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The subject of this work is 3D numerical simulations of combustion and soot emissions for a passenger car diesel engine. The CFD code STAR-CD version 3.26 [1] is used to resolve the flowfield. Soot is modeled using a detailed kinetic soot model described by Mauss [2]. The model includes a detailed description of the formation of polyaromatic hydrocarbons. The coupling between the turbulent flowfield and the soot model is achieved through a flamelet library approach, with transport of the moments of the soot particle size distribution function as outlined by Wenzel et al. [3]. In this work we extended this approach by considering acetylene feedback between the soot model and the combustion model. The model was further improved by using new gas-phase kinetics and new fitting procedures for the flamelet soot library.

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.

Low temperature combustion (LTC), is a promising alternative for combustion engines, because it combines the positive aspects of both CI and SI engines, high efficiency and low emissions. Another positive aspect of LTC is that it can operate with gasoline of different octane ratings. Still, higher octane gasolines prove to be difficult to operate at low load conditions leading to high combustion instability (COV) that leads also to high emissions. This drawback can be reduced by increasing the intake air temperature or increasing compression ratio, but it is not a viable strategy in conventional applications. For a diesel engine running under LTC conditions, a possibility is to use the existing hardware, glow plugs in this case, to increase the in-cylinder temperature at low loads and facilitate an improved combustion event.

CO2 is a greenhouse gas that is believed to be one of the main contributors to global warming. Recent studies show that a combination of methanol as a renewable fuel and advanced combustion concepts could be a promising future solution to alleviate this problem. However, high unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions can be stated as the main drawback in low load operations when using methanol under advanced combustion concepts. This issue can be mitigated by modifying the stratification of the local equivalence ratio to achieve a favorable level. The stratifications evolved, and the regimes that can simultaneously produce low emissions, and high combustion efficiency can be identified by sweeping the injection timing from homogeneous charge compression ignition (HCCI) to partially premixed combustion (PPC). Understanding how the stratification of the local equivalence ratio for methanol evolves during the sweep is essential to gain these benefits.

Methanol is not a fuel typically used in compression ignition engines due to the high resistance to auto-ignition. However, conventional diesel combustion and PPC offer high engine efficiency along with low HC and CO emissions, albeit with the trade-off of increased NOx and PM emissions. This trade-off balance is mitigated in the case of methanol and other alcohol fuels, as they bring oxygen in the combustion chamber. Thus methanol compression ignition holds the potential for a clean and effective alternative fuel proposition. Most existing research on methanol is on SI engines and very little exists in the literature regarding methanol auto-ignition engine concepts. In this study, the spray characteristics of methanol inside the optically accessible cylinder of a DI-HD engine are investigated. The liquid penetration length at various injection timings is documented, ranging from typical PPC range down to conventional diesel combustion.

Internal combustion engine (ICE) fuel efficiency is a balance between good indicated efficiency and mechanical efficiency. High indicated efficiency is reached with a very diluted air/fuel-mixture and high load resulting in high peak cylinder pressure (PCP). On the other hand, high mechanical efficiency is obtained with very low peak cylinder pressure as the piston rings and bearings can be made with less friction. This paper presents studies of a combustion engine which consists of a two stage compression and expansion cycle. By splitting the engine into two different cycles, high-pressure (HP) and low-pressure (LP) cycles respectively, it is possible to reach high levels of both indicated and mechanical efficiency simultaneously. The HP cycle is designed similar to today's turbo-charged diesel engine but with an even higher boost pressure, resulting in high PCP. To cope with high PCP, the engine needs to be rigid.

The focus has recently been directed towards the engine out soot from Diesel engines. Running an engine in PPC (Partially Premixed Combustion) mode has a proven tendency of reducing these emissions significantly. In addition to combustion strategy, several studies have suggested that using alcohol fuels aid in reducing soot emissions to ultra-low levels. This study analyzes and compares the characteristics of PM emissions from naphtha gasoline PPC, ethanol PPC, methanol PPC and methanol diffusion combustion in terms of soot mass concentration, number concentration and particle size distribution in a single cylinder Scania D13 engine, while varying the intake O2. Intake temperature and injection pressure sweeps were also conducted. The fuels emitting the highest mass concentration of particles (Micro Soot Sensor) were gasoline and methanol followed by ethanol. The two alcohols tested emitted nucleation mode particles only, whereas gasoline emitted accumulation mode particles as well.

