Enabling HCCI combustion of n-heptane through thermo-chemical recuperation

Abstract

Enabling HCCI Combustion of n-Heptane through Thermo-Chemical Recuperation Francisco Posada Sanchez Concerns over air quality, environmental regulatory requirements and the need for reducing fuel consumption on conventional internal combustion engines (ICE) have motivated the development of alternative combustion processes for ICE. One alternative, homogeneous charge compression ignition (HCCI), has shown benefits of high efficiency with low NO x emissions, but suffers load range limitations and control issues. An increase in equivalence ratio at constant speed changes the combustion timing relative to top dead center (TDC), and also increases the pressure rise rate. The opposite occurs if the equivalence ratio is reduced, delaying the combustion timing with respect to TDC and reducing the peak pressure. Excessive heat release associated with richer mixtures drives the engine to ringing conditions, which sets the upper limit for HCCI operation. This thesis aims to investigate numerically the HCCI process under dual fuel operation. The secondary fuel stream bears different autoignition characteristics to regulate combustion timing and heat release at specific operational conditions. The secondary fuel stream is produced onboard as a reformed product of the primary fuel. The reforming process, which requires additional steam, takes advantage of the exhaust gases to convey the fuel reforming reactions, process that is called thermo-chemical recuperation (TCR). This thesis contributes to understanding the effects of different system conditions on the integrated HCCI-TCR system operational range and emissions of nitrogen oxides (NO X), and carbon monoxide (CO). Using n-heptane as the main fuel for HCCI combustion and fuel reforming, two different HCCI models are developed and validated. The single-zone model allows for studying the effect of the secondary fuel on combustion timing. A more complex model, the multi-zone model, allows for studying the effect of secondary fuel on overall engine performance and emissions. Both models operate under engine steady state assumptions (constant speed-load conditions). The combustion models were linked through a cycle simulation code to predict gas exchange processes, in particular the exhaust gas process which defines reforming conditions in the TCR. A model to predict the secondary fuel stream composition on the steam-fuel reformer, under steady state conditions, is developed and validated. Using n-heptane and steam as reactants, the model is able to predict the reformed gas (RG) concentration at the reformer outlet as a function of reforming temperature and relative initial molar fractions of n-heptane and water. The model results are compared against experimental work on steam reforming of n-heptane. RG composition obtained is used to substitute n-heptane at the intake, which alters the HCCI combustion history and performance. Changes in combustion history concurrently affect the exhaust conditions, therefore affecting fuel reforming conditions. This interrelation between the...