Homogeneous Lean Combustion in a 2lt Gasoline Direct Injected Engine with an Enhanced Turbo Charging System

摘要

In the quest for a highly efficient, low emission and affordable source of passenger car propulsion system, meeting future demands for sustainable mobility, the concept of homogeneous lean combustion (HLC) in a spark ignited (SI) multi-cylinder engine has been investigated. An attempt has been made to utilize the concept of HLC in a downsized multi-cylinder production engine producing up to 22 bar BMEP in load. The focus was to cover as much as possible of the real driving operational region, to improve fuel consumption and tailpipe emissions. A standard Volvo two litre four-cylinder gasoline direct injected engine operating on commercial 95 RON gasoline fuel was equipped with an advanced two stage turbo charger system, consisting of a variable nozzle turbine turbo high-pressure stage and a wastegate turbo low-pressure stage. The turbo system was specifically designed to meet the high demands on air mass flow when running lean on higher load and speeds. Also, a dual coil ignition system was used for enhanced ignition ability and a lean NO_x emissions exhaust after-treatment system (EATS) dummy was fitted downstream the turbo to receive representative exhaust pressures and temperatures for further development purposes. The engine was mapped running lean in various load points in the operational area of interest. It was found that the engine could sustain a high degree of dilution in lower engine speeds and intermediate loads. Fuel consumption improvements of 12% were obtained running at 1500 rpm and 10 bar BMEP at lambda 1.8. At higher engine loads, above 10 bar BMEP, it was found that the combustion stability deteriorated. The ignition could not be optimized due to knocking combustion and at the same time, combustion duration, measured in crank angle degrees, increased with increasing en-leanment and engine speed, leading to late combustion phasing and large variation in cycle-to-cycle of NMEP. This is currently limiting the operational region of lean combustion of the engine used. The load limit in lean operation was investigated, assessing combustion variations and knock phenomena under different operating conditions.