Diesel engines are one of the most important power generating units these days. Increasing greenhouse gas emission level and the need for energy security has prompted increasing research into alternative fuels for diesel engines. Biodiesel is the most popular among the alternatives for diesel fuel as it is biodegradable and renewable and can be produced domestically from vegetable oils. In recent years, hydrotreated vegetable oil (HVO) has also gained popularity due to some of its advantages over biodiesel such as higher cetane number, lower deposit formation, storage stability, etc. HVO is a renewable, paraffinic biobased alternative fuel for diesel engines similar to biodiesel. Unlike biodiesel, the production process for HVO involves hydrogen as catalyst instead of methanol which removes oxygen content from vegetable oil.

Today numerical models are a major part of the diesel engine development. They are applied during several stages of the development process to perform extensive parameter studies and to investigate flow and combustion phenomena in detail. The models are divided by complexity and computational costs since one has to decide what the best choice for the task is. 0D models are suitable for problems with large parameter spaces and multiple operating points, e.g. engine map simulation and parameter sweeps. Therefore, it is necessary to incorporate physical models to improve the predictive capability of these models. This work focuses on turbulence and mixing modeling within a 0D direct injection stochastic reactor model. The model is based on a probability density function approach and incorporates submodels for direct fuel injection, vaporization, heat transfer, turbulent mixing and detailed chemistry.

A novel 0-D Probability Density Function (PDF) based approach for the modelling of Diesel combustion using tabulated chemistry is presented. The Direct Injection Stochastic Reactor Model (DI-SRM) by Pasternak et al. has been extended with a progress variable based framework allowing the use of a pre-calculated auto-ignition table. Auto-ignition is tabulated through adiabatic constant pressure reactor calculations. The tabulated chemistry based implementation has been assessed against the previously presented DI-SRM version by Pasternak et al. where chemical reactions are solved online. The chemical mechanism used in this work for both, online chemistry run and table generation, is an extended version of the scheme presented by Nawdial et al. The main fuel species are n-decane, α-methylnaphthalene and methyl-decanoate giving a size of 463 species and 7600 reactions.

Heavy-duty direct injection compression ignition (DICI) engine running on methanol is studied at a high compression ratio (CR) of 27. The fuel is injected with a common-rail injector close to the top-dead-center (TDC) with two injection pressures of 800 bar and 1600 bar. Numerical simulations using Reynold Averaged Navier Stokes (RANS), Lagrangian Particle Tracking (LPT), and Well-Stirred-Reactor (WSR) models are employed to investigate local conditions of injection and combustion process to identify the mechanism behind the trend of increasing nitrogen oxides (NOx) emissions at higher injection pressures found in the experiments. It is shown that the numerical simulations successfully replicate the change of ignition delay time and capture variation of NOx emissions.

In recent years, stricter regulations on emissions and higher demands for more fuel efficient vehicles have led to a greater focus on increasing the efficiency of the internal combustion engine. Nowadays, there is increasing interest in the recovery of waste heat from different engine sources such as the coolant and exhaust gases using, for example, a Rankine cycle. In diesel engines 15% to 30% of the energy from the fuel can be lost to the coolant and hence, does not contribute to producing work on the piston. This paper looks at reducing the heat losses to the coolant by increasing coolant temperatures within a single cylinder Scania D13 engine and studying the effects of this on the energy balance within the engine as well as the combustion characteristics. To do this, a GT Power model was first validated against experimental data from the engine.

The concept of Partially Premixed Combustion (PPC) in internal combustion engines has shown to yield high gross indicated efficiencies, but at the expense of gas exchange efficiencies. Most of the experimental research on partially premixed combustion has been conducted on compression ignition engines designed to operate on diesel fuel and relatively high exhaust temperatures. The partially premixed combustion concept on the other hand relies on dilution with high exhaust gas recirculation (EGR) rates to slow down the combustion which results in low exhaust temperatures, but also high mass flows over cylinder, valves, ports and manifolds. A careful design of the gas exchange system, EGR arrangement and heat exchangers is therefore of utter importance. Experiments were performed on a heavy-duty, compression ignition engine using a fuel consisting of 80 volume % 95 RON service station gasoline and 20 volume % n-heptane.

