion and emissions simulations. Eight CFD simulation variables concerning grid geometry and simulation time step were used as the basis for a twenty-two point DoE analysis. The results showed that for practical CFD simulations the CFD predictions were heavily dependent on local grid distribution and the calculation timestep. From the DoE statistical models, two CFD setups were predicted to give optimal combustion and emissions results with CPU times of 11 and 44 hours, model (A) and model(B), respectively. These two CFD model setups were then used to assess the accuracy of CFD combustion and emissions predictions against a series of detailed measurements performed on a single cylinder engine fitted with a common rail fuel injection system.

In the present paper some results, obtained by the use of modern numerical C.F.D tools, are presented. In particular, starting from the experimental characterization of a conventional design D.I. diesel engine, the empirical constants of the different submodels were tuned to obtain satisfactory results in some key test conditions. After that, in the same points of the engine performance map, the following parameters were systematically varied: Fuel injection system design and operating conditions Intake swirl level Exhaust gas recirculation level. The influence of each parameter on combustion evolution is discussed and the most promising trend for the engine optimization is presented. Taking into account the model formulations limits, the results obtained suggest, from a theoretical point of view, that “common rail” equipped light duty diesel engines are suitable to meet the future European emission regulations.

The specific aim is to validate an engineering model for direct injection (DI) Diesel engine emissions. Characteristic times describing the controlling fluid mechanics and chemical kinetics will be employed in the model to correlate both NOx and particulate emissions. Because the model equations are algebraic, they are suitable for implementation in a phenomenological cycle simulation program, or as an emissions model option in a computational fluid dynamics code. An original premise was that earlier work on global NO chemistry based on pollutant emissions dominated by diffusion flame contributions had adequately elucidated the kinetic aspects of the model. It is shown here that this approach is not valid for modern engines. Rather, an improved two-zone flame model for NO formation/decomposition is required. Mellor et al. [1] propose such a model, but include only qualitative preliminary model validation.

Predictive models of soot formation during Diesel combustion are of great practical interest, particularly in light of newly proposed strict regulations on particulate emissions. A modified version of the phenomenological model of soot formation developed previously has been implemented in KIVA-II CFD code. The model includes major generic processes involved in soot formation during combustion, i.e., formation of soot precursors, formation of surface growth species, soot particle nucleation, coagulation, surface growth and oxidation. The formulation of the model within the KIVA-II is fully coupled with the mass and energy balances in the system. The model performance has been tested by comparison with the results of optical in-cylinder soot measurements in a single cylinder Cummins NH Diesel engine. The predicted soot volume fraction, number density and particle size agree reasonably well with the experimental data.

An engine cycle simulation program has been developed to predict both engine performance and NOx emission with reasonable accuracy. The program is a product that combines two programs, VIPRE™ and ALAMO_ENGINE, both developed at Southwest Research Institute (SwRI). VIPRE™ is a comprehensive tool for engine systems with intake and exhaust dynamics, emphasizing engine performance [1], while ALAMO_ENGINE is a program focusing on NOx prediction [2]. Demonstrations in this paper were based on a six-cylinder, turbocharged diesel engine with a high-pressure exhaust gas recirculation (EGR) loop. The EGR system is connected upstream of the intake manifold from the exhaust manifold located in the turbine entrance. A two-way butterfly valve is used to control the EGR rate. The major challenge in this modeling work is the prediction of engine performance and NOx emission in an acceptable and systematical fashion.

Organic compound treatment was considered in two non-equilibrium atmospheric pressure discharges: a preheated pulsed corona discharge and a gliding arc discharge. The pulsed corona discharge with pulse duration of about 100 ns and total power of 10 to 300 W was arranged with possible preheating to temperatures from room to 1200 K. The gliding arc discharge was investigated with total power up to 10 kW. Plasma catalytic effect of chemical conversion of hydrocarbons at relatively low temperatures was experimentally observed.

A dielectric barrier discharge device has been built to test nonthermal plasma discharges for simulated diesel exhaust NOx removal. The device has also been tested with selected catalysts located after the plasma. Emissions are measured by conventional automotive emission analyzers, plus FTIR. Dielectric barrier discharges without catalyst convert input NO to a mix of NO2, HONO, HNO3, and organic nitrates. At 30 J/l energy deposition, approximately 26% of the input NO is “lost”. Some of the hydrocarbon input is converted to a variety of species, including CO, CO2, aldehydes, and alcohols. A Cu-ZSM catalyst after the plasma device eliminates the apparent NOx conversion seen with the bare plasma. This indicates that the apparent NOx conversion of the bare plasma is actually conversion to some (unmeasured) species which can be reconverted to NOx by the Cu-ZSM catalyst. Placing a proprietary catalyst within the plasma results in significant NOx conversion.

