Ultra-micro Gas Turbine Research Articles

Multidisciplinary Design Methodology for Micro-Gas-Turbines—Part II: System Analysis and Optimization

Abstract Owing to their high specific energy capabilities, ultramicrogas turbines (UMGT) are a high-potential technology to provide portable electric power supply for applications with demand of less than 1 kW. UMGT conceptual design is challenged by small-scale effects augmenting interdisciplinary dependencies leading to highly coupled, nonlinear component interactions. This work provides a novel approach to conceptual UMGT design by combining reduced order component and system modeling with constrained multi-objective optimization. Hereby, Part I presents integrated design and performance modeling of compressor, turbine, combustor, and generator. In Part II, the heat engine and generator modules are merged into a system framework by establishing conceptual UMGT rotor geometry and engine design. Following bearing selection and lifetime assessment, experimentally validated reduced order models are developed for heat transfer and rotordynamic analysis. Using the elaborated framework, a constrained multi-objective system optimization of a 300 W engine is performed based on ten design parameters and comparing SiAlON and Inconel 718 as potential rotor materials available for additive manufacturing. Hereby, bearing lifetime, system efficiency, and specific power are maximized while meeting rotordynamic, structural, and thermal requirements. Evaluating the results, interdisciplinary effects are highlighted, and two optimum engine configurations are suggested.

Cycle-Tempo Simulation of Ultra-Micro Gas Turbine Fueled by Producer Gas Resulting from Leaf Waste Gasification

Leaf waste has the potential to be converted into energy because of its high availability both in the world and Indonesia. Gasification is a conversion technology that can be used to convert leaves into producer gas. This gas can be used for various applications, one of which is using it as fuel for gas turbines, including ultra-micro gas ones, which are among the most popular micro generators of electric power at the time. To minimize the risk of failure in the experiment and cost, simulation is used. To simulate the performance of gas turbines, the thermodynamic analysis tool called Cycle-Tempo is used. In this study, Cycle-Tempo was used for the zero-dimensional thermodynamic simulation of an ultra-micro gas turbine operated using producer gas as fuel. Our research contributions are the simulation of an ultra-micro gas turbine at a lower power output of about 1 kWe and the use of producer gas from leaf waste gasification as fuel in a gas turbine. The aim of the simulation is to determine the influence of air-fuel ratio on compressor power, turbine power, generator power, thermal efficiency, turbine inlet temperature and turbine outlet temperature. The simulation was carried out on condition that the fuel flow rate of 0.005 kg/s is constant, the maximum air flow rate is 0.02705 kg/s, and the air-fuel ratio is in the range of 1.55 to 5.41. The leaf waste gasification was simulated before, by using an equilibrium constant to get the composition of producer gas. The producer gas that was used as fuel had the following molar fractions: about 22.62% of CO, 18.98% of H2, 3.28% of CH4, 10.67% of CO2 and 44.4% of N2. The simulation results show that an increase in air-fuel ratio resulted in turbine power increase from 1.23 kW to 1.94 kW. The generator power, thermal efficiency, turbine inlet temperature and turbine outlet temperature decreased respectively from 0.89 kWe to 0.77 kWe; 3.17% to 2.76%; 782 °C to 379 °C and 705°C to 304 °C. The maximums of the generator power and thermal efficiency of 0.89 kWe and 3.17%, respectively, were obtained at the 1.55 air-fuel ratio. The generator power and thermal efficiency are 0.8 kWe and 2.88%, respectively, with the 4.64 air-fuel ratio or 200% excess air. The result of the simulation matches that of the experiment described in the literature.

New Insights From Conceptual Design of an Additive Manufactured 300 W Microgas Turbine Toward Unmanned Aerial Vehicle Applications

