Applied Mechanics and Materials Vol. 388 (2013) pp 201-205 Online available since 2013/Aug/30 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.388.201
Controlled Auto-Ignition Combustion in a Two-Stroke Cycle Engine Using Hot Burned Gases Amin Mahmoudzadeh Andwari1, a, Azhar Abdul Aziz2, b, M.F. Muhamad Said3, c and Z. Abdul Latiff4, d 1,2,3,4
Automotive Development Center (ADC), Faculty of Mechanical Engineering, UniversitiTeknologi Malaysia (UTM), 81310, Johor Bahru, Malaysia
a
[email protected],
[email protected],
[email protected], d
[email protected]
Keywords: Two-Stroke cycle engine, Controlled Auto-Ignition (CAI), hot burned gas, auto-ignition temperature, ATAC (Active Thermo-Atmospheric Combustion), Homogeneous Charge Compression Ignition (HCCI), Exhaust gas Recirculation (EGR).
Abstract. A new combustion concept, which is viewed increasingly as a probable solution to these issues is Controlled Auto-Ignition (CAI) Combustion. In such an engine, a homogeneous mixture of air, fuel and residual gases is compressed until auto-ignition occurs. Due to its significantly low temperature combustion, NOx will be dramatically reduced while the mixture will be under ultralean fuel-air condition, thus able to achieve high efficiency and low emission. In the case of twostroke engine, problem of poor combustion efficiency and excessive white smoke emission can be addressed by the incorporation some features that will ultimately convert a typical two-stroke engine into an efficient CAI engine demonstrating the best of both features. Due to its inherent high internal residual gas rate in partial load operation, the two-stroke engine has been the first application to benefit from the unconventional CAI combustion process. This paper will concisely discuss the utilization of hot burned gas for induction thus imposing a CAI combustion feature onto two-stroke cycle engine. Among the features incorporated are the increasing in the level of Exhaust gas Recirculation and cycle-by-cycle uniformity of the air-fuel ratio (AFR) supplied to cylinder, which will be crucial in creating a suitable temperature within the engine’s combustion chamber. Introduction Two-stroke cycle engines are well known owing to their light weight, simple construction, less components, cheap to manufacturing and the potential to pack almost twice the power-density than that of a four-stroke engine having similar capacity[1]. For a longtime, the objective of the different research works on two-stroke engines optimization was to eliminate its two main drawbacks leading to high emissions of unburned hydrocarbons (uHC) and poor fuel efficiency. The first one is the unstable running operation combined with incomplete combustion, especially at light load. The second one is fuel short circuit at medium and full load. However due to the short-circuiting of the fuel before combustion, this has resulted in deterioration in overall performances especially poor combustion efficiency and high white smoke emission problem[2]. For that reason, many researchers have begun to research the new kind of alternative combustion intensively. One example of these attempts is the ATAC (Active Thermo-Atmospheric Combustion)[3], TS Combustion (Toyota-Soken)[4], ARC (Activated Radical Combustion)[5], Homogeneous Charge Compression Ignition (HCCI)[6] and CAI (Controlled Auto-Ignition) combustion concept, bulk combustion or low temperature combustion, a combustion process that has conventionally been used in two-stroke engines. It has been found that depending on the engine speed, load ratio and level of Exhaust Gas Recirculation (EGR) applied, it is possible to induce Auto-Ignited (AI) combustion in a two-stroke engine as a result of the mixing of unburned mixture gas introduced into the cylinder and hot residual (burned) gas[5]. These combustion processes can reduce emissions of unburned HCs and allow stable engine operation by lower cyclic variation. Owing to its inherent high internal residual gas rate in partial load operation, the two-stroke engine All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 161.139.95.61, Universiti Teknologi Malaysia UTM, Johor Bahru, Johor, Malaysia-05/09/13,03:14:14)
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has been the first application to take benefit of the unconventional CAI combustion process [7]. In the light load operating range, the residual gas in combustion chamber increases due to the poor scavenging. Normal flame propagation is disturbed by the large amount of residual gas, which generates irregular combustion[3]. The research works during this period to study the part load lean two-stroke combustion have led to discover that the irregularities of the combustion and the autoignition, which are considered as the weak points of the two-stroke engine, can be effectively controlled and managed to get a part load stable two-stroke combustion process for lean mixtures in which ignition occurs without spark assistance. Suitably, remarkable improvements in stability, fuel efficiency, exhaust emissions; noise and vibration will be achieved [3, 4, 8]. Basic Principle of the CAI Combustion and Its Fundamentals Similar to a conventional SI engine, in a CAI engine the fuel and air are mixed together either in the intake system or in the cylinder with direct injection. The premixed fuel and air mixture is then compressed. Towards the end of the compression stroke, combustion is initiated by auto-ignition in a similar way to the conventional CI engine. The figure 1 shows ideal representations of both SI and CAI combustion processes. In the case of SI combustion, it is the flame front that separates the burned gases from the fresh unburned gases and its velocity controls the combustion heat release. In the case of CAI, the combustion reactions take place with multiple auto-ignition sites. Even if the combustion locally can progress slowly, since it occurs spontaneously and simultaneously at several locations within the combustion chamber, the overall heat release can be as fast or even faster than with the flame front controlled SI without generating the typically high combustion temperatures of the flame front. This could contribute to explain the CAI low NOx emissions advantage.
