ISSN 1068-7998, Russian Aeronautics (Iz.VUZ), 2012, Vol. 55, No. 1, pp. 36–40. © Allerton Press, Inc., 2012. Original Russian Text © M.D. Garipov, R.Yu. Sakulin, K.N. Garipov, R.F. Zinnatullin, 2012, published in Izvestiya VUZ. Aviatsionnaya Tekhnika, 2012, No. 1, pp. 28–30.

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AIRCRAFT AND ROCKET ENGINE THEORY

Investigation of Aqueous Ethanol Combustion in the Reciprocating Internal Combustion Engine

M. D. Garipov, R. Yu. Sakulin, K. N. Garipov, and R. F. Zinnatullin Ufa State Aviation Technical University, Ufa, Russia

Received October 21, 2011

Abstract—The results of an experimental research, conducted to study a possibility of reducing nitrogen oxide emission in the reciprocating internal combustion engine (ICE) at the combustion mode close to stoichiometric one by increasing the water content in the aqueous ethanol solution are presented. It has been established that as the water quantity in aqueous ethanol grows, a 30-fold reduction in the nitrogen oxides emission level is achieved without a drop in the engine power-to-weight ratio.

DOI: 10.3103/S1068799812010072

Keywords: combustion, aqueous ethanol, toxicity, nitrogen oxides.

Nitrogen oxides are the most toxic and difficult in neutralization components of exhaust gases. Currently, main efforts of researchers are focused on suppressing the formation of nitrogen oxides directly in the combustion chamber. The analysis of this issue allows us to distinguish two directions:

—the organization of the working process with combustion of the lean premixed mixtures with the excess air coefficient in the combustion zone 2;α ≈

—the organization of the working process with combustion of aqueous fuel-air mixtures. The nitrogen oxides emissions are reduced during combustion of poor premixed mixtures by

decreasing the combustion temperature to 1700 K. At present, this principle is successfully implemented in the gas turbine installations (GTI). In addition, possibilities of using this concept in diesel engines (HCCI) are investigated now and the results obtained show a significant reduction in nitrogen oxides emission. Since the operation at full load with such air excess coefficients leads to a significant reduction in the engine power-to-weight ratio, then it is more appropriate to use this method in stationary installations. The process with the combustion of aqueous nearly stoichiometric fuel-air mixtures is free of this shortcoming. But in this case, there appear additional problems associated with the organization of the uniform water distribution in the combustion zone, which must be solved in order to avoid formation of local areas of reduced and elevated temperatures.

In reciprocating ICEs, as opposed to gas turbine ones, along with the problems of mixture formation and combustion of aqueous fuel-air mixtures, there appears a problem of their ignition. As a consequence, despite the encouraging results of some studies [1, 2], a problem of realizing the combustion of nearly stoichiometric mixtures in reciprocating engines with such a water content that reduces the nitrogen oxides concentration to the values comparable with the emission at combustion of poor, premixed mixtures still remains unsolved.

A possible option of organizing the multifuel working process, in which the combustion of aqueous nearly stoichiometric mixtures should be implemented is being investigated at Ufa State Aviation Technical University. The process is carried out due to the injection, directly into the engine working chamber, of rich fuel-air stream, which is ignited by a single spark discharge, located near its borders. Experiments showed [3] that this principle of mixing and ignition makes it possible to fire a variety of

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fuels by a discharge being formed by a standard automobile ignition system. Gasoline, diesel fuel and aqueous ethanol solutions with a high water content were used in the experiments.

Aqueous ethanol solutions were chosen as a fuel by the following reasons. It is necessary to solve additionally the problems related to the water supply and distribution in the combustion chamber in burning the water-air fuel mixtures based on traditional petroleum fuels. In this case, it is necessary to organize mixture formation in such a way as to prevent the nonuniform water distribution in the air-fuel mixture volume that may result in formation of areas with increased and reduced combustion temperatures. In its turn, it can simultaneously lead to increased emissions of both nitrogen oxides and products of incomplete fuel combustion. Ethanol forms solutions with water and this fact may solve the problem substantially. Moreover, ethanol is one of the most probable alternative fuels able to replace petroleum fuels in the future, and a large water content in the ethanol solution simplifies the technology and significantly reduces energy intensity of its production.