AbstractOwing to the high energy density of hydrocarbon fuels, ultramicrogas turbines (UMGT) with power outputs below 1 kW have clear potential as battery replacement in drones. However, previous works on gas turbines of this scale revealed severe challenges due to air bearing failures, heat transfer from turbine to compressor, rotordynamic instability, and manufacturing limitations. To overcome these obstacles, a novel gas turbine architecture is proposed based on conventional roller bearing technology that operates at up to 500,000 RPM and an additively manufactured monolithic rotor in cantilevered configuration, equipped with internal cooling blades. The optimum turbomachinery design is elaborated using diabatic cycle calculation, coupled with turbomachinery meanline design code. This approach provides new insights on the interdependencies of heat transfer, component efficiency, and system electric efficiency. Thereby, a reduced design pressure ratio of 2.5 with 1200 K turbine inlet temperature (TIT) is identified as most suitable for 300 W electric power output. In following, a review of available additive manufacturing technologies yields material properties, surface roughness, and design constraints for the monolithic rotor. Rotordynamic simulations are then conducted for four available materials using a simplified rotor model to identify valid permanent magnet dimensions that would avoid operation close to bending modes. To complete the baseline engine architecture, a novel radial inflow combustor concept is proposed based on porous inert media combustion. computational fluid dynamics (CFD) simulations are conducted to quantify compressor efficiency and conjugate heat transfer (CHT) analysis of the monolithic rotor is performed to assess the benefit of the internal cooling cavity and vanes for different rotor materials. It is demonstrated that the cavity flow absorbs large amount of heat flux from turbine to compressor, thus cooling the rotor structure and improving the diabatic cycle efficiency. Finally, the results of this conceptual study show that ultramicrogas turbine with electric efficiency of up to 5% is feasible, while energy density is increased by factor of 3.6, compared to lithium-ion batteries.

Designing and analysis of the micro-trigeneration systems based on combined proton exchange membrane fuel cell with photovoltaic and photovoltaic/thermal prime movers in portable applications

Developing micro-trigeneration systems is an important step in military applications. Therefore, suggesting new portable trigeneration systems that use several energy sources is necessary. The importance of the issue is enhanced when renewable energy sources are used in suggested systems for places with no accessibility to energy sources. In this paper, different useful micro-personal trigenertion systems are proposed based on photovoltaic/thermal (PVT) and combination of photovoltaic (PV) with proton exchange membrane fuel cell (PEMFC). The proposed systems are designed to provide all or a part of personal demand loads by photovoltaic phenomena. Using different energy sources such as solar and hydrogen electrochemical energies in the proposed systems increases the flexibility and reliability of the system. The trigeneration performance of suggested cycles is simulated in the engineering equation solver software (EES) and the results based on different prime movers are compared with the system based on ultra-micro gas turbine (UMGT), which was studied in the previous work under extremely cold/hot weather. Although utilizing the PV module and PVT collector improves the performance of the system, it increases its weight as well, which is not suitable in portable applications. Therefore, the size of elements must be selected carefully. Results showed that the system based on PVT generated more power in summer so that the maximum of 105 and 17 W electrical power were generated with overall efficiency of 14% and 9% in summer and winter at maximum solar irradiance respectively. In addiotion, the most efficient system was the system based on UMGT in which the maximum overall efficiency was 21% and 28% in summer and winter respectively. The effects of the water mass flow rate of PVT on power and overall efficiency were investigated too. We found that increasing in mass flow rate increases the power and overall efficiency slightly about 16% and 15% in summer and about 33% and 25% in winter respectively. Additionally, results showed that in the combined PV-PEM system, the PV module decreased the aluminum hydride consumption significantly and generated a maximum of 13.5 W electrical power in summer. The weight of the suggested trigeneration systems is different based on different prime movers and varies from 5.66 kg to about 37.5 kg for single PEM and PVT systems respectively.

Thermodynamic feasibility study of a suggested portable personal micro trigeneration system based on micro-gas turbine and micro-absorption chiller

Combined cooling, heating and power (CCHP) which is also called trigeneration can be designed and manufactured in diverse scales and applications. In the present paper a single person portable micro CCHP system based on a micro-gas turbine and micro-absorption chiller is proposed and its performance is analyzed under different operational conditions. People under attack of war or natural disasters such as earthquake can use this system. A thermodynamic model is introduced to evaluate the personal energy demands at different ambient conditions. Then a micro-gas turbine is designed to produce 20 W electricity at speed of 200000 rpm and ambient temperature of 50 °C. In the next step the 500 W-class ultra-micro gas turbine (UMGT), is simulated as prime mover to estimate the cycle performance. LPG is the main fuel and the CCHP performance under extremely cold /hot weather is evaluated thermodynamically for three load management strategies. The maximum efficiency and minimum fuel consumption were related to the following hybrid electrical and thermal load strategy. In this case the maximum single-person CCHP efficiency was about 29% and fuel consumption was about 0.0645gr·s-1 of LPG at ambient temperature of 50 °C. The number of people who their energy demand was provided became maximum when following electrical load. The weight of the CCHP system is estimated to be less than 15 kg that is easily portable by a single grown person.