Figure 1: Spark Ignition (SI) combustion (left) and CAI combustion (right) The heat release characteristics of the CAI combustion can be seen with using figure 1. In the case of SI combustion, a thin reaction zone or flame front separates the cylinder charge into burned and unburned regions and the heat release is confined to the reaction zone. Thus, flame front velocity controls the combustion heat release. As it can be seen in equation 1, the cumulative heat released in a SI engine is therefore the sum of the heat released by a certain mass, dm i, in the reaction zone and it can be expressed as (1) where q is the heating value per unit mass of fuel and air mixture, N is the number of reaction zones.
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Figure 2: Heat release characteristics of SI (a) and CAI combustion (b)[3] In an idealized CAI combustion process, combustion reactions take place simultaneously in the cylinder and all the mixture participates in the heat release process at any instant of the combustion process. In other word, the combustion reactions take place with multiple auto-ignition sites. Even if the combustion locally can progress slowly, since it occurs spontaneously and simultaneously at several locations within the combustion chamber, the overall heat release can be as fast or even faster than with the flame front controlled SI without generating the typically high combustion temperatures of the flame front. This could contribute to explain the CAI low NOx emissions advantage that will be described in the following section. Regarding equation 2, the cumulative heat release in such an engine is therefore the sum of the heat released from each combustion reaction, dqi, of the complete mixture in the cylinder, m, i.e. (2) where K is the total number of heat release reactions, and qi is the heat released from the ith heat release reaction involving per unit mass of fuel and air mixture. Whereas the entire heating value of each minute parcel of mixture must be released during the finite duration spend in the reaction zone in a SI engine, heat release takes place uniformly across the entire charge in an idealized CAI combustion. However, in practice, due to in-homogeneities in the mixture composition and temperature distributions in a real engine, the heat release process will not be uniform throughout the mixture[3, 4]. Two-Stroke CAI Combustion Controlling In spark ignition mode the combustion can be rather easily directly controlled by the spark advance. In the case of CAI combustion, there are a lot of relevant control parameters with, in addition, complex interactions between some parameters. Prior to examining in more detail the main relevant two-stroke CAI control parameters, it is important to define what has to be controlled: The Combustion Timing and The Combustion Heat Release Rate. A correctly controlled CAI combustion should have the best combustion timing for the highest combustion efficiency. Significant parameters that are account for combustion phasing in conjunction with the CAI combustion may be specified as follow: 1: Mixing Between Fresh Charge and Burned Gases, 2: The Engine Speed, 3: The In-cylinder Flow Velocities, 4: The In-Cylinder Pressure, 5: The Overall Temperature, 6: The Fuel Formulation and 7: Changing of Engine's Design (Elongated Transfer Duct, Transfer Duct Throttling and Exhaust Port Throttling) [3-5, 7, 9]. Effect Of Exhaust Gas As Diluent In order to achieve CAI/HCCI combustion, the temperature of the charge at the beginning of the compression stroke has to be increased to reach auto-ignition conditions at the end of the compression stroke. This can be done by heating the intake air or by keeping part of the hot
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combustion products (charge dilution) in the cylinder. Both strategies result in a higher gas temperature throughout the compression process, which in turn speeds up the chemical reactions that lead to the start of combustion of homogeneously mixed fuel and air mixtures. In-cylinder gas temperature must be sufficiently high to initiate and sustain the chemical reactions leading to autoignition processes. Substantial charge dilution is necessary to control runaway rates of the heat releasing reactions. Both of these requirements can be realized by recycling and/or trapping the burned gases within the cylinder, which the former is represented as External-EGR and the latter is known as Internal-EGR, respectively.The presence of the recycled or trapped burned gases has a number of effects on the CAI combustion and emission processes within the cylinder. The Charge Heating Effect. If hot burned gases are mixed with cooler inlet mixture of fuel and air, the temperature of the intake charge increases owing to the heating effect of the hot burned gases. This is often the case for CAI combustion with high-octane fuels, such as gasoline and alcohols[10]. The Dilution Effect. The introduction or retention of burned gases in the cylinder replaces some of the inlet air and hence causes a substantial reduction in the oxygen concentration. The reduction of air/oxygen due to the presence of burned gases is called the dilution effect[11]. The Heat Capacity Effect. The total heat capacity of the in-cylinder charge will be higher with burned gases, mainly owing to the higher specific heat capacity values of