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Extending the lean operation limit of a gasoline Wankel rotary engine using hydrogen enrichment
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Extending the lean operation limit of a gasoline Wankel rotary engine using hydrogen enrichment F. Amrouche a,*, P.A. Erickson b, J.W. Park b, S. Varnhagen b a b
Centre de Developpement des Energies Renouvelables, CDER, 16340, Algiers, Algeria Mechanical and Aerospace Engineering Department, UC Davis, CA, 95616, USA
article info
abstract
Article history:
In this paper, an experimental investigation of the effect of hydrogen enrichment on a
Received 15 February 2016
gasoline Wankel engine's Lean Operation Limit (LOL) was performed. A monorotor Wankel
Received in revised form
engine operating at 3000 rpm and wide open throttle was modified to run on gasoline and
28 June 2016
hydrogen blends. Excess air ratio varying from stoichiometric mixtures to the engine lean
Accepted 29 June 2016
burn limit with 0%, 3% and 6% of hydrogen energy fractions in the intake were investigated.
Available online 16 July 2016
The experimental results proved that adding hydrogen to gasoline extends the original engine LOL while improving the engine stability. An increased thermal efficiency and
Keywords:
reduced burn duration, brake specific energy consumption, and HC, CO and CO2 emissions
Gasoline Wankel engine
were observed with hydrogen enrichment. Furthermore, the extension of the engine LOL
Hydrogen enrichment
with hydrogen addition could potentially control the NOx emissions of a gasoline Wankel
Lean operating limit
engine to very low levels without using a three-way catalytic converter. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the global trend to reduce both air pollution and energy consumption, today, the new target of all automobile manufacturers is to develop more efficient and less polluting vehicles [1]. This framework helps support development of emerging clean technologies such as fuel cell and electrical vehicles. However, compared to conventional vehicles, pure electric vehicles such as the battery electric vehicle experience a significantly limited range because of the storage system. The Wankel rotary engine is a possible alternative to the reciprocating engine used in hybrid vehicles [2,3] or potentially as a range extender for battery electric vehicles [4,5]. Since the Wankel engine is considerably lighter, simpler and has higher power density than the same power reciprocating engine [2,6], this engine allows a larger battery package in a
vehicle. Moreover, through previous studies, the Wankel engine has given promising results while being used as a range extender by Ribaut et al. [4] and Varnhagen et al. [5]. On the other hand, the Wankel engine has an unusual geometry. The elongated and irregular combustion chamber shape acts as a large heat transfer surface decreasing the engine thermodynamic efficiency that thereby increases the fuel consumption and toxic emissions [2]. Moreover, the Wankel engine's high surface to volume ratio is not conducive to the spread of the flame front, which expands the quenching area and leads to increased emissions in the exhaust. Key emissions resulting from increased quenching area are unburned hydrocarbons (HC) and carbon monoxide (CO) [7,8]. In addition, the geometry of the trochoid housing of the Wankel engine divide the combustion chamber into leading and trailing sides. While the rotor spins, the squish flow produced accelerates the flame propagation more effectively in the leading direction than in
* Corresponding author. E-mail address: [email protected] (F. Amrouche). http://dx.doi.org/10.1016/j.ijhydene.2016.06.250 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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the trailing direction, which weakens the combustion in the trailing side. In the Wankel engine, lower efficiency and higher amounts of unburned hydrocarbons emissions are mainly attributed to this factor. The solution to overcome these drawbacks in emissions and efficiency is to improve the combustion process. Lean combustion is a practical technology used to improve fuel economy and reduce emissions from spark-ignited reciprocating engines [9,10]. Lean combustion is also an ideal control strategy for reducing NOx emissions [9]. Extending the lean operating limit serves for potentially extending the benefits of lean combustion. There are two typical methods used to extend the lean operating limit in a spark-ignition engine. These are increased turbulence, and partial stratification [9]. However, experience gained through the years has proven that hydrogen enrichment has been shown to affect the same type of advantages as those two methods [9,11e16]. Indeed, hydrogen provides the necessary features to extend a stable combustion regime for leaner mixtures that offers the potential for attaining high thermal efficiencies in an internal combustion engine and reducing engine exhaust emissions [14]. The unique characteristics of hydrogen such as wide flammability range, large diffusion coefficient, high flame propagation velocity help to improve heat and mass transfer and reduce quenching distance [9,12]. This also, improve significantly the combustion rate and reduce the combustion duration, therefore, the heat release is concentrated close to top dead center (TDC) which decreases thermal losses by more closely approaching the isochoric heat release. Moreover, increasing hydrogen enrichment in the hydrocarbon fuel increases proportionally the thermal efficiency [17e19] and decreases CO, HC and CO2 emissions while increasing the NOx emissions [14,19e23]. However, hydrogen enrichment enhances the lean burn capability and enables more stable combustion of a hydrocarbon fuel by reducing the incomplete combustion and the cyclic variation of the original engine, and potentially resolves the problem of NOx emissions of the hybrid fuel mixture through extending the lean operating limit of engines [14,15]. Furthermore, using renewable hydrogen-hydrocarbon blends in an internal combustion engine are an attractive alternative for reducing dependence on fossil fuels and could become a convenient and economically viable strategy for introducing hydrogen in the nearterm [24,25]. The design of the Wankel rotary engine is well suited for hydrogen combustion. In fact, the absence of the exhaust valve and the physical separation between combustion and exhaust chambers can prevent pre-ignition and/or backfire that are the common sources of abnormal combustion while using hydrogen in reciprocating engines. Moreover, because of the flame characteristics of hydrogen, using hydrogen as fuel additive can partially resolve the Wankel engine drawbacks. Indeed, hydrogen improves the spread of the flame front, which helps to reduce the quenching effect and decrease the thermal losses inside the working chamber of the Wankel engine. This results in an increased thermal efficiency and reduced fuel consumption as well as HC and CO emissions in the exhaust. Simultaneously, NOx emissions resulting from burning hydrogen could be reduced by running the engine with lean mixture and further reduction can be expected while extending the lean burn capability of the
engine. Moreover, the accelerated flame speed with hydrogen addition can also help to improve the combustion in the trailing edge of the Wankel engine. In the literature about spark ignition reciprocating internal combustion engines, the improvement of the lean burn capability of fuels such as Gasoline [1,26], Natural Gas [6,15,27], Landfill Gas [14], methanol [28] and Ethanol [13,16] by hydrogen enrichment has already been investigated. However, no study has yet been reported on hydrogen enrichment within the Wankel rotary engine that focuses on the lean burn capability of gasoline-hydrogen mixtures. Brown et al. [29] and Salanki [3] experimentally investigated the performance of a single rotor Wankel engine fueled on pure hydrogen through a large range of relative equivalence ratios. An absence of combustion difficulties was demonstrated by both studies. However, when compared to gasoline operation, the maximum power decreased whereas the brake thermal efficiency increased and NOx emissions were found to be low for very lean mixture. Actually, Mazda is the only manufacturer that still developing the Wankel engines for their commercial vehicles. The company has already commercialized the RX-8 pure hydrogen Wankel rotary engine. This vehicle uses a combination of direct and port injection that has brought simultaneous achievement of high power, NOx reduction and high thermal efficiency. For further improvement in fuel efficiency, lean burn operation was used in this vehicle [30]. Amrouche et al. [20] experimentally investigated the effect of hydrogen enrichment on performance and emissions of gasoline Wankel engine at lean mixture and Wide Open Throttle (WOT) conditions. They found that increasing hydrogen enrichment helps to increase proportionally the brake thermal efficiency and reduce the brake specific HC, CO and CO2 emissions, however, they observed an important rise in brake specific NOx emissions. Motived by these results, a strategy for reducing NOx emissions by extending the lean burn limit of air-fuel mixture by hydrogen addition was performed in this research.
Experimental procedure Experimental setup A simplified schematic of the experimental set up is displayed in Fig. 1. The experiments were performed on a 0.530L single rotor with a single spark plug and air cooled normally aspirated Wankel engine, manufactured by Outboard Marine Corp's (OMC). The engine Technical Specifications are listed in Table 1. The engine was coupled to a Telma CC100 eddy current dynamometer to control and measure the engine speed and torque output. Initially built with a carburetor, this OMC Wankel engine was modified to achieve real time control over the air/fuel mixture preparation as well as hydrogen addition. This was done through the installation of two separate fuel injection systems, one for gasoline and the other for hydrogen. The two injectors were mounted into an aluminum block added in the fueling system. This aluminum block is a mixing chamber that allows the gasoline vapor and fumigated hydrogen to mix
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Fig. 1 e Schematic of the experimental set up.