Numerical prediction of thermal performances in a concentric triple tube heat exchanger

In the last two decades, many research efforts have been carried out in order to scale down electronic and mechanical devices and components. Under this category also fall power generation devices such as Ultra-Micro Gas Turbines (UMGTs) which are one of the most suitable candidates as portable power supply. In many previous researches the feasibility and reliability of this type of systems have been proven, however many issues still need to be solved. Amongst these there are: excessive heat losses from the hot parts of the devices leading to low thermal efficiency, fluid dynamic losses at microscale need further investigation and the necessity of developing a material able to withstand combined thermal and mechanical stresses. A method to increase thermal efficiency and limiting fuel consumption is to recover heat from the combustion products to preheat the cold incoming mixture. Therefore, this study focuses on the interactions between combustion and heat exchanges at small scale. A numerical model of a three concentric tubes combustion chamber was developed and investigated with the aim of evaluating its thermal performances. A 17 species, 73 reactions skeletal mechanism able to describe methane oxidation was utilized for the purpose. A parametric study was carried out varying the mass flow rate and the thermal conductivity of the walls. Also, the effects of utilizing a flame holder in the combustion chamber were investigated. Results showed how the triple tube combustion chamber allows a good level of regeneration and the inclusion of the flame holder produces additional benefit in terms of heat transferred to the cold mixture.

Thermal Performances of a Small-Scale Regenerative Combustion Chamber for Ultra-Micro Gas Turbine

ABSTRACTNew manufacturing techniques and technologies have led to the development of a new research topic focusing on fluid dynamics and combustion at small scale, in particular in the last 20 years. One of the most promising technologies is represented by the ultra-micro gas turbines, which were developed with the aim of providing a portable and clean power source for devices, such as unmanned aerial vehicles and drones, global positioning system, exoskeletons for military applications, and backup emergency power supply. Currently, not many prototypes have been built because of the issues posed by scaling down the system. Among these there are excessive fluid dynamic and thermal losses decreasing the overall efficiency, elevated combined thermal and mechanical stress of components, and the necessity of developing high speed bearings. This study focuses on investigating experimentally a possible solution to effectively recover heat contained in the combustion products to preheat the unburned mixture, increasing the overall efficiency. Thus, an 18-mm internal-diameter regenerative combustion chamber surrounded by two sets of helicoidal channels was developed. The combustion chamber included a sintered steel porous medium to allow the flame to stabilize. The combustion products were recirculated in order to maximize the heat transferred to the cold mixture, with benefits in terms of flammability limits and fuel consumption. Mass flow rate and equivalence ratio were varied in the tests and the gas temperatures at different locations within the combustion chamber were measured, along with the composition of the combustion products. The heat released was included in the range 65 W and 343 W. Tests were run in both a non-insulated and insulated configuration and comparisons were made. Resulted showed the achievement of clean combustion of liquefied petroleum gas with combustion efficiency higher than 99% for lean mixtures. Also, a good level of heat recovery was achieved, reaching 45% and 23% of heat transferred to the reactants from the exhaust gases in the insulated and non-insulated combustion chamber, respectively.

Experimental Study on Flat Flame Combustion for Ultra Micro Gas Turbine Applications

ABSTRACTThe requirement for efficient power sources for portable electronics and miniature mechanical devices, such as laptops, micro robots, or micro aerial vehicles, has led to research on the ultra micro gas turbine (UMGT). The ultra micro gas turbine is one of the most promising power sources for small-scale applications due to its higher power and energy densities compared to currently used batteries. In order to realize UMGT as a viable power source, its individual components have to be developed, since downscaling introduces new problems for each component. Since the micro combustor is one of the key components of UMGT, it has to be improved. Until now, it has been very difficult for micro combustors to achieve wide flame stability, high combustion efficiency, and clean combustion with low pressure loss, due to the associated downscaling problems, such as high heat loss and small residence time. In order to achieve wide flame stability, the effect of preheating the reactants on flat flame stabilization was investigated experimentally on a 46-mm inner diameter quartz-walled flat flame combustor. The experimental results showed that preheating improves flat flame stabilization. Since preheating increases the burning velocity, flat flame can stabilize at higher incoming flow velocities as long as they are lower than the burning velocities. From the relationship between the incoming flow velocities and burning velocities, a correlation was obtained for flat flame micro combustors for micro power generation applications. With this correlation, it is possible to determine the minimum combustor diameter required for stable flat flame combustion. Also, in order to have a stable flat flame at higher mass flow rates, the correlation enables the calculation of the required reactants temperature, and is a major contribution to the design of micro combustors.

