Cold compensation calibration for diesel engines is traditionally applied in two areas. One is for cold start and cold driveaway after the engine has been allowed to cool-soak for an extended period of time between 2 and 12 hours. The other is for sustained operation in cold ambient environments when the engine reaches a stable coolant temperature after the initial start and warm-up, but for inlet air temperatures approaching cold ambient temperatures.
Cold start and cold-driveaway calibration involves adjusting fueling, injection timing, fuel injection multipulse sequence, backpressure, and EGR (exhaust gas recirculation) for soak temperatures down to -40°C (-40°F) with allowance for various starting assists per the engine design from the OEM. Calibration for sustained operation at cold ambient conditions involves similar calibration parameters, but for more fully warmed engine coolant conditions in the range of 50 to 90°C (122 to 194°F), with sustained inlet air temperatures down to -35°C (-31°F).
Roush Industries engineers have recently developed a procedure that involves cold inlet air temperatures simulated in a transient dynamometer test cell environment to facilitate cold compensation calibration.
Their transient dynamometer test room can simulate both cold-soak and sustained cold inlet air temperature vehicle drive scenarios for cold compensation calibration. To simulate the cold ambient inlet air test environment, the vehicle air-to-air intercooler is replaced with test room heat exchangers to reduce inlet air temperature upstream of the intake throttle down to -30 to -35°C (-22 to -31°F), simulating the cold ambient drive test conditions for both steady-state conditions and transient operational modes.
Important measurements conducted during the cold compensation tests consisted of a number of parameters all integrated into a single data set in the test room data acquisition system. They included wet HC and NOx emissions; exhaust opacity and exhaust smoke number; combustion analysis and heat release parameters from measured cylinder pressure; and other stability indicators such as cylinder-to-cylinder exhaust temperature and engine torque variation.
Cylinder pressure and heat release diagrams from the combustion analyzer were a key indicator when conducting cold calibration tests, with instability measures providing an early indicator to later issues with HC emissions, exhaust smoke, and the ability to follow a simulated vehicle drive cycle in the transient dynamometer test cell. DOEs were conducted to optimize calibration parameters via statistical regression models to minimize combustion instability and HC emissions while maximizing exhaust temperature for catalyst lightoff.
To validate final calibration, transient tests were conducted simulating a vehicle drive trace while inlet air temperature was maintained at -35°C (-31°F). If any issues arose while running the transient cycles, the DOE process was repeated at the particular speed/load point to produce a more robust calibration.
To validate cold compensation calibration, vehicle drive traces for the heavy-duty Federal test procedure (HD FTP) were run on the transient dynamometer while observing exhaust emissions, opacity, combustion analyzer stability parameters, peak cylinder pressures, and parameters from the engine control unit (ECU).
Matt Van Benschoten of Roush Industries wrote this article for Automotive Engineering.