The potential of dielectric barrier discharges for the reduction of NO emitted from Diesel cars has been investigated. Without additional measures the non-thermal plasma induced oxidation of NO to NO2 is favored over reduction to N2 and O2. Therefore a combination of plasma and a catalyst for the selective catalytic reduction of NO with ammonia as reducing agent has been tested: An NO conversion of about 70 % was achieved at a temperature as low as 100 °C, which cannot be explained by simply adding the reduction rates obtained by plasma and by selective catalytic reduction.

Increasing environmental awareness and regulatory pressure have motivated investigations into energy efficient methods to remove oxides of nitrogen (NOx) from diesel exhaust. Past emission requirements have been achieved by modifying the engine combustion parameters. However, engine modifications alone are not sufficient to meet the proposed 2004 EPA regulations for heavy duty diesel powered trucks. Some form of post combustion control is necessary. Conventional catalyst technologies, such as three-way automotive catalysts are ineffective under high oxygen levels. The use of non-thermal plasmas offers the potential to selectively reduce NOx in such high oxygen exhausts without the need for supplemental scavengers or additives. Plasmas produce energetic electrons which collide with the background gas molecules leading to the formation of a variety of new species including ions, metastable species, atoms and free radicals.

Plasma chemical hybrid process has been investigated for the control of NOx flue gas emissions. Previous results have shown that nonthermal plasma is able to oxidize NO to NO2 but cannot reduce NO2 to N2 effectively. As for the nonthermal plasma processes, part of the NO2 is converted to form N2O, HNO3 and NO3-. Several hydrocarbon additives, catalysts, and water film combined with the nonthermal plasma process have been investigated to enhance NOx reduction, but NOx reduction has been limited to the 70% range and byproduct formation still remains to be unsolved. As an alternative technology, the plasma chemical hybrid process was developed: plasma reactor to convert NO to NO2, and the chemical reduction process to convert NO2 to N2 with minimum byproducts. The barrier dielectric packed-bed reactor followed by the chemical reactor was able to achieve nearly 100% NOx decomposition with an extremely low power level (14 W/cfm, 30 J/L, or 40 eV/molecule) and minimum N2O formation.

Much work has been done on the application of plasmas to the treatment of NOx from power plants. In power plant applications, the purpose of the plasma is to oxidize NO to NO2, and eventually to nitric acid. The desired products, in the form of ammonium salts, are then obtained by mixing ammonia with the formed acids. Some form of scrubbing is required to collect the final products. For applications to the treatment of exhausts from cars and trucks, it is very important to make a distinction between NO removal by chemical oxidation and NO removal by chemical reduction. To avoid the need for scrubbing of plasma processing products, the desired method of NO removal is by chemical reduction; i.e. the conversion of NO to benign gaseous products like N2. This paper will discuss the results of an extensive series of experiments aimed towards understanding the effect of gas composition on the NOx conversion chemistry in a plasma.

This paper presents the results of experiments in which various parameters were varied systematically in an attempt to understand how the reactor design affects the energy efficiency for plasma processing of NOX. These parameters include the packing material, electrode diameter, and voltage frequency. It is shown that the applied voltage is not the relevant parameter when comparing the performance of different plasma reactors. The important control parameter is the input energy density. In accordance with the observations reported by Penetrante et al. [Applied Physics Letters 68, 3719-3721 (1996)], we have found that reactor design has little influence on the basic energy consumption of the plasma. Consequently, different reactor designs should yield basically the same plasma chemistry if the experiments are performed under identical gas composition and temperature conditions.

A laser-induced fluorescence (LIF) system has been developed to obtain measurements of the instantaneous lubricant film thickness in the piston-cylinder assembly of a firing single-cylinder, direct-injection diesel engine. Measurements were made at top-dead-centre (TDC), mid-stroke and bottom-dead-centre (BDC) position by means of three fibre optic probes inserted into the cylinder liner and mounted flush with its surface. Following extensive repeatability tests, the cycle-averaged lubricant film thickness was estimated for different multi-grade oils as a function of engine speed, load and temperature. The results quantified the dependence of the film thickness ahead, under and behind the piston rings on oil chemistry and viscometric properties, thus confirming the important role of the LIF technique in the development and formulation of new engine oils.

Laser induced fluorescence (LIF) was used in the cylinder liner of a firing single-cylinder direct-injection diesel engine to characterise the development of the lubricant film during the first 200 engine cycles under cold-start conditions. The results have provided information on the rate of oil film development which has proved to be a highly unsteady process due to the complicated oil transport processes through the ring-pack.