combustion rate

Study on Combustion and Emission Characteristics of HCCI Engines Fueled with MTBE/Heptane Mixtures. Original authors: Hou Yuchun, Lü Xingcai, Zu Linlin, Huang Zhen, School of Mechanical Engineering and Power, Shanghai Jiao Tong University, Shanghai 200030. Abstract: Single-cylinder HCCI combustion and emission tests were conducted using heptane and its mixtures with methyl tert-butyl ether (MTBE) in various proportions in a high-speed 4-cylinder diesel engine. The test results show that as the MTBE proportion increases, the low-temperature reactions in the HCCI combustion of the mixed fuel are weakened and delayed, which in turn suppresses the combustion during the high-temperature phase of HCCI and delays the combustion phase; therefore, the addition of MTBE has the potential to expand the HCCI combustion range of n-heptane. Keywords: HCCI, methyl tert-butyl ether, heptane, combustion characteristics, emissions 1. Introduction HCCI is a chemically kinetic combustion process controlled by factors such as temperature, pressure, and mixture composition, and it holds potential for high efficiency and low emissions. The operating range of HCCI engines is limited by misfire and deflagration, and the key issue lies in the control of ignition timing and combustion rate. One of the measures to address these problems is to use technical methods to improve the operating conditions inside the cylinder, thereby controlling HCCI combustion. Among these, measures such as exhaust gas recirculation, variable compression ratio, and variable valve timing have achieved certain progress in controlling the ignition timing and combustion rate in HCCI combustion. HCCI combustion is characterized by distinct exothermic phases at low and high temperatures, and the properties of the fuel can influence the timing and extent of the exothermic reactions at these low and high temperatures. Therefore, the introduction of a fuel design concept may be beneficial for HCCI combustion control. Since the 1990s, to gain a deeper understanding of the mechanisms behind HCCI, a range of fuels have been used in combustion experiments related to HCCI, such as diesel with a high cetane number, n-heptane, DME, gasoline with a high octane number, isooctane, LPG, ethanol, and methanol. The basic approach to fuel design at this stage is to use two fuels or additives with different properties, mixing them together to create a mixture whose combustion characteristics differ from those of the individual components. From a medium- to long-term perspective, designing fuels suitable for the characteristics of HCCI combustion may be an effective way to address existing HCCI problems. The reactions of MTBE and its volume-equivalent mixtures with n-heptane in jet-stirred reactors have been studied in the literature. Its research focuses on the reaction products of MTBE in the presence and absence of n-heptane. The author studied HCCI combustion using a high-octane MT2BE/heptane fuel, and explored from the perspective of fuel design the effect of MTBE on the ignition timing, combustion, and emission characteristics of heptane. II. Fuel Introduction To better understand the effects of MTBE addition on the combustion and emissions in HCCI of n-heptane, it is necessary to explain the chemical kinetic reactions between n-heptane and MTBE. The properties of these two fuels are shown in Table 1. Table 1 Physical and chemical properties of n-heptane and MTBE. N-heptane is one of the two basic reference fuels used for evaluating the octane rating of engine fuels; it has a cetane number of 56 and a research octane number of 0. Its properties are similar to those of diesel, and it exhibits active reaction behavior at low temperatures. The low-temperature reaction of n-heptane first involves a dehydrogenation reaction with radicals such as oxygen to form heptyl. The addition and heterolytic reactions of heptyl with oxygen molecules yield perheptyl, which then undergoes a second addition reaction with oxygen to produce OH radicals and similar substances. The OH radical is quite active; once its reactive pool is established, the dominant reactions at low temperatures will be the dehydrogenation of n-heptane by the OH radical, followed by an addition reaction with oxygen molecules to form heptyl peroxide, which in turn yields OH radicals and similar compounds. During the low-temperature phase, this entire reaction cycle is exothermic, resulting in a rapid rise in the temperature of the reaction environment; this is also manifested as a cold-flame reaction in the first stage. When the temperature reaches a certain level, the oxidation of heptyl groups to form polyolefins becomes a competitive reaction, causing the entire reaction to enter an NTC region in which the reaction rate decreases as the temperature rises. During this period, a large amount of H2O2 is generated; due to the slow action of the system itself or external influences, the temperature continues to rise. Once the temperature reaches the decomposition temperature of H2O2, a large number of OH radicals are produced, which triggers a thermal ignition that consumes most of the fuel. MTBE is an oxygenated fuel with an octane number of 108; it was added to gasoline as early as 1979 to replace lead as a