Large two-stroke marine Diesel engines have special injector geometries, which differ substantially from the configurations used in most other Diesel engine applications. One of the major differences is that injector orifices are distributed in a highly non-symmetric fashion affecting the spray characteristics. Earlier investigations demonstrated the dependency of the spray morphology on the location of the spray orifice and therefore on the resulting flow conditions at the nozzle tip. Thus, spray structure is directly influenced by the flow formation within the orifice. Following recent Large Eddy Simulation resolved spray primary breakup studies, the present paper focuses on spray secondary breakup modelling of asymmetric spray structures in Euler-Lagrangian framework based on previously obtained droplet distributions of primary breakup.

Several models for ignition, combustion and emission formation under diesel engine conditions for multi-dimensional computational fluid dynamics have been proposed in the past. It has been recognized that the use of a reasonably detailed chemistry model improves the combustion and emission prediction especially under low temperature and high exhaust gas recirculation conditions. The coupling of the combustion chemistry and the turbulent flow can be achieved with different assumptions. In this paper we investigate a selection of n-heptane spray experiments published by the Engine Combustion Network (ECN spray H) with three different combustion models: well-stirred reactor model, transient interactive flamelet model and progress variable based conditional moment closure. All models cater for the use of detailed chemistry, while the turbulence-chemistry interaction modeling and the ability to consider local effects differ.

In this paper, a parametric analysis on the main engine calibration parameters applied on gasoline Partially Premixed Combustion (PPC) is performed. Theoretically, the PPC concept permits to improve both the engine efficiencies and the NOx-soot trade-off simultaneously compared to the conventional diesel combustion. This work is based on the design of experiments (DoE), statistical approach, and investigates on the engine calibration parameters that might affect the efficiencies and the emissions of a gasoline PPC. The full factorial DoE analysis based on three levels and three factors (33 factorial design) is performed at three engine operating conditions of the Worldwide harmonized Light vehicles Test Cycles (WLTC). The pilot quantity (Qpil), the crank angle position when 50% of the total heat is released (CA50), and the exhaust gas recirculation (EGR) factors are considered. The goal is to identify an engine calibration with high efficiency and low emissions.

In this study, an investigation was made on a heavy duty diesel engine using both conventional diesel combustion mode and a partially premixed combustion (PPC) mode. A segment mesh was built up and modeled using the commercial CFD code AVL FIRE, where only the closed volume cycle, between IVC and EVO, was modeled. Both combustion modes were validated using experimental data, before a number of heat flux boundary conditions were applied. These conditions were used to evaluate the engine response in terms of engine performance and emission levels for the different percentage of heat rejection. The engine performance was measured in terms of specific fuel consumption and estimated power output, while the calculated net soot and accumulated NOx mass fractions were used for comparing the emission levels. The results showed improved efficiency for both combustion types, but only the PPC combustion mode managed that without increasing the production of NOx emissions severely.

Few previous publications investigate the possibility of combining multiple waste heat sources in a combustion engine waste heat recovery system. A waste heat recovery system for a HD truck diesel engine is evaluated for utilizing multiple heat sources found in a conventional HD diesel engine. In this type of engine more than 50% of heat energy goes futile. The majority of the heat energy is lost through engine exhaust and cooling devices such as EGRC (Exhaust gas recirculation cooler), CAC (Charge air cooler) and engine cooling. In this paper, the potential of usable heat recuperation from these devices using thermodynamic analysis was studied, and also an effort is made to recuperate most of the available heat energy that would otherwise be lost. A well-known way of recuperating this heat energy is by employing a Rankine cycle circuit with these devices as heat sources (single loop or dual loop), and thus this study is focused on using a Rankine cycle for the heat recovery system.

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.

The impact of ignition quality and chemical properties on engine performance and emissions during low load partially premixed combustion (PPC) in a light-duty diesel engine were investigated. Four fuels in the gasoline boiling range, together with Swedish diesel (MK1), were operated at loads between 2 and 8 bar IMEPg at 1500 rpm, with 50% heat released located at 6 crank angle degrees (CAD) after top dead center (TDC). A single injection strategy was used, wherein the start of injection (SOI) and the injection duration were adjusted to achieve desired loads with maintained CA50, as the injection pressure was kept constant at 1000 bar. The objective of this work was to examine the low-load limit for PPC at approximately 50% EGR and λ=1.5, since these levels had been suggested as optimal in earlier studies. The low-load limits with stable combustion were between 5 and 7 bar gross IMEP for the gasoline fuels, higher limit for higher RON values.