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Partially Premixed Combustion (PPC) is used to meet the increasing demands of emission legislation and to improve fuel efficiency. PPC with gasoline fuels have the advantage of a longer premixed duration of fuel/air mixture which prevents soot formation at higher loads. The objective of this paper is to investigate the degree of stratification for low load (towards idle) engine conditions using different injection strategies and negative valve overlap (NVO). The question is, how homogenous or stratified is the partially premixed combustion (PPC) for a given setting of NVO and fuel injection strategy. In this work PRF 55 has been used as PPC fuel. The experimental engine is a light duty (LD) diesel engine that has been modified to single cylinder operation to provide optical access into the combustion chamber, equipped with a fully variable valve train system. Hot residual gases were trapped by using NVO to dilute the cylinder mixture.

Engine knock is an abnormal phenomenon, which places barriers for modern Spark-Ignition (SI) engines to achieve higher thermal efficiency and better performance. In order to trigger more controllable knock events for study while keeping the knock intensity at restricted range, various spark strategies (e.g. spark timing, spark number, spark location) are applied to investigate on their influences on knock combustion characteristics and pressure oscillations. The experiment is implemented on a modified single cylinder Compression-Ignition (CI) engine operated at SI mode with port fuel injection (PFI). A specialized liner with 4 side spark plugs and 4 pressure sensors is used to generate various flame propagation processes, which leads to different auto-ignition onsets and knock development. Based on multiple channels of pressure signals, a band-pass filter is applied to obtain the pressure oscillations with respect to different spark strategies.

Pre-chamber combustion (PCC) is a promising engine combustion concept capable of extending the lean limit at part load. The engine experiments in the literature showed that the PCC could achieve higher engine thermal efficiency and much lower NOx emission than the spark-ignition engine. Improved understanding of the detailed flow and combustion physics of PCC is important for optimizing the PCC combustion. In this study, we investigated the gas exchange and flame jet from a narrow throat pre-chamber (PC) by only fueling the PC with methane in an optical engine. Simultaneous negative acetone planar laser-induced fluorescence (PLIF) imaging and OH* chemiluminescence imaging were applied to visualize the PC jet and flame jet from the PC, respectively. Results indicate a delay of the PC gas exchange relative to the built-up of the pressure difference (△ P) between PC and the main chamber (MC). This should be due to the gas inertia inside the PC and the resistance of the PC nozzle.

Pre-chamber combustion (PCC) allows an extension on the lean limit of an internal combustion engine (ICE). This combustion mode provides lower NOx emissions and shorter combustion durations that lead to a higher indicated efficiency. In the present work, a narrow throat pre-chamber was tested, which has a unique nozzle area distribution in two rows of six nozzle holes each. Tests were carried out in a modified heavy-duty engine for optical visualization. Methane was used as fuel for both the pre-chamber and the main chamber. Seven operating points were tested, including passive pre-chamber mode as a limit condition, to study the effect of pre- and main-chamber fuel addition on the pre-chamber jets and the main chamber combustion via chemiluminescence imaging. A typical cycle of one of the tested conditions is explained through the captured images. Observations of the typical cycle reveal a predominant presence of only six jets (from the lower row), with well-defined jet structures.

Pre-chamber spark ignition (PCSI) combustion is an emerging lean-burn combustion mode capable of extending the lean operation limit of an engine. The favorable characteristic of short combustion duration at the lean condition of PCSI results in high efficiencies compared to conventional spark ignition combustion. Since the engine operation is typically lean, PCSI can significantly reduce engine-out NOx emissions while maintaining short combustion durations. In this study, experiments were conducted on a heavy-duty engine at lean conditions at mid to low load. Two major studies were performed. In the first study, the total fuel energy input to the engine was fixed while the intake pressure was varied, resulting in varying the global excess air ratio. In the second study, the intake pressure was fixed while the amount of fuel was changed to alter the global excess air ratio.

Gasoline partially premixed combustion (PPC) has the potential of high efficiency and simultaneous low soot and NOx emissions. Running the engine in PPC mode with high octane number fuels has the advantage of a longer premix period of fuel and air which reduces soot emissions. The problem is the ignitability at low load and idle operating conditions. In a previous study it was shown that it is possible to use NVO to improve combustion stability and combustion efficiency at operating conditions where available boosted air is assumed to be limited. NVO has the disadvantage of low net indicated efficiency due to heat losses from recompressions of the hot residual gases. An alternative to NVO is the rebreathing valve strategy where the exhaust valves are reopened during the intake stroke. The net indicated efficiency is expected to be higher with the rebreathing strategy but the question is if similar improvements in combustion stability can be achieved with rebreathing as with NVO.

