Finally, regardless of their structures, simple molecules such as MB33Methyl butanoate. Is your work missing from RePEc? Here is how to contribute. Questions or problems? Page updated This article describes a study into the potential for using a fuel injection and combustion model for combustion performance prediction. The GT-Suite software can be used for a combined analysis of both fluid flow and mechanical systems. In this context, fluid flow means the intake and exhaust paths and the flow of fuel through the fuel injection system pump, high-pressure pipe, and injectors , while the mechanical systems include the crank and pistons.
The flow analysis uses a set of equations for the conservation of mass, momentum, and energy based on the Navie-Stokes equations, and a feature of the analysis is that, unlike the 3D computational flow dynamics CFD code used for detailed analysis, it is able to obtain a solution rapidly by only performing spatial discretization in the direction of flow 1. The combustion prediction model is made up of separate models for the fuel injection system and for combustion in the cylinder.
First, the fuel injection model is used to calculate the fuel injection rate for the given pump operating conditions. Next, the calculated fuel injection rate together with other parameters such as the boost and exhaust pressure are provided as inputs to the cylinder combustion model to predict the combustion process.
Note that system identification needs to be performed beforehand for both the fuel injection system and combustion models using actual data obtained under the standard operating conditions. The fuel injection system, which is for a medium-speed marine engine, with independent pump, high-pressure pipe, and injector for each cylinder is used for the model see Fig.
The fuel injection system model represents all of the functional components, from the fuel pipe at the pump inlet to the injector, as simplified pipe elements and mechanical elements. The following sections describe model development for the plunger and barrel, delivery valve constant pressure valve , high-pressure fuel pipe, and fuel injector respectively. The fuel that enters the pump via the fuel inlet pipe is temporarily charged in a space called the fuel gallery. Connected to the barrel via a communicating path and feed holes, the fuel gallery serves both as a buffer for supplying fuel to the barrel during the intake stroke and as the receiver for fuel spilled at the end of the discharge stroke.
In the modelling, the fuel gallery is finely divided into elements around the circumference, with each element being treated separately as a general branch pipe with specified volume and inlet and outlet shapes. The communicating path includes a narrowed section that controls the fuel backflow from the barrel. This is expressed in the model as an orifice in the pipe, with flow rate coefficients adjusted to match the actual situation.
Recent studies on soot modeling for diesel combustion
The plunger that discharges the fuel is pushed up by a roller tappet driven by a fuel cam located under the pump. This pressurizes the fuel in the barrel. The actual pump has a diagonal cut called a "lead" on the side of the plunger. Fuel discharge starts when the feed hole is closed by the top of the plunger and ends when the feed hole crosses to the lower surface of the lead, thereby creating a flow path between the fuel gallery and the fuel path under the lead.
This mechanism means that the fuel injection amount can be adjusted by rotating the plunger to vary the effective length of the plunger discharge stroke.
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This operation was expressed in the model by setting the effective flow path area of the feed port for the plunger lift separately for the discharge start and end steps. Furthermore, the effective plunger discharge stroke was expressed by adjusting the distance between the discharge start and end steps so as to match the actual fuel injection amount. The fuel pressurized in the barrel passes through the delivery valve to the high-pressure pipe. In addition, this function does not reflect the effects of the shape of the combustion chamber and the fuel injection rate on the history of heat release as desired in current engine development.
This however, added more number of adjustable constants that are dependent on the engine type. While such algebraic functions are easy to compute, there are various other complex models Table 2. Table 2. In addition, the turbulent energy decay in time is proportional to the total kinetic energy of the injected fuel itself. It is observed that this model predicts the trend of heat release quite closely if only there is no impingement of sprays on the piston.
This situation arises in engines of capacities less than 2 L per cylinder operating at more than half load, where majority of diesel engines belongs. Therefore, an attempt has been made in this book to enhance this model by encompassing the phenomena at the wall and the instantaneous injection rate derived from the indicated performance of fuel injection equipment Lakshminarayanan et al.
Emission Models DI diesel engines emit smoke, hydrocarbons, nitric oxides, carbon monoxide and particulate matter are mainly regulated. They are formed in different phases of combustion as described below. Hydrocarbons The fuel leaned beyond flammability limits Greeves et al. A semi-empirical phenomenological model was successfully made for HC emissions considering the fuel injected and mixed beyond the lean combustion limit during ignition delay and fuel effusing from the nozzle sac at low pressure Lakshminarayanan et al.
The oxygen-enriched fuels that attract great attention worldwide owing to its excellent combustion characteristics, exhibit different behaviour especially in case of ignition delay and HC emissions. Oxides of nitrogen Considering the heterogeneous nature of fuel-air mixture in diesel engines, NOx and particulate matter PM are important emissions.
Continuous efforts are being made to minimize the quantities of these two pollutants from the diesel engine exhaust. Vioculescu and Borman carried out gas sampling from within the cylinder of a naturally aspirated direct injection DI diesel engine using a rapid acting sampling valve. This resulted in a plot showing time history of ratio of the average cylinder NOx concentration in the exhaust during the combustion process. Similar modelling and gas sampling studies have been done with indirect injection IDI diesel engines, which suggest that prechamber is the prominent location for formation of nitrogen oxides Mansouri et al.