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Table 1 e OMC Wankel engine technical specifications. Type of engine Number of rotor Cooling Engine manufacturer Ignition source Fuel supply system Displacement (cc) Compression ratio Power output Weight Fuel Engine geometry Eccentricity generating radius width of rotor housing (mm)
Rotary Wankel engine Single rotor Air cooled Outboard Marine Corp's, USA Spark plug Fuel injection 530 cm3 8.75 to 1 26 kW @ 5500 rpm 27 kg 50 parts Gasoline, 1 part Oil 13.97 mm 92.202 mm 77.7748 mm
in the intake air stream. The fuel flow of the gasoline was metered by a calibrated Micromotion CMF010M Coriolis flow meter with an accuracy of 0.10% of rate for flow rates of 0e23 g/s. The hydrogen used in this study was bottled industrial hydrogen (99.95% purity) that is metered by a calibrated Aalborg differential pressure mass flow controller, model GFC 47 with an accuracy of ±3% (0e20% full scale), and ±1.5% (20e100% full scale). During the test, the air mass flow was metered by an orifice plat, and a large air chamber was added past the intake to damp cyclic variations in air flow caused by cyclic flow in the engine. A hybrid electronic control unit (HECU), developed in the laboratory, was used to control in a real time the spark timing, injection timing and flows of hydrogen and gasoline according to the desired air to fuel ratio and the specified hydrogen energy fractions in the intake. The exhaust emissions were sampled by three distinct analyzers. A California Analytical exhausts gas analyzer, model 300, was used to measure CO and CO2 concentrations by non dispersive Infrared (NDIR) with a linearity and repeatability of better than 1%, and a response time of 90% of full-scale in 2 s. A California Analytical heated chemiluminescence photodiode detector, model 400, was used to measure NO and NOx emissions with a resolution of 0.1 ppm and sensitivity up to 5 ppm. The HC, CO, CO2 and O2 emissions were measured through a Horiba MEXA574GE emissions analyzer, in which the HC and O2 was determined by a hydrogen flame ionization detection method, and the CO and CO2 were measured by a non-dispersive infrared method. The Horiba has a time response less than 10s, a repeatability of 0.04%, 20 ppm, 0.1%, and 0.4% respectively for CO, HC, CO2 and O2 During the test, the air to fuel ratio was calculated through both direct calculation of the ratio from the measured mass flow rate of air, gasoline and hydrogen, as well as using a Bosch Universal Exhaust Gas Oxygen (UEGO) sensor installed in the exhaust stream. An innovative Motorsports LC-1 wideband controller controlled this sensor. The UEGO sensor measures in real time the oxygen content in the exhaust stream and outputs a predicted air-to-fuel ratio. This air-tofuel ratio was used as a feedback signal ratio, in a closed loop control system, by the HECU to achieve more precise control of the air-to-fuel ratio.
The measured air to fuel ratios has shown repeatability and equivalence between the two methods during all the experiments. The excess air ratio of the gasolineehydrogeneair mixture is defined as [20,22]: l ¼ m_ air
m_ g $AFst;g þ m_ H2 $AFst;H2
(1)
_ H2 , are respectively the measured air, gaso_ air m _ g; m where m line and hydrogen mass flow rates (kg/h). AFst;g and AFst;H2 are the stoichiometric air-to-fuel ratios respectively of gasoline and Hydrogen, such as AFst;g ¼ 14.6 and AFst;H2 ¼ 34.3. A calibrated Kistler 6051B high temperature piezoelectric pressure transducer (sensitivity 20.5 pC/bar) and Kistler 5010B charge amplifier were used to record the working chamber pressures trace for over 100 consecutive cycles. A trigger wheel and a Hall effect magnetic sensor were used to obtain the crank angle position data. The crank angle signal was synchronized with the pressure trace.
Tests conditions All experiments were performed at an engine speed of 3000 rpm and at WOT conditions. This engine speed was chosen because according to Ward et al. [31], the maximum average torque produced by this OMC Wankel engine was achieved at 3000 rpm. After a warm-up period on gasoline, the appropriate hydrogen energy fraction was set by replacing the amount of gasoline and the engine was gradually leaned out to the engine lean operation limit (LOL). In this study, LOL was found as the equivalence ratio at which the coefficient of variation (COV) in IMEP reaches 10%. This is because it is generally accepted that a COVIMEP above 10% will be perceived by a driver as a poor running condition [15,32,33]. The engine was allowed to reach the steady state at each set of tests then the data was collected. For this work, three hydrogen energy fractions in the intake 0%, 3% and 6% were used. Table 2 summarizes the operating conditions used along the experiments. The spark timing was kept at 15 CA BTDC of the original engine. Such a fixed ignition timing control strategy was also adopted by other researchers to explore the performance of hydrogen-enriched engines [19e22,32]. The fraction of hydrogen in the total intake gas is defined as energy fraction, calculated as follow [20,34]: " # m_ H2 LHVH2 100 % H2 ¼ m_ g LHVg þ m_ H2 LHVH2
(2)
m_ H2 , m_ g are respectively the mass flow rate of hydrogen and gasoline (g/s).
Table 2 e Engine operating conditions during the experiments. Hydrogen energy fraction Ignition timing (degree crank angle BTDC) Engine speed (rpm) Air-to-fuel relative equivalence ratio l Throttle position
0%, 3%, 6% 15 3000 1.15 to LOL Wide open
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LHVH2 , LHVg are the lower heating value respectively of hydrogen and gasoline (MJ/Kg), such as LHVH2 ¼ 120.1, LHVg ¼ 43:5.
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improve the engine stability and help to extend the LOL of the Wankel engine.
Combustion analysis
Results and discussion Effect of hydrogen addition on LOL and the engine stability The cycle-to-cycle variation that is quantified through the calculation of the coefficient of variance in the IMEP (COVIMEP) mirrors the engine stability. Fig. 2 depicts the variations of COVIMEP as a function of relative equivalence ratio l for pure gasoline and gasoline blended with 3% and 6% hydrogen energy fraction in the intake. As shown in Fig. 2, adding hydrogen to gasoline-air mixture lowers the cyclic variations. It can also be observed from Fig. 2 that without hydrogen addition, the COVIMEP began to increase sharply above 10% for load l > 1.29. However, when 3% and 6% of hydrogen were added to the fuel mixture, the COVIMEP began to increase above 10% for load of l > 1.42 and l > 1.51 respectively. Therefore, hydrogen addition results in better stability of the engine at leaner mixtures, where the combustion is more problematic. Moreover, lean limit was extended as hydrogen fraction increased in the fuel mixture. Indeed, the addition of hydrogen extended the LOL of the original engine from lLOL ¼ 1.29 (pure gasoline) to lLOL ¼ 1.42 and lLOL ¼ 1.51 matching an extension up to 10% and 17% of LOL for an enrichment of hydrogen about 3% and 6%, respectively. The higher flame speed of hydrogen helps to increase the rate of combustion of the airegasoline mixture. While the smaller quenching distance exhibited by hydrogen in comparison to gasoline results in a more complete combustion in the Wankel engine. Furthermore, the higher diffusivity of hydrogen compared to gasoline improves the mixing and increases the homogeneity in the load. All of these mechanisms help to enhance the combustion rate in the combustion chamber of the Wankel engine and especially in the trailing side. Therefore, the higher combustion rate, the more complete combustion and the better homogeneity of the load
Fig. 2 e COVIMEP versus excess air ratio l for gasoline enriched at 0%, 3% and 6% hydrogen energy fractions.
