Fuel-Substitution Method for Investigating the Kinetics of Low-Volatility Fuels under Enginelike Operating Conditions

Abstract

The combustion properties of two low-volatility diesel fuels were characterized under engine-relevant conditions using a new fuel-substitution strategy that minimizes biases associated with unknown boundary conditions. The engine is operated in a homogeneous charge compression ignition mode with port fuel injection of the primary reference fuels, n-heptane and isooctane, and an upstream prevaporizer for the diesel-like heavy test fuels. The engine is first operated on the primary reference fuels (PRFs). The intake conditions are then fixed, and the heavy fuel is introduced in increasing amounts while the mass of port-injected fuel is reduced to maintain engine load and the n-heptane-to-isooctane ratio is adjusted to maintain constant combustion phasing. It is shown that this provides nearly constant in-cylinder thermodynamic and boundary conditions. The baseline condition, which uses well-characterized fuels, can be used to adjust the trapped-gas initial conditions and the heat transfer rates of a computational model, but these parameters do not change with the subsequent addition of heavy test fuel. The test fuel combustion characteristics are described in terms of the PRF number of the port-injected mixture. A simple linear blending method was found to adequately represent the heavy test fuel in terms of an effective PRF number. Testing under different operating conditions showed no significant change in the effective PRF number of the test fuels. The fuels investigated in this study were F-76, a high-sulfur-content diesel fuel, and HRD, a hydroprocessed renewable diesel fuel composed primarily of alkanes. Effective PRF numbers of 35 and −25 were found for F-76 and HRD, respectively.

A Model Investigation of Fuel and Operating Regime Impact on Homogeneous Charge Compression Ignition Engine Performance

“…Although these solutions enable low NOx emissions by reducing the in-cylinder temperature, excessive charge dilution leads to very low power output and unburned hydrocarbon emissions. Despite these drawbacks, the actual interest in HCCI engines is well demonstrated by the extensive and recent literature on this subject, which analyzes several different features, such as fuel effect, the thermal stratification inside the engine, optimal operating strategies with both traditional , and biofuels, − as well as the combustion control . Chemical kinetics plays a fundamental role in controlling the combustion in HCCI engines, since local conditions of the charge govern autoignition timing .…”

Screening Method for Fuels in Homogeneous Charge Compression Ignition Engines: Application to Valeric Biofuels

“…4 Characterizing the conditions in the cylinder and the coupling effect appearing when changing inlet conditions is also very important. 5 The HCCI engine has already been used to characterize various fuels. 6−9 Kalghatgi and Head have presented the octane index (OI) methodology, which predicts fairly well the combustion behavior according to the engine setup combined with the research octane number (RON) and the motor octane number (MON) of the fuel.…”

“…Given the right engine settings, HCCI can therefore be operated with any fuel. − Predicting or determining the right engine settings and the effects of fuel chemistry and properties such as octane number on ignition delay and burning rate are the key to find the envelope of HCCI operations for specific fuels . Characterizing the conditions in the cylinder and the coupling effect appearing when changing inlet conditions is also very important …”