ANÁLISIS EXPERIMENTAL DE LA COMBUSTION DE UN MOTOR DIESEL DE AUTOMOCIÓN OPERANDO CON MEZCLAS DIESEL-BIODIESEL DE PALMA

JOHN AGUDELO
Grupo de Manejo Eficiente de la Energía GIMEL, Universidad de Antioquia, jragude@udea.edu.co

ELKIN GUTIÉRREZ
Grupo de Manejo Eficiente de la Energía GIMEL, Universidad de Antioquia, elk.gtz@gmail.com

PEDRO BENJUMEA
Grupo de combustibles alternativos, Facultad de Minas, Universidad Nacional de Colombia, pbenjume@unalmed.edu.co

ABSTRACT: Differences in the chemical nature between petroleum diesel fuels and vegetable oils-based fuels lead to differences in their physical properties affecting the combustion process inside the engine. In this work a detailed combustion diagnosis was applied to a turbocharged automotive diesel engine operating with neat palm oil biodiesel (POB), No. 2 diesel fuel and their blends at 20 and 50% POB by volume (B20 and B50 respectively). To isolate the fuel effect, tests were executed at constant power output without carrying out any modification of the engine or its fuel injection system. As the POB content in the blend increased, there was a slight reduction in the fuel/air equivalence ratio from 0.39 (B0) to 0.37 (B100), an advance of injection timing and of start of combustion. Additionally, brake thermal efficiency, combustion duration, maximum mean temperature, temperature at exhaust valve opening and exhaust gas efficiency decreased; while the peak pressure, exergy destruction rate and specific fuel consumption increased. With diesel fuel and the blends B20 and B50 the same combustion stages were noticed. However, as a consequence of the differences pointed out, the thermal history of the process was affected. The diffusion combustion stage became larger with POB content. For B100 no premixed stage was observed.

KEYWORDS: Heat release, palm oil biodiesel, diesel engines, exergy analysis.

RESUMEN: Debido a las diferencias entre los combustibles diesel y biodiesel en cuanto a estructura química, se derivan diferencias entre las propiedades físicas de éstos, afectando con ello el proceso de combustión en el motor. En este trabajo se aplica un modelo de diagnóstico del proceso de combustión a un motor diesel de automoción turboalimentado operando con biodiesel puro, diesel convencional (acpm) y mezclas al 20 y 50% en volumen (B20 y B50 respectivamente), operando a la misma potencia para estudiar solo el efecto del combustible, las pruebas se realizaron sin ninguna modificación en el motor ni en el sistema de inyección de combustible.

En el estudio experimental y de diagnóstico se observó que a medida que aumentaba la concentración de biodiesel en la mezcla se obtuvo una ligera reducción de la relación combustible/aire (0.39 para B0, y 0.37 para B100), en el avance de la inyección y en el inicio de la combustión. Además disminuyeron la eficiencia térmica efectiva, la duración de la combustión, la temperatura media máxima en el interior del cilindro, la temperatura de los gases a la salida de la válvula de escape y la eficiencia de los gases de escape; mientras que la presión máxima, la tasa de exergía destruida y el consumo específico de combustible aumentaron.

PALABRAS CLAVE: Calor liberado, biodiesel de palma, motores diesel, análisis energético.

1. INTRODUCTION

Biodiesel is attracting more attention every day as an alternative fuel for diesel engines, not only for its inherent characteristics: renewable, biodegradable, non toxic, oxygenated and free of sulphur and aromatics, but also for the main

The global market for biodiesel has undergone an accelerated growth in last years. The overall biodiesel production in the European Union (EU) increased from 1.9 million tonnes in 2004

to 3.2 in 2005 and to about 4.9 in 2006. In the USA , world’s second largest biodiesel player, production in 2006 amounted to about 250 million gallons (Approx. 836000 tonnes) [1] . A rapid growth in biodiesel production is also expected in Asia and Latin America. In particular, the Colombian government has established by law the utilization of blends of 5% biodiesel mixed with diesel fuel (B5) by 2008, this target implying an annual national demand closer to 68 million gallons.