Duggal et al. There are a number of potential mechanisms responsible for NO in combustion processes. The relative importance of these different mechanisms is strongly affected by the temperature, fuel-air equivalence ratio, pressure, flame 2 Phenomenology of Diesel Combustion and Modelling 15 conditions, residence time and concentrations of key reacting species. Rapid NOx formation begins after the start of heat release.
Shortly after the end of heat release, the period of rapid NOx formation ends because temperatures of the burned gas decrease due to mixing with cool bulk gas and expansion of the charge Kitamura et al. Fuel-Air equivalence ratio is another important factor influencing NOx formation. As the equivalence ratio becomes leaner, NO and NOx decrease significantly as expected.
The NO2 peaks at an equivalence ratio near 0. Leaner equivalence ratio is indicative of lower loads and lower bulk gas temperatures that are conducive to the formation of NO2 Pipho et al. Advancing injection timing or increasing injection pressure improves combustion efficiency raises combustion temperature.
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In general, higher combustion temperatures lead to higher NOx formation Henein and Patterson Addition of diluents to the engine intake air is considered as an effective mean to reduce the NO formation rate and hence the exhaust NOx levels. The effect is primarily one of reducing the peak flame temperature, which is the driving factor for NOx formation.
Plee et al. The NO2 is formed via NO molecule. Therefore, the modelling of NOx formation is most often reduced to studying the formation of NO. It is widely accepted that in diesel engines the major portion of NO is formed via thermal path Ahmed and Plee Many multi-dimensional and multi-zone phenomenological models use extended Zeldovich mechanism Heywood This mechanism was postulated by Zeldovich and improved by Lavoie et al. Khan et al. They concluded that an increased rate of injection or increased air swirl reduces the amount of exhaust smoke and increases NOx.
All these models utilize empirical heat transfer correlation, which are mass averaged.
Combustion in Diesel Engines
During combustion, the heat loss is caused partly by convection from burned gases at high temperature and partly by radiation from soot particles formed during the diffusion flame. Due to the short distance between the nozzle and the combustion chamber wall under typical operating conditions, diesel fuel impinges on the wall in the form of liquid followed by fuel vapour and flame after onset of auto-ignition. However, it is known that in 16 Modelling Diesel Combustion diesel engines radiative heat transfer may have a significant contribution in addition to convective heat transfer.
The radiative heat transfer in diesel engines is caused by both radiation of hot gases and by radiation of soot particles within the diffusion flame. It is agreed in the literature that the latter has a significantly greater impact on the radiative heat flux, and thus most heat transfer models concentrate on the radiation of soot only.
It should be noted though, that a general difficulty in the evaluation of soot radiation exists in that the prediction of the soot concentration itself is typically subject to significant uncertainties Stiesch Therefore, the empirical heat transfer correlations focused mainly on convective mode.
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The heat transfer coefficient has been derived by many researchers by assuming an analogy with a steady turbulent flow over a solid wall. The colour pyrometer and fast response thermocouples were employed for experimental investigations. Annand developed correlation for convective heat transfer but it was based on experiments conducted on only cylinder head.
Probably, the most widely used approach in this category is the one suggested by Woschni Hohenberg improved the above correlation by using a length based on instantaneous cylinder volume and exponent of the temperature term. This approach gives an estimate of the surface-averaged heat transfer coefficient history in terms of the bulk gas temperature and a surface-averaged or total heat flux Ikegami et al.
However, this approach cannot give the kind of information necessary to design modern engines. The empirical correlations underestimate to varying degrees the heat transfer during combustion. The investigations have revealed that during the combustion period the wall heat flux is substantial locally in space and time, due to the transient nature of the flame propagation.
In particular, during combustion the heat flux increases rapidly after impingement on the wall Kleemann et al. The characteristics of injected spray and its interaction with the swirling air and the wall of the combustion chamber determine the efficiency and the exhaust emissions. In Chapter 14, a phenomenological model for NOx prediction is proposed based on spray combustion incorporating localised effect of heat transfer in wall spray and exhaust gas recirculation.
Smoke and particulate matter The characterization of diesel smoke has remained a challenge in engine development and modeling work. Effect of different parameters of combustion chamber and injection on soot and NOx emissions were investigated by De Risi et al. Kurtz and Foster identified critical time for mixing in diesel engine and its effect on emissions. Based on in-situ laser diagnostics, a conceptual model of burning jet was developed John Dec Khan et al first presented a model for the prediction of soot related to engine operating condition.
Hiroyasu et al. Later on, the model was extended up to a simple threedimensional model Nishida K, Hiroyasu H Fusco et al. There is a principal mathematical problem in the modeling of the engine-out soot emissions by using 2 Phenomenology of Diesel Combustion and Modelling 17 formation and oxidation methodology Stiesch G Since the soot mass in the exhaust is the very small difference between two nearly equal large quantities i. Magnussen et al. The burn up of the soot was related to the dissipation of turbulence. The importance of turbulent energy dissipation rate on smoke in quiescent chamber diesel engines was identified quantitatively.
Recently, Dec and Tree a, b investigated interactions of combusting fuel jet free in air and at the wall using laser diagnostics. They found that soot deposition on the wall and blow-off are not the major contributors to engine-out soot emissions. In chapter 12, a model that clearly distinguishes the free jet and wall jet regimes of a diesel-engine spray and their turbulence structure is developed to explain the smoke.
Diesel particulates consist principally of carbonaceous material soot from smoke generated by combustion on which some organic compounds have become absorbed.