Development and experimental investigation of a 500-W class ultra-micro gas turbine power generator

We experimentally investigate the feasibility of a 500-W class ultra-micro gas turbine power generator. System specifications include a design rotational speed of 400,000 rpm, a compressor ratio of 3:1, and a turbine inlet temperature of 1200 K. In a previous study, we used an integrated test rig to examine the feasibility of start-up and self-sustaining capabilities. Here, we describe our development and testing of a new integrated test rig to measure the performance of electric-power generation during self-sustaining and boosting operations. We controlled a motor-generator using an electric controller in fixed speed mode and guided the rotational speed according to the start-up sequence. We employed a radial-thrust integral metal-mesh bumper air foil bearing with which we obtained the design rotational speed of 400,000 rpm in a standalone motoring test. In the combustor, we used liquefied petroleum gas as the main fuel and kerosene as the ignition fuel and partially as the main fuel during the boost operation. The integrated test rig achieved electric power generation of 30 W at 200,000 rpm, 50% of the design rotational speed.

Premixed combustion of methane–air mixture stabilized over porous medium: A 2D numerical study

The advancing development of small portable devices and miniaturization of electronic components such as smartphones, laptops, Global Positioning Systems (GPS), Unmanned Aerial Vehicles (UAVs) and sensors have led to an increasing demand for small scale power generators, such as the Ultra Micro Gas Turbine (UMGT). This study focuses on the two dimensional numerical investigation of premixed methane–air flames stabilized over a porous medium in a cylindrical combustion chamber which is a fundamental part of an UMGT. A reduced skeletal mechanism of methane combustion including 17 species and 73 reactions was employed for the simulations. The main objective of the present work is to investigate the effect of important parameters such as incoming mixture mass flow rate, equivalence ratio, reactants’ initial temperature, thermal conductivity and porosity of flame holder on the combustion process. Results show that combustion can be considerably affected by the flame holder, especially because under certain conditions it could lead to considerable heat losses and eventually flame quenching. Generally when the burning velocity is much higher than the incoming flow velocity, the flame lays on the surface of the porous medium and heat is lost through the flame holder causing a decrease in the flame temperature. On the other hand, when the flame is slightly lifted from the flame holder, heat is dispersed to the outer wall and through axial conduction the porous medium is heated up, allowing the mixture to increase its temperature before combustion. Finally the effect of the thermal conductivity and porosity of the porous medium are analysed, showing that the more insulating the flame holder is, the less heat losses occur.

Numerical investigation of heat transfer effects in small wave rotor

Although a wave rotor is expected to enhance the performance of the ultra-micro gas turbine, the device itself may be affected by downsizing. Apart from the immediate effect of viscosity on flow dynamics when downscaled, the effects of heat transfer on flow field increase at such small scales. To gain an insight into the effects of heat transfer on the internal flow dynamics, numerical investigations were carried out with adiabatic, isothermal and conjugate heat transfer boundary treatments at the wall, and the results compared and discussed in the present study. With the light shed by the discussion of adiabatic and conjugate heat transfer boundary treatments, this work presents investigations of the heat flux distributions, as well as the effects of heat transfer on the internal flow dynamics and the consequent charging and discharging processes for various sizes. When heat transfer is taken into account, states of fluid in the cell before compression process varies, shock waves in compression process are found to be weaker, and changes in the charging and discharging processes are observed. Heat transfer differences between conjugate heat transfer boundary treatment and isothermal boundary treatment are addressed through comparisons of local wall temperature and heat flux. As a result, the difference in discharging temperature of high pressure fluid is noticeable in all sizes investigated, and the rapid increase of differences between results of isothermal and conjugate heat transfer boundary treatment in small size reveals that for certain small sizes (length of cell < 23 mm) the thermal boundary treatment should be taken care of.

