Impact of oxygen enriched combustion on the performance of a single cylinder diesel engine

Abstract

In the present experiment, a computerized single cylinder diesel engine with a data acquisition system was used to study the effects of oxygen enriched combustion technology (OECT) on the performance characteristics. The use of different levels of oxygen-enriched air was compared with respect to percentage load. Increasing the oxygen content in the air leads to faster burn rates and increases the combustibility at the same stoichiometry (oxygen-to-fuel ratio). These effects have the potential to increase the thermal efficiency and specific power output of a diesel engine. The power increases considerably with oxygen enrichment. In addition, oxygen enrichment can also be considered as a way to reduce the sudden loss in power output when the engine operates in a high load condition. Assessed high combustion temperature from the oxygen enriched combustion leads to high combustion efficiency. OECT reduces the volume of flue gases and reduces the effects of greenhouse effects. Engine tests were conducted in the above said engine for different loads and the following performance characteristics like brake power (BP), specific fuel consumption (SFC), mean effective pressure, brake thermal efficiency, mechanical efficiency, and exhaust gas temperature were studied. The objective of this paper is to address, in a systematic way, the key technical issues associated with applying OECT to single cylinder diesel engines.

Keywords

oxygen enriched combustion / exhaust gas temperature / brake power (BP) / specific fuel consumption (SFC)

Introduction

Diesel engine manufacturers face major challenges to meet the power requirement with high combustion efficiency. Moreover, the decrease of fuel consumption has forced the automobile industry to produce better engines with new technology, leading to development of new combustions systems. Lots of research was done and a number of experimental studies demonstrated the benefits of applying oxygen enriched combustion technology (OECT) in diesel engines [1]. In the present work separate oxygen cylinder was used to enrich the oxygen level in the intake air. A small mixing chamber was provided before inlet manifold. Use of oxygen enriched air was compared with different load and different level of oxygen enrichment. Other aspects of oxygen enrichment like reduction in particulates, smoke and unburned hydro carbon (HC) were not included in this paper.

Experimental work

The test engine was a single cylinder water cooled Kirloskar diesel engine with a computerized data acquisition system. The schematic diagram of the experimental setup is illustrated in Fig. 1. Detailed specifications of the test engine are listed in Table 1.

Data acquisition system

Data acquisition is the sampling of the real world to generate data that can be manipulated by the computer. It typically involves acquisition of signals and waveforms and processing of the signals to obtain desired information. The components of the data acquisition systems include appropriate sensors that convert any instrument parameter to an electrical signal, which is acquired by data acquisition hardware. Acquired data is displaced, analyzed and stored in the computer. Data acquisition begins with the physical phenomenon or physical property of an object to be measured. This property may be the temperature or temperature change of a room, the intensity or intensity change of a light source, the pressure inside a chamber, the force applied to an object or many other things. An effective data acquisition system can measure all of these different properties or phenomena.

For intake air, low levels of oxygen enrichment, not exceeding 4 L/min of the intake air were used in order to protect the engine. Higher oxygen enrichment levels need special engine modifications because of the expected higher output temperature which is expected to be produced. The intake air oxygen concentration was increased by injecting pure oxygen from a cylinder to the mixing chamber. To ensure effective oxygen enrichment, the pure oxygen was injected directly through mixing chamber in its inlet and the intake air oxygen concentration was measured properly using a gas flow meter.

Figure 2 demonstrates the performance related parameter of the engine, like fuel consumption, brake power (BP), load, speed, exhaust gas temperature and cooling water flow. In Fig. 2 the data to be entered manually are cooling water flow; fuel density; and calorific value. It can be observed that the above mentioned performance related parameters can be taken as an output with respect to the given load in kilogram.

One can switch on to a different page by pressing a button called “combustion page” at the bottom of the page which is displayed in Fig. 3. In the combustion page, performance characteristics that can be collected are mechanical efficiency, indicated power, frictional power and mean effective pressure. Results can be taken from the performance page and combustion page directly with respect to different levels of oxygen enrichment and load.

Results and discussion

The parameters to evaluate the engine performance were BP, specific fuel consumption (SFC), mean effective pressure, brake thermal efficiency, mechanical efficiency and exhaust gas temperature. The performance data were collected after stabilization of the engine conditions. Cylinder pressure traces were also collected during the experiments. The experiments were conducted for different loads with different oxygen enrichment levels.