The study of the effect of hydrogen addition on the combustion duration is essential to the analysis of hydrogen's ability to extend lean burn limit. Based on pressure trace data calculation, the CA0-10, CA10-90 and CA50 burn durations are respectively the flame development, the flame propagation and the central heat release angle of the combustion of an airfuel mixture. The CA0-10, CA10-90 and CA50 burn durations versus relative equivalence ratio profiles are shown in Fig. 3a, b and c, respectively. Thus, for the combustion of pure gasoline and gasoline enriched 3% and 6% of Hydrogen energy fractions at 3000 rpm, WOT, fixed spark timing of 15 BTDC. It can be observed from Fig. 3a, b and c that for the three fuel mixtures as the engine was leaned out, the CA0-10, CA1090 and CA50 burn durations last longer due to the slowed combustion rates. This is mainly due to the reduced energy flow rate while the engine is leaning out. However, at a given l, the CA0-10, CA10-90 and CA50 burn durations shortened proportionally as hydrogen fraction increased in the fuel mixture, which benefits reduced cooling and exhaust losses and increased mass fraction burned [19,35]. This demonstrates the ability of hydrogen addition to speed up flame development and propagation. The reduced CA0-10 is the result of lower ignition energy of hydrogen that improves the flammability of the hydrogen-gasoline fuel mixture at lean mixture in early combustion process. Moreover, according to Wang et al. [35], the addition of hydrogen promotes the chemical reaction due to the increased H, O and OH mole fractions in the flame. Therefore, hydrogen-gasoline blends can be ignited more easily than pure gasoline. The reduced CA10-90 is the result of the high flame propagation and diffusion rate of hydrogen compared to gasoline that improve the homogeneity and speed the propagation of flame front of the gasolineehydrogeneair mixture. Moreover, compared to reciprocating engine, the Wankel rotary engines is characterized by high heat losses on the wall and large quenching area that affects the engine economy and HC emissions. All of which become noticeably worse as the engine is leaned out. Therefore, the addition of hydrogen through the reduction of the burn duration increases the turbulence inside the working chamber that improves the heat transfer between the burned and unburned zones and reduces the heat losses and the quenching effect. Moreover, due to hydrogen's burn characteristics, hydrogen enrichment is able to accelerate the squish flow generated by the Wankel engine especially in the trailing side. This helps to increase the burning speed during the combustion in this disadvantageous side of the combustion chamber. These data help to demonstrate that the hydrogen enrichment strategy is well suited in the Wankel engine. The central heat release angle CA50 represents the combustion efficiency of an airefuel mixture. The ideal engine has the CA50 at TDC because the engine combustion losses are reduced. Therefore, in practice, it is suggested that the optimum efficiency of engine combustion is achieved if 50% energy conversion occurs between 8 and 10 ATDC. These
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Fig. 3 e Combustion characteristics: a) CA0-10 flame development, b) CA10-90 flame propagation and c) CA50 central heat release angle.
values of CA50 achieve a compromise between power, wall heat transfer and exhaust gas energy [11,36]. The optimum crank angle location of 50% energy conversion should be calculated and controlled by modifying the point of ignition [37]. Therefore the addition of hydrogen can also be a way to control this optimum location. It can be found from Fig. 3c that as the engine is leaned out, the CA50 of the gasoline and hydrogen-gasoline mixtures is significantly retarded. Therefore, due to the narrow flammability and low burning speed of gasoline, CA50 of the gasoline engine is perceptibly advanced with the increase of hydrogen energy fraction. The retarded CA50 translates to an increased post-combustion duration, since more fuel is burnt during the expansion stroke. Therefore, the advanced CA50 shows that the combustion is enhanced by hydrogen enrichment. This is because the improved flammability and burning rate of gasolineehydrogen fuel mixture. Fig. 4 shows the CA0-10 and CA10-90 burn durations versus lLOL profiles for pure gasoline and gasoline enriched at 3% and 6% of hydrogen energy fraction at 3000 rpm, WOT, fixed spark timing of 15 BTDC. According to Fig. 4, the crank angle intervals for the CA0-10 and CA10-90 burn durations reached limiting values at the lean limit that were independent of engine operating conditions and fuel types used. This result was also observed by Quader [38], while investigating the LOL of the gasoline reciprocating engine. Therefore, it is interesting that despite the difference between those two engines, the conclusions agree so well.
shows the variations of BMEP with excess air ratio for pure gasoline and gasoline blended at 3% and 6% hydrogen energy fractions, at 3000 rpm and WOT conditions. As seen in Fig. 5, BMEP increased with increasing hydrogen addition levels. However, since the fuel energy flow rate is reduced when the engine is leaning out, increasing the dilution results in decreased BMEP for pure gasoline as well as hydrogen enriched gasoline. The narrow flammability limit of the gasoline makes the combustion for the lean mixture incomplete and as the excess air ratio increases to be closer to the LOL, the original gasoline engine suffers from slow burning. Therefore, the fast flame propagation rate with hydrogen addition accelerates the combustion of gasolineehydrogeneair mixtures while extending the flammability limit of the air fuel mixture. Thus, the addition of hydrogen can enhance the Wankel
Brake mean effective pressure The Brake Mean Effective Pressure (BMEP) is an important performance parameter used for comparing engines. Fig. 5
Fig. 4 e Burn duration at lean operation limit of gasoline enriched at 0%, 3% and 6% hydrogen energy fractions.
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Fig. 5 e Brake mean effective pressure versus excess air ratio l for gasoline enriched at 0%, 3% and 6% hydrogen energy fractions.
engine BMEP at lean conditions. These results were also proven by Wang et al. [39] while investigation the effect of hydrogen enrichment in reciprocating engine.
Brake thermal efficiency The engine thermal efficiency is crucial for assessing the economy of an engine. This parameter can be improved by optimizing the combustion system or fuel characteristics. The variations of Brake Thermal Efficiency (BTE) with excess air ratio for pure gasoline and gasoline blended 3% and 6% hydrogen energy fractions, at 3000 rpm and WOT are displayed in Fig. 6. According to Fig. 6, the engine BTE increased with increasing hydrogen blending fraction in the fuel mixture. However, as the fuel energy flow rate diminished with increasing excess air, the thermal efficiency decreased for all air-fuel mixtures. Moreover, the original Wankel engine thermal efficiency decreased sharply close to the original LOL of the engine. However, with hydrogen addition this behavior is eased and the thermal efficiency was significantly improved close to the LOL. The extension of the LOL of the original engine through the addition of 3% and 6% of hydrogen energy
Fig. 6 e Brake thermal efficiency versus excess air ratio l for gasoline enriched at 0%, 3% and 6% hydrogen energy fractions.
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fraction in gasoline increases the thermal efficiency when compared to pure gasoline at LOL. Accordingly, hydrogen enriched gasoline Wankel engine could produce almost constant thermal efficiencies over a wide range of excess air ratio for lean mixtures. This is the result of the reduced burn duration and improved homogeneity of the fuel mixture that helps to complete the combustion and reduce the cooling and exhaust loses of the rotary engine. When it comes to Wankel engine this result is crucial, because it helps to overcome the Wankel engine low thermodynamic efficiency resulting from its geometry. In addition, the smaller quenching distance exhibited by hydrogen in comparison to gasoline means that the flame could travel closer to the combustion chamber wall and further into crevices resulting in more completed combustion and improved BTE.
Brake specific energy consumption The Brake Specific Energy Consumption (BSEC) is the energy used by the engine to produce unit power. Therefore, it is a more reliable criterion to compare different fuels since it takes in consideration the calorific value and the mass flow rate of fuel. This parameter like the thermal efficiency reflects the engine economy. Fig. 7 shows the profiles of BSEC for different air ratios, for pure gasoline and gasoline blended with 3% and 6% of hydrogen energy fractions. According to Fig. 7, the BSEC is much higher for the engine operation on pure gasoline and this rises rapidly by 31% with the increase of excess air to the LOL. This is because of the incomplete combustion in lean mixture especially close to LOL where there is a poor utilization of fuel. However, the BSEC is reduced with increasing hydrogen levels in the air-fuel mixtures for the same excess air ratio. However, BSEC increased smoothly with increasing excess air for the same hydrogen energy fraction in the mixture. The improvement in BSEC of the Wankel engine after hydrogen addition is largely attributed to the higher LHV of hydrogen and to the capability of hydrogen to reduce the combustion duration event. This helps to improve the quality of the combustion and thus the thermal efficiency, reducing the thermal loses and thus decreasing the energy consumption under lean mixture conditions.
Fig. 7 e BSEC versus excess air ratio l for gasoline enriched at 0%, 3% and 6% hydrogen energy fractions.
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Fig. 8 e The effect of the extension of LOL by hydrogen addition on BTE and BSEC.
Fig. 8 summarizes the effect of the extension of LOL through hydrogen addition on BTE and BSEC. According to Fig. 8, it is important to notice that the extension of the original LOL of the Wankel engine through the addition of 3% and 6% of hydrogen energy fraction to gasoline enables a reduction in BSEC of about 15% and 22% respectively and an improvement in BTE of about 23% and 28% respectively.
