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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.

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.

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.

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.

SI Engine knock is caused by autoignition in the unburnt part of the mixture (end-gas) ahead of the propagating flame. Autoignition of the end-gas occurs when the temperature and pressure exceeds a critical limit when comparatively slow reactions-releasing moderate amounts of heat-transform into ignition and rapid heat release. In this paper the difference in the heat released in the end-gas-by low temperature chemistry-between lean, rich, stochiometric, and stoichiometric mixtures diluted with cooled EGR was examined by measuring the temperature in the end-gas with Dual Broadband Rotational CARS. The measured temperature history was compared with an isentropic temperature calculated from the cylinder pressure trace. The experimentally obtained values for knock onset were compared with results from a two-zone thermodynamic model including detailed chemistry modeling of the end-gas reactions.

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.

This paper discusses the effects of cooled EGR on a turbo charged multi cylinder HCCI engine. A six cylinder, 12 liter, Scania D12 truck engine is modified for HCCI operation. It is fitted with port fuel injection of ethanol and n-heptane and cylinder pressure sensors for closed loop combustion control. The effects of EGR are studied in different operating regimes of the engine. During idle, low speed and no load, the focus is on the effects on combustion efficiency, emissions of unburned hydrocarbons and CO. At intermediate load, run without turbocharging to achieve a well defined experiment, combustion efficiency and emissions from incomplete combustion are still of interest. However the effect on NOx and the thermodynamic effect on thermal efficiency, from a different gas composition, are studied as well. At high load and boost pressure the main focus is NOx emissions and the ability to run high mean effective pressure without exceeding the physical constraints of the 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.

The objective of this paper is to investigate how the combustion chamber design will influence combustion parameters and emissions in a natural gas SI engine. Ten different geometries were tried on a converted Volvo TD102 engine. For the different combustion chambers emissions and the pressure in the cylinder have been measured. The pressure in the cylinder was then used in a one-zone heat-release model to get different combustion parameters. The engine was operated unthrottled at 1200 rpm with different values of air/fuel ratio and EGR. The air/fuel ratio was varied from stoichiometric to lean limit. EGR values from 0 to 30% at stoichiometric air/fuel ratio were used. The results show a remarkably large difference in the rate of combustion between the chambers. The cycle-to-cycle variations are fairly independent of combustion chamber design as long as there is some squish area and the air and the natural gas are well mixed.

The use of a spark plug as an ionization sensor in an engine, and its physical and chemical explanation has been investigated. By applying a small constant DC voltage across the electrodes of the spark plug and measuring the current through the electrode gap, the state of the gas can be probed. An analytical expression for the current as a function of temperature is derived, and an inverse relation, where the pressure is a function of the current, is also presented. It is also found that a relatively minor species, NO, seems to be the major agent responsible for the conductivity of the hot gas in the spark gap.

In order to separate the HC-emissions from two-stroke engines into short-circuit losses and emissions due to incomplete combustion, Laser Induced Fluorescence (LIF) measurements were performed on the exhaust gases just outside the exhaust ports of two engines of different designs. The difference between the two engines was the design of the transfer channels. One engine had “finger” transfer channels and one had “cup handle” transfer channels. Apart from that they were similar. The engine with “finger” transfer channels was earlier known to give more short-circuiting losses than the other engine, and that behavior was confirmed by these measurements. Generally, the results show that the emission of hydrocarbons has two peaks, one just after exhaust port opening and one late in the scavenging phase. The spectral information shows differences between the two peaks and it can be concluded that the latter peak is due to short-circuiting and the earlier due to incomplete combustion.

Emissions standards are becoming increasingly harder to reach without the use of exhaust aftertreatment systems such as Selective Catalytic Reduction and particulate filters. In order to make efficient use of these systems it is important to have accurate models of engine-out emissions. Such models are also useful for optimizing and controlling next-generation engines without aftertreatment using for example exhaust gas recirculation (EGR). Engines are getting more advanced using systems such as common rail fuel injection, variable geometry turbochargers (VGT) and EGR. With these new technologies and active control of the injection timing, more sophisticated models than simple stationary emission maps must be used to get adequate results. This paper is focused on the calculation of engine-out NOx and engine parameters such as cylinder pressure, temperature and gas flows.

Exhaust Emissions from lean burn natural gas engines may not always be as low as the potential permits, especially engines with open loop lambda control. These engines can produce much higher emissions than a comparable diesel engine without exhaust gas after treatment. Even if the engine has closed loop lambda control, emissions are often unacceptably high for future emission regulations. A three way catalyst is, today, the best way to reduce hazardous emissions. The drawback is that the engine has to operate with a stoichiometric mixture and this leads to; higher heat losses, higher pumping work at low to medium loads, higher thermal stress on the engine and higher knock tendency (requiring lower compression ratio, and thus lower brake efficiency). One way to reduce these drawbacks is to dilute the stoichiometric mixture with EGR. This paper compares lean burn operation with operation at stoichiometric conditions diluted with EGR, and using a three way catalyst.

