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A constant-volume combustion chamber was used to examine injection of a small quantity of slightly rich fuel/air mixture towards the spark plug around the time of ignition, in an overall very lean mixture rotating at velocities representative of modern spark-ignition engines. The results show that it is possible to achieve 100% ignitability with overall air-fuel ratios in excess of 50 and much faster burn rates than those with initially homogenous mixtures of the same equivalence ratio with high swirl and turbulence. The advantages of this method of local charge stratification have been demonstrated in terms of both pressure measurements and shadowgraphs of the early flame development while the transient characteristics of the injected rich mixture at the spark plug gap were monitored by a fast flame ionization detector.

The electrical characteristics of transistorized coil ignition (TCI) and capacitor discharge ignition (CDI) systems were investigated in spark-ignited quiescent and flowing propane/air mixtures within an optically-accessible, cylindrical constant-volume combustion chamber. Under quiescent flow conditions, the initial pressure, temperature and equivalence ratio of the mixture as well as the spark gap width and geometry were varied systematically in order to examine the relationship between ignition characteristics and flame initiation and development. The effect of the flow in the spark gap on the electrical characteristics of the ignition system, mixture ignitability and flame development was also examined by varying the pre-ignition mean flow and turbulence as well as the spark plug orientation relative to the mean flow.

Laser Doppler velocimetry, phase Doppler anemometry and Mie scattering were applied to a single-cylinder, four-valve, spark-ignition gasoline research engine equipped with a fully transparent liner and piston, to obtain information about the tumble flow and the droplet size and velocity distributions during induction and compression, for lean air/fuel mixture ratios of 17.5 and 24 and with closed-valve and open-valve fuel injection. The mixture distribution obtained with the two injection strategies was correlated with flame images, pressure analysis and exhaust emissions which confirmed the advantages of combining open-valve injection with tumble to allow stable and efficient engine operation at an air/fuel ratio of 24 through charge stratification and faster flame growth.

The effect of various levels of exhaust gas recirculation (EGR) on the combustion characteristics has been investigated in the four-cylinder 1.9L direct-injection optical VW diesel engine in terms of the cylinder pressure, flame development, temperature and KL-factor distributions. Images of the developing flame under twelve engine operating conditions including 1000rpm/idle, 2000rpm/2 bar bmep and 2000rpm/10 bar bmep at 0%, 30% and 50% EGR-rates were obtained by means of two CCD cameras, in the absence of external illumination, with and without interference filters in the optical path. Analysis of these images has revealed that increased EGR rates lead to increased cyclic pressure variations during the warm-up period of the engine, reduced and more fragmented high-temperature regions, reduced flame core temperatures, generally reduced soot oxidation rates but similar ignition delay times.

Direct flame imaging and pressure analysis were applied to the combustion of gasoline and compressed natural gas (CNG) in a single-cylinder, four-valve spark-ignition engine equipped with optical access via quartz windows in the cylinder liner and piston crown. Tests were performed at three engine speed/load conditions and at equivalence ratios of 1.0, 0.9 and 0.8. The four-valve head incorporated two different port geometries, with and without metal sleeves to deflect the intake air flow, in order to investigate the effect of tumble strength on combustion and engine-out emissions of unburned hydrocarbons and NOx. The results showed that sleeving of the intake ports produced a significant increase in IMEP and a reduction in CoV IMEP for both CNG and gasoline, due to the greatly reduced bum duration.

The in-cylinder flow during induction and compression in the pentroof chamber of a four-valve, single-cylinder, spark-ignition optical engine was quantified by LDV and correlated with combustion development especially under lean mixture conditions. The tumble-generating capacity of the cylinder head was first characterised by a tumble adaptor under steady flow conditions and, subsequently, enhanced by two sleeves introduced into the intake ports which generated a stronger tumbling motion.

Measurements of flow, spray, combustion and performance characteristics are reported for a Hydra direct-injection diesel, based on the Ford 2.5 L, engine and equipped with a variable-swirl port, a unit fuel injector and optical access through the liner and piston. The results provide links between the pre-combustion and combustion flow and, at the same time, between purpose-built single-cylinder optical engines and multi-cylinder production engines of nearly identical combustion chamber geometry. In particular, the spray penetration was found to depend on engine speed, rather than load, with velocities up to around 260 m/s at atmospheric pressure and temperature which are reduced by a factor of 2.5 under operating conditions and seem to be unaffected by swirl. The duration of combustion was reduced with increasing swirl and ignition delay increased linearly with engine speed.

The spatial and temporal characteristics of transient diesel sprays impinging on unheated and heated walls were investigated by phase-Doppler anemometry (PDA) and the heat-transfer distribution in the vicinity of the impingement region was determined by fast response thermocouples. The results have provided quantitative evidence about the effect that the presence of the flat wall exerts on the spray characteristics. For example, independent of the thickness of the liquid film, the wall rearranges the droplet size distribution of the free spray with droplet collision and coalescence playing an important role in both the droplet redistribution and in the development of the wall-jet. Droplet sizes were reduced and mean tangential velocities increased with wall temperature at the upstream side and at the front of the wall-jet, respectively.

The in-cylinder flow, mixture distribution, combustion and exhaust emissions in a research, five-valve purpose-built gasoline engine are discussed on the basis of measurements obtained using laser Doppler velocimetry (LDV), fast spark-plug hydrocarbon sampling, flame imaging and NOx/HC emissions using fast chemiluminescent and flame ionisation detectors/analysers. These measurements have been complemented by steady flow testing of various cylinder head configurations, involving single- and three-valve operation, in terms of flow capacity and in-cylinder tumble strength.

