P.G. Aleiferis

Characteristics of Ethanol, Butanol, Iso-Octane and Gasoline Sprays and Combustion from a Multi-Hole Injector in a DISI Engine

Recent pressures on vehicle manufacturers to reduce their average fleet levels of CO2 emissions have resulted in an increased drive to improve fuel economy and enable use of fuels developed from renewable sources that can achieve a net reduction in the CO2 output of each vehicle. The most popular choice for spark-ignition engines has been the blending of ethanol with gasoline, where the ethanol is derived either from agricultural or cellulosic sources such as sugar cane, corn or decomposed plant matter. However, other fuels, such as butanol, have also arisen as potential candidates due to their similarities to gasoline, e.g. higher energy density than ethanol. To extract the maximum benefits from these new fuels through optimized engine design and calibration, an understanding of the behaviour of these fuels in modern engines is necessary. In particular, the use of direct injection spark-ignition technology requires spray formation and combustion characteristics to be quantified in order to improve both injector design and operating strategies. To this end an optical investigation of spray development and combustion was undertaken in a single-cylinder direct-injection spark-ignition engine with a centrally mounted multi-hole injector. Specifically, crank-angle resolved imaging studies were performed and batches of images from 100 consecutive cycles were acquired with synchronised in-cylinder pressure logging. The engine was motored and fired at 1500 RPM stoichiometrically under part load (0.5 bar intake pressure), with injection timing set early in the intake stroke to promote homogeneous mixture formation. The effects were investigated at engine coolant temperatures of 20 °C and 90 °C using gasoline, iso-octane, ethanol and butanol. Projected spray areas as seen through the piston crown were calculated to reveal information about the atomization and evaporation processes for each fuel. Additionally, flame areas and centroids were calculated to analyse the combustion process relative to measured in-cylinder pressure histories. INTRODUCTION The application of alcohol fuels in spark ignition engines is not a modern development. As far back as the early 1900s Henry Ford intended his first model T and most other new road vehicles to run on ethanol from renewable sources. The discovery of numerous oil fields and the higher compatibility of gasoline with engine materials, among other factors, meant that ethanol take up was slow and was eventually replaced by gasoline. Today’s increasing awareness of the human contribution to global warming, as result of excess CO2 emissions, and the question marks surrounding the sustainability of an oil-based world economy, have lead to a renewed urgency and increased research efforts to find alternative ‘carbon neutral’ sources of energy for the transportation sector, which in the UK alone accounted for ~28% of all CO2 emissions in 2005 [1]. There is significant potential to reduce CO2 emissions using direct-injection spark-ignition engines by adopting new technologies such as turbocharging and variablevalve actuation with downsizing concepts [2, 3]. However, unless operation with alternative renewable fuels is incorporated, it will not be possible to meet future CO2 emissions targets. Moreover, unless these fuels are compatible with existing distribution infrastructure and engine components, voluntary take up by vehicle manufacturers is likely to be slow and dictated only by the requirement to meet new waves of legislation. For these reasons the current study investigates the spray development and combustion characteristics of ethanol and butanol in direct comparison to gasoline and isooctane fuels. The former alcohol is currently the most common and widespread alternative to fossil fuels but still faced with significant obstacles for use in concentrations higher than 10–15% with gasoline, whilst the latter alcohol is a more compatible alcohol fuel for use with current vehicle and engine technologies, as well as with existing supply and distribution infrastructures. In terms of engine performance, the benefits of using ethanol are generally viewed as positive due its higher octane rating – increasing its knocking resistance and allowing the use of higher compression ratios which improves thermal efficiency. This is compounded by significantly higher charge cooling capability compared to gasoline. However, the low energy density of ethanol impacts a vehicle range between re-fuelling periods and therefore is a set-back from the perspective of the consumer. In this respect, the much smaller difference between energy densities of gasoline and butanol, make this an interesting alternative, particularly given its compatibility with current vehicle technologies. Recent work has concentrated on investigating the potential benefits of ethanol or ethanol/gasoline blends using thermodynamic engines and quantifying the effects in terms of engine performance and particulate, unburned HC and NOX emissions [4–6]. However, there is little existing literature on the fundamental differences between spray formation of alcohol-based fuels versus standard gasoline, particularly for direct-injection configurations. Moreover, very few reports are available on optical studies of mixture preparation and combustion processes using alcohols and all but none featuring new generation multi-hole injectors or other alcohols than ethanol, especially butanol. Analysis of spray and atomization phenomena with reference to fluid properties is also rare but it is vital in providing assessment and interpretation of the fundamental behaviour of these fuels inside engines. Optical studies of sprays and combustion also provide an essential database for developers and modelers due to the very limited data available on spray break-up and flame speeds of alcohol fuels [7–10]. This work seeks to examine the effect of fuel type on incylinder spray development and combustion from a DISI multi-hole injector operating under realistic engine conditions of temperatures and load by: • Characterising the effects of engine temperature on spray development in a motoring engine for four fuel types, namely, chemically pure ethanol, butanol and iso-octane, as well as a standard multi-component commercial gasoline. • Investigating the behaviour of these different fuels in a firing engine by studying in-cylinder flame growth and motion on a crank-angle resolved basis for different engine temperatures. • Carrying out analysis of spray and flame parameters relative to in-cylinder pressure parameters to link observed spray effects to flame growth behavior. In order to study such interactions, a single-cylinder optical DISI engine was used for high-speed imaging of the fuel spray and flame growth through the piston crown, along with acquisition of in-cylinder pressure via a piezoelectric transducer mounting in the cylinder head. EXPERIMENTAL ARRANGEMENT

