Development of a New 1.8L Down-Speeding Turbocharged Gasoline Engine with Miller Cycle 2018-01-1712
Upcoming China 4th stage of fuel consumption regulation and China 6a emission legislation require improvement of many existing engines. This paper summarizes an upgrade of combustion system and mechanical layout for a four-cylinder engine family. Based on an existing production process for a naturally aspirated 2.0-liter gasoline engine, a 1.8-liter down-speeded and turbocharged gasoline engine is derived. Starting development by analysis of engine base geometry, a layout for a Miller-Cycle gas exchange with early closing of intake valves is chosen. Requirements on turbocharger configuration are investigated with one-dimensional gas exchange simulation and combustion process will be analyzed by means of 3D-CFD simulation. Challenging boundary conditions of a very moderate long-stroke layout with a stroke/bore-ratio of only 1.037 in combination with a cost efficient port fuel injection system and fixed valve lift profiles are considered. To compensate reduced in-cylinder charge motion of small valve lift three measures are taken. Firstly, the intake port is modified for significantly increased tumble motion at higher valve lift. This is combined with a masking of the intake valve area in the combustion chamber and enables a high turbulence even at very small valve lift. Finally, the valve lift profile is optimized by changing cylinder head layout from tappet valve train to roller-finger follower. This has also positive impact on friction behavior. Further reduction of parasitic losses by reduced main bearing diameter, adapted piston ring layout and introduction of an oil pump with two pressure stages accompanies the increase of combustion efficiency. The development targets with almost ninety-two percent of the maximum torque achieved at an engine speed 1250 rpm and a minimum specific fuel consumption of 230 g/kWh are reached, which means maximum thermal efficiency of 36.8% normalizing to caloric value of 42.5 MJ/kg.