A hybrid propulsion system for a high-endurance UAV: configuration selection, aerodynamic study, and gas turbine bench tests

In recent years, renewed interest in the development of unmanned aerial vehicles (UAVs) has led to a wide range of interesting applications in reconnaissance and surveillance. In these missions, the noise produced by propeller-driven UAVs is a major drawback, which can be partially solved by installing an electric motor to drive the propeller. While the evolution of high performance brushless motors makes electric propulsion particularly appealing, at least for small and medium UAVs, all electric propulsion systems developed to date are penalized by the limited range and endurance that can be provided by a reasonably sized battery pack. In this paper we propose a hybrid propulsion system based on a recently developed ultramicro gas–turbine (UMGT), which can be used to power an electric generator, providing a significant range and (or) mission time extension. The UMGT is undergoing operational testing in our laboratory, to identify the most suitable configuration and to improve performance: a new compact regenerative combustion chamber was developed and several tests are being carried out to reduce its weight and size so as to increase, all other things being equal, the vehicle payload. This paper aims to propose a high endurance UAV, by a preliminary configuration selection and aerodynamic study of its performance.

Start-up and Self-sustain Test of 500 W Ultra-Micro Gas Turbine Generator

This paper provides the performance test for start-up and self-sustaining of 500W ultra-micro gas turbine (UMGT) generator. Each component of UMGT, a centrifugal compressor, a radial turbine, an annular combustor and a shaft is already designed, manufactured and tested to meet design requirements in previous researches. However, they are not tested to work in an integrate system. Currently, integrated test unit with a compressor, a combustor and a turbine, is developed to find the proper condition of start-up and self-sustain. Ignition sequence depending on rotating speed is designed. Performance test for start-up and self-sustain is designed based on the ignition possible condition. An air impingement starter and a hot bulb inginer are applied. LPG is used as main fuel.

Flame stability and emission characteristics of propane-fueled flat-flame miniature combustor for ultra-micro gas turbines

An engineering model of a propane-fueled miniature combustor was developed for ultra-micro gas turbines. The combustion chamber had a diameter of 20mm, height of 4mm, and volume of 1.26cm3. The flat-flame burning method was applied for lean-premixed propane–air combustion. To create the stagnation flow field for a specific flat-flame formation, a flat plate was set over the porous plate in the combustion chamber. A burning experiment was performed to evaluate the combustion characteristics. The flame stability limit was sufficiently wide to include the design operation conditions of an equivalence ratio of 0.55 and air mass flow rate of 0.15g/s, and the dominant factors affecting the limit were clarified as the heat loss and velocity balance between the burning velocity and the premixture flow velocity at the porous plate. CO, total hydrocarbons (THC), and NOx emission characteristics were established based on the burned gas temperatures in the combustion chamber and the temperature distribution in the combustor. At an air mass flow rate of less than 0.10g/s, CO and THC emissions were more than 1000ppm due to large heat loss. As the air mass flow rate increased, the heat loss decreased, but CO emissions remained large due to the short residence time in the combustion chamber. NOx emission depended mainly on the burned gas temperature in the combustion chamber as well as on the residence time. To reduce emissions despite the short residence time, a platinum mesh was placed after the combustion chamber, which drastically decreased the CO emissions. The combustor performance was compared with that of other miniature combustors, and the results verified that the present combustor has suitable combustion characteristics for a UMGT, although the overall combustor size and heat loss need to be reduced.

Testing of the Ultra-Micro Gas Turbine Devices (1 - 10 kW) for Portable Power Generation at University of Roma 1: First Tests Results

The ever increasing development of portable electronics has led to a higher demand for compact and reliable power sources. Significant resources are being presently dedicated to the study of micro machined gas turbines, because of their remarkable power density. The paper reports the procedures and the results of a series of tests conducted at the Department of Mechanical & Aerospace Engineering of University of Roma 1, to obtain the map of an ultra-micro gas turbine device, and the head losses and the combustion efficiency of the corresponding ultra-micro combustion chamber, fed by a mixture of butane and propane. This work is a part of a research aimed at the conception, design and prototyping of an ultra-micro thermo-electrical device for portable power generation. The novelty of the research consists in the fact that the thermal engine is a (ultra-micro) gas turbine set. In a subsequent stage, several different configurations have been assessed to select the most proper geometry and structural characteristics of the most relevant components (radial compressor, radial turbine, combustion chamber, electric motor and generator, bearings, regenerative heat exchanger).

