A. Megaritis

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.

Research on expansion of operating windows of controlled homogeneous auto-ignition engines

Abstract A major effort has been made to surmount the current obstacles to expanding the operating window of homogeneous charge compression ignition (HCCI) engines. The research involves extensive experimental studies on single-cylinder and multi-cylinder engines and the work is devoted to the development of on-board fuel-reforming technology and to the application of supercharging combined with trapping of residual gases in the cylinder. Fuel reforming yields significant quantities of hydrogen and is used to extend the lower load boundary while supercharging and internal exhaust gas recirculation (EGR) trapping are used to increase the upper load limit of HCCI engines. The present paper highlights the main findings of the research to date; in particular it reveals that using a combination of technical elements for effective control of auto-ignition in a typical passenger car engine configuration is possible and promising.

AN EXPERIMENTAL STUDY OF BIOETHANOL HCCI

ABSTRACT Bioethanol has been successfully used in conventional spark ignition (SI) internal combustion engines. Homogeneous charge compression ignition (HCCI) combustion, a novel combustion method, has shown the potential for low nitric oxides (NOx) emissions with no particulate matter formation. This paper explores two different approaches to achieve HCCI with bioethanol; namely, trapping of internal residual gas and intake temperature heating with a high compression ratio. For naturally aspirated HCCI operation with residual gas trapping on a spark ignition engine, although the NOx emissions were low, the load range was unacceptably small. When inlet manifold pressurisation was employed, a substantial increase in the upper load boundary could be achieved without any substantial increase in NOx emissions. With forced induction, the feasibility of using boost control as the main method of load control for higher engine loads during HCCI operation has been explored with possible methods of utilizing boost control. One possible strategy is a map based strategy where fuelling rates are correlated versus boost pressure and trapped residual amounts. A proof of concept using this strategy showed that transient operation from a low load to a much higher load, using boost control might be possible without engine misfire.

An Investigation into the Operating Mode Transitions of a Homogeneous Charge Compression Ignition Engine Using EGR Trapping

While Homogeneous Charge Compression Ignition (HCCI) is a promising combustion mode with significant advantages in fuel economy improvement and emission reductions for vehicle engines, it is subject to a number of limitations, for example, hardware and control complexity, or NOx and NVH deterioration near its operating upper load boundary, diminishing its advantages. Conventional spark-ignition combustion mode is required for higher loads and speeds, thus the operating conditions near the HCCI boundaries and their corresponding alternatives in Sl mode must be studied carefully in order to identify practical strategies to minimise the impact of the combustion mode transition on the performance of the engine. This paper presents the results of an investigation of the combustion mode transitions between Sl and HCCI, using a combination of an engine cycle simulation code with a chemical kinetics based HCCI combustion code. It investigates and discusses the key issues concerning the combustion mode transition and its control. A new approach has been adopted in the modelling of the HCCI engine gas exchange and combustion, which allowed a parametric study of the transient process in the transition on a cycle-by-cycle basis and provided guidelines in the design of the engine hardware and its control strategies.

Fuel reforming for diesel engines

Abstract: This chapter reviews the application of fuel reforming in diesel engines for on-board generation of hydrogen-rich gas (reformate). The chapter first provides background information about engine fuel reforming applications. It then presents theoretical aspects of hydrogen production by fuel reforming including reforming thermodynamics and modelling, followed by discussion about diesel fuel reforming process parameters and reforming catalyst screening and evaluation. Finally, issues related to different applications of diesel fuel reforming and the corresponding requirements in terms of reforming catalysts and reactor designs are discussed. This discussion includes reforming applications aiming to produce reformate for utilisation as diesel combustion and emissions improver, and as diesel exhaust aftertreatment improver.

Experimental investigation of the effect of combined hydrogen and diesel combustion on the particulate size distribution from a high speed direct injection diesel engine

The effects of hydrogen addition and exhaust gas recirculation (EGR) levels on the exhaust particulate matter size distribution in a diesel engine have been investigated. The experiments were performed on a 2.0 litre, 4-cylinder, direct injection engine equipped with a modern high-pressure common rail. A nano-Micro-Orifice Uniform Deposit Impactor (nano-MOUDI) was used in this work to study the particulate matter size distribution. All tests were conducted at the set operating point of 1,500 rpm. The experimental work showed that the particulate matter size distribution was not dramatically altered by the addition of EGR, but the main peak was shifted towards the nucleation mode with the addition of hydrogen. The addition of hydrogen increases the emissions of nitrogen oxides (NOx), but reduces the emissions of unburnt hydrocarbons (THC). Conversely, the addition of EGR reduces NOx, but can increase THC. Hydrogen addition increases the peak cylinder pressure and the maximum rate of pressure rise.