High-pressure isobaric combustion used in the double compression expansion engine (DCEE) concept was proposed to obtain higher engine brake thermal efficiency than the conventional diesel engine. Experiments on the metal engines showed that four consecutive injections delivered by a single injector can achieve isobaric combustion. Improved understanding of the detailed fuel-air mixing with multiple consecutive injections is needed to optimize the isobaric combustion and reduce engine emissions. In this study, we explored the fuel spray characteristics of the four-consecutive-injections strategy using high-speed imaging with background illumination and fuel-tracer planar laser-induced fluorescence (PLIF) imaging in a heavy-duty optical engine under non-reactive conditions. Toluene of 2% by volume was added to the n-heptane and served as the tracer. The fourth harmonic of a 10 Hz Nd:YAG laser was applied for the excitation of toluene.

Experiments on a modern DI Diesel engine were carried out: The engine was fuelled with standard Diesel fuel, RME and a mixture of 85% standard Diesel fuel, 5% RME and 10% higher alcohols under low load conditions (4 bar IMEP). During these experiments, different external EGR levels were applied while the injection timing was chosen in a way to keep the location of 50% heat release constant. Emission analysis results were in accordance with widely known correlations: Increasing EGR rates lowered NOx emissions. This is explained by a decrease of global air-fuel ratio entailing longer ignition delay. Local gas-fuel ratio increases during ignition delay and local combustion temperature is lowered. Exhaust gas analysis indicated further a strong increase of CO, PM and unburned HC emissions at high EGR levels. This resulted in lower combustion efficiency. PM emissions however, decreased above 50% EGR which was also in accordance with previously reported results.

The structure of HCCI (homogeneous charge compression ignition) combustion flames was quantitatively analyzed by measuring the two-dimensional gas temperature distribution using phosphor thermometry. It was found from the relation between a turbulent Reynolds number and Karlovitz number that, when compared with the flame propagation in an S.I. engine, HCCI combustion has a wider flame structure with respect to the turbulence scale. As a result of our experimentation for the influence of low temperature reaction (LTR) using two types of fuel, it was also confirmed that different types of fuel produce different histories of flame kernel structure.

This paper is a direct continuation of a previous study that addressed the performance and design of a variable compression engine, the Alvar-Cycle Engine [1]. The earlier study was presented at the SAE International Conference and Exposition in Detroit during February 23-26, 1998 as SAE paper 981027. In the present paper test results from a single cylinder prototype are reviewed and compared with a similar conventional engine. Efficiency and emissions are shown as function of speed, load, and compression ratio. The influence of residual gas on knock characteristics is shown. The potential for high power density through heavy supercharging is analyzed.

An engine can be run in Homogenous Charge Compression Ignition (HCCI) mode by applying a negative valve overlap, thus trapping hot residuals so as to achieve an auto-ignition temperature. By employing spark assistance, the engine can be operated in what is here called Spark Assisted Compression Ignition (SACI) with ethanol as fuel. The influence of fuel stratification by means of port fuel injection as well as in combination with direct injection was investigated. A high-speed multi-YAG laser system and a framing camera were utilized to capture planar laser-induced fluorescence (PLIF) images of the fuel distribution. The charge homogeneity in terms of fuel distribution was evaluated using a homogeneity index calculated from the PLIF images. The homogeneity index showed a higher stratification for increased proportions of direct-injected fuel. It was found that charge stratification could be achieved through port fuel injection in a swirling combustion system.

The paper describes a novel method for calculating the residual gas fraction and the trapping efficiency in a 2 stroke engine. Assuming one dimensional compressible flow through the inlet and exhaust ports, the method estimates the instantaneous mass flowing in and out from the combustion chamber; later the residual gas fraction and trapping efficiency are estimated combining together the perfect displacement and perfect mixing scavenging models. It is assumed that when the intake port opens, the fresh mixture is pushing out the burned charge without any mixing and after a multiple of the time needed for the largest eddy to perform one rotation, the two gasses are instantly mixed up together and expelled. The result is a very simple algorithm that does not require much computational time and is able to estimate with high level of precision the trapping efficiency and the residual gas fraction in 2 stroke engines.

Phosphor thermometry was used during fuel injection in an optical engine with the glass piston of reentrant type. SiO2 coated phosphor particle was used for the gas-phase temperature measurements, which gave much less background signal. The measurements were performed in motored mode, in combustion mode with injection of n-heptane and in non-combustion mode with injection of iso-octane. In the beginning of injection period, the mean temperature of each injection cases was lower than that of the motored case, and temperature of iso-octane injection cases was even lower than that of n-heptane injection cases. This indicates, even if vaporization effect seemed to be the same at both injection cases, the effect of temperature decrease changed due to the chemical reaction effect for the n-heptane cases. Chemical reaction seems to be initiated outside of the fuel liquid spray and the position was moving towards the fuel rich area as the time proceeds.

