The effect of increased intake air pressure of a naturally aspirated diesel engine on performance an

Abstract

Keywords: Natural aspiration; boost pressure; diesel engine; performance; exhaust emissions.

1. Introduction

Environmental awareness is increasing day by day and a large part of the pollutant emissions are due to motor vehicles. Therefore, it is desired to continuously lower the emission limits. In terms of global warming and climate change, the improvement of the combustion in the engines and therefore the reduction of negative emissions in terms of the environment has a significant importance [1]. To meet these demands, a number of methods such as fuelimprovement, enginebased technologies (combustion chamber design, fuel injection strategies, HCCI etc.) and postcombustion technologies are being investigated. Almost similar fuel improvement and enginebased strategies are applied for spark ignition engines (SIE) and compression ignition engines (CIE). However, different methods are applied to improve post-combustion exhaust emissions. This is because the CO, HC and NOx emissions can be improved by the three-way catalytic converter because of SIEs homogeneous mixture and approximate stoichiometric air/fuel ( = 1) ratio. However, since diesel engines work with heterogeneous fuel mixture and the high excess air ratio, the three-way catalytic converter is not sufficient alone to convert three harmful gases. Therefore, more expensive devices are needed to improve diesel engine emissions. For example; oxidation catalyst for CO and HC emissions, diesel particulate filter (DPF) for PM, and selective catalytic reduction (SCR), lean nitrogen oxide trap (LNT) and EGR are being used for NOx emissions. This increases price and fuel consumption of a diesel engine. Researches have been *

Corresponding Author: e-mail: yusufozbakis@hotmail.com, tel: (507) 960 35 55 361

carried out recently on operating diesel engines by stoichiometric air/fuel ratio (SDCStoichiometric Diesel Combustion or SCI-Stoichiometric Compression Ignition) in order to convert NOx emissions to similar to SIEs [2-11]. It is understood from the studies done that the NOx emission decreases, the combustion efficiency, and the effective pressure decrease, and the specific fuel consumption (SFC) increase. For example; Lee et al. [3.4] found that SFC increases 7% in an engine with SDC at low load. In stoichiometric diesel combustion, the main cause of dete rioration of engine efficiency is attributed to insufficient air flow around the injected fuel assembly [11]. Cha et al. [9] in their study, have found that NOx emissions are reduced, the induced effective pressure decreases, and the soot emission increases. Junepyo et al. [10], in their study, showed that it increased total HC and CO emissions and significantly increased soot emission level. It has been stated that to improve SFC and combustion efficiency in SDC, some researchers have applied single spraying [2,3] while some researchers have applied multiple spraying strategies [4], but have not achieved a satisfactory improvement [11]. The reason for this is, although it is given that λ=1.4, mixture preparation time to be short, not sufficiently benefitting from the air for reasons such as homogeneous mixture etc., and the air to be even more choked in SDC. As the air is further reduced, the advantages of diesel engines operating with excess air (such as high volumetric efficiency) will be obscured, the combustion will become worse as the heterogeneous and rich mixing zones will increase, and especially the emissions will become worse. The fact that there is little mention of CO and HC emissions in SDC studies has further reinforced this opinion. Unlike the SDC, it is thought that increasing the amount of air entering the cylinders by increasing the air pressure entering naturally aspirated diesel engine cylinders may improve the performance and emissions of diesel engines that burn the heterogeneous mixture. When reviewing the literature, it is seen that engine performance and emissions are improved in a homogeneous combustion-related study [12]. Yoshimoto et al. [13], in their study, noted that increasing the input air pressure by supercharging significantly improves the combustion characteristics. Yang et al. [14]. Studied the compliance of the turbocharger system to the altitude and showed in their study that in spite of the turbocharger system, the lack of air increases the brake specific fuel consumption and reduces the output torque. In his study, Karabektaş [15] has shown that turbocharging reduces CO emissions and somewhat increases the NOx emissions. A similar study by Luft and Skrzek [16] investigated the effect of a dual-fuel compression ignition input air pressure on NOx, soot and non-HC emissions. At first glance, it seems that there is a limited number of studies to be aimed at this, but a closer scrutiny of the studies suggests that they are more about the effect of a turbocharger or supercharger on the performance and emissions of a naturally aspirated diesel engine without changing the fuel system [13-15] and it is also understood that the charging pressures applied (1.4, 1.8 bar, etc.) without any modification are very high [17]. The purpose of this study is to investigate the effects of different pressures (NA, 5, 7.5, 10, 12.5, 15, 17.5 and 20 kPa) in the air intake on a single-cylinder, naturally aspirated and compression-ignited engine, with different engine loads (25%, 50%, and 100%) at constant engine speed (2800 rpm).

