Otto Cycle Research Articles

Performance of Universal Reciprocating Heat-Engine Cycle with Variable Specific Heats Ratio of Working Fluid.

Considering the finite time characteristic, heat transfer loss, friction loss and internal irreversibility loss, an air standard reciprocating heat-engine cycle model is founded by using finite time thermodynamics. The cycle model, which consists of two endothermic processes, two exothermic processes and two adiabatic processes, is well generalized. The performance parameters, including the power output and efficiency (PAE), are obtained. The PAE versus compression ratio relations are obtained by numerical computation. The impacts of variable specific heats ratio (SHR) of working fluid (WF) on universal cycle performances are analyzed and various special cycles are also discussed. The results include the PAE performance characteristics of various special cycles (including Miller, Dual, Atkinson, Brayton, Diesel and Otto cycles) when the SHR of WF is constant and variable (including the SHR varied with linear function (LF) and nonlinear function (NLF) of WF temperature). The maximum power outputs and the corresponding optimal compression ratios, as well as the maximum efficiencies and the corresponding optimal compression ratios for various special cycles with three SHR models are compared.

Comparative Analysis Between Different Configurations in a Biomass Gasifier and a Genset for Electrical Energy Production

Abstract The effect of different gensets coupled to a downdraft, fixed bed gasifier was investigated, pruning residue from five different tree species was used. This gasifier was designed to power a dual (produced gas/diesel) engine, with a maximum power electric generation of 20 kWe. This engine was first replaced by a four-cylinder engine, and later by a six-cylinder diesel engine, which underwent a conversion process for the Otto cycle; it was thus possible to operate the genset only with produced gas. This modification allowed an increase in the produced electric power from 20 kWe to 32 kWe. To operate this engine only with the produced gas, without harming its capacity to generate electric power, it required many modifications, as an increase in motor supply pipe diameter. Also, due to excessive accumulation of ash on the grate that separates the reactor core from the ash deposit, a mechanical system that clears this grate has been installed. This device allows gases produced in the gasification to have a continuous flow through the grate, being manually operated whenever required. In order to evaluate the viability of this new set gasifier/genset, some parameters such as consumption, power output, and system efficiency were studied, and a comparison was made with the data generated by works previously done by the same laboratory team and using the same gasifier.

Finite-time quantum Otto engine: Surpassing the quasistatic efficiency due to friction.

In finite-time quantum heat engines, some work is consumed to drive a working fluid accompanying coherence, which is called "friction." To understand the role of friction in quantum thermodynamics, we present a couple of finite-time quantum Otto cycles with two different baths: Agarwal versus Lindbladian. We solve them exactly and compare the performance of the Agarwal engine with that of the Lindbladian engine. In particular, we find remarkable and counterintuitive results that the performance of the Agarwal engine due to friction can be much higher than that in the quasistatic limit with the Otto efficiency, and the power of the Lindbladian engine can be nonzero in the short-time limit. Based on additional numerical calculations of these outcomes, we discuss possible origins of such differences between two engines and reveal them. Our results imply that, even with an equilibrium bath, a nonequilibrium working fluid brings on the higher performance than what an equilibrium working fluid does.

Holographic heat engines as quantum heat engines

Certain solutions of Einstein’s equations in anti-de Sitter spacetime can be engineered, using extended gravitational thermodynamics, to yield ‘holographic heat engines’, devices that turn heat into useful mechanical work. On the other hand, there are constructions (both experimental and theoretical) where a series of operations is performed on a small quantum system, defining what are known as ‘quantum heat engines’. We propose that certain holographic heat engines can be considered models of quantum heat engines, and the possible fruitfulness of this connection is discussed. Motivated by features of quantum heat engines that take a quantum system through analogues of certain classic thermodynamic cycles, some black hole Otto and Diesel cycles are presented and explored for the first time. In the expected regime of overlap, our Otto efficiency formulae are of the form exhibited by quantum and classical heat engines.

Quasistatic and quantum-adiabatic Otto engine for a two-dimensional material: The case of a graphene quantum dot

In this work, we study the performance of a quasistatic and quantum-adiabatic magnetic Otto cycles with a working substance composed of a single graphene quantum dot modeled by the continuum approach with the use of the zigzag boundary condition. Modulating an external or perpendicular magnetic field, in the quasistatic approach, we found a constant behavior in the total work extracted that is not present in the quantum-adiabatic formulation. We find that, in the quasistatic approach, the engine yielded a greater performance in terms of total work extracted and efficiency as compared with its quantum-adiabatic counterpart. In the quasistatic case, this is due to the working substance being in thermal equilibrium at each point of the cycle, maximizing the energy extracted in the adiabatic strokes.

