The lean-burn capability of the Diesel-ignited gas engine combined with its potential for high efficiency and low CO ${}_2$ emissions makes this engine concept one of the most promising alternative fuel converters for passenger cars. Instead of using a spark plug, the ignition relies on the compression-ignited Diesel fuel providing ignition centers for the homogeneous air-gas mixture. In this study the amount of Diesel is reduced to the minimum amount required for the desired ignition. The low-load operation of such an engine is known to be challenging, as hydrocarbon (HC) emissions rise. The objective of this study is to develop optimal low-load operation strategies for the input variables equivalence ratio and exhaust gas recirculation (EGR) rate. A physical engine model helps to investigate three important limitations, namely maximum acceptable HC emissions, minimal CO ${}_2$ reduction, and minimal exhaust gas temperature. An important finding is the fact that the high HC emissions under low-load and lean conditions are a consequence of the inability to raise the gas equivalence ratio resulting in a poor flame propagation. The simulations on the various low-load strategies reveal the conflicting demand of lean combustion with low CO ${}_2$ emissions and stoichiometric operation with low HC emissions, as well as the minimal feasible dual-fuel load of $3.2$ bar brake mean effective pressure.

This paper investigates the combustion phasing control of natural gas-diesel engines. In this study, the combustion phasing is influenced by manipulating the start and the duration of the diesel injection. Instead of using both degrees of freedom to control the center of combustion only, we propose a method that simultaneously controls the combustion phasing and minimizes the amount of diesel used. Minimizing the amount of diesel while keeping the center of combustion at a constant value is formulated as an optimization problem with an equality constraint. A combination of feedback control and extremum seeking is used to solve this optimization problem online. The necessity to separate the different time scales is discussed and a structure is proposed that facilitates this separation for this specific example. The proposed method is validated by experiments on a test bench.

The operation of dual fuel engines, operated with natural gas as main fuel, offers the potential of substantial savings in CO2. Nevertheless, the operating map area where low pollutant emissions are produced is very narrow. Especially at low load, the raw exhaust gas contains high concentrations of unburned methane and, with high pilot fuel portions due to ignition limitations, also soot. The analysis of the combustion in those conditions in particular is not trivial, since multiple combustion modes are present concurrently.The present work focuses on the evaluation of the individual combustion modes of a dual fuel engine, operated with natural gas as main and diesel as pilot fuel, using a combustion model. The combustion has been split in two partwise concurrent combustion phases: the auto-ignition phase and the premixed flame propagation phase. In order to calculate the amount of fuel and equivalence ratio distribution of the diesel and substrate at ignition, a simplified spray model is used. The trapped natural gas within the spray volume provides the basis for the distribution of fuel burnt in the auto-ignition phase and in the premixed flame propagation. The sum of the auto-ignition combustion mode and the premixed flame propagation combustion mode represents the total of the heat released, disregarding the cool flame, heat losses and unburned fuel.The description has been calibrated on a 4 cylinder, 2 litre Volkswagen Industrial Diesel Engine, modified to include natural gas port injection. The engine was operated with a wide variation of settings, ranging from low to high load, globally lean and stoichiometric with pilot fuel mass ratio from 2% to 50% and different EGR rates. The heat release rate derived from the pressure curve shows very good agreement with the sum of the individual combustion modes in all the abovementioned engine operating conditions. The ability to distinguish between the individual combustion modes, offered by the developed combustion description herein aids in the understanding of the engine operation limitations. The predictive model shows a strong dependency on the ignition delay which is itself challenging to model. Consequently the accuracy of predicted combustion characteristics are limited to the accuracy of the modelled ignition delay.

In this work, the potential for improving the trade-off between fuel consumption and tailpipe NO_x emissions through variable engine calibration (VEC) is demonstrated for both conventional and hybrid electric vehicles (HEV). First, a preoptimization procedure for the engine operation is proposed to address the challenge posed by the large number of engine control inputs. By excluding infeasible and suboptimal operation offline, an engine model is developed that can be evaluated efficiently during online optimization. Next, dynamic programming is used to find the optimal trade-off between fuel consumption and tailpipe NO_x emissions for various vehicle configurations and driving missions. Simulation results show that for a conventional vehicle equipped with VEC and gear optimization run on the worldwide harmonized light vehicles test cycle (WLTC), the fuel consumption can be reduced by 5.4% at equivalent NO_x emissions. At equivalent fuel consumption, the NO_x emissions can be reduced by 80%. For an HEV, the introduction of VEC, in addition to the optimization of the torque split and the gear selection, drastically extended the achievable trade-off between fuel consumption and tailpipe NO_x emissions in simulations. Most notably, the region with very low NO_x emissions could only be reached with VEC.