NOx emissions in direct injection diesel engines: Part 2: model performance for conventional, prolonged ignition delay, and premixed charge compression ignition operating conditions

Abstract

Investigations from recent years have shown that at operating conditions characterized by long ignition delays and resulting large proportions of premixed combustion, the NOx emission trend does not correspond to the (usually) postulated correlation with an appropriately defined (adiabatic) burnt flame temperature. This correlation, however, is the cornerstone of most published NOx models for direct injection diesel engines. In this light, a new phenomenological NOx model has been developed in Brückner et al. (Part 1), which considers NOx formation from products of premixed and diffusion combustion and accounts for compression heating of post-flame gases, and describes NOx formation by thermal chemistry. In this study (Part 2), the model is applied to predict NOx emissions from two medium-speed direct injection diesel engines of different size and at various operating conditions. Single parameter variations comprising sweeps of injection pressure, start of injection, load, exhaust gas recirculation rate, number of injections, and end-of-compression temperature are studied on a single-cylinder engine. In addition, different engine configurations (valve timing, turbocharger setup) and injection parameters of a marine diesel engine are investigated. For both engines and all parameter variations, the model prediction shows good agreement. Most notably, the model captures the turning point of the NOx emission trend with increasing ignition delay (first decreasing, then increasing NOx) for both engines. The differentiation in the physical treatment of the products of premixed and diffusion with increasing ignition delay showed to be essential for the model to capture the trend-reversal. Specifically, the model predicted that peak NOx formation rates in diffusion zones decrease with increasing ignition delay, whereas for the same change in ignition delay, peak formation rates in premixed zones increase. This is caused by the high energy release in short time, causing a strong compression of existing premixed combustion product zones that mix at a slower rate and have less time to mix, significantly increasing their temperature. In contrast, the model under-predicts NOx emissions for very low oxygen concentrations, in particular below 15 vol.%, which is attributed to the simple thermal NOx kinetic mechanism used. It is concluded that the new model is able to predict NOx emissions for conventional diesel combustion and for long ignition delay operating conditions, where a substantial amount of heat is released in premixed mode.

Effect of Injection Pressure and Air–Fuel Ratio on the Self-Ignition Properties of 1-Butanol–Diesel Fuel Blends: Study Using a Constant-Volume Combustion Chamber

“…59,88,89,103 Engine operation becomes noisy and the crank−piston system load increases, accelerating engine wear and increasing nitrogen oxide emissions. 59,89,104,105 The significant effect of ID on engine operation and the resulting character of the combustion process are also confirmed by the work on the effect of CN on the engine performance parameters. 106−117 The large correlation between ID and the CN and DCN, which are the standard measures of self-ignition, is indicated in various papers, such as refs 112,113 and, 118−120 and relevant standards.…”

Research of Nitrogen Oxides Concentrations in Exhaust Gas of Compression Ignition Engine Fuelled with Alternative Fuel

“…Although there are many methods to determine the concentration of nitrogen oxides, there are no comparative studies of methods for alternative fuels [9,15,17]. This paper uses previous studies to determine which method gives the most similar results to the real ones.…”

“…These are fuels made from animal waste, fatty acids and burned vegetable oils [9][10][11]. In order to reach appropriate demands, it is essential to use the system which optimally uses the potential of the structure to gain the output parameters [12][13][14][15]. One of the substances harmful to health is NOx.…”

Autoignition Comparison of n-Dodecane/Benzene and n-Dodecane/Toluene Blends in a Constant Volume Combustion Chamber

“…Autoignition propensity is an important property for the fuels used for diesel combustion engines because the combustible mixture in diesel engines is prepared through direct fuel injection in the compression stroke and ignited by piston compression. Fuel autoignition propensity is particularly of importance in the advanced diesel combustion strategies, for example, low-temperature combustion, , premixed charge compression ignition, , and stratified charge combustion ignition. , In these advanced combustion concepts that require a more uniform mixture distribution, the fuel and charge mixing quality highly depends on the length of the ignition induction period that is significantly influenced by fuel autoignition propensity. Therefore, researches have been conducted to investigate the diesel autoignition characteristics and chemical kinetics − and the influential factors to control the fuel ignition delay (ID) times in the engine cylinder. − …”