1. Introduction
As is well known, aviation piston engines (APEs) are developing toward high power density [
1]. Compared with four-stroke engines, the high-speed two-stroke engine for light aircraft has higher specific power per weight and displacement volume, as well as better thermal efficiency [
2]. In recent years hybrid power systems are gradually developed in general aviation due to their excellent power performance and economic performance [
3]. However, the series-parallel hybrid system is not suitable for small UAVs because it requires transmission systems [
4,
5]. In addition, the current controller cannot fulfill all the tasks faultlessly in the energy management process [
6]. Therefore, small internal combustion (IC) engines are still the main powerplants of model airplanes [
7]. The design parameters of the combustion chamber and the operating conditions of APEs have a direct impact on combustion and emission characteristics [
8]. It can be predicted that HF-APEs will gradually replace the existing aviation gasoline engines and become the main power system of general aviation aircraft, as well as long-endurance UAVs in the future [
9]. For an aircraft diesel engine, the priorities for optimization are reliability and performance [
10,
11]. Due to the small displacement and high-speed reciprocating motion of the cylinder, the fuel-air mixing, combustion space, and time are greatly limited during direct injection in the cylinder, and combustion organization is difficult [
12]. Excessive fuel accumulation on the surface of the combustion chamber due to the contradiction between the fuel spray penetration distance and the combustion chamber diameter, increased smoke emissions at high loads [
13]. Therefore, the analysis of scavenging and combustion processes of heavy fuel direct injection small APEs is the key to improving the comprehensive performance of the engine [
14].
It was found that the DI combustion system yields several advantages: better take-off performance (higher power output), lower fuel consumption at cruise conditions, improved altitude performance, and reduced cooling requirements [
15]. Carlos J. used the heat release analysis method to investigate the factor, which influences pilot-injection combustion [
16]. Busch S et al. from Sandia National Laboratories, compared the combustion performance of three different types of pistons, which showed that the stepped-lip piston led to higher thermal efficiency [
17]. Yang et al. analyzed the interactions between sprays and sprays with swirls using Flame Image Velocimetry (FIV), which showed that swirls could enhance the mixing [
18]. Le M K et al. investigated the flame-moving process in a small heavy fuel engine by optical method, which showed that the spray hit the combustion chamber wall soon after the start of injection and a part of them would rebound while others would expand along the combustion chamber wall [
19]. Pradeep M. summarized several conclusions and analyzed the trend of development in a small naturally aspirated CRDI diesel engine [
20]. Based on the particle sampling method of thermophoresis, Zhang Y L et al. found that half of the combustion flame propagated against the swirl, while the other half developed in the same direction due to the influence of the swirl in the small-bore diesel engine [
21]. Francesco B et al. proposed a method in which the fuel is injected into the combustion chamber intermittently, which leads to the fresh air being less diluted by the residual gas and the combustion efficiency increasing [
22]. Xue M established the simulation model of in-cylinder combustion of piston aviation and verified by experiments, which state that the combustion center of gravity will increase by increasing the ignition advance angle [
23]. Pan Z J et al. analyzed the impact of combustion chamber diameter and depth on emissions, which showed that properly reducing combustion chamber diameter and diameter-depth ratio can reduce emissions [
24].
Zhou Y et al. analyzed the trends of cutting-edge technologies and the theory of gas exchange [
25,
26]. James W. G. compared different scavenging systems for 2-stroke engines, which showed that the opposed-piston has the best scavenging effect and the highest thermal efficiency, because the structure can achieve maximum thermal expansion and minimum heat loss [
27]. In order to increase the engine braking power during take-off and reduce the engine fuel consumption under cruise conditions, Carlucci A P et al. analyzed the characteristics of a two-stroke uniflow diesel engine and compared the simulations to the test data [
28]. In order to improve combustion efficiency, Hu CM et al. proposed the application of the AADI system in spark-ignition piston aero-engine [
29]. Xu Z et al. predicted the high altitude performance of Poppet-valves 2-Stroke (PV2S) aircraft diesel engine, and the results showed that the high altitude power loss of the PV2S engine was more serious than the traditional two-stroke engine [
30,
31]. Chen Y L et al. used different swirl ratio tests to study the effect of intake swirl on engine combustion performance, the results showed that the best fuel/air mixture and combustion performance were obtained in the direct injection diesel engine without intake swirl [
32]. Pavel B et al. proposed a method to analyze the correlation between the residual gas fraction in the exhaust port and the residual gas fraction in the cylinder using the scavenging curve [
33]. Despite a large number of experimental investigations that have been implemented to explore the small two-stroke engine performance, there is no systematic treatment of combustion characteristics and altitude characteristics with a small two-stroke engine.
