Application of exhaust gas fuel reforming in diesel and homogeneous charge compression ignition (HCCI) engines fuelled with biofuels

This is the post-print version of the final paper published in Energy. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2007 Elsevier B.V.This paper documents the application of exhaust gas fuel reforming of two alternative fuels, biodiesel and bioethanol, in internal combustion engines. The exhaust gas fuel reforming process is a method of on-board production of hydrogen-rich gas by catalytic reaction of fuel and engine exhaust gas. The benefits of exhaust gas fuel reforming have been demonstrated by adding simulated reformed gas to a diesel engine fuelled by a mixture of 50% ultra low sulphur diesel (ULSD) and 50% rapeseed methyl ester (RME) as well as to a homogeneous charge compression ignition (HCCI) engine fuelled by bioethanol. In the case of the biodiesel fuelled engine, a reduction of NOx emissions was achieved without considerable smoke increase. In the case of the bioethanol fuelled HCCI engine, the engine tolerance to exhaust gas recirculation (EGR) was extended and hence the typically high pressure rise rates of HCCI engines, associated with intense combustion noise, were reduced

Effect of inlet valve timing and water blending on bioethanol HCCI combustion using forced induction and residual gas trapping

This is the post-print version of the final paper published in Fuel. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2007 Elsevier B.V.It has been shown previously that applying forced induction to homogeneous charge compression ignition (HCCI) combustion of bioethanol with residual gas trapping, results in a greatly extended engine load range compared to normal aspiration operation. However, at very high boost pressures, very high cylinder pressure rise rates develop. The approach documented here explores two ways that might have an effect on combustion in order to lower the maximum pressure rise rates and further improve the emissions of oxides of nitrogen (NOx); inlet valve timing and water blending. It was found that there is an optimal inlet valve timing. When the timing was significantly advanced or retarded away from the optimal, the combustion phasing could be retarded for a given lambda (excess air ratio). However, this would result in higher loads and lower lambdas for a given boost pressure, with possibly higher NOx emissions. Increasing the water content in ethanol gave similar results as the non-optimal inlet valve timing

Optical study of flow and combustion in an HCCI engine with negative valve overlap

One of the most widely used methods to enable Homogeneous Charge Compression Ignition (HCCI) combustion is using negative valve overlapping to trap a sufficient quantity of hot residual gas. The characteristics of air motion with specially designed valve events having reduced valve lift and durations associated with HCCI engines and their effect on subsequent combustion are not yet fully understood. In addition, the ignition process and combustion development in such engines are very different from those in conventional spark-ignition or diesel compression ignition engines. Very little data has been reported concerning optical diagnostics of the flow and combustion in the engine using negative valve overlapping. This paper presents an experimental investigation into the in-cylinder flow characteristics and combustion development in an optical engine operating in HCCI combustion mode. PIV measurements have been taken under motored engine conditions to provide a quantitative flow characterisation of negative valve overlap in-cylinder flows. The ignition and combustion process was imaged using a high resolution charge coupled device (CCD) camera and the combustion imaging data was supplemented by simultaneously recorded in-cylinder pressure data which assisted the analysis of the images. It is found that the flow characteristics with negative valve overlapping are less stable and more valve event driven than typical spark ignition in-cylinder flows, while the combustion initiation locations are not uniformly distributed. © 2006 IOP Publishing Ltd

Physics of puffing and microexplosion of emulsion fuel droplets

The physics of water-in-oil emulsion droplet microexplosion/puffing has been investigated using high-fidelity interface-capturing simulation. Varying the dispersed-phase (water) sub-droplet size/location and the initiation location of explosive boiling (bubble formation), the droplet breakup processes have been well revealed. The bubble growth leads to local and partial breakup of the parent oil droplet, i.e., puffing. The water sub-droplet size and location determine the after-puffing dynamics. The boiling surface of the water sub-droplet is unstable and evolves further. Finally, the sub-droplet is wrapped by boiled water vapor and detaches itself from the parent oil droplet. When the water sub-droplet is small, the detachment is quick, and the oil droplet breakup is limited. When it is large and initially located toward the parent droplet center, the droplet breakup is more extensive. For microexplosion triggered by the simultaneous growth of multiple separate bubbles, each explosion is local and independent initially, but their mutual interactions occur at a later stage. The degree of breakup can be larger due to interactions among multiple explosions. These findings suggest that controlling microexplosion/puffing is possible in a fuel spray, if the emulsion-fuel blend and the ambient flow conditions such as heating are properly designed. The current study also gives us an insight into modeling the puffing and microexplosion of emulsion droplets and sprays.This article has been made available through the Brunel Open Access Publishing Fund