An Optical Study of Spray Development and Combustion of Ethanol, Iso-Octane and Gasoline Blends in a DISI Engine

In recent times regulatory pressure to reduce CO 2 emissions has driven research towards looking at blending fossil fuels with alternatives such as crop-produced alcohols. The alcohol of interest in this paper is ethanol and it was studied in mixtures with gasoline and iso -octane in an optical sparkignition engine, running at 1500 RPM at low-load operation with 0.5 bar absolute intake plenum pressure. Specifically, tests involved fuels of 100% gasoline and 100% iso -octane, so that differences between multi and single-component fuels could be compared within this environment. A mixture of 25% ethanol with 75% iso -octane was also tested and compared. Finally, mixtures of highpercentage of ethanol (85% ethanol) in gasoline and in iso -octane were used in the study and compared. Tests were undertaken using a standard port injection system as well as a direct injection system so an appraisal of both mixture preparation methods could be made. Initially, a high-speed imaging study of the in-cylinder spray formation was undertaken with the direct injection system for different injection timings and engine-head temperatures under motoring engine conditions. The engine was also run with continuous firing using all fuels. In-cylinder pressure data were collected at 0.2° crank angle resolution for each cycle and synchronized with simultaneous high-speed flame imaging at 1° crank angle resolution for a series of 100 consecutive cycles for all test points. The flame images were processed to quantify the evolution of an equivalent flame radius.

Deposit Formation in the Holes of Diesel Injector Nozzles: A Critical Review

Current developments in fuels and emissions regulations are resulting in increasingly severe operating environment for the injection system. Formation of deposits within the holes of the injector nozzle or on the outside of the injector tip may have an adverse effect on overall system performance. This paper provides a critical review of the current understanding of the main factors affecting deposit formation. Two main types of engine test cycles, which attempt to simulate field conditions, are described in the literature. The first type involves cycling between high and low load. The second involves steady state operation at constant speed either at medium or high load. A number of influences on the creation of deposits are identified. This includes fouling through thermal condensation and cracking reactions at nozzle temperatures of around 300°C. Also the design of the injector holes is an influence, because it can influence cavitation. The implosion of cavitation bubbles is believed to limit nozzle deposits. Field and laboratory tests showed that small amounts (around 1ppm) of zinc tend to increase the formation of deposits and are therefore another influence. But it is not clear whether zinc acts catalytically to accelerate deposit formation or if it becomes part of the solid deposits. Bio-diesel has been observed to lead to higher deposit formation in the injector nozzle. The chemical and physical processes that lead to deposit formation are not known or well understood, due to their complexity. A physical mechanism put forward focuses on the role of the residual fuel that remains in the nozzle holes after the end of the injection process.

