Simulation of a Two-stroke Dimethyl-ether Free Piston Engine Operating on HCCI Combustion
|Course||Power Machinery and Engineering|
|Keywords||Free piston engine DME HCCI Knock|
In this paper a dimethyl-ether (DME) hydraulic free piston engine operating on HCCI combustion was proposed based on the technical characteristics of DME, HCCI and hydraulic free piston engine (HFPE). The high burning rate of HCCI solve the problem that the HFPE stays less time around TDC and low-pressure spray of DME outside the c ylinder was used in the HPFE thus the wear and leak of DME high pressure s pray s ystem w as a voided. D ue t o t he f lexible a nd c ontrollable compression ratio of the HFPE we solved the problem of the accurate control of the start of combustion of HCCI. And the cetane number of DME is high, thus can easily compression ignition and achieve a reliable combustion. Through combined the three technologies ab ove w e expected t o a n ew t ype o f clean, e fficient alternative f uel engine.This pa per i nvestigated t he c ombustion pr ocess a nd c haracteristics of a two-stroke D ME f ree piston e ngine ope rating on H CCI combustion, us ing a one-dimensional dynamics model in AMESIM and a three-dimensional CFD model in AVL-FIRE. The feasibility of the proposed scheme, the key factors that influence the HFPE performance and the mechanism had been studied. Through the simulation we got the main conclusions as follows:The c ompression s troke a nd t he e xpansion s troke of t he H FPE w ere non-symmetrical in terms of the top dead centre compared to a crank engine that had the same displacement and same frequency. The HFPE stayed less time around the nominal top dead center (TDC).The acceleration of the HFPE was very large and the velocity changed rapidly around the nominal TDC.When the engine operating on HCCI combustion the combustion phase of the HFPE was closer to the TDC, the negative work was less and the knocking intensity was lower than the crank engine.EGR rate and equivalent ratio affected the combustion phase .Through adjusting the EGR rate and equivalent ratio the engine efficiency and knock intensity can reach a desired level.The us e of boos t c ompensated t he pow er o f f ree pi ston engine un der l ow equivalence ratio and high EGR rate. When the equivalent ratio was 0.4, the EGR rate was 0.4 a nd the boost l evel is 2.6, t he mean effective p ressure of the HFPE was 6.72bar; hydraulic output power was 26.64KW and heat-work conversion efficiency 51.7% was achieved. And the knock intensity was 5.84WM / m2, below the knock limit.When the HFPE was controlled on the operating condition, equivalent ratio 0.4, the EGR rate 0.4 and the boost level 2.6 a unique structure of the rate of the heat release appeared. There were three peaks in the curve of the rate of the heat release. The third exothermic peak, there is no i nvolvement of DME .The main exothermic ingredient is CO. During the third peak of the rate of the heat release, the rate of energy contribution was 38.7%.Through the research above the theoretical basis for prototype design of a HCCI free piston engine was provided.