An ionization sensor has been used to study the combustion process in a six-cylinder lean burn, truck-sized engine fueled with natural gas and optimized for low emissions of nitric oxides. The final goal of the investigations is to study the prospects of using the ionization sensor for finding the optimal operating position with respect to low NOx emission and stable engine operation. The results indicate that unstable combustion can be detected by analyzing the coefficient of variation (CoV) of the detector current amplitude. Close relationships between this measure and the CoV of the indicated mean effective pressure have been found during an air-fuel ratio scan with fixed ignition advance.

A theoretical investigation is presented concerning how the heat transfer process from the gas in the combustion chamber, burned as well as the unburned gas regions, to the walls is affected by the autoignition phenomenon in SI engines. The zonal model in ref. [1] is adapted for the calculations. The radiative heat flux during the heat release period and the heat transfer in the thermal boundary layer by convection are predicted for situations when autoignition has occurred. The cylinder wall temperature is also used as a parameter in this study. The effects of engine operating parameters such as engine speed, timing of ignition, duration time of flame propagation and the fuel parameter Research Octane Number, i.e., RON, on the heat flux to the walls have been studied. The heat release is calculated for a detailed chemical kinetic model, refs. [1, 2 and 3].

This paper reports an one–dimensional modeling procedure of the hot spot autoignition with a detailed chemistry and multi–species transport in the end gas in an SI engine. The governing equations for continuity of mass, momentum, energy and species for an one–dimensional, unsteady, compressible, laminar, reacting flow and thermal fields are discretized and solved by a fully implicit method. A chemical kinetic mechanism is used for the primary reference fuels n–heptane and iso–octane. This mechanism contains 510 chemical reactions and 75 species. The change of the cylinder pressure is calculated from both flame propagation and piston movement. The turbulent velocity of the propagating flame is modeled by the Wiebe function. Adiabatic conditions, calculated by minimizing Gibb's free energy at each time step, are assumed behind the flame front in the burned gas.

Exhaust Gas Recirculation, EGR, is one of the most effective means of reducing NOx emissions from diesel engines and is likely to be used in order to meet future emissions standards. Exhaust gases can either be used to replace some of the air that enters the engine or can be added to the intake flow. The former case has been studied in this paper. One advantage of air replacement is that the exhaust mass flow is reduced in addition to the decreased NOx formation. The objective of this study has been to take a closer look at the factors affecting NOx emissions at different EGR rates. This is done by combining heat release analysis, based on measured pressure traces and NO formation in a multi zone combustion model. The model used is an improved version of an earlier presented model [1]. One feature in the new model is the possibility to separate the NO formation during the premixed combustion from NO formed during the diffusive combustion.

A FeCrAlloy mesh-type catalyst has been used to reduce hydrocarbons (HC) and carbon monoxide (CO) emissions from a 4-stroke HCCI engine. Significant for the HCCI engine is a high compression ratio and lean mixtures, which leads to a high efficiency, low combustion temperatures and thereby low NOx emissions, <5 pmm, but also low exhaust temperatures, around 300°C. It becomes critical to: 1. Ensure that the HCCI-combustion generates as low HC emissions as possible, this can be done by very precise control of engine inlet conditions and, if possible, compression ratio. 2. Ensure that the exhaust temperature is high enough, without loosing efficiency or producing NOx; in order to get an oxidizing catalyst to work. 3. Select proper catalyst material for the catalyst so that the exhaust temperature can be as low as possible.

In an attempt to extend the upper load limit for Homogeneous Charge Compression Ignition (HCCI), supercharging in combination with Exhaust Gas Recirculation (EGR) have been applied. Two different boost pressures were used, 1.1 bar and 1.5 bar. High EGR rates were used in order to reduce the combustion rate. The highest obtained IMEP was 16 bar. This was achieved with the higher boost pressure, at close to stoichiometric conditions and with approximately 50 % EGR. Natural gas was used as the main fuel. In the case with the higher boost pressure, iso-octane was used as pilot fuel, to improve the ignition properties of the mixture. This made it possible to use a lower compression ratio and thereby reducing the maximum cylinder pressure. The tests were performed on a single cylinder engine operated at low speed (1000 rpm). The test engine was equipped with a modified cylinder head, having a Variable Compression Ratio (VCR) mechanism.