Effect of Fuel Properties on Spray Development from a Multi-Hole DISI Engine Injector

Extensive literature exists on spray development, mixing and combustion regarding engine modeling and diagnostics using single-component and model fuels. However, often the variation in data between different fuels, particularly relating to spray development and its effect on combustion, is neglected or overlooked. By injecting into a quiescent chamber, this work quantifies the differences in spray development from a multi-hole direct-injection spark-ignition engine injector for two single-component fuels (iso-octane and n-pentane), a non-fluorescing multi-component model fuel which may be used for in-cylinder Laser Induced Fluorescence experiments, and several grades of pump gasoline (with and without additives). High-speed recordings of the sprays were made for a range of fuel temperatures and gas pressures. It is shown that a fuel temperature above that of the lowest boiling point fraction of the tested fuel at the given gas pressure causes a convergence of the spray plumes. Increasing the fuel temperature increases this convergence, whilst an associated increased rate of evaporation tends to reduce the penetration of individual plumes. The convergence increases gradually with increasing fuel temperature until all plumes combine to form a single wider plume with a penetration rate greater than that of the individual plumes. When all plumes are converged to form a single plume along a central axis to all the plumes, any further increase in fuel temperature at the given gas pressure acts to increase the rate of evaporation of the fuel. At experiments up to 180 °C fuel temperature and down to 0.3 bar absolute gas pressure, none of the tested fuels were found to spontaneously vaporize; all observed spray formations being a gradual evolution. Increasing the gas pressure at any given fuel temperature, leads to an increase in the boiling temperature of all components of that fuel and, hence, diminishes these effects.

Investigations on Deposit Formation in the Holes of Diesel Injector Nozzles

Current developments in fuels and emissions regulations are resulting in an increasingly severe operating environment for diesel fuel injection systems. The formation of deposits within the holes or on the outside of the injector nozzle can affect the overall system performance. The rate of deposit formation is affected by a number of parameters, including operating conditions and fuel composition. For the work reported here an accelerated test procedure was developed to evaluate the relative importance of some of these parameters in a high pressure common rail fuel injection system. The resulting methodology produced measurable deposits in a custom made injector nozzle on a single cylinder engine. The results indicate that fuels containing 30%v/v and 100% Fatty Acid Methyl Ester (FAME), that does not meet EN 14214 produced more deposit than an EN590 petroleum diesel fuel. Overall, the addition of zinc to the fuel had the biggest effect on deposit formation and resulted in a 12.2% decrease in Indicated Mean Effective Pressure (IMEP). The effects of zinc were unexpectedly reduced when it was added to fuel containing 30%v/v biodiesel. Reducing the common-rail pressure with 30%v/v biodiesel (no added zinc) increased the loss in IMEP. Raising the air and fuel temperatures by 40°C and 30°C respectively showed no bigger loss in IMEP. The results indicate that deposit formation may continue after engine shut down.