Biodiesel is similar to conventional petroleum based diesel fuel in its main characteristics and so it can be used neat or blended in existing diesel applications without significant modifications to the engine. However, differences in the chemical nature of both fuels lead to differences in their physical properties, affecting engine performance, combustion process and pollutant emissions. The extent of this effect mainly depending on biodiesel production raw material, and engine type and operating parameters [2, 3] .

Hamasaki et al. [4] studied the combustion characteristics of a turbocharged direct injection automotive diesel engine fuelled by waste vegetable oil biodiesel and conventional diesel fuel, reporting slight differences in the shape of the heat release rate curve and in the main combustion parameters. Similar results have been obtained by other researchers testing biodiesel from several vegetable oils and animal fats [5-9] . The development of similar stages in the combustion process using biodiesel and diesel fuel resulted in small differences on thermal efficiency, and so the lower energy content of biodiesel led to an increase in specific fuel consumption [10-12] . Several researchers testing biodiesel-diesel blends have reported reductions in the peak temperature as well as in the maximum pressure gradient reached inside the combustion chamber, as the biodiesel content increased. As a consequence of this, a lower heat release rate for the high biodiesel content blends was obtained, especially in the premixed combustion stage [6, 11, 13-15] . There are few reported works related with the combustion diagnosis of diesel engines operating with palm oil biodiesel (POB). Kinoshita et al. [16] compared the combustion performance of a naturally aspirated direct injection single-cylinder diesel engine fuelled by neat POB (B100) and diesel fuel. Tests carried out with B100, at constant crankshaft rotational speed varying the load from 0 to 100%, showed reductions in ignition delay and the maximum heat released without affecting indicated brake thermal efficiency. In a later work, Kinoshita et al. [17] , testing POB-diesel fuel blends at 25, 50 and 75% biodiesel by volume, using the same engine operating conditions, reported a decrease in smoke opacity and nitrogen oxides emissions as biodiesel content increased. In the mentioned work, samples of biodiesel prepared as blends of palmitic and oleic acid methyl esters, the two main fatty acid methyl esters making POB, were also tested. The results showed reductions in nitrogen oxides emissions and ignition delay as the content of palmitic acid methyl esters increased. In both reported works, the authors calculated the thermodynamic properties of combustion products by means of a chemical equilibrium code, reducing in this way the uncertainties associated with the use of correlations developed for diesel fuels.

Abdul et al. [18] studied the performance of palm oil-based fuels in a single cylinder direct injection diesel engine. With crude palm oil-diesel fuel blends at 2, 5 and 10% palm oil by volume, significant differences in the in-cylinder pressure curves were not found in spite of reductions detected in the heat release rate and ignition delay.

Rakopoulos and Giakoumis [19, 20] carried out an up-to-date revision of works related to second-law analyses applied to reciprocating internal combustion engines operation. They reported that exergy analyses of the combustion process has been applied to engines operating with alternative fuels such as ethanol, methanol, butanol, natural gas, hydrogen enriched gases, and water-diesel emulsions; but not with biodiesel.

In this work a detailed combustion diagnosis model, including second law analysis, was applied to a turbocharged automotive diesel engine operating with neat POB, commercial grade No.2 diesel fuel and their blends at 20 and 50% biodiesel by volume (B20 and B50 respectively). The diagnosis model developed, making difference from other approaches followed in the reviewed works, takes into account the effect of the combustion products composition for each tested fuel. The study carried out allowed determining the effect of POB content on the main parameters characterizing the heat release process and also on the specific fuel consumption and the energy and exergy balances for the closed-valve period.

2. METHODOLOGY

In order to determine exergy, dead state was defined by a pressure of 101.325 kPa, a temperature of 298.15 K, and an ambient environment composition equal to that assumed for the air. The in-cylinder exergy balance, considering the blow-by as the only mass exchanged, is given by the following equation:

(1)

The terms of this equation, from left to right, account for the exergy related to the system (in-cylinder gas mixture) (dE_cyl), heat transfer (dE_Q), work (dE_W), blow-by (dE_bb), fuel (dE_f) and exergy destruction (dE_d). The specific exergy of the in-cylinder gas mixture (e_cyl) was obtained as [28-30] :

(2)

where u, s and v are the internal energy, entropy and specific volume, respectively. The subscript 0 refers to the dead state condition. The thermodynamic properties of the in-cylinder gas mixture were calculated assuming it as a mixture of ideal gases [31] .