Emissions In general, the Wankel rotary engine is well known for its excessive hydrocarbon and reduced NOx emissions compared to reciprocating engine [7]. However, in this paper, because of the hydrogen enrichment, the NOx, HC, CO and CO2 emissions were measured and analyzed and were found to improve with hydrogen addition. The NOx emissions are mainly related to the working chamber temperature and air to fuel ratio. As illustrated in Fig. 9, the brake specific NOx emissions decreased with
increasing excess air for gasoline and hydrogen enriched gasoline mixtures. Moreover, according to Fig. 9, leaning out the Wankel engine to the LOL of the pure gasoline and gasoline blended with 3% and 6% hydrogen energy fraction, drops the brake specific NOx emissions by 81%, 90% and 92% respectively with a minimum emitted by each fuel mixture at lLOL. This is because, when the air-fuel mixture is leaned out the combustion temperature is lowered, which reduces the thermal NOx formation. For a given excess air ratio, the brake specific NOx emissions rise with increasing hydrogen energy fraction in airgasoline mixtures up to the LOL of the original engine configuration. This is a consequence of hydrogen addition that reduces the combustion duration and increases the combustion chamber temperature. Also, according to the Fig. 9, the extension of LOL of the engine by the addition of 3% and 6% of hydrogen energy fraction further reduces the minimum NOx emissions of the original engine by 51% and 61% respectively. This result confirms the strategy of the reduction of NOx specific emissions by extending the LOL of the original engine with the addition of hydrogen. It should be noted that after LOL extension due to hydrogen addition, the engine-out emissions will meet the proposed 0.22 g/kWh BACT standard [14], without being treated by a three way catalytic converter. Because of the geometry of the Wankel engine, the HC emissions are the most predominant exhaust emissions. Fig. 10 depicts the variation of the brake specific HC emissions with excess air ratio for pure gasoline and gasoline blends of 3% and 6% hydrogen energy fractions at WOT condition. As shown in Fig. 10, when the engine is leaned out, the brake specific HC emissions first decreased, and then increased close to the LOL of each fuel mixture. However, the HC emissions increased more sharply for pure gasoline than for hydrogen enriched gasoline. The combustion quality is directly related to energy content in the intake and it is reflected by HC emissions. Close to the LOL, the increased HC emissions are the result of the degradation of the combustion
Fig. 9 e Brake specific NOx emissions versus excess air ratio l for gasoline enriched at 0%, 3% and 6% hydrogen energy fractions.
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Fig. 10 e Brake specific HC emission versus excess air ratio l for gasoline enriched at 0%, 3% and 6% hydrogen energy fractions.
that is caused by the deceleration of combustion rate and thus the lengthening of the combustion duration, which causes erratic and incomplete combustion at LOL. This is more obvious for the Wankel engine due to its high surface to volume ratio that is not helpful for the flame propagation. However, the addition of hydrogen increases the combustion rate and reduces the combustion duration, which improves the overall combustion quality especially close to LOL of the original engine and reduces the problematic Wankel engine quenching distance. Moreover, the carbon content in fuel mixture is reduced as hydrogen was added to air-gasoline mixture. Fig. 11 shows the variation of the CO emissions versus air ratio for pure gasoline and gasoline blended at 3% and 6% hydrogen energy fractions. As displayed in Fig. 11, for pure gasoline and gasoline blended with 3% and 6% hydrogen, CO emissions decrease first as excess air increased, then, when the engine approaches the LOL, the emissions increase sharply due to the incomplete combustion as the engine was further leaned out. Furthermore, for a given excess air ratio, CO emissions decreased as hydrogen fraction increases in the air-gasoline mixture. As the engine LOL was extended by hydrogen addition, the hydrogen-enriched gasoline Wankel
Fig. 11 e CO emissions versus excess air ratio l for gasoline enriched at 0%, 3% and 6% hydrogen energy fractions.
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engine could adopt larger excess air ratios to control CO emissions. Thereby, the hydrogen is effective for reducing CO emissions in Wankel engine. This result can be attributed to the improved combustion rate and thus more complete combustion of the air-gasoline mixture was achieved [40]. Beyond that, carbon concentration of the fuel blends is decreased due to hydrogen addition. The combustion of fossil fuels emits CO2 that is a greenhouse gas emission. The impact of hydrogen addition on CO2 emissions was studied through Fig. 12 that displays the profiles of CO2 emissions versus air ratio for pure gasoline and gasoline blended at 3% and 6% of hydrogen energy fraction. According to Fig. 12, the CO2 emissions decreased with increasing excess air for both gasoline and hydrogen enriched gasoline. For a given air ratio, as hydrogen energy fraction increases, CO2 emissions decrease consistently. It can be seen from Fig. 12 that increasing the excess air ratio to the LOL, reduces CO2 emissions for pure gasoline and gasoline blended with hydrogen. Furthermore, the extension of LOL of the original engine by the addition of 3% and 6% of hydrogen energy fraction further reduces the emissions of CO2 from the original engine by 8% and 24% respectively. This reduction is due to the decrease in fuel flow rate of the fuel-air mixture with increasing excess air beyond an increased hydrogen/ carbon ratio and improved engine efficiency.
Conclusion This experimental study investigated the effects of the extension of a gasoline Wankel engine LOL through hydrogen addition on the engine combustion and emissions performance. These experiments were performed at 3000 rpm, fixed spark timing and WOT conditions. The main conclusions resulting from this paper are summarized as follows: 1. The addition of hydrogen to gasoline demonstrates the capability to extend the LOL of the original engine. Moreover, results indicate that greater amounts of hydrogen addition can maintain lower COVIMEP for larger range of
Fig. 12 e CO2 emissions versus excess air ratio l for gasoline enriched at 0%, 3% and 6% hydrogen energy fractions.
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2.
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excess of air, which means that the engine became more stable for leaner mixtures. Hydrogen addition decreases both flame development and propagation durations and advances the central heat release angle proportionally to the amount of hydrogen. This reduction became more obvious at lean combustion condition. In this study, the BMEP for hydrogen enriched gasoline is higher than that of the original gasoline Wankel engine for all lean mixtures. The increase of dilution decreases the BMEP for pure gasoline as well as hydrogen enriched gasoline since the fuel energy flow rate was reduced. The addition of hydrogen to the gasoline fueled Wankel engine improves the thermal efficiency significantly. Moreover, the extension of the LOL of the original engine by the addition of 3% and 6% of hydrogen energy fraction to gasoline increases the BTE by 23% and 28% respectively. Also, the extension of the lean operation limit of the Wankel engine through the addition of 3% and 6% of hydrogen energy fraction to the gasoline enables a reduction in the specific energy consumption of about 15% and 22% respectively. Furthermore, the extension of LOL of the engine by the addition of 3% and 6% of hydrogen energy fraction reduces the brake specific NOx emissions of the original Wankel engine by 51% and 61% respectively. These levels of NOx meet the proposed 0.22 g/kWh BACT standard, without being treated by a three way catalytic converter. The carbon based emissions such as HC, CO, CO2 were reduced with hydrogen addition due to the improved combustion and increased H/C ratio of the hybrid fuel. Furthermore, as the engine LOL was extended by hydrogen addition, the Wankel engine could adopt larger excess air ratios to control HC and CO emissions.
Acknowledgments The authors thank Moller International for their donation of the Wankel research engine. The University of California, Davis, Green Transportation Laboratory and the Energy Research Laboratory and all their associated members made this work possible.
Nomenclature AFst;g AFst;H2 BMEP BSEC BTDC BTE CA CA0-10
Stoichiometric air-to-fuel ratio of gasoline (AFst,g ¼ 14.6) Stoichiometric air-to-fuel ratio of hydrogen (AFst;H2 ¼ 34.3) Brake mean effective pressure, bar Brake specific energy consumption, MJ/kW.h Before top dead center Brake thermal efficiency, % Crank angle Flame development, it is the crank angle duration from spark discharge to 10% heat release of the total fuel, CA
CA50 Central heat release angle, CA CA10-90 Flame propagation, it is the crank angle duration from 10% to 90% heat release of the total fuel, CA CO Carbon monoxide Carbon dioxide CO2 COVimep Coefficient of variance of indicated mean effective pressure, % HC Hydrocarbon HECU Hybrid electronic control unit Lower heating value of gasoline, MJ/Kg LHVg Lower heating value of hydrogen, MJ/Kg LHVH2 LOL Lean operation limit NOx Nitrogen oxide OMC Outboard Marine Corporation Rpm Rotation per minutes TDC Top dead center WOT Wide open throttle l Excess air ratio Hydrogen energy fraction, % % H2
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Available online at www.sciencedirect.com
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Extending the lean operation limit of a gasoline Wankel rotary engine using hydrogen enrichment F. Amrouche a,*, P.A. Erickson b, J.W. Park b, S. Varnhagen b a b
Centre de Developpement des Energies Renouvelables, CDER, 16340, Algiers, Algeria Mechanical and Aerospace Engineering Department, UC Davis, CA, 95616, USA
article info
abstract
Article history:
In this paper, an experimental investigation of the effect of hydrogen enrichment on a
Received 15 February 2016
gasoline Wankel engine's Lean Operation Limit (LOL) was performed. A monorotor Wankel
Received in revised form
engine operating at 3000 rpm and wide open throttle was modified to run on gasoline and
28 June 2016
hydrogen blends. Excess air ratio varying from stoichiometric mixtures to the engine lean
Accepted 29 June 2016
burn limit with 0%, 3% and 6% of hydrogen energy fractions in the intake were investigated.