2. Experimental Setup And Procedure

The experimental study was carried out at different speeds (25%, 50%, 100%) at a constant speed of 2800 rpm with a naturally aspirated, single-cylinder compression ignition engine given in table 1; both naturally aspirated, and intake air pressures were increased by 5, 7.5, 10, 12.5, 15, 17.5 and 20 kPa, respectively. The characteristics of the fuels used in the experiments are given in table 2. In the experimental study, data such as fuel consumption and duration, engine torque, air pressure and temperature entering the cylinder, exhaust gas temperature, excess air ratio (λ) and exhaust emissions were measured. The amount of fuel consumed, and its duration is calculated by the electronic scale and digital stopwatch. The engine power was determined with an electric D.C

dynamometer capable of absor bing 10 kW of power. the exhaust gas temperatures were measured with a TES 1320 K thermocouple digital thermometer and the intake air temperature and the pressure was measured with the help of a TECHNI-C A930C type digital thermometer and a KELLER PA 21SR / 25/80 520.5 type pressure sensor&computer software, respectively. An intercooler turbocharged engine with four cylinders and a total cylinder volume of 1.9 liters was used for compressed air supply. The air hose connected to the intake manifold of this motor has been removed and connected to the test motor intake via the damping tank. The reason for using the damping tank in between is to stabilize the air flow and the pressure. The intake air pressure has been controlled by gassing the turbocharged engine. Emissions of CO (% v), HC (ppm) and NOx (ppm) have been measured with the MRU DELTA 1600 L exhaust gas analyzer, and the soot emissions, (%) with the MRU Optrans 1600 opacimeter. Figure 1 shows the general view and schematic diagram of the experiment set. The characteristics of gas and soot emission devices are given in table 3. Power and brake specific fuel consumption has been calculated from the obtained data. The sensitivities of the measuring instruments and the uncertainties of the calculated quantities are

  analysis of torque, power, and brake specific fuel consumption

calculated based on the equation [18]. Figure 1 b) Schematic view of the test setup (1- Dynamometer, 2- Test engine, 3- intake air temperature sensor, 4- Air damping tank, 5- intake air pressure sensor, 6- Intercooler, 7Turbocharger, 8- Turbine, 9- Exhaust Manifold, 10- Intake Manifold, 11- Air filter, 12- Digital thermometer, 13- Computer (for the intake air pressure reading software), 14- Electronic scale, 15- Fuel container, 16- Exhaust gas analyzer, 17- Soot measuring indicator, 18- soot measuring device, 19- Digital 20- Dynamometer control panel, 21- Load cell indicator) Table 1. Experiment engine specifications Model Specifications Cylinder number Bore (mm) x stroke (mm) Cylinder size (cm3) Compression ratio Engine power (kW)/rpm İnjection advance timing İnjection pressure (bar) İnjector nozzle hole number İnjector nozzle hole diameter (mm)

Katana KM 170 F Four strokes, air cooled, direct injection One cylinder 70 x 50 298 18:1 4/3600 31 deg 200 ± 5 4 0,1

Figure 1. a) General Appearance of the Experiment Set and Air Damping Tank

Table 3. Technical features of the exhaust gas analysis devices.

Table 4. Measurement precision and the uncertainties of calculated results. Measure Force Weight arm Speed Time CO (%vol) NOx (ppm) HC (ppm) Temperature (°C) Smoke tone (%) Calculated result Torque Force Specific fuel consumption

Accuracy %0,6 N %0,1 m 1 rpm %1 s ±0,06% ±5 ±12 ±1° ±%2 Uncertainty ±%0,4 ±%0,4 ±%0,7

3. Results And Discussion

The numerical values of all findings in the experimental study are given in table 5.

Lamda

İntake air pressure (kPa) Figure 2. Variation of brake specific fuel consumption (bsfc) 366

The change in excess air ratio due to intake air pressure increase is shown in Figure 3. In the study, increasing the air intake pressure of the diesel engine, which started to operate at 2800 rpm constant speed with natural aspiration, increased the air excess coefficient by approximately 40%, full load 50% load 21% and 25% load 28%.

Figure 3. Excess air ratio (λ) change due to an increase in engine air intake pressure. The change in the exhaust gas temperature depending on the air intake pressure increase is given in Figure 4. In the experiments performed, it can be observed that at full load, the EGT falls as the intake air pressure increases, and as the pressures increased by 20 kPa, the EGT decline rates have been by 4.5% at 25%, 6.5% at 50%, and up to 8% at 100%, respectively. It is thought that the main cause of the decrease of the exhaust gas temperature is the result of the improvement in the combustion.

İntake air pressure (kPa) Figure 4. Exhaust gas temperature change due to an increase in engine intake air pressure.

The change in temperature depending on the increase in air pressure given to the engine, is shown in Figure 5. As can be seen, in each load case, the temperature increases linearly with increasing pressure. It has been understood that the incoming air temperature is affected considerably by the incoming air pressure and is not affected much by the load. It is thought that the reason why the intake air temperature is not very affected by the load is that the air is aspirated unrestrictedly in diesel engines.

İntake air pressure (kPa) Figure 5. Change in the intake air temperature by the intake air pressure.

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