Research On The Controllability Of High Speed Marine Diesel Engine Premixed Combustion

Premixed compression ignition engine can achieve low NOx and Soot emissions and high thermal efficiency by low temperature combustion of lean premixture,so intensive and extensive research has been done in recent years.The technical difficulties of premixed compression ignition engine include following issues:Preparation of the lean premixture,especially for those diesel-like fuels which are difficult to evaporate,robust control of ignition timing,and extension of operation range.The robust control of ignition timing is the key issue.The preparation of lean premixture needs long enough fuel and air mixing time,so the connection between fuel supply and ignition becomes weak.Due to the extreme lean premixture,traditional ignition device such like spark plug cannot control the ignition timing effectively.Jet Controlled Compression Ignition(JCC1)technology was proposed to solve the ignition timing control problem of premixed compression ignition engine.A small pre-chamber is installed in the cylinder head of JCCI engine.Natural Gas(NG)is fed to the pre-chamber and ignited by a spark plug.High temperature jets issued from the pre-chamber control effectively the combustion of lean diesel premixture inside main chamber.The JCCI technology is best suitable for marine dual fuel high speed engine.A marine high-speed diesel engine was modified to a JCCI prototype engine on which experimental and simulation research was performed.Test bench experiments showed following things.Combustion phasing was related closely to spark timing.Intake manifold air temperature up to 80℃ would not make premixed combustion uncontrollable at low and mediate loads.Diesel injection timing earlier than 70℃ A before top dead center had little impact on ignition timing.Later diesel injection,however,could decrease premixture homogeneity and increase NOx emission and ringing tendency.Introduction of Exhaust Gas Recirculation(EGR)could decrease the combustion rate and ringing intensity and reduce NOx formation further.The engine could operate in JCCI combustion mode in E3 cycle Mode 4,Mode 3 and Mode 2(25%load,50%load and 75%load).The premixed combustion was out of jet’s control in Mode 1(100%load)due to insufficient EGR.By premixed combustion,E3 cycle weighted NOx raw emission meeting IMO Tier Ⅲ limit was realized without compromising engine fuel efficiency.3D simulation of JCCI combustion was performed and three-stage heat release characteristic of the combustion was revealed.During compression stroke,low temperature reaction of the diesel premixture in main chamber happened and formed the first stage heat release.Hydrogen peroxide(H2O2)produced and accumulated.The NG mixture inside the pre-chamber was ignited by spark plug,and high temperature jet formed consequently ignited partial main chamber diesel premixture and formed the second stage heat release.As in-cylinder temperature increasing,the H2O2 decomposed quickly into hydroxy(OH)which triggered the autoignition of the rest of diesel premixture,so the third stage heat release formed.The combustion of partial rich diesel premixture contributed largely to the NOx formation.A new JCCI combustion concept was proposed to reduce NOx emission and increase thermal efficiency further.Diesel enriched lean NG premixture combustion was controlled by pre-chamber jets in the new JCCI system.3D simulations of diesel and NG premixed flame and new JCCI combustion revealed that moderate diesel addition to NG premixture could increase combustion rate and extend lean combustion limit,so engine thermal efficiency increased and NOx,HC and CO emissions reduced.Indicated thermal efficiency of 51%and indicated NOx emission of 0.7 g/kWh were achieved at mediate load by new JCCI combustion with optimized parameters.The NOx emission of new JCCI combustion with high air fuel ratio was mainly from the combustion of mixture inside the pre-chamber.