Available online 16 July 2016
The experimental results proved that adding hydrogen to gasoline extends the original engine LOL while improving the engine stability. An increased thermal efficiency and
Keywords:
reduced burn duration, brake specific energy consumption, and HC, CO and CO2 emissions
Gasoline Wankel engine
were observed with hydrogen enrichment. Furthermore, the extension of the engine LOL
Hydrogen enrichment
with hydrogen addition could potentially control the NOx emissions of a gasoline Wankel
Lean operating limit
engine to very low levels without using a three-way catalytic converter. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the global trend to reduce both air pollution and energy consumption, today, the new target of all automobile manufacturers is to develop more efficient and less polluting vehicles [1]. This framework helps support development of emerging clean technologies such as fuel cell and electrical vehicles. However, compared to conventional vehicles, pure electric vehicles such as the battery electric vehicle experience a significantly limited range because of the storage system. The Wankel rotary engine is a possible alternative to the reciprocating engine used in hybrid vehicles [2,3] or potentially as a range extender for battery electric vehicles [4,5]. Since the Wankel engine is considerably lighter, simpler and has higher power density than the same power reciprocating engine [2,6], this engine allows a larger battery package in a
vehicle. Moreover, through previous studies, the Wankel engine has given promising results while being used as a range extender by Ribaut et al. [4] and Varnhagen et al. [5]. On the other hand, the Wankel engine has an unusual geometry. The elongated and irregular combustion chamber shape acts as a large heat transfer surface decreasing the engine thermodynamic efficiency that thereby increases the fuel consumption and toxic emissions [2]. Moreover, the Wankel engine's high surface to volume ratio is not conducive to the spread of the flame front, which expands the quenching area and leads to increased emissions in the exhaust. Key emissions resulting from increased quenching area are unburned hydrocarbons (HC) and carbon monoxide (CO) [7,8]. In addition, the geometry of the trochoid housing of the Wankel engine divide the combustion chamber into leading and trailing sides. While the rotor spins, the squish flow produced accelerates the flame propagation more effectively in the leading direction than in
* Corresponding author. E-mail address: [email protected] (F. Amrouche). http://dx.doi.org/10.1016/j.ijhydene.2016.06.250 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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the trailing direction, which weakens the combustion in the trailing side. In the Wankel engine, lower efficiency and higher amounts of unburned hydrocarbons emissions are mainly attributed to this factor. The solution to overcome these drawbacks in emissions and efficiency is to improve the combustion process. Lean combustion is a practical technology used to improve fuel economy and reduce emissions from spark-ignited reciprocating engines [9,10]. Lean combustion is also an ideal control strategy for reducing NOx emissions [9]. Extending the lean operating limit serves for potentially extending the benefits of lean combustion. There are two typical methods used to extend the lean operating limit in a spark-ignition engine. These are increased turbulence, and partial stratification [9]. However, experience gained through the years has proven that hydrogen enrichment has been shown to affect the same type of advantages as those two methods [9,11e16]. Indeed, hydrogen provides the necessary features to extend a stable combustion regime for leaner mixtures that offers the potential for attaining high thermal efficiencies in an internal combustion engine and reducing engine exhaust emissions [14]. The unique characteristics of hydrogen such as wide flammability range, large diffusion coefficient, high flame propagation velocity help to improve heat and mass transfer and reduce quenching distance [9,12]. This also, improve significantly the combustion rate and reduce the combustion duration, therefore, the heat release is concentrated close to top dead center (TDC) which decreases thermal losses by more closely approaching the isochoric heat release. Moreover, increasing hydrogen enrichment in the hydrocarbon fuel increases proportionally the thermal efficiency [17e19] and decreases CO, HC and CO2 emissions while increasing the NOx emissions [14,19e23]. However, hydrogen enrichment enhances the lean burn capability and enables more stable combustion of a hydrocarbon fuel by reducing the incomplete combustion and the cyclic variation of the original engine, and potentially resolves the problem of NOx emissions of the hybrid fuel mixture through extending the lean operating limit of engines [14,15]. Furthermore, using renewable hydrogen-hydrocarbon blends in an internal combustion engine are an attractive alternative for reducing dependence on fossil fuels and could become a convenient and economically viable strategy for introducing hydrogen in the nearterm [24,25]. The design of the Wankel rotary engine is well suited for hydrogen combustion. In fact, the absence of the exhaust valve and the physical separation between combustion and exhaust chambers can prevent pre-ignition and/or backfire that are the common sources of abnormal combustion while using hydrogen in reciprocating engines. Moreover, because of the flame characteristics of hydrogen, using hydrogen as fuel additive can partially resolve the Wankel engine drawbacks. Indeed, hydrogen improves the spread of the flame front, which helps to reduce the quenching effect and decrease the thermal losses inside the working chamber of the Wankel engine. This results in an increased thermal efficiency and reduced fuel consumption as well as HC and CO emissions in the exhaust. Simultaneously, NOx emissions resulting from burning hydrogen could be reduced by running the engine with lean mixture and further reduction can be expected while extending the lean burn capability of the
engine. Moreover, the accelerated flame speed with hydrogen addition can also help to improve the combustion in the trailing edge of the Wankel engine. In the literature about spark ignition reciprocating internal combustion engines, the improvement of the lean burn capability of fuels such as Gasoline [1,26], Natural Gas [6,15,27], Landfill Gas [14], methanol [28] and Ethanol [13,16] by hydrogen enrichment has already been investigated. However, no study has yet been reported on hydrogen enrichment within the Wankel rotary engine that focuses on the lean burn capability of gasoline-hydrogen mixtures. Brown et al. [29] and Salanki [3] experimentally investigated the performance of a single rotor Wankel engine fueled on pure hydrogen through a large range of relative equivalence ratios. An absence of combustion difficulties was demonstrated by both studies. However, when compared to gasoline operation, the maximum power decreased whereas the brake thermal efficiency increased and NOx emissions were found to be low for very lean mixture. Actually, Mazda is the only manufacturer that still developing the Wankel engines for their commercial vehicles. The company has already commercialized the RX-8 pure hydrogen Wankel rotary engine. This vehicle uses a combination of direct and port injection that has brought simultaneous achievement of high power, NOx reduction and high thermal efficiency. For further improvement in fuel efficiency, lean burn operation was used in this vehicle [30]. Amrouche et al. [20] experimentally investigated the effect of hydrogen enrichment on performance and emissions of gasoline Wankel engine at lean mixture and Wide Open Throttle (WOT) conditions. They found that increasing hydrogen enrichment helps to increase proportionally the brake thermal efficiency and reduce the brake specific HC, CO and CO2 emissions, however, they observed an important rise in brake specific NOx emissions. Motived by these results, a strategy for reducing NOx emissions by extending the lean burn limit of air-fuel mixture by hydrogen addition was performed in this research.
Experimental procedure Experimental setup A simplified schematic of the experimental set up is displayed in Fig. 1. The experiments were performed on a 0.530L single rotor with a single spark plug and air cooled normally aspirated Wankel engine, manufactured by Outboard Marine Corp's (OMC). The engine Technical Specifications are listed in Table 1. The engine was coupled to a Telma CC100 eddy current dynamometer to control and measure the engine speed and torque output. Initially built with a carburetor, this OMC Wankel engine was modified to achieve real time control over the air/fuel mixture preparation as well as hydrogen addition. This was done through the installation of two separate fuel injection systems, one for gasoline and the other for hydrogen. The two injectors were mounted into an aluminum block added in the fueling system. This aluminum block is a mixing chamber that allows the gasoline vapor and fumigated hydrogen to mix
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Fig. 1 e Schematic of the experimental set up.
