Homogeneous charge compression ignition

Homogeneous Charge Compression Ignition (HCCI) is a form of internal combustion in which well-mixed fuel and oxidizer (typically air) are compressed to the point of auto-ignition. As in other forms of combustion, this exothermic reaction produces heat that can be transformed into work in a heat engine. HCCI combines characteristics of conventional gasoline engine and diesel engines. Gasoline engines combine homogeneous charge (HC) with spark ignition (SI), abbreviated as HCSI. Modern direct injection diesel engines combine stratified charge (SC) with compression ignition (CI), abbreviated as SCCI. As in HCSI, HCCI injects fuel during the intake stroke. However, rather than using an electric discharge (spark) to ignite a portion of the mixture, HCCI raises density and temperature by compression until the entire mixture reacts spontaneously. Stratified charge compression ignition also relies on temperature and density increase resulting from compression. However, it injects fuel later, during the compression stroke. Combustion occurs at the boundary of the fuel and air, producing higher emissions, but allowing a leaner and higher compression burn, producing greater efficiency. Controlling HCCI requires microprocessor control and physical understanding of the ignition process. HCCI designs achieve gasoline engine-like emissions with diesel engine-like efficiency. HCCI engines achieve extremely low levels of oxides of nitrogen emissions (NOx) without a catalytic converter. Hydrocarbons (unburnt fuels and oils) and carbon monoxide emissions still require treatment to meet automobile emissions control regulations. Recent research has shown that the hybrid fuels combining different reactivities (such as gasoline and diesel) can help in controlling HCCI ignition and burn rates. RCCI, or reactivity controlled compression ignition, has been demonstrated to provide highly efficient, low emissions operation over wide load and speed ranges. HCCI engines have a long history, even though HCCI has not been as widely implemented as spark ignition or diesel injection.

Kintetic modelling of natural gas auto-ignition under engine-like conditions

Daniel Favrat, Jan-Bernhard Vos, Stefan Heyne

The modelling of natural gas combustion with detailed as well as reduced kinetic mechanisms has been subject to many publications already. Most of the mechanisms are validated at higher temperatures and ambient pressures. Only recently - in parallel to a gain of interest in the homogeneous charge compression ignition (HCCI) engine concept - there are more efforts trying to reproduce the low temperature chemistry, and in particular autoignition of natural gas under engine-like conditions. This work is part of a project on stationary natural gas engines equipped with unscavanged prechambers. It has been demonstrated that it is possible to reduce the emissions below the Swiss limits both with natural gas and biogas with the spark ignition prechamber, while keeping high efficiencies of around 38 %. We are now aiming at converting the prechamber from spark ignition to autoignition leading to longer maintenance intervals in the case of stationary natural gas engines and to a probable cleaner combustion - HCCI like combustion inside the prechamber and faster combustion in the main chamber due to an earlier arrival of the flame front - compared to prechamber spark ignition. In order to be able to model the fluid dynamics and the chemistry inside the cylinder, a good chemical model is one of the most important aspects. To obtain an appropriate mechanism - by validating and/or optimising existing mechanisms - experiments have been conducted in the rapid compression machine facility at the University of Lille with different mixtures of methane, ethane and propane. The relative air-to-fuel ratio λ varied from 1 to 1.6, the temperature and pressure at the end of compression varied from 850 to 925 K and 13 to 21 bar respectively. The compression ratio was constantly kept at 9.2 for all experiments and the final temperature and pressure were defined by varying the charge and the composition of the inert gases (Ar/N2). The compression time was between 50 and 60 ms for all experiments. For modelling the ignition delay a zero-dimensional model assuming homogeneity was used, based on the measured pressure and calculated core temperature at the end of compression of each experiment. The chemical solver package used for the simulations is CHEMKIN. Different mechanisms (PRF, GRI3.0, Konnov, …) have been tested. Most of the mechanisms are mainly validated at higher temperature ranges and therefore their applicability to ignition delay prediction in our conditions is questionable. Several attempts to adjust the GRI mechanism (version 1.2) to the lower temperature regime exist, whereof we chose the UBCMech and Petersen mechanism (GRI+RAMEC) to test on our simulations. Both mechanisms initially were developed based on pure methane mixtures while the UBCMech recently was extended to model natural gas as well. The experimentally obtained ignition delays vary from 40 ms at high pressures and temperatures to more than 300 ms for the lower regime of the experimental space. For the ignition delays up to 60 ms the best fit was obtained by a reduced version of PRF accounting only for the C1 to C3 compounds. Konnov’s mechanism and UBCMech are under predicting the experimental ignition delays while the GRI3.0 mechanism (with and without RAMEC sub module) over predicts the ignition delay for the natural gas mixtures. It has to be mentioned that the GRI3.0+RAMEC mechanism as well as the initial mechanism of Petersen (GRI1.2+RAMEC) on the other hand under predict the experimental ignition delay for pure methane mixtures. This indicates that the low temperature chemistry for methane that has been added in the RAMEC sub module is enhancing ignition too strong. Replacing part of methane in pure methane mixtures by equivalent amounts of ethane and propane - know as ignition promoters - lead to longer ignition delays in the simulations. But as the GRI3.0+RAMEC mechanism (without nitrogen chemistry) is the smallest of all tested mechanisms considering number of species and reactions, it still is an interesting option for coupling it to a computational fluid dynamics code as we are planning to do. We though performed a sensitivity analysis to obtain a set of the most dominating reactions and used a multi-objective optimisation algorithm to optimize the reaction rates within their limits of uncertainty. The results show a good fit for the short ignition delays of natural gas mixtures but still an under prediction of the methane delays. For the longer ignition delays the assumption of a constant core temperature during the post compression phase becomes questionable and the under prediction of all mechanisms of the experimental delays is an obvious consequence. A heat loss model predicting the temperature evolution has to be implemented in the simulations to properly simulate the ignition delays.

