An Idea to improve the Thermal Efficiency of the Internal Combustion Engine

The exhaust gas from an internal combustion engine contains approximately 30% of the thermal energy of combustion. Waste heat-recovery systems aim to reclaim a proportion of this energy in a bottoming thermodynamic cycle to raise the overall system thermal efficiency. The inverted Brayton cycle (IBC) considered as a potential exhaust-gas heat-recovery system is a little-studied approach, especially when applied to small automotive power plants. Hence, a model of an air-standard, irreversible Otto cycle and the IBC using finite-time thermodynamics (FTT) is presented to study heat recovery applied to an automotive internal combustion engine. The other two alternatives power cycles, the pressurized Brayton cycle and the turbocompounding system (TS), are compared with the IBC to specify the strengths and weaknesses of three alternative cycles. In the current paper, an irreversible Otto-cycle model with an array of losses is used as a base for the bottoming cycle. The deviation of the turbomachinery from the idealized behavior is described by the isentropic component efficiencies. The performance of the system as defined as the specific power output and thermal efficiency is considered using parametric studies. The results show that the performance of the IBC can be positively affected by five critical parameters—the number of compression stages, the cycle inlet temperature and pressure, the isentropic efficiency of the turbomachinery, and the effectiveness of the heat exchanger. There exists an optimum pressure ratio across the IBC turbine that delivers the maximum specific power. In view of the specific power, installing a single-stage of the IBC appears to be the best balance between performance and complexity. Three alternative cycles are compared in terms of the thermal efficiency. The results indicate that the pressurized and IBCs can improve the performance of the turbocharged engine (TCE) only when the turbomachinery efficiencies are higher than a value which changes with the operating condition. High performance of the IBC turbomachinery is required to ensure that the TCE with the IBC is superior to that with TS.

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The Benefits of an Inverted Brayton Bottoming Cycle as an Alternative to Turbo-Compounding

Volume 8: Microturbines, Turbochargers and Small Turbomachines; Steam Turbines ◽

10.1115/gt2015-42623 ◽

2015 ◽

Cited By ~ 5

Author(s):

Colin D. Copeland ◽

Zhihang Chen

Keyword(s):

Internal Combustion Engine ◽

Thermal Efficiency ◽

Heat Recovery ◽

Combustion Engine ◽

Internal Combustion ◽

Critical Parameters ◽

Specific Power ◽

Brayton Cycle ◽

Exhaust Gas ◽

Otto Cycle

The exhaust gas from an internal combustion engine contains approximately 30% of the thermal energy of combustion. Waste heat recovery systems aim to reclaim a proportion of this energy in a bottoming thermodynamic cycle to raise the overall system thermal efficiency. The inverted Brayton cycle considered as a potential exhaust-gas heat-recovery system is a little-studied approach, especially when applied to small automotive power-plants. Hence, a model of an air-standard, irreversible Otto-cycle and the inverted Brayton cycle using finite-time thermodynamics (FTT) is presented to study heat recovery applied to an automotive internal combustion engine. The other two alternatives power cycles, the pressurized Brayton cycle and the turbo-compounding system, are compared with the inverted Brayton cycle (IBC) to specify the strengths and weaknesses of three alternative cycles. In the current paper, an irreversible Otto-cycle model with an array of losses is used as a base for the bottoming cycle. The deviation of the turbomachinery from the idealized behavior is described by the isentropic component efficiencies. The performance of the system as defined as the specific power output and thermal efficiency is considered using parametric studies. The results show that the performance of the inverted Brayton cycle can be positively affected by five critical parameters — the number of compression stages, the cycle inlet temperature and pressure, the isentropic efficiency of the turbomachinery and the effectiveness of the heat exchanger. There exists an optimum pressure ratio across the IBC turbine that delivers the maximum specific power. In the view of the specific power, installing a single-stage of the inverted Brayton cycle appears to be the best balance between performance and complexity. Three alternative cycles are compared in terms of the thermal efficiency. The results indicate that the pressurized and inverted Brayton cycles can improve the performance of the turbocharged engine only when the turbomachinery efficiencies are higher than a value which changes with the operating condition. High performance of the IBC turbomachinery is required to ensure that the turbocharged engine with the inverted Brayton cycle is superior to that with turbo-compounding system.

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Exhaust gas heat recovery through secondary expansion cylinder and water injection in an internal combustion engine

Thermal Science ◽

10.2298/tsci150915282n ◽

2017 ◽

Vol 21 (1 Part B) ◽

pp. 729-743

Author(s):

Toosi Nassiri ◽

Amir Kakaee ◽

Hazhir Ebne-Abbasi

Keyword(s):

Internal Combustion Engine ◽

Thermal Efficiency ◽

Combustion Engine ◽

Water Injection ◽

Internal Combustion ◽

Exhaust Gas ◽

Exhaust Gases ◽

Injection Process ◽

Direct Water Injection ◽

Novel Concept

To enhance thermal efficiency and increase performance of an internal combustion engine, a novel concept of coupling a conventional engine with a secondary 4-stroke cylinder and direct water injection process is proposed. The burned gases after working in a traditional 4-stroke combustion cylinder are transferred to a secondary cylinder and expanded even more. After re-compression of the exhaust gases, pre-heated water is injected at top dead center. The evaporation of injected water not only recovers heat from exhaust gases, but also increases the mass of working gas inside the cylinder, therefore improves the overall thermal efficiency. A 0-D/1-D model is used to numerically simulate the idea. The simulations outputs showed that the bottoming cycle will be more efficient at higher engines speeds, specifically in a supercharged/turbocharged engine, which have higher exhaust gas pressure that can reproduce more positive work. In the modeled supercharged engine, results showed that brake thermal efficiency can be improved by about 17%, and brake power by about 17.4%.

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