BP

One of the objectives of this study was to demonstrate the ability of OECT to increase the power output in a diesel engine, due to the potential to burn more fuel at a given stoichiometry. According to Perz and Boehman, BP output increases by oxygen enrichment [2]. Assanis et al. proved the potential of BP enhancement with oxygen enrichment at full load conditions [3]. Rigopoulos et al. made experimental studies concerning the oxygen enrichment of intake air. They reported that oxygen enrichment leads to increased power output [1]. Unfortunately this method also increases NO_x emission and particulate matter (PM).

Table 2 and Fig. 4 indicate that BP significantly increases when the intake air oxygen concentrations increases from 1 to 4 L/min. A considerable output improvement can be achieved as a result of a small increase in oxygen content to 2 L/min. Up to 70% of improvements can be achieved when oxygen concentration is increased to 4 L/min.

SFC

SFC is an important parameter that reflects the performance of the engine [4]. SFC is affected by oxygen enrichment due to the change in the equivalence ratio as oxygen concentration in the intake air changes. Considerable SFC may be achieved from OECT [1]. The fuel consumption in the case of 2 to 3 L/min enriched air is reduced by 5.9%-9.1% as compared with the case of 21% oxygen air at exhaust temperature 773 K [5]. Oxygen enrichment minimized the drop in SFC [6]. Fuel savings are at least 15% compared to the conventional firing [7]. Addition of oxygen leads to complete combustion and thereby decreases SFC [8].

With reference to Table 3 and Fig. 5 an average of a 15% reduction in SFC can be observed when the intake oxygen level is increased from 1 to 4 L/min.

Mean effective pressure (p_mef)

A four percent increase in peak cylinder pressure can result in an approximately 10% increase in net engine power when the intake oxygen concentration of 4 L/min is used [™].p_mef is a valuable measure of an IC engines capacity to do work that is independent of the engine displacement. Manifold pressure increase is one of the factors that improve the mean effective pressure [4].

OECT leads to a higher brake mean effective pressure. From Table 4 and Fig. 6 it can be observed that the peak cylinder pressure was higher by approximately 5% to 25% when the intake air oxygen level is increased from 1 to 4 L/min. However the increase in peak pressure is smaller than the increase in power output.

Mechanical efficiency (η_m)

Oxygen enriched combustion also plays a role in increasing mechanical efficiency [1]. Since η_m is the ratio of output divided by input, when the power output of the system increases automatically, mechanical efficiency also increases.

A set of mechanical efficiency were computed with different load for varies levels of oxygen enrichment. From Table 5 and Fig. 7 it can be seen that an average of 5% increase in mechanical efficiency is obtained for an enrichment of 2 L/min when compared with 1 L/min of oxygen level. Moreover the maximum of 20% can be obtained for the enrichment of 4 L/min oxygen when compared with 1 L/min of oxygen enriched level.

Exhaust gas temperature

Exhaust gas temperature is one of the parameters indicating combustion efficiency. So a higher output temperature also plays a role in increasing combustion efficiency. Having higher oxygen content in the fuel leads to more complete combustion and in turn more energy release that results in high peak temperature than that of diesel fueled engine [9].

The results of exhaust gas temperature with different percentage of oxygen for different load were investigated. From Table 6 and Fig. 8 it can be noticed that a higher percentage of oxygen enrichment produces higher output temperature. It occurs as a result of additional percentage of oxygen which enters the engine with the intake air. From the tabulated values 2 L/min of oxygen enrichment can increase the exhaust output temperature more than 10ºC. An average increase of 30ºC can be achieved for an enrichment of 4 L/min. As a conclusion, use of oxygen enrichment in the four stroke internal combustion engine improves the combustion efficiency. This is due to higher oxygen concentration.

Conclusions

A single cylinder four stroke diesel engine was used to study the effects of different intake air oxygen enrichment levels on the performance characteristics. The following conclusions can be drawn from the investigations:

1)　When the intake oxygen concentration was increased from 1 to 4 L/min, an improvement of up to 70% in BP can be achieved.

2)　An average increase of 15% in SFC can be obtained for the enrichment of 4 L/min oxygen.

3)　Mean effective pressure increases up to 25% for the oxygen enrichment of 4 L/min.

4)　An average of 5% to 20% increase in mechanical efficiency can be obtained from this OECT for the maximum enrichment level of 4 L/min.

5)　An increased exhaust gas temperature of up to 30ºC can be achieved for the enrichment level of 4 L/min , which leads to an increase in combustion efficiency.

6)　Since the required modifications in the existing intake system are not complex, a small mixing chamber is necessary for the mixing of atmospheric air with the oxygen. OECT provides a power boost continuously with high combustion efficiency and less SFC.

References

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