octane booster. Dlay and Wentrup were the first scholars to study the thermal cracking of MTBE, investigating the kinetic reactions of homogeneous MTBE at low temperatures. The first-order chemical kinetic reaction involves the decomposition of MTBE through the cleavage by OH radicals to form isobutylene and methanol; this can be expressed as… In fact, the oxidative cleavage of MTBE with oxygen to produce isobutylene and formaldehyde can be represented as… Studies have shown that reaction (1) is much more active than reaction (2). It is generally accepted that the reaction between the OH group and MTBE occurs at the C—H bond of the methoxy group, as this C—H bond has a much weaker bonding strength than the C—H bonds in the butyl group; as a result, the methoxy group is easily formed. The further decomposition product of the methoxy group is HO2, which has very low reactivity. Compared to other reaction pathways, this series of reactions is dominant, which leads to a reduced reactivity of MTBE at lower temperatures. III. Test Equipment and Test Conditions: A 4-cylinder, 4-stroke high-speed direct-injection diesel engine was modified into a single-cylinder engine for HCCI combustion research; its technical parameters are shown in Table 2. The 4th cylinder of the original engine operates in HCCI combustion mode, with the HCCI fuel pump and its injection system injecting fuel directly into the separate gas passage of the 4th cylinder to achieve pre-mixed HCCI combustion in the intake passage. The other 3-cylinder engines still operate in direct injection mode using diesel. The entire test system is shown in Figure 1. To obtain test results that are comparable and reproducible, the following operating conditions were determined for the engine tests: rotational speed of 1800 r/min, intake air temperature of (23±2)°C (without EGR), and cooling water temperature of (83±2)°C. Table 2 Parameters of the single-cylinder HCCI engine. Figure 1 Test setup system. Cylinder pressure is measured using a Kistler 6125A sensor; the charge signal generated by this sensor is converted into a voltage signal via a Kistler 5015 charge amplifier. 50 cycles are sampled for each operating condition, at a sampling frequency of 100 kHz, and the data are stored and analyzed using a computer. Unburned HC, CO, CO2, and NOx are measured using an AVL gas analyzer. IV. HCCI Combustion Analysis The combustion in HCCI engines is characterized by dual-peak heat release at low and high temperatures, with the low-temperature reactions being closely related to the properties of the fuel. The author studied the combustion characteristics of different ratios of MTBE/n-heptane at low and high temperatures, as well as the influence of different air-fuel equivalence ratios on them. Here, M10 denotes a mixture of 10% by volume of MTBE and 90% by volume of n-heptane; the same naming principles apply to the others. (1) Comparison of cylinder pressure and heat release rate curves. Figure 2 shows the pressure curves of n-heptane and its mixtures with varying volumes of MTBE under specific φ conditions. The results showed that, at a constant φ, as the MTBE content increased, combustion was delayed and the maximum in-cylinder pressure decreased, with M40 and M50 showing the most pronounced effects. At φ=0129, the HCCI combustion of n-heptane, M10, M20, and M30 was quite intense, leading to detonation, which is characterized by fluctuations in the cylinder pressure curve near top dead center, as shown in Figure 2(a). However, the M40 and M50 can operate stably under these conditions without detonation, which indicates that MTBE/heptane with a high MTBE content has good anti-knock properties and holds potential for extending HCCI combustion to higher loads. As φ decreases, the maximum in-cylinder pressure for all fuels generally decreases. In Figure 2(c), at φ=0.11, the pressure curves for M20, M30, M40, and M50 differ little from those of pure compression, indicating that their HCCI operation tends toward incomplete combustion or misfire. However, the pressure and compression lines of n-heptane and M10 diverged and rose, indicating that mixtures with low MTBE content, especially pure n-heptane, possess good low-temperature reaction properties. Figure 3 shows the heat release rate curve calculated from the cylinder pressure in Figure 2. The results indicate that, for a given φ, the timing of heat release is delayed and the extent of heat release also decreases. Under high-load conditions, the large fluctuations in the heat release rate curve later on are caused by the response issues of the pressure sensor. To analyze in detail the effects of MTBE on the ignition timing and combustion rate of n-heptane in HCCI, the crankshaft angles corresponding to 10% of the low-temperature heat release peak and 10% of the high-temperature heat release were defined as the start time of low-temperature heat release, θL, and the start time of high-temperature heat release, θH, respectively. At the same time, the heat release rates at low and high temperatures are studied, and the crankshaft angles corresponding to their peak heat release values are defined as θLMAX and θHMAX respectively. (II) Combustion characteristics of low-temperature reactions. Figure 2 shows the cylinder pressure