Interest has grown in the use of compression-ignition or diesel engines for small (&lt;300 kW) unmanned aircraft systems (UAS). Ignition assist devices such as glow plugs have emerged for augmenting the control authority of fuel injection timing to manage combustion phasing during operation at low temperatures and with low cetane number fuels that may be found in remote airfields.A significant obstacle to flexible and safe UAS operation is the sensitivity of combustion phasing, and hence torque response, to fuel quality and air temperature. Combustion in state-of-the-art engines is currently controlled via open-loop fuel injection strategies that are calibrated to a few fuel blends and inlet temperatures. Open-loop strategies do not interpolate smoothly even when the fuel’s cetane value is known due to the dependency of the ignition behavior on many fuel properties. Such strategies are also difficult to coordinate with secondary actuators such as glow plugs to ensure robust performance with fuels of unknown or changing properties.This work presents the first phase of a model-based closed-loop controller that coordinates actuation of an ignition assist device alongside fuel injection to optimize combustion phasing features for stability and efficiency. The feedback controller must shift phasing in the mean value sense without amplifying the cycle-to-cycle variability induced by low cetane fuels and non-ideal ambient conditions. Few studies have identified combustion mean and variability distributions that facilitate feedback control design with variability management. In this study, a physics-based and data-driven characterization of combustion phasing statistics in response to fuel injection and ignition assist inputs is presented for this purpose.&nbsp;Experiments were performed in a commercial diesel engine equipped with off-the-shelf metal glow plugs at a throttled and chilled low-speed condition that resembles UAS operation at high altitude and low inlet air temperature conditions. The tests were repeated to quantify the uncertainty caused by the time-varying thermal state of the engine (despite the controlled cooling and air-inlet conditions). The transient response of phasing to glow plug actuation during the warm-up was also characterized. No other work has examined transient trends in combustion statistics and ignition assist actuator authority.The experimental data were used to determine a low order model for combustion phasing and to characterize the statistics of cyclic variability as a function of both actuators and the changing thermal state of the engine.&nbsp;&nbsp;The model consists of four parameters for the mean static combustion phasing, two parameters for the transient response, and five parameters for combustion variability. The control-oriented model will be leveraged to develop a closed-loop control strategy that maintains optimal combustion behavior while robustly managing changing statistics for variability.

Control-Oriented Model of the Mean and Dispersion of Diesel Combustion Phasing With Ignition Assist

Paper Proceedings: https://asmedigitalcollection.asme.org/ICEF/ICEF2021/volume/85512

ASME 2021 Internal Combustion Engine Division Fall Technical Conference (ICEF2021)

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