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Table 1 e OMC Wankel engine technical specifications. Type of engine Number of rotor Cooling Engine manufacturer Ignition source Fuel supply system Displacement (cc) Compression ratio Power output Weight Fuel Engine geometry Eccentricity generating radius width of rotor housing (mm)
Rotary Wankel engine Single rotor Air cooled Outboard Marine Corp's, USA Spark plug Fuel injection 530 cm3 8.75 to 1 26 kW @ 5500 rpm 27 kg 50 parts Gasoline, 1 part Oil 13.97 mm 92.202 mm 77.7748 mm
in the intake air stream. The fuel flow of the gasoline was metered by a calibrated Micromotion CMF010M Coriolis flow meter with an accuracy of 0.10% of rate for flow rates of 0e23 g/s. The hydrogen used in this study was bottled industrial hydrogen (99.95% purity) that is metered by a calibrated Aalborg differential pressure mass flow controller, model GFC 47 with an accuracy of ±3% (0e20% full scale), and ±1.5% (20e100% full scale). During the test, the air mass flow was metered by an orifice plat, and a large air chamber was added past the intake to damp cyclic variations in air flow caused by cyclic flow in the engine. A hybrid electronic control unit (HECU), developed in the laboratory, was used to control in a real time the spark timing, injection timing and flows of hydrogen and gasoline according to the desired air to fuel ratio and the specified hydrogen energy fractions in the intake. The exhaust emissions were sampled by three distinct analyzers. A California Analytical exhausts gas analyzer, model 300, was used to measure CO and CO2 concentrations by non dispersive Infrared (NDIR) with a linearity and repeatability of better than 1%, and a response time of 90% of full-scale in 2 s. A California Analytical heated chemiluminescence photodiode detector, model 400, was used to measure NO and NOx emissions with a resolution of 0.1 ppm and sensitivity up to 5 ppm. The HC, CO, CO2 and O2 emissions were measured through a Horiba MEXA574GE emissions analyzer, in which the HC and O2 was determined by a hydrogen flame ionization detection method, and the CO and CO2 were measured by a non-dispersive infrared method. The Horiba has a time response less than 10s, a repeatability of 0.04%, 20 ppm, 0.1%, and 0.4% respectively for CO, HC, CO2 and O2 During the test, the air to fuel ratio was calculated through both direct calculation of the ratio from the measured mass flow rate of air, gasoline and hydrogen, as well as using a Bosch Universal Exhaust Gas Oxygen (UEGO) sensor installed in the exhaust stream. An innovative Motorsports LC-1 wideband controller controlled this sensor. The UEGO sensor measures in real time the oxygen content in the exhaust stream and outputs a predicted air-to-fuel ratio. This air-tofuel ratio was used as a feedback signal ratio, in a closed loop control system, by the HECU to achieve more precise control of the air-to-fuel ratio.
The measured air to fuel ratios has shown repeatability and equivalence between the two methods during all the experiments. The excess air ratio of the gasolineehydrogeneair mixture is defined as [20,22]: l ¼ m_ air
m_ g $AFst;g þ m_ H2 $AFst;H2
(1)
_ H2 , are respectively the measured air, gaso_ air m _ g; m where m line and hydrogen mass flow rates (kg/h). AFst;g and AFst;H2 are the stoichiometric air-to-fuel ratios respectively of gasoline and Hydrogen, such as AFst;g ¼ 14.6 and AFst;H2 ¼ 34.3. A calibrated Kistler 6051B high temperature piezoelectric pressure transducer (sensitivity 20.5 pC/bar) and Kistler 5010B charge amplifier were used to record the working chamber pressures trace for over 100 consecutive cycles. A trigger wheel and a Hall effect magnetic sensor were used to obtain the crank angle position data. The crank angle signal was synchronized with the pressure trace.
Tests conditions All experiments were performed at an engine speed of 3000 rpm and at WOT conditions. This engine speed was chosen because according to Ward et al. [31], the maximum average torque produced by this OMC Wankel engine was achieved at 3000 rpm. After a warm-up period on gasoline, the appropriate hydrogen energy fraction was set by replacing the amount of gasoline and the engine was gradually leaned out to the engine lean operation limit (LOL). In this study, LOL was found as the equivalence ratio at which the coefficient of variation (COV) in IMEP reaches 10%. This is because it is generally accepted that a COVIMEP above 10% will be perceived by a driver as a poor running condition [15,32,33]. The engine was allowed to reach the steady state at each set of tests then the data was collected. For this work, three hydrogen energy fractions in the intake 0%, 3% and 6% were used. Table 2 summarizes the operating conditions used along the experiments. The spark timing was kept at 15 CA BTDC of the original engine. Such a fixed ignition timing control strategy was also adopted by other researchers to explore the performance of hydrogen-enriched engines [19e22,32]. The fraction of hydrogen in the total intake gas is defined as energy fraction, calculated as follow [20,34]: " # m_ H2 LHVH2 100 % H2 ¼ m_ g LHVg þ m_ H2 LHVH2
(2)
m_ H2 , m_ g are respectively the mass flow rate of hydrogen and gasoline (g/s).
Table 2 e Engine operating conditions during the experiments. Hydrogen energy fraction Ignition timing (degree crank angle BTDC) Engine speed (rpm) Air-to-fuel relative equivalence ratio l Throttle position
0%, 3%, 6% 15 3000 1.15 to LOL Wide open
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LHVH2 , LHVg are the lower heating value respectively of hydrogen and gasoline (MJ/Kg), such as LHVH2 ¼ 120.1, LHVg ¼ 43:5.
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improve the engine stability and help to extend the LOL of the Wankel engine.
Combustion analysis
Results and discussion Effect of hydrogen addition on LOL and the engine stability The cycle-to-cycle variation that is quantified through the calculation of the coefficient of variance in the IMEP (COVIMEP) mirrors the engine stability. Fig. 2 depicts the variations of COVIMEP as a function of relative equivalence ratio l for pure gasoline and gasoline blended with 3% and 6% hydrogen energy fraction in the intake. As shown in Fig. 2, adding hydrogen to gasoline-air mixture lowers the cyclic variations. It can also be observed from Fig. 2 that without hydrogen addition, the COVIMEP began to increase sharply above 10% for load l > 1.29. However, when 3% and 6% of hydrogen were added to the fuel mixture, the COVIMEP began to increase above 10% for load of l > 1.42 and l > 1.51 respectively. Therefore, hydrogen addition results in better stability of the engine at leaner mixtures, where the combustion is more problematic. Moreover, lean limit was extended as hydrogen fraction increased in the fuel mixture. Indeed, the addition of hydrogen extended the LOL of the original engine from lLOL ¼ 1.29 (pure gasoline) to lLOL ¼ 1.42 and lLOL ¼ 1.51 matching an extension up to 10% and 17% of LOL for an enrichment of hydrogen about 3% and 6%, respectively. The higher flame speed of hydrogen helps to increase the rate of combustion of the airegasoline mixture. While the smaller quenching distance exhibited by hydrogen in comparison to gasoline results in a more complete combustion in the Wankel engine. Furthermore, the higher diffusivity of hydrogen compared to gasoline improves the mixing and increases the homogeneity in the load. All of these mechanisms help to enhance the combustion rate in the combustion chamber of the Wankel engine and especially in the trailing side. Therefore, the higher combustion rate, the more complete combustion and the better homogeneity of the load
Fig. 2 e COVIMEP versus excess air ratio l for gasoline enriched at 0%, 3% and 6% hydrogen energy fractions.