500W급 초소형 가스터빈 개발을 위한 압축기 성능 평가

Performance evaluation of a compressor is conducted to develop 500W class ultra-micro gas turbine (UMGT) for power generation. The performance evaluation is essential to check the performance of the components of UMGT, a radial turbine, a centrifugal compressor, an angular combustor and a shaft, which have been already designed in previous researches. The purpose of this study is to introduce the development process of the performance testing equipments of the UMGT and to present the results of compressor performance test. For the performance evaluation of the compressor, two test equipments are developed and the initial test equipment uses commercial static air bearings with long shaft. In the improved test equipment, static air bearing is improved to increase rotating speed and compressed nitrogen gas is used for utility gas of the static air bearing to supply compressed air in a stable and steady way. To increase rotating speed to 320,000 rpm, 80% speed of design speed, compressed air is provided to the turbine. The performance map of the compressor with the 50%, 60%, 70%, 80% speed of design point is presented. The results of the performance test of compressor show a good agreement with the results of 3D CFD.

500W급 발전용 초소형 가스터빈 설계

Design of 500W class UMGT(Ultra Micro Gas Turbine) for power generation is conducted. Basic design parameters are obtained by cycle analysis. Off-design performances are predicted by 1D aerodynamic design and 1D performance analysis of compressor and turbine. 3D impellers are designed and 3D performance analysis is carried out to predict the performance characteristics of UMGT. 1D and 3D performance analysis show similar results. Structure analysis is conducted to select materials. Titanium Alloy is proposed for structural stability.

Fundamental Characteristics of Ultra-Micro Gas Turbine Combustor Using Liquid Fuel

Micro gas turbine systems have been developed for a portable power supply. In this study kerosene sprays with pre-vaporized system for the ultra-micro gas turbine combustor were tested. Fundamental characteristics of the kerosene spray with fuel vaporization on a hot wall were investigated experimentally. A model combustor of ultra-micro gas turbine was used for combustion test of kerosene spray. In the combustion test, an extension tubes were set at the exits of fuel injectors for the enhancement of pre-vaporization of the kerosene spray. The effect of the extension tube on flame stability was discussed. As a result, the blow off limit became wider when the extension tube was set at fuel nozzle exit.

5-3-2 油焚き超小型マイクロガスタービンの性能改善(5-3 微粒子と微粒化,Session 5 燃焼・微粒化,研究発表)

5-3-2 油焚き超小型マイクロガスタービンの性能改善(5-3 微粒子と微粒化,Session 5 燃焼・微粒化,研究発表)

Heat loss reduction and hydrocarbon combustion in ultra-micro combustors for ultra-micro gas turbines

For the development of ultra-micro combustors for Ultra-Micro Gas Turbines (UMGT), heat loss reduction and hydrocarbon fuel use are the key issues. An approach for reducing the effect of heat loss in ultra-micro combustors was proposed. The heat loss ratio ( HLR), which was defined as the ratio of heat loss rate from a combustor to heat release rate in the combustor, was related to the space heating rate ( SHR), and experiments using some flat-flame ultra-micro combustors with hydrogen/air premixture exhibited the relation of HLR ∼ SHR −0.92/ℓ (ℓ, characteristic length of combustor). From the viewpoint of heat loss reduction, burning at high SHR in compact ultra-micro combustors is essential for a practical UMGT combustor. As for hydrocarbon combustion, the flat-flame burning method with and without catalyst was applied to propane fuel. The flat-flame combustor, having an inner diameter of 18.5 mm, a height of 3.5 mm, and a volume of 0.806 cm 3, could form a propane flame successfully in the chamber without a catalyst and achieved an extremely high SHR of 3370 MW/(MPa m 3). Flame stable region was wide enough, and the combustion efficiency achieved was more than 99.4% between the equivalence ratios of 0.5 and 0.7 at m ˙ a = 0.06 g / s . The flat-flame combustor using a Pt-impregnated porous plate showed catalytic combustion, but did not improve the combustion characteristic. On the other hand, the flat-flame combustor using a nozzle whose surface was covered with Pt showed a combination of catalytic and gas-phase combustion with improved combustion efficiency for a wider range of equivalence ratios, due to CO oxidation in the burned gas after gas-phase combustion in the chamber.