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Unburned hydrocarbon (UHC) and carbon monoxide (CO) emission sources are examined in an optical, light-duty diesel engine operating under low load and engine speed, while employing a highly dilute, partially premixed low-temperature combustion (LTC) strategy. The impact of engine load and charge dilution on the UHC and CO sources is also evaluated. The progression of in-cylinder mixing and combustion processes is studied using ultraviolet planar laser-induced fluorescence (UV PLIF) to measure the spatial distributions of liquid- and vapor-phase hydrocarbon. A separate, deep-UV LIF technique is used to examine the clearance volume spatial distribution and composition of late-cycle UHC and CO. Homogeneous reactor simulations, utilizing detailed chemical kinetics and constrained by the measured cylinder pressure, are used to examine the impact of charge dilution and initial stoichiometry on oxidation behavior.

Power supply systems play a very important role in applications of everyday life. Mainly, for low power generation, there are two ways of producing energy: electrochemical batteries and small engines. In the last few years many improvements have been carried out in order to obtain lighter batteries with longer duration but unfortunately the energy density of 1 MJ/kg seems to be an asymptotic value. If the energy source is an organic fuel with an energy density of around 29 MJ/kg and a minimum overall efficiency of only 3.5%, this device can surpass the batteries. Nowadays the most efficient combustion process is HCCI combustion which is able to combine high energy conversion efficiency and low emission levels with a very low fuel consumption. In this paper, an investigation has been carried out concerning the effects of the compression ratio on the performance and emissions of a mini, Vd = 4.11 [cm3], HCCI engine fueled with diethyl ether.

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.

High-speed video imaging in a swirl-supported (Rs = 1.7), direct-injection heavy-duty diesel engine operated with moderate-to-high EGR rates reveals a distinct correlation between the spatial distribution of luminous soot and mean flow vorticity in the horizontal plane. The temporal behavior of the experimental images, as well as the results of multi-dimensional numerical simulations, show that this soot-vorticity correlation is caused by the presence of a greater amount of soot on the windward side of the jet. The simulations indicate that while flow swirl can influence pre-ignition mixing processes as well as post-combustion soot oxidation processes, interactions between the swirl and the heat release can also influence mixing processes. Without swirl, combustion-generated gas flows influence mixing on both sides of the jet equally. In the presence of swirl, the heat release occurs on the leeward side of the fuel sprays.

Nowadays for small-scale power generation there are electrochemical batteries and mini engines. Many efforts have been done for improving the power density of the batteries but unfortunately the value of 1 MJ/kg seems to be asymptotic. If the energy source is an organic fuel which has an energy density of around 29 MJ/kg with a minimum overall efficiency of only 3.5%, this device would surpass the batteries. This paper is the fifth of a series of publications aimed to study the HCCI combustion process in the milli domain at high engine speed in order to design and develop VIMPA, Vibrating Microengine for Low Power Generation and Microsystems Actuation. Previous studies ranged from general characterization of the HCCI combustion process by using metal and optical engines, to more specific topics for instance the influence of the boundary layer and quenching distance on the quality of the combustion.

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.

In this work, constant volume combustion is studied using a zero-dimensional FORTRAN code, which is a wide-ranging chemical kinetic simulation that allows a closed system of gases to be described on the basis of a set of initial conditions. The model provides an engine- or reactor-like environment in which the engine simulations allow for a variable system volume and heat transfer both to and from the system. The combustion chamber is divided into two zones as burned and unburned ones, which are separated by an assumed thin flame front in the combustion model used for this work. Equilibrium assumptions have been adopted for the modeling of the thermal ionization, where Saha's equation was derived for singly ionized molecules. The investigation is focused on the thermal ionization of NO as well as for other species. The outputs generated by the model are temperature profiles, species concentration profiles, ionization degree and an electron density for each zone.