The study of the effect of hydrogen addition on the combustion duration is essential to the analysis of hydrogen's ability to extend lean burn limit. Based on pressure trace data calculation, the CA0-10, CA10-90 and CA50 burn durations are respectively the flame development, the flame propagation and the central heat release angle of the combustion of an airfuel mixture. The CA0-10, CA10-90 and CA50 burn durations versus relative equivalence ratio profiles are shown in Fig. 3a, b and c, respectively. Thus, for the combustion of pure gasoline and gasoline enriched 3% and 6% of Hydrogen energy fractions at 3000 rpm, WOT, fixed spark timing of 15 BTDC. It can be observed from Fig. 3a, b and c that for the three fuel mixtures as the engine was leaned out, the CA0-10, CA1090 and CA50 burn durations last longer due to the slowed combustion rates. This is mainly due to the reduced energy flow rate while the engine is leaning out. However, at a given l, the CA0-10, CA10-90 and CA50 burn durations shortened proportionally as hydrogen fraction increased in the fuel mixture, which benefits reduced cooling and exhaust losses and increased mass fraction burned [19,35]. This demonstrates the ability of hydrogen addition to speed up flame development and propagation. The reduced CA0-10 is the result of lower ignition energy of hydrogen that improves the flammability of the hydrogen-gasoline fuel mixture at lean mixture in early combustion process. Moreover, according to Wang et al. [35], the addition of hydrogen promotes the chemical reaction due to the increased H, O and OH mole fractions in the flame. Therefore, hydrogen-gasoline blends can be ignited more easily than pure gasoline. The reduced CA10-90 is the result of the high flame propagation and diffusion rate of hydrogen compared to gasoline that improve the homogeneity and speed the propagation of flame front of the gasolineehydrogeneair mixture. Moreover, compared to reciprocating engine, the Wankel rotary engines is characterized by high heat losses on the wall and large quenching area that affects the engine economy and HC emissions. All of which become noticeably worse as the engine is leaned out. Therefore, the addition of hydrogen through the reduction of the burn duration increases the turbulence inside the working chamber that improves the heat transfer between the burned and unburned zones and reduces the heat losses and the quenching effect. Moreover, due to hydrogen's burn characteristics, hydrogen enrichment is able to accelerate the squish flow generated by the Wankel engine especially in the trailing side. This helps to increase the burning speed during the combustion in this disadvantageous side of the combustion chamber. These data help to demonstrate that the hydrogen enrichment strategy is well suited in the Wankel engine. The central heat release angle CA50 represents the combustion efficiency of an airefuel mixture. The ideal engine has the CA50 at TDC because the engine combustion losses are reduced. Therefore, in practice, it is suggested that the optimum efficiency of engine combustion is achieved if 50% energy conversion occurs between 8 and 10 ATDC. These
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Fig. 3 e Combustion characteristics: a) CA0-10 flame development, b) CA10-90 flame propagation and c) CA50 central heat release angle.
values of CA50 achieve a compromise between power, wall heat transfer and exhaust gas energy [11,36]. The optimum crank angle location of 50% energy conversion should be calculated and controlled by modifying the point of ignition [37]. Therefore the addition of hydrogen can also be a way to control this optimum location. It can be found from Fig. 3c that as the engine is leaned out, the CA50 of the gasoline and hydrogen-gasoline mixtures is significantly retarded. Therefore, due to the narrow flammability and low burning speed of gasoline, CA50 of the gasoline engine is perceptibly advanced with the increase of hydrogen energy fraction. The retarded CA50 translates to an increased post-combustion duration, since more fuel is burnt during the expansion stroke. Therefore, the advanced CA50 shows that the combustion is enhanced by hydrogen enrichment. This is because the improved flammability and burning rate of gasolineehydrogen fuel mixture. Fig. 4 shows the CA0-10 and CA10-90 burn durations versus lLOL profiles for pure gasoline and gasoline enriched at 3% and 6% of hydrogen energy fraction at 3000 rpm, WOT, fixed spark timing of 15 BTDC. According to Fig. 4, the crank angle intervals for the CA0-10 and CA10-90 burn durations reached limiting values at the lean limit that were independent of engine operating conditions and fuel types used. This result was also observed by Quader [38], while investigating the LOL of the gasoline reciprocating engine. Therefore, it is interesting that despite the difference between those two engines, the conclusions agree so well.
shows the variations of BMEP with excess air ratio for pure gasoline and gasoline blended at 3% and 6% hydrogen energy fractions, at 3000 rpm and WOT conditions. As seen in Fig. 5, BMEP increased with increasing hydrogen addition levels. However, since the fuel energy flow rate is reduced when the engine is leaning out, increasing the dilution results in decreased BMEP for pure gasoline as well as hydrogen enriched gasoline. The narrow flammability limit of the gasoline makes the combustion for the lean mixture incomplete and as the excess air ratio increases to be closer to the LOL, the original gasoline engine suffers from slow burning. Therefore, the fast flame propagation rate with hydrogen addition accelerates the combustion of gasolineehydrogeneair mixtures while extending the flammability limit of the air fuel mixture. Thus, the addition of hydrogen can enhance the Wankel
Brake mean effective pressure The Brake Mean Effective Pressure (BMEP) is an important performance parameter used for comparing engines. Fig. 5
Fig. 4 e Burn duration at lean operation limit of gasoline enriched at 0%, 3% and 6% hydrogen energy fractions.
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Fig. 5 e Brake mean effective pressure versus excess air ratio l for gasoline enriched at 0%, 3% and 6% hydrogen energy fractions.
engine BMEP at lean conditions. These results were also proven by Wang et al. [39] while investigation the effect of hydrogen enrichment in reciprocating engine.
Brake thermal efficiency The engine thermal efficiency is crucial for assessing the economy of an engine. This parameter can be improved by optimizing the combustion system or fuel characteristics. The variations of Brake Thermal Efficiency (BTE) with excess air ratio for pure gasoline and gasoline blended 3% and 6% hydrogen energy fractions, at 3000 rpm and WOT are displayed in Fig. 6. According to Fig. 6, the engine BTE increased with increasing hydrogen blending fraction in the fuel mixture. However, as the fuel energy flow rate diminished with increasing excess air, the thermal efficiency decreased for all air-fuel mixtures. Moreover, the original Wankel engine thermal efficiency decreased sharply close to the original LOL of the engine. However, with hydrogen addition this behavior is eased and the thermal efficiency was significantly improved close to the LOL. The extension of the LOL of the original engine through the addition of 3% and 6% of hydrogen energy
Fig. 6 e Brake thermal efficiency versus excess air ratio l for gasoline enriched at 0%, 3% and 6% hydrogen energy fractions.
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fraction in gasoline increases the thermal efficiency when compared to pure gasoline at LOL. Accordingly, hydrogen enriched gasoline Wankel engine could produce almost constant thermal efficiencies over a wide range of excess air ratio for lean mixtures. This is the result of the reduced burn duration and improved homogeneity of the fuel mixture that helps to complete the combustion and reduce the cooling and exhaust loses of the rotary engine. When it comes to Wankel engine this result is crucial, because it helps to overcome the Wankel engine low thermodynamic efficiency resulting from its geometry. In addition, the smaller quenching distance exhibited by hydrogen in comparison to gasoline means that the flame could travel closer to the combustion chamber wall and further into crevices resulting in more completed combustion and improved BTE.
Brake specific energy consumption The Brake Specific Energy Consumption (BSEC) is the energy used by the engine to produce unit power. Therefore, it is a more reliable criterion to compare different fuels since it takes in consideration the calorific value and the mass flow rate of fuel. This parameter like the thermal efficiency reflects the engine economy. Fig. 7 shows the profiles of BSEC for different air ratios, for pure gasoline and gasoline blended with 3% and 6% of hydrogen energy fractions. According to Fig. 7, the BSEC is much higher for the engine operation on pure gasoline and this rises rapidly by 31% with the increase of excess air to the LOL. This is because of the incomplete combustion in lean mixture especially close to LOL where there is a poor utilization of fuel. However, the BSEC is reduced with increasing hydrogen levels in the air-fuel mixtures for the same excess air ratio. However, BSEC increased smoothly with increasing excess air for the same hydrogen energy fraction in the mixture. The improvement in BSEC of the Wankel engine after hydrogen addition is largely attributed to the higher LHV of hydrogen and to the capability of hydrogen to reduce the combustion duration event. This helps to improve the quality of the combustion and thus the thermal efficiency, reducing the thermal loses and thus decreasing the energy consumption under lean mixture conditions.
Fig. 7 e BSEC versus excess air ratio l for gasoline enriched at 0%, 3% and 6% hydrogen energy fractions.
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Fig. 8 e The effect of the extension of LOL by hydrogen addition on BTE and BSEC.
Fig. 8 summarizes the effect of the extension of LOL through hydrogen addition on BTE and BSEC. According to Fig. 8, it is important to notice that the extension of the original LOL of the Wankel engine through the addition of 3% and 6% of hydrogen energy fraction to gasoline enables a reduction in BSEC of about 15% and 22% respectively and an improvement in BTE of about 23% and 28% respectively.
