Georgios C. Mavropoulos

In recent years, numerous studies have focused on reducing emissions from modern reciprocating engines without compromising their performance characteristics. One promising approach is to use Mild-Hybrid engines as an alternative proposal to the conventional reciprocating ones. This study aims to investigate how the hybridization will impact the main performance variables and the pollutant emissions of a turbocharged, modern aircraft spark-ignition (SI) engine—specifically, the ROTAX 914. The engine is analyzed under three distinct operating conditions at three different altitudes, defined by different combinations of engine speed and throttle position, using conventional aviation fuel (AVGAS 100LL). The analysis is conducted using GT-POWER, an advanced engine simulation software that allows for complete engine modeling and parameterization across a wide range of operating conditions. The accuracy of the simulated engine model is validated by comparing its output to experimental data obtained from the engine’s technical manuals. Key performance indicators examined in this study include brake power (We), brake torque (Mσ), brake specific fuel consumption (BSFC), and emissions of nitrogen monoxide (NO) and carbon monoxide (CO). Therefore, the proposed model can be employed to investigate the operational behavior of the ROTAX 914 UL aircraft engine when integrated into a hybrid aircraft propulsion system, in which the engine is connected in series with an electric battery. In particular, the model enables parametric studies on the effects of varying engine–battery hybridization levels—defined as the respective contributions of the engine and the battery to the total propulsive power available at the aircraft propeller—on the main performance variables and emissions of the ROTAX 914 UL engine in different altitudes. The primary objective is to assess the effects of series hybridization on engine operation and its most significant emissions. This is accomplished by operating the ICE at a lower operating condition, because it is connected with a battery, which helps the engine deliver the required output power sooner. The results suggest that increasing the power output delivered by the battery, so the ICE is operating in lower loads, can significantly enhance the performance and environmental efficiency of a turbocharged aircraft SI engine at different flight altitudes. In conclusion, series hybridization presents a promising solution for improving present-day reciprocating SI engines.

This paper presents a study aiming to provide insight into the complex flow and heat transfer processes in the exhaust manifold of a four-stroke, compression ignition engine. An experimental system has been constructed capable of capturing temperature and heat flux high-frequency signals as they develop in the exhaust pipe wall during the engine cycle, under its steady-state operation. The values of the Heat Transfer Coefficient obtained by applying the classic convection relations have been correlated in the form of a Nusselt–Reynolds number relationship for local and spatially averaged steady-state heat transfer and compared with available experimental data obtained at the same position of the exhaust manifold. It has been shown that the use of conventional steady-state heat transfer relationships for fully developed steady-state turbulent flow in pipes underpredicts heat transfer rates when compared with those experimentally observed. Periodic flow of high frequency and geometrical effects at the exhaust entrance are expected to affect the validity of the application of the classic steady-state correlations for the exhaust manifold. To overcome this problem it is developed and presented a new correlation for the time-averaged heat transfer rates. To verify the heat transfer mechanism, the thermal field of the whole engine cylinder head, including the intake and exhaust manifolds, was analyzed using FEA (Finite Element Analysis), and the results are compared and verified with available experimental data.

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.

In the present paper, it is examined the potential application of a Rankine cycle exhaust gas heat recovery system which recovers and utilises energy from the exhaust gas of four-stroke marine diesel auxiliary unit before it is released to the atmosphere, increasing thus its total efficiency. For this purpose, a thermodynamic simulation model of the Rankine bottoming cycle is used. Heat utilisation is initially considered only from the main exhaust gas stream after the T/C turbine. As reference for the investigation are used the values from the official shop tests at various loads ranging from 25% up to 100%. From these using the Rankine cycle simulation model, it is investigated the potential for increase of generated cycle power output and optimisation of its efficiency. In addition, it is examined the effect of working media by considering steam and an organic. The results derived reveal that there exists a significant potential for fuel saving by utilisation of exhaust gas energy of auxiliary engines installed on commercial vessels.

In the present paper a new method is proposed for the analysis of the two main phases of the engine exhaust stroke blowdown and displacement. The method is based on the processing of fast-response experimental temperatures obtained from the exhaust manifold wall during the engine cycle. A novel experimental installation has been developed, which separates the engine transient temperature signals into two groups, namely the long- and the short- term response ones. This has been achieved by processing the respective signals acquired from 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 applied. For the experimental procedure a direct injection (DI), air-cooled diesel engine is used. One-dimensional heat conduction theory with Fourier analyses techniques are implemented to calculate the instantaneous local heat flux in the exhaust manifold wall surfaces. To estimate the duration of the different phases of exhaust stroke a numerical method is developed which makes use of first and second temporal derivatives of exhaust manifold wall temperature. The analysis reveals the effect of engine speed and load on the initiation (engine crank angle) and duration of both blowdown and displacement exhaust phases. In addition, the distribution of instantaneous wall temperatures and heat fluxes between the different exhaust phases is analyzed indicating several important characteristics. The results reveal that duration of blowdown phase is correlated adequately with cylinder pressure at the point of Exhaust Valve Opening.