Development of a real-size optical injector nozzle for studies of cavitation, spray formation and flash-boiling at conditions relevant to direct-injection spark-ignition engines

High-pressure multi-hole injectors for direct-injection spark-ignition engines have shown enhanced fuel atomisation and flexibility in fuel targeting by selection of the number and angle of the nozzle holes. The nozzle internal flow is known to influence the characteristics of spray formation; hence, understanding its mechanisms is essential for improving mixture preparation. However, currently, no data exist for fuel temperatures representative of real engine operation, especially at low-load high-temperature conditions with early injection strategies that can lead to phase change due to fuel flash-boiling upon injection. This challenge is further complicated by the predicted fuel stocks, which may include new (e.g. bio-derived) components. The physical/chemical properties of such components can differ markedly from gasoline, and it is important to have the capability to study their effects on in-nozzle flow and spray formation, taking under consideration their different chemical compatibilities with optical materials as well. The current article presents the design and development of a real-size quartz optical nozzle, 200 µm in diameter, suitable for high-temperature applications and also compatible with new fuels such as alcohols. First, the internal geometry of a typical real multi-hole injector was analysed by electron microscopy. Mass flow was measured, and relevant fluid mechanics dimensionless parameters were derived. Laser and mechanical drilling of the quartz nozzle holes were compared. Abrasive flow machining of the optical nozzles was also performed and analysed by microscopy in comparison to the real injector. Initial validation results with a high-speed camera showed successful imaging of microscopic in-nozzle flow and cavitation phenomena, coupled to downstream spray formation, under a variety of conditions including high fuel temperature flash-boiling effects. The current work used gasoline and iso-octane to provide proof-of-concept images of the optical nozzle, and future work will include testing of a range of fuels, some of which will also be bio-derived.

A Study of Alcohol Blended Fuels in an Unthrottled Single Cylinder Spark-Ignition Engine

This work involved study of the effects of alcohol blends on combustion, fuel economy and emissions in a single cylinder research engine equipped with a mechanical fully variable valvetrain on the inlet and variable valve timing on the exhaust. A number of splash blends of gasoline, iso-octane, ethanol and butanol were examined during port fuel injected early inlet valve closing operation, both with and without variable valve timing. Under low valve overlap conditions, it was apparent that the inlet valve durations/lifts required for full unthrottled operation were remarkably similar for the wide range of blends studied. However, with high valve overlap differences in burning velocities and internal EGR tolerances warranted changes in these valve settings. In turn, it was concluded that high ethanol content blends facilitated minimum throttling at the inlet valve itself and the largest relative savings in terms of fuel consumption, engine-out emissions of NOx and (corrected) unburned hydrocarbons.

Hydrogen SI and HCCI Combustion in a Direct-Injection Optical Engine

Hydrogen has been largely proposed as a possible alternative fuel for internal combustion engines. Its wide flammability range allows higher engine efficiency with leaner operation than conventional fuels, for both reduced toxic emissions and no CO2 gases. Independently, Homogenous Charge Compression Ignition (HCCI) also allows higher thermal efficiency and lower fuel consumption with reduced NOX emissions when compared to Spark-Ignition (SI) engine operation. For HCCI combustion, a mixture of air and fuel is supplied to the cylinder and autoignition occurs from compression; engine is operated throttle-less and load is controlled by the quality of the mixture, avoiding the large fluid-dynamic losses in the intake manifold of SI engines. HCCI can be induced and controlled by varying the mixture temperature, either by Exhaust Gas Recirculation (EGR) or intake air pre-heating. A combination of HCCI combustion with hydrogen fuelling has great potential for virtually zero CO2 and NOX emissions. Nevertheless, combustion on such a fast burning fuel with wide flammability limits and high octane number implies many disadvantages, such as control of backfiring and speed of autoignition and there is almost no literature on the subject, particularly in optical engines. Experiments were conducted in a singlecylinder research engine equipped with both Port Fuel Injection (PFI) and Direct Injection (DI) systems running at 1000 RPM. Optical access to in-cylinder phenomena was enabled through an extended piston and optical crown. Combustion images were acquired by a highspeed camera at 1° or 2° crank angle resolution for a series of engine cycles. Spark-ignition tests were initially carried out to benchmark the operation of the engine with hydrogen against gasoline. DI of hydrogen after intake valve closure was found to be preferable in order to overcome problems related to backfiring and air displacement from hydrogen’s low density. HCCI combustion of hydrogen was initially enabled by means of a pilot port injection of n-heptane preceding the main direct injection of hydrogen, along with intake air preheating. Sole hydrogen fuelling HCCI was finally achieved and made sustainable, even at the low compression ratio of the optical engine by means of closed-valve DI, in synergy with air-pre-heating and negative valve overlap to promote internal EGR. Various operating conditions were analysed, such as fuelling in the range of air excess ratio 1.2–3.0 and intake air temperatures of 200–400 °C. Finally, both single and double injections per cycle were compared to identify their effects on combustion development.