Research on performance optimization and fuel-saving mechanism of an Atkinson cycle gasoline engine at low speed and part load

The Atkinson cycle engine plays a key role in the development of hybrid electronic vehicle (HEV) because of greater fuel economy than Otto cycle engine. The thermodynamic analysis shows Atkinson cycle has the significant advantage of cycle efficiency and the ratio of expansion ratio to effective compression ratio and pressure rise ratio have great influence on efficiency. A performance optimization strategy for the Atkinson cycle engine is proposed through analysis of compression ratio range according to cycle efficiency at small pressure rise ratio and optimization of the high compression ratio and the late intake valve closure (LIVC) under the compression pressure constraint. The experimental results show significant improvements in pumping losses and fuel economy are achieved, thus verifying verifies the effectiveness of the optimization strategy and brake specific fuel consumption (BSFC) of is improved by 9% at 2000 rpm@2 bar and 8% at 3000 rpm@3 bar, respectively. The fuel-saving mechanism is investigated in depth. The increase of mechanical efficiency due to the reduction in pumping mean effective pressure (PMEP) is the main reason for the fuel economy improvement at low and medium load, while the increase of indicated thermal efficiency is the main reason at high load. The increase in intake pressure is the main reason for the decrease in PMEP. The Atkinson cycle engine can achieve more obvious constant volume combustion and the combustion quality has been greatly improved. These findings can be used as guidelines for the development of new Atkinson cycle engines.

Unruh quantum Otto heat engine with level degeneracy

We investigate the Unruh quantum Otto heat engine with level degeneracy. An effectively two-level system, where the ground state is non-degenerate and the excited state is n-fold degenerate, is acting as the working substance, and the vacuum of massless free scalar field serves as a thermal bath via the Unruh effect. We calculate the heat and work at each step of the Unruh quantum Otto cycle and study the features of the heat engine. The efficiency of the heat engine depends only on the excited energy values of the two-level system, not on its level degeneracy. However, the degeneracy acts as a kind of thermodynamic resource and helps us to extract more work than in the non-degenerate case. The extractable work has a finite upper bound, corresponding to n→∞.

Алгоритм применения водородного топлива на колесных транспортных средствах

The use of hydrogen as fuel in wheeled vehicles with an internal combustion engine is considered, and an algorithm for its effective use is developed. This algorithm will minimize the time and cost of re-equipping the power system to obtain an environmental and economic effect on vehicles equipped with gasoline or gas internal combustion engines operating on the Otto cycle with various power systems.

Variable Valve Timing for a Camless Stepping Valve Engine

Variable valve timing (VVT) enables the valve events to vary with RPM for optimum engine performance. It alters the timing on the camshaft using pneumatic, hydraulic or electromechanical means. The Cameless Stepping Valve system uses software controlled valve events. Here, VVT is achieved by advancing/retarding valve events using only the direction of the piston and interpolated virtual angles on the flywheel. The engine model used has an optimum RPM of 24000, which translates to a period of 2.5mS. A 10 angle increment on the flywheel corresponds to 6.9µS. As a result, a retardation/advance of 10 would mean that the valve will open/close 6.9µS after the current/previous TDC. A 9bit real-time digital counter was triggered to count the calculated angles at every Otto Cycle so that advance/retard is updated at every RPM. In this project, it was demonstrated that valve event control is possible using interpolation of virtual angles on the flywheel, replacing a multi-tooth timing wheel. The proposed technique is more accurate and robust.

Energy efficiency of automotive gasoline engine: current approaches

Introduction. To meet the promising requirements fuel consumption and CO 2 emissions of passenger cars and commercial vehicles of 2025-2030 further improvement of the design and workflowof a gasoline internal combustion engine (ICE) is required in the full range of the working map, especially at high loads. The purpose of the study was to review and analyze ways to improve the indicator efficiency of a gasoline ICE and approaches aimed at increasing efficiency by reducing heat losses. Methodology and research methods . The review of barriers to increasing the indicator efficiency of a gasoline ICE was based on the analysis of the ideal and real Otto cycle and the results of experimental and calculated foreign and domestic studies of recent years aimed at increasing fuel efficiency by reducing heat losses. Scientific novelty and results . It was shown that effective new approaches to reduce the heat losses of the ICE of the future were: organization of combustion of a stoichiometric mixture diluted with a large amount of cooled recirculated exhaust gases (up to 25-35%) at high loads; increasing the ratio of the piston stroke to the cylinder diameter S/D to a value of the order of 1.5; the use of thin thermal barrier coatings that provide a “temperature swing” of the combustion chamber surface. Combined with proven modern technologies (direct fuel injection, variable valve drive, etc.), they can significantly increase the optimal geometric compression ratio, significantly reduce heat loss to the walls of the combustion chamber and the tendency of the ICE to detonate, and provide an increase in the indicator efficiency up to 49-53%. The practical significance lies in the possibility of using the results of the work when choosing a scheme and design solutions for a promising gasoline ICE with reduced fuel consumption and CO 2 emissions.

