Search Results

The combustion timing in internal combustion engines affects the fuel consumption, in-cylinder peak pressure, engine noise and emission levels. The combination of an in-cylinder pressure sensor together with a direct injection fuel system lends itself well for cycle-to-cycle control of the combustion timing. This paper presents a method of controlling the combustion timing by the use of a cycle-to-cycle injection-timing algorithm. At each cycle the currently estimated heat-release rate is used to predict the in-cylinder pressure change due to a combustion-timing shift. The prediction is then used to obtain a cycle-to-cycle model that relates combustion timing to gross indicated mean effective pressure, max pressure and max pressure derivative. Then the injection timing that controls the combustion timing is decided by solving an optimization problem involving the model obtained.

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.

There is growing global interest in using renewable alcohols to reduce the greenhouse gases and the reliance on conventional fossil fuels. Recent studies show that methanol combined with partially premixed combustion provide clear performance and emission benefits compared to conventional diesel diffusion combustion. Nonetheless, high unburned hydrocarbon (HC) and carbon monoxide (CO) emissions can be stated as the main PPC drawback in light load condition when using high octane fuel such as Methanol with single injection strategy. Thus, the present experimental study has been carried out to investigate the influence of multiple injection strategies on the performance and emissions with methanol fuel in partially premixed combustion. Specifically, the main objective is to reduce HC, CO and simultaneously increase the gross indicated efficiency compared to single injection strategy.

Methods for detection of the spatial position and timing of diesel ignition with improved accuracy are demonstrated in an optically accessible constant-volume chamber at engine-like pressure and temperature conditions. High-speed pressure measurement using multiple transducers, followed by triangulation correction for the speed of the pressure wave, permits identification of the autoignition spatial location and timing. Simultaneously, high-speed Schlieren and broadband chemiluminescence imaging provides validation of the pressure-based triangulation technique. The combined optical imaging and corrected pressure measurement techniques offer improved understanding of diesel ignition phenomenon. Schlieren imaging shows the onset of low-temperature (first-stage) heat release prior to high-temperature (second-stage) ignition. High-temperature ignition is marked by more rapid pressure rise and broadband chemiluminescence.

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.

Partially Premixed Combustion (PPC) provides the potential of simultaneous reduction of NOx and soot for diesel engines. This work attempts to characterize the operating range and conditions required for PPC. The characterization is based on the evaluation of emission and in-cylinder measurement data of engine experiments. It is shown that the combination of low compression ratio, high EGR rate and engine operation close to stoichiometric conditions enables simultaneous NOx and soot reduction at loads of 8bar, 12bar, and 15bar IMEP gross. The departure from the conventional NOx-soot trade-off curve has to be paid with a decline in combustion efficiency and a rise in HC and CO emissions. It is shown that the low soot levels of PPC come along with long ignition delay and low combustion temperature. A further result of this work is that higher inlet pressure broadens the operating range of Partially Premixed Combustion.

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.

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.

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.

Laser induced fluorescence (LIF) imaging during the expansion stroke, exhaust gas emissions, and cylinder pressure measurements were used to investigate the influence on combustion and CO/UHC emissions of variations in squish height and fuel spray targeting on the piston. The engine was operated in a highly dilute, partially premixed, low-temperature combustion mode. A small squish height and spray targeting low on the piston gave the lowest exhaust emissions and most rapid heat release. The LIF data show that both the near-nozzle region and the squish volume are important sources of UHC emissions, while CO is dominated by the squish region and is more abundant near the piston top. Emissions from the squish volume originate primarily from overly lean mixture. At the 3 bar load investigated, CO and UHC levels in mixture leaving the bowl and ring-land crevice are low.

An estimation model which uses the gross heat release data and the fuel energy to estimate the total amount of emissions and unburned Hydro Carbon (HC) is developed. Gross heat release data is calculated from a self-tuned heat release method which uses in-cylinder pressure data for computing the energy released during combustion. The method takes all heat and mass losses into account. The method estimates the polytropic exponent and pressure offset during compression and expansion using a nonlinear least square method. Linear interpolation of polytropic exponent and pressure offset is then performed during combustion to calculate the gross heat release during combustion. Moreover the relations between the emissions specifically HC and Carbon Monoxide (CO) are investigated. The model was validated with experimental data and promising results were achieved.

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.

Soot emissions from diesel internal combustion engines are strictly regulated nowadays. Laser extinction measurement (LEM) and natural luminosity (NL) of sooty flames are commonly applied to study soot. LEM measures soot along the laser beam path and it can probe soot regardless of temperature. NL integrates the whole field of view and relies on soot temperature. In this work, a comparison of simultaneously recorded LEM and NL data has been performed in a heavy-duty optical engine. A 685 nm laser beam is used for LEM. The laser was modulated at 63 kHz, which facilitated subtraction of the background NL signal from the raw LEM data. By Beer-Lambert’s law, KL factor can be calculated and used as a metric to describe soot measurements. A compensation of transmitted laser intensity fluctuation and soot deposits on optical windows has been performed in this work.