2006

Prechamber autoignition in natural gas engines

Daniel Favrat, Jan-Bernhard Vos, Stefan Heyne

Unscavanged prechamber ignition has been developed at the Laboratory of Industrial Energy Systems to reduce the emissions from cogeneration spark ignition gas engines, operating with both natural gas and biogas. Demonstration has been made that a lean burn engine unit equipped with optimized prechambers allows efficient operation, while remaining below the Swiss limits [1,2]. Further development aims at using prechambers operating in autoignition mode similar to HCCI (Homogeneous charge compression ignition). The advantages are on the one hand that there is no need for maintenance and replacement of the spark plugs which usually have a limited life time while on the other hand we are expecting to benefit from the HCCI characteristics, e.g. low NOx emission levels. The prechamber configuration though should be a system easier to control than an engine operating completely in HCCI mode. The investigation of the applicability of this technique for engines used in the transport sector with rapidly changing operating conditions might be a further step in the development. In order to better understand the phenomena, simulations are done taking into account both fluid dynamics and chemical kinetics. Ignition delay measurements with mixtures of methane, ethane and propane have been conducted at the Rapid compression machine facility at the University of Lille at varying stoichiometric ratios. These measurements are used as a basis to choose an appropriate chemical mechanism for combustion simulation. The kinetic solver CHEMKIN has been used for zero-dimensional simulations at TDC conditions of the rapid compression machine to reproduce the ignition delays. Several detailed combustion mechanisms have been applied for these simulations, e.g. GRI 3.0 + RAMEC, n-Heptane and PRF mechanisms (LLNL) and Konnov’s mechanism. The aim is to obtain a reduced mechanism that will be used in combination with computational fluid dynamics. In order to do this, sensitivity and reaction pathway analysis is performed, as well as multi- objective optimization of the kinetic parameters. In cooperation with CFS engineering a coupling between the CFD code NSMB (Navier Stokes Multi Block) and the chemical solver CHEMKIN has been established enabling a simulation of the experiments conducted with the rapid compression machine. Further steps will be the design of a prechamber to be used with an experimental mono-cylinder engine and the simulation of the in-cylinder flow with prechamber to determine the influencing factors for controlling the autoignition event. Behaviour at part load as well as in transient conditions will also be investigated.

2005

Kintetic modelling of natural gas auto-ignition under engine-like conditions

Daniel Favrat, Jan-Bernhard Vos, Stefan Heyne

2006

Prechamber autoignition in natural gas engines

Daniel Favrat, Jan-Bernhard Vos, Stefan Heyne

2005