The control volume exchanges heat only with the combustion chamber walls at the gas mean temperature. The exergy related to this process was calculated considering that heat is leaving the system [32, 33] :

(3)

where is the heat transfer to the walls. The exergy of work was obtained assuming that the compression-expansion processes are internally reversible [33-35] and taking into account the expansion work against the atmosphere:

(4)

The blow-by mass was obtained considering one-dimensional, compressible, isentropic flow [21] and its exergy was taken equal to that of the in-cylinder gases.

When a fraction of fuel is burned, its chemical exergy is released according to the next expression [20, 36, 37] :

(5)

where the fuel-burning rate ( ) was calculated from the fuel lower heating value (LHV) and the heat release rate ( ) obtained from the diagnosis model:

(6)

The chemical exergy of the fuel, , was estimated from its composition and lower heating value [38, 39] .

Carrying out a mass balance, replacing terms and solving (1) for the in-cylinder exergy destruction, the following equation is obtained:

(7)

The chemical exergy of the exhaust gases was neglected because it is too low and very difficult to recover [29, 36, 40-43] .

Table 1. Fuel properties

Measurements were carried out in duplicate in order to guarantee their repeatability. Tests were executed without carrying out any modification on the engine or its fuel injection system (mass injected and injection timing). The engine was tested at 2000 rpm and 100 Nm. This mode was chosen because it was the point of minimum air-fuel ratio and maximum smoke opacity.

The air consumption was measured with a hot-wire sensor (Magnetrol TA2, Accuracy ±0.5% full scale), and fuel consumption with a Danfoss Masflo 6000 Coriolis-type mass flow sensor (Accuracy ±0.1% of actual flow). For recording the instantaneous in-cylinder pressure a Kistler 6056A piezoelectric pressure transducer installed in the glow plug and a Kistler 5011B charge amplifier were used. Injection pressure was recorded with an AVL 41DP 1200K piezoresistive pressure transducer, installed at the exit of the injection pump. In order to guarantee confidence in the combustion diagnosis results, 100 pressure curves were registered at each operation mode [44] . The instantaneous position of the piston was determined using an angular encoder with a resolution of 1024 pulses/revolution (Heidennhain ROD 426) coupled to the crankshaft at the opposite extreme of the fly-wheel. The engine was coupled to a hydraulic dynamometer (GO-Power D512, Accuracy ±1Nm). The crankshaft rotational speed was measured using the sensor coupled to the injection pump (Accuracy ±1 rpm). High speed data were acquired using Labview^TM software and National Instruments^TM data acquisition system (Model PCI-MIO-16E-4 board).

3. RESULTS AND DISCUSSION

3.1 Combustion diagnosis
As observed in Fig. 1, the absolute fuel/air ratio (left axis) increased with POB content, this trend being a consequence of a corresponding increase in fuel consumption. However, the oxygen content of POB and its blends led to a slight reduction of the fuel/air equivalence ratio from 0.39 (B0) to 0.37 (B100) (Fig. 1, right axis).

Figure 1. Fuel/air ratio (Left: absolute, Right: equivalence)

The higher quantity of fuel injected per stroke of the volumetric injection pump may be related to the higher density of POB (Table 1). A similar behaviour has been reported for rape seed oil biodiesel [9] and cooking oil biodiesel [47] .

As seen in Fig. 2, as POB content increased the injection timing was advanced. This behaviour being related with biodiesel higher density, speed of sound and bulk modulus leading to a faster increase in injection pressure [2, 6, 9, 48, 49] . In order to guarantee the same break power with all fuels, it was necessary to increase the acceleration (due to the lower heating value of biodiesel).

Figure 2. Injection-line pressure

It means that the injection pump (volumetric type) had to change its operating point although the engine speed and torque were maintained constant (maintaining the engine operation mode) since it had to inject more fuel at the same engine speed and so it had to start delivering fuel before.