curves for different MTBE/n-heptane ratios under specific φ conditions. Figure 3 shows the heat release rate curves for different MTBE/n-heptane ratios under specific φ conditions. The effects of the air-fuel equivalence ratio φ and MTBE content on the low-temperature heat release of MTBE/n-heptane are shown in Figure 4. For a specific fuel, whether it is n-heptane or an MTBE/n-heptane mixture, as φ decreases, the onset time and the peak time of low-temperature heat release are delayed, as shown in Figure 4(a) and Figure 4(b). Meanwhile, the low-temperature exothermic peak decreases as φ decreases, indicating that the low-temperature reaction slows down. Under the same φ conditions, as the MTBE content increases, the onset time of the low-temperature reaction for MTBE/n-heptane is delayed. For n-heptane, the low-temperature reaction begins at 20°–22°C ABTDC, whereas that for M50 begins at 16°–17.5°C ABTDC. It can be seen that MTBE is more difficult to ignite than n-heptane; moreover, the peak time of heat release at low temperatures is also delayed, as shown in Figure 4(b). More importantly, there is a significant difference in the degree of low-temperature heat release; when φ is constant, the peak value of low-temperature heat release decreases as the content of MTBE in the mixture increases, as shown in Figure 4(c). For φ = 0.29, the peak value of low-temperature heat release for M50 is only about 3.5 J/°, whereas n-heptane can reach 21.8 J/°. Thus, the exothermic reaction of n-heptane at low temperatures is suppressed by the addition of MTBE, mainly because the decomposition of MTBE consumes a large amount of OH radicals, and more importantly, no more OH radicals are generated in subsequent reactions. Therefore, the low-temperature HCCI reaction of MTBE is harmful rather than beneficial to the establishment of a mixed-fuel reaction cell. Moreover, this trend becomes more pronounced as MTBE increases. Especially at φ=0.11, the low-temperature heat release of fuels such as M50 is extremely weak, to the point that the relevant detailed information cannot be discerned. Although the addition of MTBE has a significant effect on the phase and extent of the exothermic reaction at low temperatures, the overall temperature range of this exothermic reaction does not change significantly; it begins to occur in the range of 750–850 K. Generally, the temperature environment required for low-temperature reactions is mainly provided by the temperature rise resulting from piston compression of the mixture, while the low-temperature exothermic chemical reaction of fuel/air also plays a certain role. As can be seen from Figure 4(d), as the φ value increases, the initial reaction temperature decreases slightly; however, for the same φ, there is no significant change in the low-temperature initiation temperatures of different MTBE/heptane mixtures. (III) Combustion characteristics in the high-temperature stage: Figure 5 compares the combustion characteristics in the high-temperature stage of different MTBE/heptane blend fuels under specific φ conditions. The θH of n-heptane is 13° before top dead center, and most of its HCCI combustion occurs before top dead center. The onset time of high-temperature heat release, θH, is significantly delayed as MTBE increases, as shown in Figure 5(a); simultaneously, the peak time of high-temperature heat release is also postponed, as shown in Figure 5(b). Thus, when the MTBE content reaches a certain level, the entire HCCI combustion is delayed to near the top dead center. For φ=0.29, θH for M40 is 2° before top dead center, and θH for M50 is 1° after top dead center. Therefore, it can be seen that MTBE has a significant effect on delaying HCCI combustion. The addition of MTBE significantly reduced the heat release during the high-temperature stage; Figure 5(c) shows that the peak value of the high-temperature heat release rate decreased markedly as the MTBE content increased, and this trend was particularly evident at high φ values. On the one hand, this is related to MTBE’s own lack of reactivity ； On the other hand, intermediate products of the low-temperature reaction of MTBE, such as isobutylene and methanol, have low reactivity at low temperatures, which also plays a significant role in suppressing the high-temperature combustion in HCCI. Different high-temperature heat releases result in varying degrees of combustion, which is also reflected in the combustion temperature within the cylinder. As shown in Figure 5(d), at φ=0.29, the combustion temperatures of M10, M20, and M30 are between 1450 and 1500 K, while that of n-heptane is close to 1900 K. As the MTBE content increases, the highest temperature inside the cylinder gradually decreases; the highest heat release temperatures for M40 and M50 are 1320 K and 1250 K respectively. It can be seen from Figure 5 that φ has a significant effect on high-temperature heat release. As φ decreases, the high-temperature ignition time is delayed, and both the phase and amount of heat release during the entire high-temperature stage are **delayed and reduced. It can be seen from the variations in the heat release rate peak and the highest temperature of combustion inside the cylinder as φ changes that the HCCI combustion of n-heptane is affected by