Energy distributions in a diesel engine using low heat rejection (LHR) concepts
Introduction
Energy efficiency is one of the most significant assessment factors concerned with the internal combustion engine. From the perspective of thermodynamics, the IC engine may theoretically approach 100% efficiency since the IC engine converts chemical energy to mechanical energy; chemical energy is fully available to do useful work. The IC engine will still be limited, however, by second law considerations such as irreversible processes [1], [2]. Actual IC engines generally convert only approximately one-third of the fuel energy to useful work; the rest is rejected typically in the form of thermal energy to the coolant and exhaust.
The LHR concept has been of interest since the 1980s, when a substantial number of programs investigated the “adiabatic engine” [3], [4], [5]. These programs aimed to improve engine efficiency with partial/complete suppression of heat loss through the combustion chamber walls. Thermal barrier coatings have been extensively used in LHR engine designs, which increase the thermal resistance of the combustion chamber walls and consequently increase the level of temperatures inside the cylinder. From the first law of thermodynamics, it can be expected that any retained energy by reducing heat losses through the chamber walls can be converted to useful work and consequently improve the fuel conversion efficiency. The energy balance study on a partially stabilized zirconia (PSZ) coated diesel engine by Modi et al. [6] reported that the heat loss to the cooling system was decreased by 2%, 7%, and 7% for low, medium, and high loads respectively compared to the baseline engine. Prarath et al. [7] claimed 30–40% reduction in coolant load was achieved in coated engine, and a marginal improvement on the brake thermal efficiency was observed. Srithar et al. [8] investigated the effects of thermal barrier coated combustion chamber on a single cylinder diesel engine with dual biodiesel, the specific fuel consumption showed to be lower than that of the baseline engine by 13.9% and the reduction in coolant losses was found to be around 5–25%. Panneerselvam et al. [9] reported a review on the application of LHR concept in bio-diesel engines, stating that the energy of bio-diesel can be released more efficiently under LHR operation. Investigations by Kamo et al. [10] and Modi and Gosai [11] also indicated promising improvements in fuel efficiency at high load conditions using multi-cylinder diesel engines. Jafarmadar et al. [12] used a 3-D computational fluid dynamics code to further study the combustion processes within the chamber of a LHR diesel engine from the perspective of the second law of thermodynamics, which indicated that the exergy efficiency can be improved by LHR concept at all the studied load conditions.
In addition, Reddy et al. [13] summarized that all of the simulation work proved the fuel economy superiority of LHR engines over conventionally cooled engines. However, these facts do not necessarily substantiate a conclusion that LHR engines will outperform the conventional engine. In fact, some of the previous experimental work where the results are mixed show that a number of engine operating parameters are inter-related which can negatively affect the efficiency of a LHR engine. For instance, the higher operating temperature conditions decrease the volumetric efficiency, which in turn adversely influences the energy conversion efficiency [14]. Also, Caton [15], [16] and Tunér [17] show that the resulting high gas temperatures in LHR engines yield lower values of the specific heats ratio causing less effective conversion of energy to work energy. Those conflicting results are mostly concluded from studies on ceramic-coated engines; so far, relatively little attention has been devoted to investigating the possibility of LHR engines with the approach of altering coolant temperature, which does not require significant modifications on engine. Reductions in heat losses to the coolant jacket can be supposedly implemented by raising the engine coolant temperature (ECT), due to the smaller temperature difference between the coolant and the wall. Only a few works, however, have reported the benefits of operating engines at higher ECT conditions from the perspective of heat transfer [18], [19]. The main finding was that a higher ECT reduced the heat rejection to the coolant or the net in-cylinder heat transfer rate, the associated gains also include enhancing fuel evaporation and therefore mitigating hydrocarbon (HC) and CO emissions [20], [21]. It seems therefore logical to expand on the investigations of LHR concept through varying engine coolant temperature.
Typically, the operation conditions with ECT beyond 110 °C are unlikely to be realized due to the needs of avoiding excessive oil temperature [22]. The maximum ECT is also restricted by the occurrence of nucleate boiling causing strong convection when the wall surface temperature goes beyond the coolant saturation temperature. In this case, the simulation method allows for extended studies at ECTs beyond the limited values on an actual engine. The current work is aimed at providing a detailed consideration of the energy balance for a LHR engine using elevated coolant temperatures. As described above, this is an area that has only received limited attention in the past.
This paper describes a study that lays the foundation for simulation-based analysis of LHR engines. The work complements a prior experimental study [23] of conventional and low-temperature combustion with LHR and offers analytical tools to explore more deeply the associated thermodynamics. The current study relies mostly on a one-dimensional engine simulation using a pseudo-predictive combustion approach to predict the energy distributions for a multi-cylinder light-duty diesel engine. This article begins with a brief introduction on the engine simulation model, which is followed by the theoretical work on the energy balance methodology. Then, the results and discussion section presents the comparative study of energy distributions between simulation and experiment, which is further applied to correlate the improvements in fuel conversion efficiency to the changes in engine coolant temperature.
Section snippets
Model setup and validation
The diesel engine was modeled using a one-dimensional engine simulation software package (GT-Power) [24]. Table 1 lists the main specifications of the engine. In general, such a system-level engine model is developed with built-in templates providing detailed representation of the engine components, such as intake and exhaust manifolds, cylinders, valve train, and engine crank train.
The simulation model was validated using the steady-state measurements from the instrumented engine where five
Energy path analysis
This study attempts to compare the energy distributions between the simulation and experiment, the lack of information on turbocharger, intercooler and exact engine external structures (e.g., engine block surfaces) prevents the simulation from capturing the unaccounted energy, i.e. the heat rejected to the oil plus convection and radiation dissipated to the environment through the engine’s external surfaces. To ensure the experimental energy balance analyses follow a consistent fashion of
Results and discussion
The methods described in the previous section are now applied to the experimental energy balance analyses by using the control volume shown by Fig. 3. These results are subject to the constraints of the experimental measurements and to the approximations involved in the methodology. First, this section compares the experimental results to the predictions from simulation and attempts to explain the possible factors causing the disparities in the relevant energy terms. Then, the ultimate
Summary and conclusions
The first law of thermodynamics was implemented to examine the strategy of altering engine coolant temperature to devise a version of LHR application in a conventional light-duty diesel engine. With the combustion chamber based control volume, a comparative study between simulation and experiment was carried out to investigate the disposition of initial input energy at five different ECTs. Due to the constraints involved in the measurements, the methodology used for the experimental energy
Acknowledgements
This work was supported by National Science Foundation under Grant # 1343255 and General Motors Research & Development. Additionally, the authors wish to acknowledge Dr. Stephen Busch and Dr. Kan Zha, researchers at Sandia National Laboratories, for their assistance in providing the injection rate data. Any opinions or views expressed in this manuscript are not necessarily those of the sponsoring agency.
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