Study On The Key Technologies Of The Opposed Piston Opposed Cylinder Two Stroke Diesel Engine

Opposed piston and opposed cylinder two stroke engine (OPOC engine) is a new type of engine. The idea was not new but many innovative ideas were derived in the last decade and now it becomes a hot spot of interest both overseas and domestic because its large power density, low fuel consumption and great potential for improvement to the traditional two stroke engine.In this thesis paper the special structural features and performance advantage s of the OPOC engine is introduced, then it is focused on the study of the key technologies of the OPOC engine: the inner relationship between the piston motion profile and the port opening area/timing is derived; The engine performance，scavenging process, fuel spray and the in-cylinder combustion process, are systematically studied via both numerical computation and bench testing.The piston movement profile is the premise and foundation of other study. The study demonstrates that:①TDC is not located at the maximum position of either the inner or the outer piston, because the inner and outer piston do not reach the extreme position of their displacement at the same time.②Phase difference between the TDC and BDC is not, although close to, necessarily180crankshaft angle.③The larger the intake and exhaust port heights are, the greater the maximum opening area is，the earlier the ports are opened, the later they are closed, and the smaller the effective compression ratio and the effective expansion ratio are. The OPOC engine performance is closely related to the boost level, port heights and other parameters. The turbocharged OPOC engine has its own uniquecharacteristics:①The inlet pressure at the intake ports must be greater than the that at the exhaust ports to form a pressure difference for positive scavenging, to ensure the normal operation of the engine;②Introducing ECT (Electrically Controlled Turbocharger) provides assistance for the compressor to guarantee a pressure difference for positive scavenging. For the matching between the OPOC engine with the turbocharger, this research revealed the inner-relationship between the engine performance goals with the required boost pressure level, derived the mathematical models for pre-matching of turbochargers of both one and two-stage turbocharging systems, and developed a engine/turbocharger pre-matching software V2.0. One-stage turbocharger without external EGR and two-stage turbochargers with large scale external EGR (up to40%by mass) are preselected and identified by application of the software, and verified by engine dyno tests. It is proven that the proposed pre-matching models with both one and two-stage turbochargers are reliable and the turbocharger pre-matching software V2.0has a high value in engineering. In addition, analysis results of the intake/exhaust port heights indicate that: with fixed intake port heights, the longer the exhaust ports are, the better the engine performance is at high speeds, but the worse at low speeds; Other the other hands, with fixed exhaust port heights, the longer the intake port is, the better the engine performance is at low speeds while the performance at high speeds get worse. Scavenging performance directly affects engine power, fuel economy andemissions performance of the engine. The article details the main evaluation indicators of scavenging performance: scavenging efficiency, delivery ratio, capture rate. Based on the engine bench test data, firstly focusing on the simulation calculation and analysis of the scavenging performance of the external characteristics, It comes to a conclusion that:①When the engine structure is determined, the scavenging efficiency depends on the different pressure and absolute time of scavenging.When pressure difference is large, the scavenging effect is better; When scavenging absolute time is long, the scavenging efficiency is higher.②The scavenging efficiency and delivery ratio are good, but the trapping ratio is low at low speed;the delivery ratio is moderate and the scavenging effect is good, but trapping ratio is low in middle speed;the delivery ratio is too small (less than1) and scavenging efficiency is poor, but the trapping ratio is high in high speed. Secondly, in full-load conditions at3500r·min-1,analyzing and simulated calculating the tangential angle of intake port, concluding that:①With the tangential angle is increasing, scavenging efficiency increases first and then decreases; the trends of fresh air changing with tangential angle increasing in cylinder is the same with scavenging efficiency when the intake port angle changes, but residual burned mass fraction shows just the opposite; delivering ratio decreases as the inclination angle increases (due to increasing angle, the intake port resistance increases and the flow coefficient will decrease, resulting in reduction of the intake port air mass flow in a equal time); Trapping ratio continues to increase with increasing inclination.②For the scavenging efficiency and fresh charge, the best value of the opening angle of the tangential scavenging is30o. The power, economy, emissions performance of the OPOC are directlydetermined by spray and combustion in cylinder. This paper introduces a relatively new parameters of evaluating thermal-power conversion process: equivalent effective expansion ratio (E.E.R), and it is revised and test validated, then appling to the analysis and evaluating combustion in cylinder; and making unique nozzle arrangement and the shape of the chamber about the OPOC engine, and studying the influence of the main parameters of the fuel injection system such as the number of nozzles, fuel injection timing, injection characteristics on the processes of spray and combustion, the following conclusions can be drawn:①The larger the number of nozzle holes is utilized, the smaller the nozzle hole diameter becomes, the smaller the spray Sauter diameter is, the better is the atomization quality (also the interferences between the fuel sprays can be avoided); the larger the number of nozzle holes is, the faster the burning rate is, the higher the cylinder pressure and temperature are; the larger the number of nozzle holes is, the higher the raw NOx emission is; as for the soot formation, because the speed of the post-oxidation is much faster with larger number of nozzle holes, the emission would become better; Lastly, the larger the number of nozzle holes is, the better the fuel economy is. The reason behind is that the atomizing effect and the combustion process are both improved with larger number of nozzles holes, producing a faster and more complete combustion and heat release process.②The larger the injection advance angle is, the sooner the fuel atomization happens, and the more combustible premixed gas is generated during the ignition delay period, the higher the cylinder pressure rise rate is, so is the higher peak cylinder pressure, the higher cylinder temperature, with much more raw NOX emissions and much lower Soot emissions. If the injection timing is reasonable, the combustion heat release curve would be ideal, the best E.E.R value can be achieved, with the best fuel economy.③The effects of pre-injection and main injection period: with increased pre-injection quantity, NOX formation decreases at the beginning but then increases in the end; since more compression power is consumed in the compression stroke, the economy is getting worse with too much pre-injection quantity.④Comparing between the different fuel spray angles, i.e.50,60,70,80and90degrees, indicates that when the beam angle is at60o, the mount of Soot will be the minimum, mainly because of the lower rate of Soot generates and the higher rate of oxidation. This phenomenon is different from conventional engines, mainly due to higher fuel beam angle, the amount of fuel which hits the piston top is reduced, this changed the law that the fuel spray speed increases with the spray beam angle in a conventional engine.