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In order to meet the requirements in the stringent emission regulations, more and more research work has been focused on homogeneous charge compression ignition (HCCI) and partially premixed combustion (PPC) or partially premixed compression ignition (PCCI) as they have the potential to produce low NOx and soot emissions without adverse effects on engine efficiency. The mixture formation and charge stratification influence the combustion behavior and emissions for PPC/PCCI, significantly. An ultra-high speed burst-mode laser is used to capture the mixture formation process from the start of injection until several CADs after the start of combustion in a single cycle. To the authors’ best knowledge, this is the first time that such a high temporal resolution, i.e. 0.2 CAD, PLIF could be accomplished for imaging of the in-cylinder mixing process. The capability of resolving single cycles allows for the influence of cycle-to-cycle variations to be eliminated.

A phosphor thermometry, for measurements of two-dimensional gas-phase temperature was examined in turbulent combustion in an engine. The reasonable temperature deviation and the agreement with calculated data within 5% precision were achieved by single-shot images in the ignition process of compression ignition engine. Focusing on the local flame kernel, the flame structure could be quantitatively given by the temperature. It became evident that the HCCI flame kernels had 1-3 mm diameter and the isolated island structures. Subsequently, the HTR zone consisted of the combined flame kernels near TDC.

An experiment was conducted to investigate the effect of charge stratification on the combustion phasing in a single cylinder, heavy duty (HD) compression ignition (CI) engine. To do this the start of injection (SOI) was changed from -180° after top dead centre (ATDC) to near top dead centre (TDC) during which CA50 (the crank angle at which 50% of the fuel energy is released) was kept constant by changing the intake temperature. At each SOI, the response of CA50 to a slight increase or decrease of either intake temperature or SOI were also investigated. Afterwards, the experiment was repeated with a different intake oxygen concentration. The results show that, for the whole SOI period, the required intake temperature to keep constant CA50 has a “spoon” shape with the handle on the -180° side.

The partially premixed combustion (PPC) concept is regarded as an intermediate process between the thoroughly mixed Homogeneous charge compression ignition (HCCI) combustion and compression ignition (CI) combustion. It’s a combination of auto-ignition mode, a fuel-rich premixed combustion mode, and a diffusion combustion mode. The concept has both high efficiency and low soot emission due to low heat losses and less stratified fuel and air mixtures compared to conventional diesel CI. The mechanisms behind the combustion process are not yet very well known. This work focuses on the efficiency and the in-cylinder process in terms of fuel distribution and the initial phase of the combustion. More specifically, double injection strategies are compared with single injection strategies to achieve different levels of stratification, ranging from HCCI to PPC like combustion as well as poor (43%) to good (49%) of gross indicated efficiency.

In a previous study, in order to investigate the effect of charge stratification on combustion behavior such as combustion efficiency and combustion phasing which also largely affects the emissions, an experiment was conducted in a heavy-duty compression ignition (CI) metal engine. The engine behavior and emission characteristics were studied in the transition from HCCI mode to PPC mode by varying the start of injection (SOI) timing. To gain more detailed information of the mixing process, in-cylinder laser diagnostic measurements, namely fuel-tracer planar laser induced fluorescence (PLIF) imaging, were conducted in an optical version of the heavy-duty CI engine mentioned above. To the authors’ best knowledge, this is the first time to perform fuel-tracer PLIF measurements in an optical engine with a close to production bowl in piston combustion chamber, under transition conditions from HCCI to PPC mode.

Partially premixed combustion (PPC) is a promising way to achieve high efficiency and low engine-out emissions simultaneously in a heavy-duty engine. Compared to Homogeneous Charge Compression Ignition (HCCI), it can be controlled by injection events and much lower HC and CO emissions can be achieved. This work focuses on the transition from HCCI to PPC and combustion and emissions characteristics during the process are investigated. Injection strategies, EGR and boost pressure were the main parameters used to present the corresponding effect during the transition.

Partially premixed combustion 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, even at higher loads. The problem is the ignitability at low load and idle operating conditions. The objective is to investigate the usefulness of negative valve overlap on a light duty diesel engine running with gasoline partially premixed combustion at low load operating conditions. The idea is to use negative valve overlap to trap hot residual gases to elevate the global in-cylinder temperature to promote auto-ignition of the high octane number fuel. This is of practical interest at low engine speed and load operating conditions because it can be assumed that the available boost is limited. The problem with NVO at low load operating conditions is that the exhaust gas temperature is low.