Homogeneous Charge Compression Ignition (HCCI) holds great promises for good fuel economy and low emissions of NOX and soot. The concept of HCCI is premixed combustion of a highly diluted mixture. The dilution limits the combustion temperature and thus prevents extensive NOX production. Load is controlled by altering the quality of the charge, rather than the quantity. No throttling together with a high compression ratio to facilitate auto ignition and lean mixtures results in good brake thermal efficiency. However, HCCI also presents challenges like how to control the combustion and how to achieve an acceptable load range. This work is focused on solutions to the latter problem. The high dilution required to avoid NOX production limits the mass of fuel relative to the mass of air or EGR. For a given size of the engine the only way to recover the loss of power due to dilution is to force more mass through the engine.

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.

Although there seems to be a consensus regarding the low emission potential of DME, there are still different opinions about why the low NOx emissions can be obtained without negative effects on thermal efficiency. Possible explanations are: The physical properties of DME affecting the spray and the mixture formation Different shape and duration of the heat release in combination with reduced heat losses In this paper an attempt is made to increase the knowledge of DME in relation to diesel fuel with respect to heat release and NOx formation. The emphasis has been to create injection conditions as similar as possible for both fuels. For that purpose the same injection system (CR), injection pressure (270 bar), injection timing and duration have been used for the two fuels. The only differences were the diameters of the nozzle holes, which were chosen to give the same fuel energy supply, and the physical properties of the fuels.

During development of an air assisted, direct injection combustion system, it was found that an air pulse during the late part of compression stroke significantly shortened the combustion duration and extended the lean limits of the engine. The effect of an injection of pure air through an air assist direct injector was studied with Particle Image Velocimetry, PIV. Results showed that an air pulse during the compression stroke significantly speeded up in-cylinder velocities, which also was showed in the heat release analysis. A system to use low density seeding particles was developed and is presented in the paper.

Laser-induced incandescence (LII) is a sensitive diagnostic technique capable of making exhaust particulate-matter measurements during transient operating conditions. This paper presents measurements of LII signals obtained from the exhaust gas of a 1.9-L TDI diesel engine. A scanning mobility particle sizer (SMPS) is used in fixed-size mode to obtain simultaneous number concentration measurements in real-time. The transient studies presented include a cranking-start/idle/shutdown sequence, on/off cycling of EGR, and rapid load changes. The results show superior temporal response of LII compared to the SMPS. Additional advantages of LII are that exhaust dilution and cooling are not required, and that the signal amplitude is directly proportional to the carbon volume fraction and its temporal decay is related to the primary particle size.

“Hot Bulb engines” was the popular name of the early direct injected 2-stroke oil engine, invented and patented by Carl W. Weiss 1897. This paper covers engines of this design, built under license in Sweden by various manufacturers. The continuous development is demonstrated through examples of different combustion chamber designs. The material is based on official engine performance evaluations on stationary engines and farm tractors from 1899 to 1995 made by the National Machinery Testing Institute in Sweden (SMP). Hot-bulb, diesel and spark ignited engines are compared regarding efficiency, brake mean effective pressure and specific power (power per displaced volume). The evaluated hot-bulb engines had a fairly good efficiency, well matching the contemporary diesel engines. At low mean effective pressures, the efficiency of the hot-bulb engines was even better than that of subsequent diesel engines.

A specially designed cylinder head, enabling unthrottled operation with a standard cam-phasing mechanism, was tested in an optical single-cylinder engine. The in-cylinder flow was measured with particle image velocimetry (PIV) and the results were compared with heat release and emission measurements. The article also discusses effects of residual gas and effective compression ratio on heat-release and emissions. The special design of the cylinder head, with one inlet and one exhaust valve per camshaft, made it possible to operate the engine unthrottled at part load. Cam phasing led to late inlet valve closing, but also to increased valve overlap. The exhaust valve closing was late in the intake stroke, resulting in high amounts of residual gases. Two different camshafts were used with late inlet valve closing. One of the camshafts had shorter valve open duration on the phased exhaust cam lobe.