Análisis del comportamiento de las emisiones de CO2, CO y del factor lambda de un vehículo con sistema de inyección convencional con catalizador y sin catalizador

El análisis del comportamiento de emisiones de CO, CO2 sirve para determinar el comportamiento del ciclo de trabajo del motor, además de la verificación de la gráfica del factor lambda, para lo cual se realiza el estudio de la importancia de un catalizador porque en algunas ocasiones los propietarios de los vehículos deciden eliminar el convertidor catalítico de la línea de salida de los gases combustionados del motor y así circulan por las vías del Ecuador desconociendo la afectación hacia la salud de los ciudadanos y la contaminación directa hacia el medioambiente. Con el análisis del funcionamiento y características de las emisiones contaminantes de un motor de combustión interna ciclo Otto se generan modelos de control para la proyección de la cantidad de gases contaminantes que se emiten al eliminar el convertidor catalítico y de tal forma establecer los niveles de emisiones que un vehículo sin catalizador genera, a pesar de que el motor se encuentre en óptimas condiciones de funcionamiento en diferentes regímenes de giro.

Achieve higher efficiency at maximum power with finite-time quantum Otto cycle.

The optimization of heat engines was intensively explored to achieve higher efficiency while maintaining the output power. However, most investigations were limited to a few finite-time cycles, e.g., the Carnot-like cycle, due to the complexity of the finite-time thermodynamics. In this paper, we propose a class of finite-time engine with quantum Otto cycle, and demonstrate a higher achievable efficiency at maximum power. The current model can be widely utilized, benefitting from the general C/τ^{2} scaling of extra work for a finite-time adiabatic process with long control time τ. We apply the adiabatic perturbation method to the quantum piston model and calculate the efficiency at maximum power, which is validated with an exact solution.

Performance study of the hypocycloid gear mechanism for internal combustion engine applications

Fuel conversion efficiency is one of the main concerns in the field of internal combustion engine systems. Although the Otto cycle delivers the maximum efficiency possible in theory, the kinematics of the slider–crank mechanism of the conventional internal combustion engines makes it difficult to reach this level of efficiency in practice. This study proposes using the unique hypocycloid gear mechanism instead of the conventional slider–crank mechanism for the internal combustion engines to increase engine efficiency and minimize frictional power losses. The hypocycloid gear mechanism engine’s kinematics provides the means for the piston-rod assembly to reciprocate in a straight-line motion along the cylinder axis besides achieving a nonlinear rate of piston movement. As a result, this characteristic allows for a true constant-volume combustion, which in turn would lead to higher work output. An in-cylinder gas volume change model of the hypocycloid gear mechanism engine was developed and incorporated into the thermodynamic model for the internal combustion engine cycle. The thermodynamic model of the hypocycloid gear mechanism engine was developed and simulated using MATLAB/Simulink software. A comparison between the conventional engine and the hypocycloid gear mechanism engine in terms of engine performance characteristics showed the enhancements achieved using hypocycloid gear mechanism for internal combustion engine applications. The hypocycloid gear mechanism engine analysis results indicated higher engine efficiency approaching that of the Otto cycle.

Experimental Characterization of a Spin Quantum Heat Engine

Developments in the thermodynamics of small quantum systems envisage nonclassical thermal machines. In this scenario, energy fluctuations play a relevant role in the description of irreversibility. We experimentally implement a quantum heat engine based on a spin-1/2 system and nuclear magnetic resonance techniques. Irreversibility at a microscope scale is fully characterized by the assessment of energy fluctuations associated with the work and heat flows. We also investigate the efficiency lag related to the entropy production at finite time. The implemented heat engine operates in a regime where both thermal and quantum fluctuations (associated with transitions among the instantaneous energy eigenstates) are relevant to its description. Performing a quantum Otto cycle at maximum power, the proof-of-concept quantum heat engine is able to reach an efficiency for work extraction (η≈42%) very close to its thermodynamic limit (η=44%).