An Analysis of the Combustion Behavior of Ethanol, Butanol, Iso-Octane, Gasoline, and Methane in a Direct-Injection Spark-Ignition Research Engine

Future automotive fuels are expected to contain significant quantities of bio-components. This poses a great challenge to the designers of novel low-CO2 internal combustion engines because biofuels have very different properties to those of most typical hydrocarbons. The current article presents results of firing a direct-injection spark-ignition optical research engine on ethanol and butanol and comparing those to data obtained with gasoline and iso-octane. A multihole injector, located centrally in the combustion chamber, was used with all fuels. Methane was also employed by injecting it into the inlet plenum to provide a benchmark case for well-mixed “homogeneous” charge preparation. The study covered stoichiometric and lean mixtures (λ = 1.0 and λ = 1.2), various spark advances (30–50° CA), a range of engine temperatures (20–90°C), and diverse injection strategies (single and “split” triple). In-cylinder gas sampling at the spark-plug location and at a location on the pent-roof wall was also carried out using a fast flame ionization detector to measure the equivalence ratio of the in-cylinder charge and identify the degree of stratification. Combustion imaging was performed through a full-bore optical piston to study the effect of injection strategy on late burning associated with fuel spray wall impingement. Combustion with single injection was fastest for ethanol throughout 20–90°C, but butanol and methane were just as fast at 90°C; iso-octane was the slowest and gasoline was between iso-octane and the alcohols. At 20°C, λ at the spark plug location was 0.96–1.09, with gasoline exhibiting the largest and iso-octane the lowest value. Ethanol showed the lowest degree of stratification and butanol the largest. At 90°C, stratification was lower for most fuels, with butanol showing the largest effect. The work output with triple injection was marginally higher for the alcohols and lower for iso-octane and gasoline (than with single injection), but combustion stability was worse for all fuels. Triple injection produced a lower degree of stratification, with leaner λ at the spark plug than single injection. Combustion imaging showed much less luminous late burning with tripe injection. In terms of combustion stability, the alcohols were more robust to changes in fueling (λ = 1.2) than the liquid hydrocarbons.

Characterisation of Flow Structures in a Direct-Injection Spark-Ignition Engine using PIV, LDV and CFD

In-cylinder air flow structures are known to play a major role in mixture preparation and engine operating limits for DISI engines. In this paper PIV was undertaken on in-cylinder flow fields for three different planes of measurement in the intake and compression strokes of a DISI engine for a lowload engine operating condition at 1500 RPM, 0.5 bar inlet plenum pressure (World Wide Mapping Point). One of these planes was vertical, cutting through the centrally located spark plug (tumble plane); the other two planes were horizontal, one close to TDC (10 mm below fire face) and the other one close to mid stroke (50 mm below fire face). Statistical analysis was undertaken on the numbers of cycles needed to determine ensemble average flow-field and turbulent kinetic energy maps with up to 1200 cycles considered. The effect of engine head temperature was also examined by obtaining flow fields using PIV with the engine head coolant held at 20 °C and 80 °C. LDV measurements were also performed and compared to the data obtained by PIV. Finally comparisons were made between the experimental data and results from CFD simulations using two different turbulence models on a grid of 1 million cells.