A comparison of scanning mobility particle sizer (SMPS) and laser-induced incandescence (LII) measurements of diesel particulate matter (PM) was performed. The results reveal the significance of the aggregate nature of diesel PM on interpretation of size and volume fraction measurements obtained with an SMPS, and the accuracy of primary particle size measurements by LII. Volume fraction calculations based on the mobility diameter measured by the SMPS substantially over-predict the space-filling volume fraction of the PM. Correction algorithms for the SMPS measurements, to account for the fractal nature of the aggregate morphology, result in a substantial reduction in the reported volume. The behavior of the particulate volume fraction, mean and standard deviation of the mobility diameter, and primary particle size are studied as a function of the EGR for a range of steady-state engine speeds and loads for a turbocharged direct-injection diesel engine.

Ion current measurements can give information useful for controlling the combustion stability in a multi-cylinder engine. Operation near the dilution limit (air or EGR) can be achieved and it can be optimized individually for the cylinders, resulting in a system with better engine stability for highly diluted mixtures. This method will also compensate for engine wear, e.g. changes in volumetric efficiency and fuel injector characteristics. Especially in a port injected engine, changes in fuel injector characteristics can lead to increased emissions and deteriorated engine performance when operating with a closed-loop lambda control system. One problem using the ion-current signal to control engine stability near the lean limit is the weak signal resulting in low signal to noise ratio. Measurements presented in this paper were made on a turbocharged 9.6 liter six cylinder natural gas engine with port injection.

In this paper a fast NOx model is presented which can be used for engine optimization, aftertreatment control or virtual mapping. A cylinder pressure trace is required as input data. High calculation speed is obtained by using table interpolation to calculate equilibrium temperatures and species concentrations. Test data from a single-cylinder engine and from a complete six-cylinder engine have been used for calibration and validation of the model. The model produces results of good agreement with emission measurements using approximately 50 combustion product zones and a calculation time of one second per engine cycle. Different compression ratios, EGR rates, injection timing, inlet pressures etc. were used in the validation tests.

High-speed laser diagnostics was utilized for single-cycle resolved studies of the formaldehyde distribution in the combustion chamber of an HCCI engine. A multi-YAG laser system consisting of four individual Q-switched, flash lamp-pumped Nd:YAG lasers has previously been developed in order to obtain laser pulses at 355 nm suitable for performing LIF measurements of the formaldehyde molecule. Bursts of up to eight pulses with very short time separation can be produced, allowing capturing of LIF image series with high temporal resolution. The system was used together with a high-speed framing camera employing eight intensified CCD modules, with a frame-rate matching the laser pulse repetition rate. The diagnostic system was used to study the combustion in a truck-size HCCI engine, running at 1200 rpm using n-heptane as fuel. By using laser pulses with time separations as short as 70 μs, cycle-resolved image sequences of the formaldehyde distribution were obtained.

The main benefit of HCCI engines compared to SI engines is improved fuel economy. The drawback is the diluted combustion with a substantially smaller operating range if not some kind of supercharging is used. The reasons for the higher brake efficiency in HCCI engines can be summarized in lower pumping losses and higher thermodynamic efficiency, due to higher compression ratio and higher ratio of specific heats if air is used as dilution. In the low load operating range, where HCCI today is mainly used, other parameters as friction losses, and cooling losses have a large impact on the achieved brake efficiency. To initiate the auto ignition of the in-cylinder charge a certain temperature and pressure have to be reached for a specific fuel. In an engine with high in-cylinder cooling losses the initial charge temperature before compression has to be higher than on an engine with less heat transfer.

The most economical way to convert truck and bus DI-diesel engines to natural gas operation is to replace the injector with a spark plug and modify the combustion chamber in the piston crown for spark ignition operation. The modification of the piston crown should give a geometry well suited for spark ignition operation with the original swirling inlet port. Ten different geometries were tried on a converted VOLVO TD102 engine and a remarkably large difference in the rate of combustion was noted between the chambers. To find an explanation for this difference a cycle resolved measurement of the in-cylinder mean velocity and turbulence was performed with Laser Doppler Velocimetry (LDV). The results show a high correlation between in cylinder turbulence and rate of heat release in the main part of combustion.