Current and future emission legislations require a significant reduction of engine-out emissions for Diesel engines. For a further reduction of engine-out emissions, different measures are necessary such as:
Especially an advanced emission and closed-loop combustion control has gained increased significance during the past years.
The field of application for a closed-loop combustion control, for short-, mid- and long term applications, can be identified as:
Precise correction of ambient conditions such as cold, hot, high altitude
Adaptation to different fuel specifications (e.g. variation in cetane number, bio diesel fuel, potential for JP8 or kerosene operation)
Mandatory for the support of pHCCI combustion strategies
Adaptive compensation of production tolerances and drift during lifetime (e.g. injectors, boosting system, EGR system)
Precise torque control
General support of emission control
Implementation of fast, real-time combustion modulation allowing optimized combustion conditions through the modulation of the available air and fuel handling parameters
Adequate substitution or diagnostics of engine and/or aftertreatment sensors/actuators for example for OBD purposes
A highly premixed combustion allows an overall reduction in exhaust emissions. This premixed combustion is achievable in low and medium engine speed and load areas where sufficient time permits the preparation of the mixture before combustion occurs. In combination with an optimized combustion system in form of hardware components that are harmonized to their corresponding mode of operation (e.g. bowl and injector nozzle design are matched to provide the best possible air utilization during combustion), there is a further emission reduction potential by applying an appropriate calibration and injection strategy. But it has to be taken into account that during transient and also steady-state operation under all ambient conditions an acceptable combustion noise level is maintained and engine misfiring must be avoided.
This paper discusses a combustion control which considers the aspects of a conventional control of the center of combustion [7], [8] combined with a self-learning control algorithm of the main injection timing. The main issue is a closed-loop control of the global injection characteristic which describes the number of pre-injections used. The injection characteristic as the control mechanism is controlled by combustion stability and combustion noise as feedback signals.
With this control mechanism, one or more pre-injections are activated for very short time periods to stabilize the combustion during transient operation and also for stationary operation with challenging combustion conditions such as low load operation.
This paper explains the control strategy and presents results of vehicle emission tests.