Alternative fuels for spark-ignition engines: mixing rules for the laminar burning velocity of gasoline-alcohol blends

Experimental measurements of the laminar burning velocity are mostly limited in pressure and temperature and can be compromised by the effects of flame stretch and instabilities. Computationally, these effects can be avoided by calculating one-dimensional, planar adiabatic flames using chemical oxidation mechanisms. Chemical kinetic models are often large, complex and take a lot of computation time, and few models exist for multi-component fuels. The aim of the present study is to investigate if simple mixing rules are able to predict the laminar burning velocity of fuel blends with a good accuracy. An overview of different mixing rules to predict the laminar burning is given and these mixing rules are tested for blends of hydrocarbons and ethanol. Experimental data of ethanol/n-heptane and ethanol/n-heptane/iso-octane mixtures and modeling data of an ethanol/n-heptane blend and blends of ethanol and a toluene reference fuel are used to test the different mixing rules. Effects of higher temperature and pressure on the performance of the mixing rules are investigated. It was found that simple mixing rules that consider only the change in composition are accurate enough to predict the laminar burning velocity of ethanol/hydrocarbon blends. For the blends used in this study, a Le Chatelier's rule based on energy fractions is preferable because of the similar accuracy in comparison to other mixing rules while being more simple to use

A fuel independent heat transfer correlation for premixed spark ignition engines

Simulation models for internal combustion engines are an indispensable development tool, since engines have become increasingly complex, with many control variables and conflicting optimization targets. The heat transfer model, which computes the heat losses to the cylinder walls inside the engine is one of the most important sub-models, because it has an influence on the main three optimization targets (efficiency, emissions and power output). Although it is an important sub-model, little progress was reported in recent years due to the lack of accurate heat loss measurements inside an engine. Consequently, heat transfer models have not followed the evolution in engine technology and fuels and there is a need to improve them. This doctoral research investigated the heat losses in spark-ignition engines and focused on the effect of different alternative fuels. First, an accurate heat flux sensor was implemented in a test engine in the lab to measure the heat losses to the walls. This was not a straightforward step, since it is not a mature technology due to the required response and robustness. Second, the sensor was used to build a database of heat flux measurements in the engine which focuses on the effect of the fuel. Special care was devoted to the experimental design, resulting in a unique database which shows that the fuel can significantly affect the amount of heat losses. The obtained database allowed to better understand the heat transfer mechanism inside an internal combustion engine. Finally, these insights resulted in a new heat transfer model which is shown to be a significant improvement compared to the existing ones

On the applicability of empirical heat transfer models for hydrogen combustion engines

Hydrogen-fuelled internal combustion engines are being investigated as an alternative for current drive trains because they have a high efficiency, near-zero noxious and zero tailpipe greenhouse gas emissions. A thermodynamic model of the engine cycle would enable a cheap and fast optimization of engine settings for operation on hydrogen, facilitating the development of these engines. The accuracy of the heat transfer submodel within the thermodynamic model is important to simulate accurately the emissions of oxides of nitrogen which are influenced by the maximum gas temperature. These emissions can occur in hydrogen internal combustion engines at high loads and they are an important constraint for power and efficiency optimization. The most common heat transfer models in engine research are those from Annand and Woschni. These models are developed for fossil fuels, which have different combustion properties. Therefore, they need to be evaluated for hydrogen. We have measured the heat flux and the wall temperature in an engine that can run on hydrogen and methane. This paper describes an evaluation of the models of Annand and Woschni, using those heat flux measurements and assesses if the models capture the effect of changing combustion and fuel properties. The models fail on all the tests, so they need to be improved to accurately model the heat transfer generated by hydrogen combustion

Heat transfer in premixed spark ignition engines part I : identification of the factors influencing heat transfer

The heat transfer from the combustion gases to the cylinder walls inside a spark ignition engine is a key factor in an engine's design, due to its influence on the engine's effciency, power and emissions. Therefore a lot of research has been conducted in order to accurately model the heat transfer, for engine design and optimization purposes. These models have been found to provide inaccurate predictions for fuels which have significantly different gas properties compared to traditional fossil fuels. This indicates that the models either do not properly include gas properties, or are missing some important properties in their formulation. In order to construct a general (fuel-independent) heat transfer model, new measurements need to be executed, with multiple fuels that have different properties. Designing such an experiment requires a thorough understanding of the factors influencing the heat transfer and their interactions. In this paper a literature review is presented of heat transfer measurements in spark ignition engines in order to investigate the effect of the engine factors on the heat transfer. Based on this review, a root cause analysis is conducted to identify the independent factors that affect heat transfer. These factors are then used to set up two experiments according to a Design of Experiments methodology that allows the investigation of the effect of different gas properties and engine settings on the heat transfer in a consistent way. The results of these measurements for motored operation are discussed in [1] and for fired operation in a companion paper [2]

Investigation of the influence of engine settings on the heat flux in a hydrogen- and methane-fueled spark ignition engine

Hydrogen-fueled internal combustion engines are a possible solution to make transportation more ecological. Only emissions of oxides of nitrogen (NOx) occur at high loads, being a constraint for power and efficiency optimization. A thermodynamic model of the engine cycle enables a cheap and fast optimization of engine settings. It needs to accurately predict the heat transfer in the engine because the NOx emissions are influenced by the maximum gas temperature. However, the existing engine heat transfer models in the literature are developed for fossil fuels and they have been cited to be inaccurate for hydrogen. We have measured the heat transfer inside a spark ignited engine with a thermopile to investigate the heat transfer process of hydrogen and to find the differences with a fossil fuel. This paper describes the effects of the compression ratio, ignition timing and mixture richness on the heat transfer process, comparing hydrogen with methane. A convection coefficient is used to separate the effect of the temperature difference between the gas and the wall from the influence of the gas movement and combustion. The paper shows that the convection coefficient gives more insight in the heat transfer process in a combustion engine despite the assumptions involved in its definition. The comparison between hydrogen and methane demonstrates, in contrast to what is believed, that the heat loss of hydrogen can be lower