As observed in Fig. 3, no significant differences in the in-cylinder pressure were registered during the compression and expansion strokes. The peak pressure increased about 12 bar when B100 was used instead of diesel fuel.

Figure 3. In-cylinder pressure

Figure 4. Heat release rate

The fraction of fuel burned during the premixed combustion stage decreased with POB content. A similar behaviour with soybean oil biodiesel was reported by Zhang and Van Gerpen [2, 6, 9-11, 50] who argued that this reduction was a consequence of the lesser biodiesel volatility (lower evaporation rate). As POB content was increased in the blend, the diffusion combustion stage increased. For B100 no premixed stage was observed as a consequence of the fewer mass burned during this period.

The heat release fraction shown in Fig. 5 can be used as an indicator of the overall rate of the combustion process and also as a way to verify the strength of the developed diagnosis model.

Figure 5. Heat release fraction

As POB content increased, combustion process became faster (Fig. 6), leading to a reduction in combustion duration which was estimated as the angular distance from 10 to 90% of the heat release fraction (Fig. 7). At 60 CA after TDC all fuels were completely burned. Crank angles for 25, 50 and 75% of heat release were reduced with POB content, causing that the combustion process remained centred in spite of injection timing and start of combustion advances. This behaviour may be favoured by the content of molecular oxygen in biodiesel.

Figure 6. Angle for 25, 50 and 75% of heat release

Figure 7. Combustion duration

The mechanisms by which biodiesel undergoes combustion is not well understood yet, but it is likely that its long carbon chains bonded to the ester functional group behave similarly to long chain aliphatic hydrocarbons [6] . The molecular structure of the fatty acid methyl esters making POB, together with its lower final boiling point may be the reasons for the faster combustion process obtained (Fig. 7).

As shown in Fig. 8, the maximum mean in-cylinder temperature decreased with biodiesel content due to the pressure gradient reduction, which affected the premixed combustion stage causing that the angle corresponding to the maximum mean temperature moved towards TDC. Since this behaviour remained along the expansion stroke, the temperature at exhaust valve opening (EVO) decreased with POB content. These results being in agreement with those reported by Canakci [50] for soybean oil biodiesel.

Figure 8. In-cylinder temperature

Although POB underwent the same combustion stages as No. 2 diesel fuel, the differences pointed out above, affected the thermal history of the process, causing a decrease in the brake thermal efficiency (η_b) and an increase in the brake specific fuel consumption (g_f) as POB content was increased in the blend.

Comparing the performance of B100 and the reference fuel, a reduction of 2% in η_b (Fig. 9 Left) and an increase of about 15% in g_f (Fig. 9 Right) were obtained. The g_f increase was also related with the lower LHV of POB. Similar results have been reported by several researchers for soybean oil, yellow grease and cooking oil biodiesel [11, 47, 50] .

Figure 9. Left: Brake thermal efficiency, Right: Brake specific fuel consumption

Figure 10. Dimensionless cumulative exergy destruction

4. CONCLUSIONS

The objective of this work was to study the combustion process in an automotive diesel engine operating with neat palm oil biodiesel, conventional diesel fuel and their B20 and B50 blends. Based on the experimental results, the following conclusions can be drawn as POB content was increased in the blend:

The injection timing and the start of combustion were advanced as reported for other types of biodiesel obtained from different raw materials. All fuels tested underwent the same combustion stages; however, small differences in start of injection and start of combustion affected the process thermal history.

The combustion process became faster and was maintained centred. However the maximum pressure increased while the maximum mean temperature decreased as a consequence of a pressure gradient reduction close to TDC.

Brake thermal efficiency underwent a slight decrease, while specific fuel consumption increased about 16% comparing B100 with diesel fuel.

The exergy destruction was higher for greater biodiesel content due to the faster combustion and to the chemical structure differences with diesel fuel, which may have led to greater entropy of mixing of the combustion products. As a consequence, the cumulative exergy destruction was increased.

5. ACKNOWLEDGEMENTS

The authors wish to acknowledge the financial support of COLCIENCIAS (Colombian Institute for Science and Technology Development Francisco José de Caldas) to the research project 1115-05-16882. They also acknowledge the collaboration of ICP (Colombian Institute of Petroleum).

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