Two optical single-cylinder spark-ignition engines equipped with two- and four-valve cylinder heads were used to examine the flow and flame interaction under lean mixture conditions. Images of the developing flame under quiescent, swirl, low tumble and high tumble flow conditions corresponding to a wide range of mean velocity and turbulence levels around the time of ignition were obtained with an image-intensified CCD camera using the light radiated by the flame and the flow in the vicinity of the spark plug was quantified by laser Doppler velocimetry. In the case of the tumbling flow, the flame images were software-processed to allow estimation of the total flame area, the displacement of its centre as a function of crank angle and their correlation with the cylinder pressure.

A transparent enlarged model of a six-hole injector used in the development of emerging gasoline direct-injection engines was manufactured with full optical access. The working fluid was water circulating through the injector nozzle under steady-state flow conditions at different flow rates, pressures and needle positions. Simultaneous matching of the Reynolds and cavitation numbers has allowed direct comparison between the cavitation regimes present in real-size and enlarged nozzles. The experimental results from the model injector, as part of a research programme into second-generation direct-injection spark-ignition engines, are presented and discussed. The main objective of this investigation was to characterise the cavitation process in the sac volume and nozzle holes under different operating conditions. This has been achieved by visualizing the nozzle cavitation structures in two planes simultaneously using two synchronised high-speed cameras.

The diesel engine is the most efficient user of fossil fuels for vehicle propulsion and seems to best fulfill the requirements of the future. It is for this reason that Volkswagen has initiated a very broad research programme for diesels. The purpose of this paper is to build a bridge between fundamental research and technical developments which could allow evaluation of the prospects of direct- injection diesels as powerplants of choice for passenger cars in the turn of the century. The current knowledge on mixture formation, combustion and pollutant formation in diesel engines is presented and discussed with special emphasis given to the concept of the direct-injection diesel engine.

An optical single-cylinder four-valve high speed DI Diesel engine equipped with a high-pressure electronic fuel injection system has been used to obtain information about the spray development, combustion and exhaust emissions (NOx and smoke levels) for a range of operating conditions corresponding to engine speeds between 600 and 1800 rpm, injection pressures up to 1200 bars and fuel injection quantities from idle to full load. Two six-hole vertical mini-sac type injection nozzles with different hole sizes have been employed in order to investigate the effect of nozzle hole diameter on spray formation, combustion and exhaust emissions. Parallel to the experimental programme, a computational investigation of the fuel flow distribution inside the injection system and of the subsequent spray characteristics has been performed in order to assist in the interpretation of the results.

The spatial and temporal characteristics of a diesel spray injected into the atmosphere through a multi-hole nozzle used in small DI Diesel engines have been investigated by laser techniques as a function of pump speed and load. The results showed that spray tip penetration and velocity depend on injection frequency rather than injected volume and the spray is asymmetric during the early and main part of the injection period. In the time/space domain different structures have been identified within the injection period, with the early injection period characterized by a well atomized cloud of droplets, the main period by the spray head and a dense core and the late injection period by the disintegrating dense core and the spray tail. IN DIRECT-INJECTION DIESEL ENGINES for passenger cars, fuel is injected through multi-hole nozzles at high pressure to promote mixing with the rapidly swirling air inside the combustion chamber.

Diesel fuel was injected through a pintle nozzle into quiescent ambient air and the transient characteristics of the spray were examined as a function of injection pump speed. The laser-based techniques characterised the spray in terms of its transient structure, tip penetration, droplet axial mean and rms velocities and average droplet size. The results, when correlated with the fuel line pressure and nozzle exit conditions, revealed the presence of four regimes in the transient spray development: an early injection period representing the first stage of droplet formation, the main injection period associated with the formation and break up of a dense core and representing the second stage of droplet formation, a late injection period corresponding to the collapse of the dense core and a post injection period where, depending on the injection conditions, liquid ligaments and/or large droplets are present near the nozzle and may give rise to a third stage of droplet formation.

The ability of certain induction systems to enhance turbulence levels at the time of ignition, through formation of long-lived tumbling vortices on the plane of the valve and cylinder axes, has been investigated in a two-valve spark-ignition engine by rotating the intake port at 90° and 45° to the orientation of production directed ports. Detailed measurements of the three velocity components, obtained by laser velocimetry, revealed that the 90° port generated a pure tumble motion, with a maximum tumbling vortex ratio of 1.5 at 295°CA, zero swirl, and 42% turbulence enhancement relative to the standard configuration, while the 45° port gave rise to a combined tumble/swirl structure with a maximum tumbling vortex ratio of 0.5 at 285°CA, swirl ratio of 1.0 at TDC, and turbulence enhancement of 24%. The implications of the two types of flow structures for combustion are discussed.

A visualization study using shadowgraphy was performed in an optically-accessible, cylindrical constant-volume combustion chamber to identify the mechanism of flow/flame interaction in spark-ignited, lean propane-air mixtures. The effect of the flow on flame initiation and propagation was examined by varying the pre-ignition mean flow and turbulence within a range typical of modern four-valve spark-ignition (SI) engines, as well as the spark plug orientation relative to the mean flow. The initial flame development was quantified in terms of 2-D images which provided information about the projected flame area and the displacement of the flame center as a function of flow conditions, time from the spark initiation and spark plug orientation. The results showed that high mean flow velocities and turbulence levels can shorten combustion duration in lean mixtures and that the positioning of the ground electrode can have an important effect on the initial kernel formation.