Karin Munch

Comparison of Working Fluids in Both Subcritical and Supercritical Rankine Cycles for Waste-Heat Recovery Systems in Heavy-Duty Vehicles

In a modern internal combustion engine, most of the fuel energy is dissipated as heat, mainly in the form of hot exhaust gas. A high temperature is required to allow conversion of the engine-out emissions in the catalytic system, but the temperature is usually still high downstream of the exhaust gas aftertreatment system. One way to recover some of this residual heat is to implement a Rankine cycle, which is connected to the exhaust system via a heat exchanger. The relatively low weight increase due to the additional components does not cause a significant fuel penalty, particularly for heavy-duty vehicles. The efficiency of a waste-heat recovery system such as a Rankine cycle depends on the efficiencies of the individual components and the choice of a suitable working fluid for the given boundary conditions. Commonly used pure working fluids have the drawback of an isothermal evaporation and condensation, which increases irreversibility, and consequently decreases the efficiency during the heat transfer. Previous work has suggested that one way to overcome this problem is to use zeotropic mixed working fluids. These have already been applied in several stationary systems and refrigerant cycles but not yet in waste-heat recovery systems for portable applications. This theoretical study compares different pure working fluids and zeotropic mixtures in both subcritical and supercritical Rankine cycles. The main objective was to analyze the respective energy and exergy efficiencies by modeling the Rankine cycles. The results suggested that the final fluid and cycle choice is limited by the exhaust-gas temperature range of a heavy-duty diesel engine and realistic condensation conditions for the fluid. Further, environmental and safety concerns over working fluids in portable applications are important challenges, which need to be taken into account in selecting an appropriate fluid. Copyright

An Experimental Study on the Use of Butanol or Octanol Blends in a Heavy Duty Diesel Engine

Global warming driven by “greenhouse gas” emissions is an increasingly serious concern of both the public and legislators. A potentially potent way to reduce these emissions and conserve fossil fuel resources is to use n-butanol, iso-butanol or octanol (2-ethylhexanol) from renewable sources as alternative fuels in diesel engines. The effects of adding these substances to diesel fuel were therefore tested in a single-cylinder heavy duty diesel engine operated using factory settings. These alcohols have better calorific values, flash points, lubricity, cetane numbers and solubility in diesel than shorter-chain alcohols. However, they have lower cetane numbers than diesel, so either hydrotreated vegetable oil (HVO) or Di-tertiary-butyl peroxide (DTBP) was added to the diesel-alcohol mixtures to generate blends with the same Cetane Number (CN) as diesel. Blends containing 10 and 20% of n-butanol or iso-butanol, or 30 % octanol were tested at four operating points from the European Stationary Cycle. The same engine settings were used in all cases. The average engine performance in tests with the blends was similar to that achieved with pure diesel but the blends generated less cycle-to-cycle variation. The brake thermal efficiency was similar for all the fuels but the brake specific fuel consumption was slightly higher for the blends due to their lower calorific value. Because of their oxygen content, the blends produced much lower emissions of soot and carbon monoxide than were achieved with diesel. Blends yielded slightly higher NOx emissions than pure diesel and all the fuels produced similar hydrocarbon emissions. Possibly because of its branched molecular structure, the iso-butanol blends yielded slightly higher soot emissions than the n-butanol blends. Because HVO contains no aromatics, its addition to fuel blends reduced soot emissions. Overall, these results confirm the substantial potential of renewable longer-chain alcohols as components of blended diesel fuels.

Selecting an Expansion Machine for Vehicle Waste-Heat Recovery Systems Based on the Rankine Cycle

An important objective in combustion engine research is to develop strategies for recovering waste heat and thereby increasing the efficiency of the propulsion system. Waste-heat recovery systems based on the Rankine cycle are the most efficient tools for recovering energy from the exhaust gas and the Exhaust Gas Recirculation (EGR) system. The properties of the working fluid and the expansion machine have significant effects on Rankine cycle efficiency. The expansion machine is particularly important because it is the interface at which recovered heat energy is ultimately converted into power. Parameters such as the pressure, temperature and mass-flow conditions in the cycle can be derived for a given waste-heat source and expressed as dimensionless numbers that can be used to determine whether displacement expanders or turbo expanders would be preferable under the circumstances considered. The goal of this theoretical study was to use this approach to analyze waste-heat recovery systems for a heavy-duty diesel engine and a light-duty gasoline engine. Given the different waste-heat rates of these two engines, the relationships between Rankine cycle performance and design aspects such as the expansion ratio and the locations of pinch points in the heat exchanger were evaluated. The calculated values of these parameters were used as inputs in a dimensionless analysis to identify an optimal expansion machine for each case. The impact of varying the working fluid used was investigated, since it had a large impact on the results obtained and provided insights into design dependencies in these systems.

Thermodynamic Cycle and Working Fluid Selection for Waste Heat Recovery in a Heavy Duty Diesel Engine

Thermodynamic power cycles have been shown to provide an excellent method for waste heat recovery (WHR) in internal combustion engines. By capturing and reusing heat that would otherwise be lost to the environment, the efficiency of engines can be increased. This study evaluates the maximum power output of different cycles used for WHR in a heavy duty Diesel engine with a focus on working fluid selection. Typically, only high temperature heat sources are evaluated for WHR in engines, whereas this study also considers the potential of WHR from the coolant. To recover the heat, four types of power cycles were evaluated: the organic Rankine cycle (ORC), transcritical Rankine cycle, trilateral flash cycle, and organic flash cycle. This paper allows for a direct comparison of these cycles by simulating all cycles using the same boundary conditions and working fluids. To identify the best performing cycle, a large number of working fluids were evaluated with regards to the maximum power output of the power cycle for each heat source. Taking into account the constraints and boundary conditions, this study shows that the ORC gives the best performance with a power output of around 1.5 kW for the coolant, 2.5 kW for the exhaust gas recirculation cooler, and 5 kW for the exhaust with acetone, cyclopentane and methanol as the best performing working fluids.

Combustion Characteristics for Partially Premixed and Conventional Combustion of Butanol and Octanol Isomers in a Light Duty Diesel Engine

Reducing emissions and improving efficiency are major goals of modern internal combustion engine research. The use of biomass-derived fuels in Diesel engines is an effective way of reducing well-to-wheels (WTW) greenhouse gas (GHG) emissions. Moreover, partially premixed combustion (PPC) makes it possible to achieve very efficient combustion with low emissions of soot and NOx. The objective of this study was to investigate the effect of using alcohol/Diesel blends or neat alcohols on emissions and thermal efficiency during PPC. Four alcohols were evaluated: n-butanol, isobutanol, n-octanol, and 2-ethylhexanol. The alcohols were blended with fossil Diesel fuel to produce mixtures with low cetane numbers (26-36) suitable for PPC. The blends were then tested in a single cylinder light duty (LD) engine. To optimize combustion, the exhaust gas recirculation (EGR) level, lambda, and injection strategy were tuned. The measured emissions and thermal efficiencies for PPC with the blends were compared to those for conventional combustion with production engine settings. The study showed a viable way to achieve PPC by low CN alcohol/Diesel blends in a single cylinder LD engine. Because of its lower combustion temperature and increased fuel-air mixing, PPC produced very low soot and NO emissions, independently of the fuels used. High HC and CO emissions were observed when the ignition dwell cross zero from negative to positive value. Properties of the individual component would influence the combustion behavior even compare to the fuel with similar CN. Compared to conventional diffusion-controlled combustion, PPC generated a high indicated thermal efficiency up to 50% in all tested conditions for both low CN level blends.