Selective Non-Catalytic Reduction (SNCR), used to reduce the emissions of nitrogen oxides (NOx), has been a well-established technology in the power plant industry for several decades. The SNCR technique is an aftertreatment strategy based on thermal reduction of NOx at high temperatures. In the compression ignition engine application, the technology has not been applicable due to low exhaust temperatures, which makes the SCR (Selective Catalytic Reduction) system essential for efficient nitrogen oxide reduction to fulfill the environment legislation. For a general Double Compression Expansion Engine (DCEE) the complete expansion cycle is split in two separate cycles, i.e. the engine is a split cycle engine. In the first cylinder the combustion occurs and in the second stage the combustion gas is introduced and further expanded in a low-pressure expansion cylinder. The combustion cylinder is connected with the expansion cylinder through a large insulated high-pressure tank.

Auto ignition with SI compression ratio can be achieved by retaining hot residuals, replacing some of the fresh charge. In this experimental work it is achieved by running with a negative valve overlap (NVO) trapping hot residuals. The experimental engine is equipped with a pneumatic valve train making it possible to change valve lift, phasing and duration, as well as running with valve deactivation. This makes it possible to start in SI mode, and then by increasing the NVO, thus raising the initial charge temperature it is possible to investigate the intermediate domain between SI and HCCI. The engine is then running in spark assisted HCCI mode, or spark assisted compression ignition (SACI) mode that is an acronym that describes the combustion on the borderline between SI and HCCI. In this study the effect of changing the in-cylinder flow pattern by increased swirl is studied. This is achieved by deactivating one of the two intake valves.

The main losses in internal combustion engines are the heat losses to the cylinder walls and to the exhaust gases. Adiabatic, or low heat rejection engines, have received interest and been studied in several periods in history. Typically, however, these attempts have had to be abandoned when problems with lubrication and overheating components could not be solved satisfactorily. The latest years have seen the emerging of low temperature combustion in engines as well as computational powers that provide new options for highly efficient engines with low heat rejection. Stochastic Reactor Models (SRM) are highly efficient in modeling the kinetics decided low temperature combustion in HCCI and PPC engines. Containing the means to define the variations within the cylinder while employing detailed chemistry, micro mixing and heat transfer modeling, the interaction between heat transfer, exhaust gas energy and the combustion process can be studied with the SRM.

In-cylinder heat losses in diesel engines decrease engine efficiency significantly and account for approximately 14-19% [1, 2, 3] of the injected fuel energy. A great part of the heat losses during diesel combustion presumably arises from the flame impingement onto the piston. Therefore, the present study investigates the heat losses during flame impingement onto the piston bowl wall experimentally. The measurements were performed on a full metal heavy-duty diesel engine with a small optical access through a removed exhaust valve. The surface temperature at the impingement point of the flame was determined by evaluating a phosphor’s temperature dependent emission decay. Simultaneous cylinder pressure measurements and high-speed videos are associated to the surface temperature measurements in each cycle. Thus, surface temperature readings could be linked to specific impingement and combustion events.

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.

A stochastic model based on a probability density function (PDF) was developed for the investigation of different conditions that determine knock in spark ignition (SI) engine, with focus on the turbulent mixing. The model used is based on a two-zone approach, where the burned and unburned gases are described as stochastic reactors. By using a stochastic ensemble to represent the PDF of the scalar variables associated with the burned and the unburned gases it is possible to investigate phenomena that are neglected by the regular existing models (as gas non-uniformity, turbulence mixing, or the variable gas-wall interaction). Two mixing models are implemented for describing the turbulent mixing: the deterministic interaction by exchange with the mean (IEM) model and the stochastic coalescence/ dispersal (C/D) model. Also, a stochastic jump process is employed for modeling the irregularities in the heat transfer.