carbon dioxide (CO 2) and water vapor (H2O). This rise in the heat capacity of the cylinder charge is responsible for the heat capacity effect of the burned gases[1]. The Chemical Effect. Combustion products present in the burned gases can participate in the chemical reactions leading to auto-ignition and subsequent combustion. This potential effect is classified as the chemical effect. It should be noted that the chemical effects are influenced by active species or partially oxidized hydrocarbons or activated radical[12]. It should be noted that the overall effect of hot burned gases on the CAI combustion process is to charge heating effect, to advance the Auto-Ignition timing and to shorten the combustion. By hot burned gas incorporation, the initial charge temperature of the total in-cylinder charge will be increased owing to the heating effect of hot burned gases, and the relative air fuel ratio λ will be reduced as burned gases would be replaced some of the air.The presence of hot burned gases initially causes the CAI combustion process to accelerate. Both experiments and analytical studies have shown that the overall effect of hot burned gases is to advance the start of CAI combustion due to their charge heating effect. Ignition is dominated by the charge heating effect but the combustion duration is dominated by the dilution and heat capacity effect[13]. The maximum rate of heat release is equally affected by the charge heating effect and by the combined dilution and heat capacity effect. From a macroscopic point of view of the heat balance, i.e. the relationship between the calorific value supplied in a cycle and the total heat capacity of the in-cylinder gases; a larger heat capacity will take a longer time to heat up and the maximum combustion temperature will be lower. Thus, combustion of a larger heat capacity generates a slower heat release while that of a smaller heat capacity permits a quicker heat release. For high-octane fuels, like gasoline, alcohols, natural gas, etc., it will be advantageous to retain burned gases at as high temperature as possible to promote auto-ignition of fuel/air mixture, particularly at low load operations. Conclusion Two-stroke cycle engines can be more efficient and clean by CAI combustion mode operation. Hot burned gas utilization in order to induce this unique combustion has been always interested as result of some specific advantages, which are included: the charge heating effect, the dilution effect, the heat capacity effect and the chemical effect. In general, hot burned gases that are used in two-stroke engine either trapped or recycled, have considerable effect upon combustion phenomenon and its characteristics which will be led to induce and control of the CAI combustion as follow:
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Hot burned gases are preferred in most cases in order to increase cylinder charge temperature without external heating source Overall effect of hot burned gases is to advance the start of CAI combustion due to their charge heating effect. Start of ignition is dominated by the charge heating effect Combustion duration is dominated by the dilution and heat capacity effect. Maximum rate of heat release is equally affected by the charge heating effect and by the combined dilution and heat capacity effect. The larger the heat capacity, the slower the heat release in combustion. It is most desired for gasoline, alcohols and natural gas as high-octane fuels to use the hot burned gases at as high temperature as possible to promote auto-ignition of mixture, specifically at low load. References [1]
J. B. Heywood, et al., The Two-Stroke Cycle Engine: Its Development, Operation, and Design: Taylor & Francis, 1999.
[2]
G. P. Blair and S. P. A. S. P. Committee, Advances in Two-Stroke Cycle Engine Technology: Society of Automotive Engineers, 1989.
[3]
S. Onishi, et al., Active Thermo-Atmosphere Combustion (ATAC) - A New Combustion Process for Internal Combustion Engines, 1979.
[4]
M. Noguchi, et al., A Study on Gasoline Engine Combustion by Observation of Intermediate Reactive Products during Combustion, 1979.
[5]
Y. Ishibashi, Basic Understanding of Activated Radical Combustion and Its Two-Stroke Engine Application and Benefits, 2000.
[6]
R. H. Thring, Homogeneous-Charge Compression-Ignition (HCCI) Engines, 1989.
[7]
P. Duret, et al., The Air Assisted Direct Injection ELEVATE Automotive Engine Combustion System, 2000.
[8]
A. Cairns and H. Blaxill, The Effects of Combined Internal and External Exhaust Gas Recirculation on Gasoline Controlled Auto-Ignition, 2005.
[9]
N. Iida, et al., Study on Auto-Ignition and Combustion Mechanism of HCCI Engine, 2004.
[10]
J. B. Heywood, Internal combustion engine fundamentals: McGraw-Hill, 1988.
[11]
P. Duret, A New Generation of Two-stroke Engines for the Future?: Proceedings of the International Seminar Held in Rueil-Malmaison, France, November 29-30, 1993: Éditions Technip, 1993.
[12]
J. Lavy, et al., Towards a Better Understanding of Controlled Auto-Ignition (CAI™) Combustion Process From 2-Stroke Engine Results Analyses, 2001.
[13]
K. Tsuchiya, et al., A Study of Irregular Combustion in 2-Strote Cycle Gasoline Engines, 1983.
Advances in Thermofluids 10.4028/www.scientific.net/AMM.388
Controlled Auto-Ignition Combustion in a Two-Stroke Cycle Engine Using Hot Burned Gases 10.4028/www.scientific.net/AMM.388.201