Effects of molar expansion ratio of fuels on engine efficiency

Fuel properties have a strong impact on the efficiency of internal combustion engines. Contrary to other physical and thermochemical fuel properties, the molar expansion ratio is normally ignored. Molar expansion ratio is the ratio of number of moles of the products to the reactants. In this work, the impact of the fuel’s molar expansion ratio on engine efficiency is investigated. Findings are based on simulations of a spark ignition engine using different fuels (standard fuels and user-defined fuels) and different dilution ratios. Simulations without heat transfer and friction were performed first. The combustion then takes place at top dead center with a very short combustion duration to approach the ideal Otto cycle. The heat transfer and friction were then added step by step. From this analysis, it could be concluded that the heat loss and friction work decrease as molar expansion ratio increases. The gross indicated and brake thermal efficiencies thus increase. User-defined fuels with different molar expansion ratio, but the same physical and thermochemical properties were then employed. The simulated results showed that the brake thermal efficiency increases by around 1.15% with an increase in molar expansion ratio of 0.02 compared to a fuel with a molar expansion ratio of unity. The simulation was also done with air and exhaust gas recirculation dilution.

Approaching Carnot efficiency at maximum power in linear response regime

We construct an example of a heat engine whose efficiency at maximum power breaks down the previously derived bounds in the linear response regime. Such an example takes a classical harmonic oscillator as the working substance undergoing a finite-time Otto cycle. Using a specific kind of shortcut to adiabaticity, valid only in the linear response regime, quasistatic work is performed at arbitrarily short times. The cycle duration is then reduced to the sum of relaxation times during the thermalization strokes exclusively. Thus, the power is at maximum since the work is at maximum (quasistatic work) and the cycle duration is minimal. Efficiency at maximum power can be made arbitrarily close to Carnot efficiency with an appropriate choice of the ratio between the temperatures of the two heat baths.

Comparative performance evaluations of various optimization functions for irreversible Otto cycles

A comparative performance analysis for irreversible Otto cycles has been carried out with consideration of overall and effective power loss for various optimization functions available in the literature. Basically, maximum power, maximum power density, ecological coefficient of performance and the ecological function are taken into consideration, and their optimal operating conditions were compared with respect to each other in terms of various engine parameters. The irreversible cycle model operating between two isothermal high and low temperature heat reservoirs is considered and combined with various irreversibilities for power loss analysis. In the irreversible model, non-isentropic compression and expansion, heat leak through the cylinder head and power loss due to friction are taken into account. Effects of engine volumetric efficiency, engine speed, friction and extreme cycle temperature on the overall performance for the respective optimal operating conditions have been investigated. It is seen that the overall power loss increases for increasing engine speed, decreasing volumetric efficiency, increasing friction coefficient and decreasing high temperature of the cycle. It is also observed that the maximum power conditions have higher overall power loss but lower internal irreversibility losses among the considered optimization functions.

Collective performance of a finite-time quantum Otto cycle.

We study the finite-time effects in a quantum Otto cycle where a collective spin system is used as the working fluid. Starting from a simple one-qubit system we analyze the transition to the limit cycle in the case of a finite-time thermalization. If the system consists of a large sample of independent qubits interacting coherently with the heat bath, then the super-radiant equilibration is observed. We show that this phenomenon can boost the power of the engine. Mutual interaction of qubits in the working fluid is modeled by the Lipkin-Meshkov-Glick Hamiltonian. We demonstrate that in this case the quantum phase transitions for the ground and excited states may have a strong negative effect on the performance of the machine. Conversely, by analyzing the work output we can distinguish between the operational regimes with and without a phase transition.

Thermodynamics of a Phase-Driven Proximity Josephson Junction

We study the thermodynamic properties of a superconductor/normal metal/superconductor Josephson junction in the short limit. Owing to the proximity effect, such a junction constitutes a thermodynamic system where phase difference, supercurrent, temperature and entropy are thermodynamical variables connected by equations of state. These allow conceiving quasi-static processes that we characterize in terms of heat and work exchanged. Finally, we combine such processes to construct a Josephson-based Otto and Stirling cycles. We study the related performance in both engine and refrigerator operating mode.

Quantum heat engine with a quadratically coupled optomechanical system

We propose a quantum heat engine based on a quadratically coupled optomechanical system. The optical component of the system is driven periodically with an incoherent thermal drive, which induces periodic oscillations in the mechanical component. Under the action of the quadratic optomechanical interaction, the mechanical mode evolves from an initial thermal state to a thermal-squeezed steady state, as verified by calculating the Wigner functions. The dynamics of the system is identified as an effective four-stroke Otto cycle. We investigated the performance of the engine by evaluating the dissipated power, the maximum power under a load, and the maximum extractable work. It is found that the engine operating with quadratic optomechanics is more powerful than the one operating with linear optomechanics. The effect is explained by the presence of squeezing in the quantum state of the mechanical mode.