Emissions In general, the Wankel rotary engine is well known for its excessive hydrocarbon and reduced NOx emissions compared to reciprocating engine [7]. However, in this paper, because of the hydrogen enrichment, the NOx, HC, CO and CO2 emissions were measured and analyzed and were found to improve with hydrogen addition. The NOx emissions are mainly related to the working chamber temperature and air to fuel ratio. As illustrated in Fig. 9, the brake specific NOx emissions decreased with
increasing excess air for gasoline and hydrogen enriched gasoline mixtures. Moreover, according to Fig. 9, leaning out the Wankel engine to the LOL of the pure gasoline and gasoline blended with 3% and 6% hydrogen energy fraction, drops the brake specific NOx emissions by 81%, 90% and 92% respectively with a minimum emitted by each fuel mixture at lLOL. This is because, when the air-fuel mixture is leaned out the combustion temperature is lowered, which reduces the thermal NOx formation. For a given excess air ratio, the brake specific NOx emissions rise with increasing hydrogen energy fraction in airgasoline mixtures up to the LOL of the original engine configuration. This is a consequence of hydrogen addition that reduces the combustion duration and increases the combustion chamber temperature. Also, according to the Fig. 9, the extension of LOL of the engine by the addition of 3% and 6% of hydrogen energy fraction further reduces the minimum NOx emissions of the original engine by 51% and 61% respectively. This result confirms the strategy of the reduction of NOx specific emissions by extending the LOL of the original engine with the addition of hydrogen. It should be noted that after LOL extension due to hydrogen addition, the engine-out emissions will meet the proposed 0.22 g/kWh BACT standard [14], without being treated by a three way catalytic converter. Because of the geometry of the Wankel engine, the HC emissions are the most predominant exhaust emissions. Fig. 10 depicts the variation of the brake specific HC emissions with excess air ratio for pure gasoline and gasoline blends of 3% and 6% hydrogen energy fractions at WOT condition. As shown in Fig. 10, when the engine is leaned out, the brake specific HC emissions first decreased, and then increased close to the LOL of each fuel mixture. However, the HC emissions increased more sharply for pure gasoline than for hydrogen enriched gasoline. The combustion quality is directly related to energy content in the intake and it is reflected by HC emissions. Close to the LOL, the increased HC emissions are the result of the degradation of the combustion
Fig. 9 e Brake specific NOx emissions versus excess air ratio l for gasoline enriched at 0%, 3% and 6% hydrogen energy fractions.
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Fig. 10 e Brake specific HC emission versus excess air ratio l for gasoline enriched at 0%, 3% and 6% hydrogen energy fractions.
that is caused by the deceleration of combustion rate and thus the lengthening of the combustion duration, which causes erratic and incomplete combustion at LOL. This is more obvious for the Wankel engine due to its high surface to volume ratio that is not helpful for the flame propagation. However, the addition of hydrogen increases the combustion rate and reduces the combustion duration, which improves the overall combustion quality especially close to LOL of the original engine and reduces the problematic Wankel engine quenching distance. Moreover, the carbon content in fuel mixture is reduced as hydrogen was added to air-gasoline mixture. Fig. 11 shows the variation of the CO emissions versus air ratio for pure gasoline and gasoline blended at 3% and 6% hydrogen energy fractions. As displayed in Fig. 11, for pure gasoline and gasoline blended with 3% and 6% hydrogen, CO emissions decrease first as excess air increased, then, when the engine approaches the LOL, the emissions increase sharply due to the incomplete combustion as the engine was further leaned out. Furthermore, for a given excess air ratio, CO emissions decreased as hydrogen fraction increases in the air-gasoline mixture. As the engine LOL was extended by hydrogen addition, the hydrogen-enriched gasoline Wankel
Fig. 11 e CO emissions versus excess air ratio l for gasoline enriched at 0%, 3% and 6% hydrogen energy fractions.
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engine could adopt larger excess air ratios to control CO emissions. Thereby, the hydrogen is effective for reducing CO emissions in Wankel engine. This result can be attributed to the improved combustion rate and thus more complete combustion of the air-gasoline mixture was achieved [40]. Beyond that, carbon concentration of the fuel blends is decreased due to hydrogen addition. The combustion of fossil fuels emits CO2 that is a greenhouse gas emission. The impact of hydrogen addition on CO2 emissions was studied through Fig. 12 that displays the profiles of CO2 emissions versus air ratio for pure gasoline and gasoline blended at 3% and 6% of hydrogen energy fraction. According to Fig. 12, the CO2 emissions decreased with increasing excess air for both gasoline and hydrogen enriched gasoline. For a given air ratio, as hydrogen energy fraction increases, CO2 emissions decrease consistently. It can be seen from Fig. 12 that increasing the excess air ratio to the LOL, reduces CO2 emissions for pure gasoline and gasoline blended with hydrogen. Furthermore, the extension of LOL of the original engine by the addition of 3% and 6% of hydrogen energy fraction further reduces the emissions of CO2 from the original engine by 8% and 24% respectively. This reduction is due to the decrease in fuel flow rate of the fuel-air mixture with increasing excess air beyond an increased hydrogen/ carbon ratio and improved engine efficiency.
Conclusion This experimental study investigated the effects of the extension of a gasoline Wankel engine LOL through hydrogen addition on the engine combustion and emissions performance. These experiments were performed at 3000 rpm, fixed spark timing and WOT conditions. The main conclusions resulting from this paper are summarized as follows: 1. The addition of hydrogen to gasoline demonstrates the capability to extend the LOL of the original engine. Moreover, results indicate that greater amounts of hydrogen addition can maintain lower COVIMEP for larger range of
Fig. 12 e CO2 emissions versus excess air ratio l for gasoline enriched at 0%, 3% and 6% hydrogen energy fractions.
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excess of air, which means that the engine became more stable for leaner mixtures. Hydrogen addition decreases both flame development and propagation durations and advances the central heat release angle proportionally to the amount of hydrogen. This reduction became more obvious at lean combustion condition. In this study, the BMEP for hydrogen enriched gasoline is higher than that of the original gasoline Wankel engine for all lean mixtures. The increase of dilution decreases the BMEP for pure gasoline as well as hydrogen enriched gasoline since the fuel energy flow rate was reduced. The addition of hydrogen to the gasoline fueled Wankel engine improves the thermal efficiency significantly. Moreover, the extension of the LOL of the original engine by the addition of 3% and 6% of hydrogen energy fraction to gasoline increases the BTE by 23% and 28% respectively. Also, the extension of the lean operation limit of the Wankel engine through the addition of 3% and 6% of hydrogen energy fraction to the gasoline enables a reduction in the specific energy consumption of about 15% and 22% respectively. Furthermore, the extension of LOL of the engine by the addition of 3% and 6% of hydrogen energy fraction reduces the brake specific NOx emissions of the original Wankel engine by 51% and 61% respectively. These levels of NOx meet the proposed 0.22 g/kWh BACT standard, without being treated by a three way catalytic converter. The carbon based emissions such as HC, CO, CO2 were reduced with hydrogen addition due to the improved combustion and increased H/C ratio of the hybrid fuel. Furthermore, as the engine LOL was extended by hydrogen addition, the Wankel engine could adopt larger excess air ratios to control HC and CO emissions.
Acknowledgments The authors thank Moller International for their donation of the Wankel research engine. The University of California, Davis, Green Transportation Laboratory and the Energy Research Laboratory and all their associated members made this work possible.
Nomenclature AFst;g AFst;H2 BMEP BSEC BTDC BTE CA CA0-10
Stoichiometric air-to-fuel ratio of gasoline (AFst,g ¼ 14.6) Stoichiometric air-to-fuel ratio of hydrogen (AFst;H2 ¼ 34.3) Brake mean effective pressure, bar Brake specific energy consumption, MJ/kW.h Before top dead center Brake thermal efficiency, % Crank angle Flame development, it is the crank angle duration from spark discharge to 10% heat release of the total fuel, CA
CA50 Central heat release angle, CA CA10-90 Flame propagation, it is the crank angle duration from 10% to 90% heat release of the total fuel, CA CO Carbon monoxide Carbon dioxide CO2 COVimep Coefficient of variance of indicated mean effective pressure, % HC Hydrocarbon HECU Hybrid electronic control unit Lower heating value of gasoline, MJ/Kg LHVg Lower heating value of hydrogen, MJ/Kg LHVH2 LOL Lean operation limit NOx Nitrogen oxide OMC Outboard Marine Corporation Rpm Rotation per minutes TDC Top dead center WOT Wide open throttle l Excess air ratio Hydrogen energy fraction, % % H2
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