• Energy Research

Abstract Considering continuously rising fuel prices and the global warming problem it is significantly important to reduce fuel consumption of engines used in various applications. Of specific importance is the HD diesel engine used in large haul trucks because these vehicles have an extensive operating schedule, their engines have a high power output in the range of 200–400 kW and their number is significantly high. Considering current achievements, it appears that HDDI diesel engine bsfc cannot be significantly reduced in the future unless new ideas or techniques are employed. Under this framework the utilization of exhaust heat becomes inevitable because approximately 30–40% of fuel energy is rejected to the environment. A promising technique for the recovery of energy from the exhaust gas is the use of a Rankine bottoming cycle. This technical solution has been examined in the past with very positive indications and a strong potential for significant improvement. However various technical challenges have to be solved among which most important are packaging and rejection of excess heat from the engine cooling system. For this reason in the present work a simulation model which has been developed to describe the operation of a Rankine bottoming cycle is utilized to estimate the potential efficiency gain from its application on a heavy duty truck powered by a diesel engine. Using the simulation special attention is given to the utilization of EGR cooler and CA cooler (Charge Air) heat to increase the Rankine expander power output and thus improve bsfc reduction potential. Furthermore the utilization of both EGR and CAC heat amounts is used to minimize the negative impact of the Rankine cycle on the engine cooling system, the capacity of which is exceeded at high load. The last results to installation difficulties (larger engine radiator, etc.) and in some cases a significant amount of generated power is consumed to drive the cooling fan which obviously has a strong negative impact on the bsfc reduction potential. For this reason several scenarios are proposed and examined in the present work to avoid or minimize this problem. Results are produced for both organic and steam working media that reveal a very good potential for the application of Rankine bottoming cycles in HD engine applications. Furthermore it is revealed that the utilization of both EGR cooler and CA cooler heat beyond its positive effect on bsfc reduction potential is also beneficial for overall system packaging allowing the serious reduction of primary heat exchanger dimensions.

The paper presents the results from the analysis of an experimental investigation with the aim to provide insight to the cyclic, instantaneous heat transfer phenomena occurring in both the cylinder head and exhaust manifold wall surfaces of a direct injection (DI), air-cooled diesel engine. The mechanism of cyclic heat transfer is investigated during engine transient events, viz. after a sudden change in engine speed and/or load, both for the combustion chamber and exhaust manifold surfaces. These results are then compared with relevant experimental data from steady state operation which in the present case are used as reference helping to reveal any potential influences of each transient event on cyclic heat transfer. The experimental installation allowed both long- and short-term signal types to be recorded on a common time reference base during the transient event. Processing of experimental data was accomplished using a modified version of one-dimensional heat conduction theory with Fourier analysis, capable to cater for the special characteristics of transient engine operation. Based on this model, the evolution of local surface heat flux during a transient event was calculated. Two engine transient events are examined, which present a key difference in the way the load and speed changes are imposed on each one of them. From the analysis of experimental results it is confirmed that each thermal transient event consists of two distinguished phases the “thermodynamic” and the “structural” one which are appropriately configured and analyzed. In the case of a severe variation, in the first 20 cycles after the beginning of the transient event, the wall surface temperature amplitude on cylinder head was almost three times higher than the one observed at the “normal” temperature oscillations occurring during the steady state operation. At the same time, peak pressure values in the same cycles are increased by almost 15% above their corresponding values at the final steady state. The same phenomena are valid for the exhaust manifold surfaces but on a moderated scale.

An experimental investigation is conducted to examine the effect of the main parameters influencing the compression stroke of a direct injection Diesel engine. The aim is to develop a methodology that can be used as a diagnostic tool to determine the compression condition of DI Diesel engines. However, conclusions derived from the present investigation can be extrapolated to other types of reciprocating internal combustion engines. The compression stroke itself is an important index concerning engine operation since engine conditions at the end of the compression stroke have a major effect on its overall performance. Such information is especially important for large scale Diesel engines used for stationary or marine applications. In these engines, it is important to develop non-catastrophic methods for estimating the cylinder compression condition without dismantling the engine cylinder. The outcome could be a serious reduction of maintenance costs, since unnecessary labour required for inspection could be avoided. When using measurement techniques, what is usually available is the cylinder pressure trace during the compression stroke. However, it is widely recognized that the compression stroke and peak compression pressure is strongly affected, beyond heat losses, by the initial pressure at the inlet valve closure, the compression ratio and the blowby rate. The last three parameters can vary significantly during engine operation, while the heat losses vary mainly due to engine operating conditions and their effect on the compression stroke can be considered. Thus, the knowledge of the peak compression pressure resulting from the cylinder compression pressure diagram is not adequate to define the cylinder compression condition. For this reason, an experimental investigation is conducted to examine the effect of the initial pressure at the inlet valve closure, the compression ratio and the blowby on the cylinder pressure trace. From analysis of the measured data, it is revealed that each parameter has a different effect on the different parts of the compression pressure trace. As revealed, it is possible to determine the compression condition of an engine cylinder based on the measured cylinder pressure trace.

Abstract An experimental analysis is performed to study the instantaneous heat fluxes, during the engine cycle, in the combustion chamber walls of a direct injection (DI), air cooled, four stroke, Diesel engine located at the authors laboratory. For this purpose, a novel experimental installation has been developed, which separates the engine transient temperature signals into two parts, namely the ‘long’ and the ‘short’ term response ones, followed by their discrete processing in two independent data acquisition systems. Furthermore, a new pre-amplification unit for fast response thermocouples, appropriate heat flux sensors and an innovative, object oriented, control code for fast data acquisition have been designed and developed for the needs of the study. One dimensional heat conduction with Fourier analysis of the raw temperature data are implemented in order to calculate the instantaneous engine cylinder and exhaust pipe heat fluxes. Analysis of the experimental results reveal many interesting aspects of transient engine heat transfer. The effect of engine speed on cylinder head and exhaust manifold heat losses is presented. The simultaneous presentation of heat fluxes on the cylinder head and exhaust manifold, together with the engine indicator diagram, sheds light into the mechanisms governing the transient heat transfer. This is very important, since especially for air cooled Diesel engines, limited information seems to exist in the relevant literature.

In recent years, the internal combustion engine has been the subject of debate mainly concerning its environmental impact. Despite all the discussion it becomes clear day by day that combustion engines will continue to occupy their dominant role over the following decades, especially in the mid- and large-size power spectrum ranges and retain a large share of the market in the smaller-size segment of their application. In this context, in the present paper, a novel engine kinematic mechanism is introduced, which converts rotary to reciprocating motion, and aims to become a potential replacement for the traditional crankshaft mechanism of piston engines. Following a description of the fundamental principles of the new design, we detail the main problems with the application of the new design in the first prototype SI engine and the actions and improvements implemented to overcome them. The actual measurement data from basic engine performance parameters are provided and evaluated, leading to conclusions and decisions for further action which should be implemented in the next improvement steps. Overall, the new SI engine, implementing the novel kinematic mechanism, seems to be quite promising especially in hybrid automotive applications, a fact that encourages the implementation of further improvement plans.