Optical investigation of the combustion behaviour inside the engine operating in HCCI mode and using alternative diesel fuel

doi:10.1016/j.expthermflusci.2009.10.010

Experimental Thermal and Fluid Science

Volume 34, Issue 3, April 2010, Pages 346-351

https://doi.org/10.1016/j.expthermflusci.2009.10.010 Get rights and content

Abstract

In order to understand the effect of both the new homogeneous charge compression ignition (HCCI) combustion process and the use of biofuel, optical measurements were carried out into a transparent CR diesel engine. Rape seed methyl ester was used and tests with several injection pressures were performed. OH and HCO radical were detected and their evolutions were analyzed during the whole combustion. Moreover, soot concentration was measured by means the two colour pyrometry method. The reduction of particulate emission with biodiesel as compared to the diesel fuel was noted. Moreover, this effect resulted higher increasing the injection pressure. In the case of RME the oxidation of soot depends mainly from O₂ content of fuel and OH is responsible of the NO formation in the chamber as it was observed for NO_x exhaust emission. Moreover, it was investigated the evolution of HCO and CO into the cylinder. HCO was detected at the start of combustion. During the combustion, HCO oxidizes due to the increasing temperature and it produces CO. Both fuels have similar trend, the highest concentrations are detected for low injection pressure. This effect is more evident for the RME fuel.

Introduction

The growing transport sector is considered to be one of the main reasons for failing to meet the Kyoto targets. In combination with the emission limits and new emission standards, the already very low CO₂ emission levels have to be further reduced both for spark ignition and diesel engines. Particularly, in Europe the transport sector accounts for more than 30% of the total energy consumption in the Community. It is 98% dependent on fossil fuels with the crude oil feedstock being largely imported and thus extremely vulnerable to any market disturbance. Biofuel use has to increase from its present low usage – less than 2% of overall fuel – to a substantial fraction of the transportation fuel consumption in Europe (target of 25% in 2030). However, biofuels will mostly be used in compressed ignition and spark engines and/or if it is possible specialized engines will be used in certain applications or in dedicated fleets. The most commonly known biofuels are ethanol and biodiesel.

Biodiesel is a renewable fuel that can be produced from a variety of vegetable oils including rapeseed oil, soybean oil, sunflower oil and palm oil. Waste fryer oil, which is considered toxic waste by the Environmental Protection Agency (EPA), can also be transesterified into biodiesel. Rapeseed Methyl Ester (RME) is the more widely used in Europe and Soybean Methyl Ester (SME) or imported palm oil in the USA they are collectively known as Fatty Acid Methyl Esters (FAME). The benefits of the biodiesel–FAME are well known: higher cetane number, low sulphur content, reduction of HC and CO emission, reduction of PM emission. Even if the materials compatibility, the impact on low-temperature operability, the increase in NO_x emission, the reduction of power and the fuel economy, and the higher cost of production must be evaluated [1], [2], [3].

In order to overcome these limitations and contribute further to the emission reduction, the use of homogeneous charge compression ignition (HCCI) mode must be considered in direct injection diesel engine using biodiesel fuel because some future production engines will adopt this combustion mode. HCCI mode reduces PM and NO_x emissions without penalize the performances. This occurs because the combustion develops with low temperature and burns a premixed air/fuel mixture. On the other hand, CO and HC emissions increase because the low temperature doesn’t favour their complete reduction in the cylinder. Another limit is the autoignition that remains a process not usually perfect causing knock phenomena [4], [5], [6], [7]. The combustion process and the pollutants formation depend on the compression ratio, the quality of fuel, the intake air temperature, the use of external and internal exhaust gas recirculation (EGR), and the oxygen concentration.

Many papers deal with the chemical and physical phenomena involved in the cylinder applying HCCI combustion mode [4], [5], [6], [7], nevertheless few papers report the effect of the RME fuel on HCCI mode for light duty diesel engine [8], [9]. The goal of this paper is the evaluation of the HCCI combustion in an optically accessible diesel engine. The homogeneous mixture was realized with four small early injections using diesel fuel as reference and RME fuel. The effect of fuel and injection strategy was analyzed investigating the combustion process in terms of the chemical and physical phenomena involved. Typical species and precursors of pollutants were detected by spectroscopic techniques. The OH, CH, HCO^*, and soot concentration evolutions were evaluated with high spatial and temporal resolution using chemiluminescence measurements. The effect of injection pressure on pollutants emission in the exhaust and their correlation with in-cylinder species were evaluated.

Section snippets

Optical engine

The optically accessible engine used during experiments was a single cylinder, direct injection, four-stroke, diesel engine, with EURO IV multi valves production head. Table 1 reports specifications of the engine.

The production head was designed for the four cylinders engine thus it was necessary to modify it for the single cylinder research engine with particular attention to the water-cooling and lubricating oil head ducts. The design of the engine utilizes a classic extended piston with

Results and discussion

In Fig. 2, the cylinder pressure and the rate of heat release curves varying the injection pressures are reported for REF fuel and RME, respectively. In both figures, the drive injector current is referred to the strategy with 400 bar injection pressure in order to avoid the overlap of the signals and to have a confused figure. The current signals have the same start of injection and shorter durations at increasing of injection pressure, as reported in Table 3. It can be noted that the rates of

Conclusion

HCCI combustion strategy at 1500 rpm engine speed varying the injection pressures was investigated in an optically accessible diesel engine by means of UV–visible imaging spectroscopy. Multi-pulse early injections were employed to modulate the homogeneity history of the cylinder charge. RME and diesel commercial fuel were tested.

The positive effect in the use of RME fuel using HCCI strategy was observed for the further reduction of pollutant emission. In particular, the use of RME fuel decreases

Acknowledgments

The authors wish to thank Mr. Carlo Rossi and Bruno Sgammato for maintaining the experimental apparatus and for their precious help.

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