The aim of this work is to study a possibility to reduce nitrogen oxides emission in the working process being considered at close to stoichiometric combustion mode by increasing the water content in the aqueous ethanol solution.

EXPERIMENTAL SETUP AND RESEARCH TECHNIQUE

Figure 1 shows a scheme of structural realization of the working process being proposed [5]. Fuel with a small amount of air enters injector-compressor cavity 5, where a preliminary mixing stage takes its place, namely, heating, crushing, mixing, and partial fuel evaporation. Injector-compressor piston 4 is driven by the engine crankshaft. Injection of air-fuel flame 2 into the engine working chamber where the fuel air-mixture is finally formed takes place due to the movement of the injector-compressor piston at the compression stroke. Mixture ignition is realized by a spark discharge from spark plug 6 at the air-fuel flame periphery. The compressor-injector is equipped with necessary devices for dispensing fuel and air 3 depending on the engine operation mode. The ignition system has a traditional design and discharge parameters, typical for modern gasoline engines. There is a possibility to regulate ignition advance and injection angles in the engine.

Fig. 1. Scheme of the experimental setup: (1) piston; (2) flame of rich air-fuel mixture; (3) devices for fuel and air dispensing; (4) piston of compressor-injector; (5) compressor-injector; (6) spark plug; (7) compressor-injector drive of engine shaft (symbolically); (8) coil of ignition system

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The working process was implemented in one section of the four-cylinder diesel D-65N engine (the cylinder diameter is 110 mm, the piston stroke is 130 mm, the mean indicated pressure at the maximum torque mode is 0.83 MPa with the nitrogen oxide emission level 1125 ppm). A standard injector was replaced with the unconventional compressor-injector. The compression ratio was reduced from the standard 17.3 to 12.5.

The Infralight-11P gas analyzer corresponding to the OIML R99, 1998E, class 0 class was used to analyze the exhaust gases of the experimental engines. Indicating was performed using the AVL IndiModul-621 electronic system of experimental analysis.

Aqueous ethanol solutions of different concentrations (95, 70 and 45% by volume) were used in the experiments. Engine indicator diagrams were recorded and the composition of exhausted gases was determined while working on each fuel type. Measurements were carried at the engine shaft rotational speed close to that at the maximum torque (1200 rev/min).

Additional heating of air or fuel at the inlet was not used while working on aqueous ethanol solutions. There were no problems with cold engine starting at the concentration of ethanol in the fuel of 95 and 70%. When the concentration of ethanol in the fuel was 45%, engine warming was required before refueling by the fuel being tested. Tests were carried out at an ambient temperature equal to T ≈ 293 K.

Emission of toxic components was measured directly after the exhaust gas output from the engine cylinder without using any neutralizing or after-burning devices.

Injection in the experimental working process was carried out at the beginning of the compression stroke. Preservation of power at the level of a base engine which was controlled by measuring the mean indicated pressure at the given engine shaft rotational speed was the main criterion in selecting the mode of loading. The closest value was achieved with the O2 content about 5% in the exhaust gases while working with 95% ethanol. Further depletion was impossible under experimental conditions (when the engine worked on the homogenous fuel-air mixture). The ignition advance angle was chosen such that the maximum mean indicated pressure was obtained while working with 95% ethanol. Its value was 15°of crack-shaft rotation to the top dead center. This angle was preserved for all fuels studied.

RESULTS AND DISCUSSION

Figure 2 shows curves of heat emission and rate of heat supply for the different water content in alcohol-water solution.

With the growth of the water concentration, a period of intense heat emission shifts to the end of combustion, but the total combustion duration remains constant. The duration of combustion for 95% ethanol was 62° of the crack shaft angle, for 70% ethanol it was 50° and for 45% it was 55°. The value corresponding to 95% of the maximum value of heat emission was taken as the end of combustion.

Figure 3a presents the dependences of the mean indicated pressure and nitrogen oxide emission on the fuel type. It can be seen that there is a significant decrease in the nitrogen oxide emission (more than 30 times) as the water quantity in the aqueous ethanol increases.