In order to reduce nitrogen oxides (NOx) and soot emissions while maintaining high thermal efficiency, more advanced combustion concepts have been developed over the years, such as Homogeneous Charge Compression Ignition (HCCI) and Partially Premixed Combustion (PPC), as possible combustion processes in commercial engines. Compared to HCCI, PPC has advantages of lower unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions; however, due to increased fuel stratifications, soot emissions can be a challenge when adding Exhaust-Gas Recirculation (EGR) gas. The current work presents particle size distribution measurements performed from HCCI-like combustion with very early (120 CAD BTDC) to PPC combustion with late injection timing (11 CAD BTDC) at two intake oxygen rates, 21% and 15% respectively. Particle size distributions were measured using a differential mobility spectrometer DMS500.

Low temperature combustion modes, such as Homogeneous Charge Compression Ignition (HCCI) and Partially Premixed Combustion (PPC), have been researched over recent decades since the concepts show promise for high efficiency and low emissions compared to conventional diesel combustion. PPC is an intermediate combustion type ranging from HCCI-like combustion to diesel-like combustion. The purpose of this paper is to study optically how the combustion and ignition are affected by different start of injection (SOI) timings. The study is carried out in an optically accessible heavy-duty single-cylinder engine with swirl. The intake pressure was kept constant while the intake temperature was varied to keep the combustion phasing (CA50) constant at ∼3 CAD atdc during an SOI sweep. The fuel used is a mix of primary reference fuels with octane number 81. To determine where the combustion starts, high-speed combustion imaging is used to detect the natural luminosity.

Partially premixed combustion (PPC) can be applied to decrease emissions and increase fuel efficiency in direct injection, compression ignition (DICI) combustion engines. PPC is strongly influenced by the mixing of fuel and oxidizer, which for a given fuel is controlled mainly by (a) the fuel injection, (b) the in-cylinder flow, and (c) the geometry and dynamics of the engine. As the injection timings can vary over a wide range in PPC combustion, detailed knowledge of the in-cylinder flow over the whole intake and compression strokes can improve our understanding of PPC combustion. In computational fluid dynamics (CFD) the in-cylinder flow is sometimes simplified and modeled as a solid-body rotation profile at some time prior to injection to produce a realistic flow field at the moment of injection. In real engines, the in-cylinder flow motion is governed by the intake manifold, the valve motion, and the engine geometry.

The three-way catalytic converter (TWC) is the most common catalyst for gasoline engine exhaust gas after treatment. The reduction of carbon monoxide (CO), nitrogen oxides (NOx) and unburned hydrocarbons (HC) is achieved via oxidation of carbon monoxide and hydrocarbons, and reduction of nitrogen oxides. These conversion effects were simulated in previous works using single-channel approaches and detailed kinetic models. In addition to the single-channel model multiple representative catalyst channels are used in this work to take heat transfer between the channels into account. Furthermore, inlet temperature distribution is considered. Each channel is split into a user given number of cells and each cell is treated like a perfectly stirred reactor (PSR). The simulation is validated against an experimental four-stroke engine setup with emission outputs fed into a TWC.

Partially Premixed Combustion (PPC) is a promising combustion concept for future IC engines. However, controllability of PPC is still a challenge and needs more investigation. The scope of the present study is to investigate the ignition sensitivity of PPC to the injection timing at different injection pressures. To better understand this, high-speed shadowgraphy is used to visualize fuel injection and evaporation at different Start of Injections (SOI). Spray penetration and injection targeting are derived from shadowgraphy movies. OH* chemiluminescence is used to comprehensively study the stratification level of combustion which is helpful for interpretation of ignition sensitivity behavior. Shadowgraphy results confirm that SOI strongly affects the spray penetration and evaporation of fuel. However, spray penetration and ignition sensitivity are barely affected by the injection pressure.

A previously presented robust and fast diagnostic NOx model was modified into a predictive model. This was done by using simple yet physically-based models for fuel injection, ignition delay, premixed heat release rate and diffusion combustion heat release rate. The model can be used both for traditional high temperature combustion and for high-EGR low temperature combustion. It was possible to maintain a high accuracy and calculation speed of the NOx model itself. The root mean square of the relative model error is 16 % and the calculation speed is around one second on a PC. Combustion characteristics such as ignition delay, CA50 and the general shape of the heat release rate are well predicted by the combustion model. The model is aimed at real time NOx calculation and optimization in a vehicle on the road.