Temperature stratification plays an important role in HCCI combustion. The onsets of auto-ignition and combustion duration are sensitive to the temperature field in the engine cylinder. Numerical simulations of HCCI engine combustion are affected by the use of wall boundary conditions, especially the temperature condition at the cylinder and piston walls. This paper reports on numerical studies and experiments of the temperature field in an optical experimental engine in motored run conditions aiming at improved understanding of the evolution of temperature stratification in the cylinder. The simulations were based on Large-Eddy-Simulation approach which resolves the unsteady energetic large eddy and large scale swirl and tumble structures. Two dimensional temperature experiments were carried out using laser induced phosphorescence with thermographic phosphors seeded to the gas in the cylinder.

A naturally aspirated in-line six-cylinder 2.9-litre Volvo engine is operated in Homogeneous Charge Compression Ignition (HCCI) mode, using camshafts with low lift and short duration generating negative valve overlap. This implementation requires only minor modifications of the standard SI engine and allows SI operation outside the operating range of HCCI. Standard port fuel injection is used and pistons and cylinder head are unchanged from the automotive application. A heat exchanger is utilized to heat or cool the intake air, not as a means of combustion control but in order to simulate realistic variations in ambient temperature. The combustion is monitored in real time using cylinder pressure sensors. HCCI through negative valve overlap is recognized as one of the possible implementation strategies of HCCI closest to production. However, for a practical application the intake temperature will vary both geographically and from time to time.

Combustion initiation in an HCCI engine is dependent of several parameters that are not easily controlled like the temperature and pressure history in the cylinder. So achieving the same ignition condition in all the cylinders in a multi-cylinder engine is difficult. Factors as gas exchange, compression ratio, cylinder cooling, fuel supply, and inlet air temperature can differ from cylinder-to-cylinder. These differences cause both combustion phasing and load variations between the cylinders, which in the end affect the engine performance. Operating range in terms of speed and load is also affected by the cylinder imbalance, since misfiring or too fast combustion in the worst cylinders limits the load. The cylinder-to-cylinder variations are investigated in a multi-cylinder Variable Compression Ratio (VCR) engine, and the effect it has on the engine performance.

The Homogeneous Charge Compression Ignition (HCCI) combustion progress has been characterized by means of high-speed fuel tracer Planar Laser Induced Fluorescence (PLIF) combined with simultaneous chemiluminescence imaging. Imaging has been conducted using a high-speed laser and detector system. The system can acquire a sequence of eight images within less than one crank angle. The engine was run at 1200 rpm on iso-octane or ethanol and a slight amount of acetone was added as a fuel tracer, providing a marker for the unburned areas. The PLIF sequences showed that, during the first stage of combustion, a well distributed decay of fuel concentration occurs. During the later parts of the combustion process the fuel concentration images present much more structure, with distinct edges between islands of unburned fuel and products.

The location of the peak pressure can serve as a control parameter to adjust ignition timing and optimize engine performance. The ionization sensor, an electrical probe for combustion diagnostics, can provide information about the peak pressure location. However, the reliability of such information is rather poor. In-cylinder gas flow at the electrodes may be one reason for this. We present results from an investigation of the relationship between ionization sensor current and pressure under various gas flow conditions. The gas flow velocity in the vicinity of the electrode gap was measured by LDA. From the results one may infer how the in-cylinder gas flow affects the reliability of the prediction of pressure peak location from the ionization sensor signal. One finding is that high bulk gas flow impairs the precision of the prediction in certain configurations.

Homogenous Charge Compression Ignition (HCCI) is a promising part load combustion concept for future power train applications. Different approaches to achieve and control HCCI combustion are today investigated and compared, especially concerning operating range. The HCCI operating range for vehicle applications should at least cover contemporary emissions drive cycles. The operating range in terms of speed and load is investigated with a Naturally Aspirated (NA) four-stroke multi-cylinder engine with Port Fuel Injection (PFI). HCCI combustion control is achieved with Variable Compression Ratio (VCR) and inlet air preheating with exhaust heat. Both primary reference fuels and commercial gasoline are used in the tests. HCCI combustion with commercial gasoline is achieved over a load range from 0 to 3.6bar BMEP, and over a speed range from 1000 to 5000rpm. Maximum load is at 1000rpm and decreases with an approximately straight slope to zero at 5000rpm.

Full cycle simulations of KAUST optical diesel engine were conducted in order to provide insights into the details of fuel spray, mixing, and combustion characteristics at different start of injection (SOI) conditions. Although optical diagnostics provide valuable information, the high fidelity simulations with matched parametric conditions improve fundamental understanding of relevant physical and chemical processes by accessing additional observables such as the local mixture distribution, intermediate species concentrations, and detailed chemical reaction rates. Commercial software, CONVERGE™, was used as the main simulation tool, with the Reynolds averaged Navier-Stokes (RANS) turbulence model and the multi-zone (SAGE) combustion model to compute the chemical reaction terms. SOI is varied from late compression ignition (CI) to early partially premixed combustion (PPC) conditions.