The double compression-expansion engine concepts (DCEE) are split-cycle concepts where the compression, combustion, expansion and gas exchange strokes occur in two or more different cylinders. Previous simulation studies reveal there is a potential to improve brake efficiency with these engine concepts due to improved thermodynamic and mechanical efficiencies. As a continuation of this project this paper studies an alternative layout of the DCEE-concept. The concept studied in this paper has three different cylinders, a compression, a combustion and an expansion cylinder. Overall system indicated and brake efficiency estimations were based on both engine experiments and simulations. The engine experiments were carried out at 10 different operating points and 5 fuelling rates (between 98.2 and 310.4 mg/cycle injection mass) at an engine speed of 1200 rpm. The inlet manifold pressure was varied between 3 and 5 bar.

This article presents a study related to application of pre-chamber ignition system in heavy duty natural gas engine which, as previously shown by the authors, can extend the limit of fuel-lean combustion and hence improve fuel efficiency and reduce emissions. A previous study about the effect of pre-chamber volume and nozzle diameter on a single cylinder 2 liter truck-size engine resulted in recommendations for optimal pre-chamber geometry settings. The current study is to determine the dependency of those settings on the engine size. For this study, experiments are performed on a single cylinder 9 liter large bore marine engine with similar pre-chamber geometry and a test matrix of similar and scaled pre-chamber volume and nozzle diameter settings. The effect of these variations on main chamber ignition and the following combustion is studied to understand the scalability aspects of pre-chamber ignition. Indicated efficiency and engine-out emission data is also presented.

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.

Some fuels with the same research octane number (RON) have different HCCI engine performance. Therefore RON alone cannot be used for determining auto-ignition in HCCI combustion. The current research focuses on creating an HCCI fuel index suitable for comparing different fuels for HCCI operation. More thorough studies are needed to map the fuel effects. One way to characterize a fuel is by using the Auto-Ignition Temperature (AIT). The AIT and the amount of Low Temperature Heat Release (LTHR) together describe the auto-ignition properties of the fuel. Both can be extracted from the pressure trace. The assumption is that the pressure and temperature are known at inlet valve closing (IVC) and that the mass in the cylinder does not change after IVC. The purpose of this study was to map the AIT of different Primary Reference Fuels (PRF) for HCCI combustion at different cylinder pressures.

An index to relate fuel properties to HCCI auto-ignition would be valuable to predict the performance of fuels in HCCI engines from their properties and composition. The indices for SI engines, the Research Octane Number (RON) and Motor Octane Number (MON) are known to be insufficient to explain the behavior of oxygenated fuels in an HCCI engine. One way to characterize a fuel is to use the Auto-Ignition Temperature (AIT). The AIT can be extracted from the pressure trace. Another potentially interesting parameter is the amount of Low Temperature Heat Release (LTHR) that is closely connected to the ignition properties of the fuel. A systematic study of fuels consisting of gasoline surrogate components of n-heptane, iso-octane, toluene, and ethanol was made. 21 fuels were prepared with RON values ranging from 67 to 97.

This paper is the follow up of a previous work and its target is to demonstrate that the best fuel for a Compression Ignition engine has to be with high Octane Number. An advanced injection strategy was designed in order to run Gasoline in a CI engine. At high load it consisted in injecting 54 % of the fuel very early in the pilot and the remaining around TDC; the second injection is used as ignition trigger and an appropriate amount of cool EGR has to be used in order to avoid pre-ignition of the pilot. Substantially lower NOx, soot and specific fuel consumption were achieved at 16.56 bar gross IMEP as compared to Diesel. The pressure rise rate did not constitute any problem thanks to the stratification created by the main injection and a partial overlap between start of the combustion and main injection. Ethanol gave excellent results too; with this fuel the maximum load was limited at 14.80 bar gross IMEP because of hardware issues.

The concept of double compression, and double expansion engine (DCEE) for improving the efficiency of piston reciprocating engines was introduced in SAE Paper 2015-01-1260. This engine configuration has separate high, and low pressure units thereby effectively reducing friction losses for high effective compression ratios. The presence of an additional expander stage also theoretically allows an extra degree of freedom to manipulate the combustion heat release rate so as to achieve better optimum between heat transfer, and friction losses. This paper presents a 1-D modeling study of the engine concept in GT-Power for assessing the sensitivity of engine losses to heat release rate. The simulations were constrained by limiting the maximum pressure to 300 bar.