The values of the nitrogen oxide emission obtained (water content in fuel is 55%) are comparable with the emission in stationary gas turbine installations and diesel engines with a homogeneous charge where the combustion of lean premixed mixtures is organized. However, unlike the latter, the method being described entails no reduction of the engine power-to-weight ratio. Hydrocarbon emissions increase with the growth of the water content in fuel (from ≈500 ppm on 95% ethanol to 2000 ppm on 45% ethanol). At the same time, as the water content increases, no significant growth in the CO emission is observed (Fig. 3b). On the one hand, it can be explained by the fact that the equipment being used is not able to adequately analyze the emission of unburned hydrocarbons with a high content of water vapors in the exhaust gases. The measurement module of a gas analyzer, which uses the infrared method (NDIR), identifies water vapors as hydrocarbons (the gas analyzer showed a high CH level when analyzing samples with pure water vapor).

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Fig. 2. Heat emission (a) and heat supply rate (b) in the cycle of the experimental engine: (1) 95 % 2 5C H OH, 5 % 2H O

(vol.); (2) 70 % 2 5C H OH, 30 % 2H O (vol.); (3) 45 % 2 5C H OH, 55 % 2H O (vol.).

On the other hand, high emissions of unburned hydrocarbons may be a result of the fact that the combustion chamber of a base engine was not updated for the working process being considered, and the injection was carried out at the beginning of the compression stroke.

(a) (b)

Fig. 3. Mean indicated pressure and emission of nitrogen oxides (a) and carbon oxides (b) depending on the fuel type.

At this early injection the fuel is supplied to the environment where the pressure is not much higher than the atmospheric one; therefore, a jet range is sufficiently high. A larger part of the fuel at the piston position near the bottom dead center can fall to the walls of the cylinder liner, piston and into the restrained volumes.

(a)

(b)

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The unburned hydrocarbon emission indices can be improved by refining the working process, for example, by transition to the late fuel injection advance angles, by matching the sprayed fuel flame with the engine combustion chamber.

At the same time, neutralization of incomplete fuel oxidation products is not so difficult as for nitrogen oxides, and they can be effectively eliminated in modern catalytic afterburners [2].

Thus, a possibility of burning aqueous ethanol solutions with a water fraction of up to 55% at the mode close to stoichiometric one (content of 2O in exhaust gases – about 5%) in the reciprocating ICE

was experimentally demonstrated. During the experiments it was established that: 1. Reduction of nitrogen oxide emission from 1941 to 61 ppm is observed with an increase of water

amount in aqueous ethanol (from 5 to 55% of water in volume). 2. The engine power-to-weight ratio does drop as the water amount grows. 3. As the water amount grows, no significant increase of the CO emission and time of the heat supply

is observed.

REFERENCES 1. Beyerlein, S., McIlroy, D., Blackketter, D., Steciak, J., Clarke, E., and Morton, A., Homogeneous Charge

Combustion of Aqueous Ethanol, National Institute for Advanced Transportation Technology University of Idaho., URL: ntl.bts.gov/data/letter_ak/KLK316.pdf .

2. Cordon, D., Clarke, E., Beyerlein, S., Steciak, J., and Cherry, M., Catalytic Igniter to Support Combustion of Ethanol-Water/Air Mixtures in Internal Combustion Engines, Society of Automotive Engineers, URL: papers.sae.org/2002-01-2863.

3. Garipov, M.D., Garipov K.N., and Khafizov, A.G., Spark Ignition under Conditions of Deep Air-Fuel Charge Stratification in the ICE Combustion Chamber, Vestnik UGATU, 2007, vol. 9, no. 6 (24), pp. 114–120.

4. Mack, J.H., Flowers, D.L., Aceves, S.M., and Dibble R.W., Direct Use of Wet Ethanol in a Homogeneous Charge Compression Ignition (HCCI) Engine: Experimental and Numerical Results, 2007 Fall Meeting of the Western States Section of the Combustion Institute. URL: escholarship.org/uc/item/6cd5b6vq.

5. Enikeev, R.D. and Garipov, M.D., Work Process of Prospective Reciprocating ICE, Vestnik UGATU, 2006, vol. 7, no. 3 (16), pp. 12–22.