Yusuf A.A. et al. studied the effects of spark timing and alternative fuels on engine performance, combustion, and tailpipe emissions [
34]. Shirvani S. et al. have conducted extensive research. He examined the effects of a number of nozzles, injection pressure, fuel line angle, and the start of injection (SOI) in two-stroke engines. Results revealed that advancing the start of injection by two crank angle degrees (CAD) can reduce soot emissions by 16%, and with the strategy of pre-injection, NOx is reduced by 37% [
35]. Mitianiec W. studied the combustion behavior with different piston positions and asymmetric scavenging timing. They reported that earlier opening of exhaust ports can decrease CO emission because of a lower combustion temperature [
36]. Chang C. and Wei M.X.’s experiments on two-stroke APEs studied the impact of the mixture concentration on the cylinder pressure, cylinder temperature, and engine power, the results show that a rich mixture with early Injection advance angle has a better effect on the knock suppression [
37]. Zhenfeng Z. et al. through CFD to analyze aviation kerosene combustion characteristics, and reported that a worse equivalence ratio led to knocking combustion [
38].
Therefore, to orderly organize the airflow and accelerate spray and air mixing in the cylinder of the two-stroke heavy fuel direct injection engine [
39]. A detailed study into the performance of the small two-stroke HF-APEs is presented in this article, which incorporates experimental and computational fluid dynamics (CFD) analysis to explore the optimization direction of scavenging and combustion systems for small heavy fuel direct injection engines.
5. Conclusions
The two-stroke HF-APEs have the characteristics of high power density and simple structure, which makes them widely used in motorboats, unmanned aerial vehicles, and other fields. In this study, the scavenging and combustion processes of the engine at 2400 rpm, 298 K intake air temperature, and −8 °CA injection advance angle were adopted. The calculation results of different altitudes, different injection advance angles, and different operating conditions are analyzed, and the following conclusions are obtained.
- (1)
the multi-ports cross-flow scavenging scheme can generate unbalanced aerodynamic torque in the cylinder, and with the piston moving upward, a high intake swirl ratio will be generated in the combustion chamber, and the peak swirl ratio (SR) reaches 15.
- (2)
The small two-stroke heavy fuel direct injection engine mainly uses diffusion combustion, and the swirl intensity has an important influence on the in-cylinder atomization and combustion process of the small two-stroke heavy fuel engine. When the engine speed increased from 1200 rpm to 2400 rpm, the combustion duration extended by 57%. Moreover, when the engine load is increased from 25% to 100%, the HRR is increased by about four times.
- (3)
The internal EGR of small two-stroke APEs increases the intake air temperature, accelerates the fuel atomization and evaporation process, and has a positive impact on shortening the ignition delay period and improving the combustion speed.
- (4)
At different altitudes, the combustion center can be adjusted by adjusting the injection advance angle to ensure the power and economy of the engine. When the injection advance angle moves forward by 4 °CA, the maximum pressure increases by 2 MPa, and the rising rate decreased gradually.
At present work, the influence of injection strategy and combustion chamber shape on the combustion and scavenging process is not studied. Obviously, the combustion time and space of small HF-APEs are greatly limited. The current research does not consider the changes in flow field intensity and scavenging efficiency caused by altitude changes. In future work, we should pay attention to the design of the combustion chamber shape, and improve the matching of the fuel injection system and combustion system. A control strategy suitable for small HF-APEs should be studied, which can flexibly adjust the injection timing and match the external environment. It is of great significance to optimize the performance of small HF-APEs.