Stock market volatility and jumps in times of uncertainty

In this paper we examine the predictive power of latent macroeconomic uncertainty on US stock market volatility and jump tail risk. We find that increasing macroeconomic uncertainty predicts a subsequent rise in volatility and price jumps in the US equity market. Our analysis shows that the latent macroeconomic uncertainty measure of Jurado et al. (2015) has the most significant and long-lasting impact on US stock market volatility and jumps in the equity market when compared to the respective impact of the VIX and other popular observable uncertainty proxies. Our study is the first to show that the latent macroeconomic uncertainty factor outperforms the VIX when forecasting volatility and jumps after the 2007 US Great Recession. We additionally find that latent macroeconomic uncertainty is a common forecasting factor of volatility and jumps of the intraday returns of S&P 500 constituents and has higher predictive power on the volatility and jumps of the equities which belong to the financial sector. Overall, our empirical analysis shows that stock market volatility is significantly affected by the rising degree of unpredictability in the macroeconomy, while it is relatively immune to shocks in observable uncertainty proxies

Financial Market Risk and the Macroeconomy

The first chapter of this thesis is about the predictive power of latent macroeconomic uncertainty on U.S. stock market volatility and price jumps. I find that increasing macroeconomic uncertainty predicts a subsequent rise in volatility and price jumps in the U.S. stock market. Specifically, the latent macroeconomic uncertainty measures of Jurado et al. (2015) have the most significant impact on U.S. stock market volatility and jumps in the equity market when compared to the respective impact of popular observable uncertainty proxies. In the second chapter, I empirically verify the predictive information content of U.S. Treasury yield curve volatility on stock market volatility. Given the fact that, the yield curve reflects expectations about future interest rates and economic activity, then the rising volatility in the shape of the yield curve will probably result to rising stock market volatility. I document that the volatility of the slope of the yield curve is a statistically significant predictor of stock market volatility in both in-sample and out-of-sample settings. Lastly, I examine the dynamic impact of monetary policy on option-implied expectations. I use option contracts of S&P 500 index in order to estimate the higher order moments of the risk neutral option-implied distribution of the U.S. stock market. I find that an expansionary monetary policy shock revises upwardly the investors’ expectations about the future path of the U.S. equities prices, that is in line with the empirical results of a vast relevant literature (Rigobon and Sack, 2003; Kurov et al., 2019; amongst others). Furthermore, I find that option-implied expectations with long-term horizon are more responsive to lax monetary policy compared to those with short-term horizon. My findings are in line and provide further insights to a recently growing literature on the field (Hanson and Stein, 2015; Kontonikas and Zekaite, 2018; Nakamura and Steinsson, 2018)

τ-metrizable spaces

In [1], A. A. Borubaev introduced the concept of τ-metric space, where τ is an arbitrary cardinal number. The class of τ-metric spaces as τ runs through the cardinal numbers contains all ordinary metric spaces (for τ = 1) and thus these spaces are a generalization of metric spaces. In this paper the notion of τ-metrizable space is considered

Modeling temperature distribution inside an emulsion fuel droplet under convective heating: A key to predicting microexplosion and puffing

© 2016 by Begell House, Inc. Microexplosion/puffing is rapid disintegration of a water-in-oil emulsion droplet caused by explosive boiling of embedded superheated water sub-droplets. To predict microexplosion/puffing, modeling the temperature distribution inside an emulsion droplet under convective heating is a prerequisite, since the temperature field determines the location of nucleation (vapor bubble initiation from superheated water). In the first part of the present study, convective heating of water-in-oil emulsion droplets under typical combustor conditions is investigated using high-fidelity simulation in order to accurately model inner-droplet temperature distribution. The shear force due to the ambient air flow induces internal circulation inside a droplet. It has been found that for droplets under investigation in the present study, the liquid Peclet number PeL is in a transitional regime of 100 < PeL < 500. The temperature field is therefore somewhat distorted by the velocity field, but the distortion is not strong enough to form Hill's vortex for the temperature field. In the second part of the present study, a novel approach is proposed to model the temperature field distortion by introducing angular dependency of the thermal conductivity and eccentricity of the temperature field. The model can reproduce the main features of the temperature field inside an emulsion droplet, and can be used to predict the nucleation location, which is a key initial condition of microexplosion/puffing