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Technical Paper

CFD Investigation of Heat Transfer in a Diesel Engine with Diesel and PPC Combustion Modes

2011-08-30
2011-01-1838
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.
Technical Paper

Modeling and Investigation of Exothermic Centers in HCCI Combustion

2009-04-20
2009-01-0131
The formation of exothermic centers was modeled with a Stochastic Reactor Model (SRM) to investigate their impact on HCCI combustion. By varying the exhaust valve temperature, and thus assigning more realistic wall temperatures, the formation of exothermic centers and the ignition timing was shifted in time. To be able to study the exothermic centers, their formation and their distribution, Scatter plots, standard deviation plots and Probability Density Function (PDF) plots were constructed on the basis of the data the SRM calculations provided. The standard deviation for the particle temperatures was found to be an useful indicator of the degree of homogeneity within the combustion chamber, and thus of how efficient the combustion process was. It was observed that when the standard deviation of the temperature was higher, the emissions of CO and of hydrocarbons present at the end of the closed cycle were higher.
Technical Paper

Close to Stoichiometric Partially Premixed Combustion -The Benefit of Ethanol in Comparison to Conventional Fuels

2013-04-08
2013-01-0277
Partially Premixed Combustion, PPC, with 50% Exhaust Gas Recirculation (EGR) at lean combustion conditions λ =1.5, has shown good efficiency and low emissions in a heavy-duty single-cylinder engine. To meet emission requirements in all loads and transient operation, aftertreatment devices are likely needed. Reducing λ to unity, when a three-way catalyst can be applied, extremely low emissions possibility exists for stoichiometric PPC. In this study, the possibility to operate clean PPC from lean condition to stoichiometric equivalence ratio with reasonable efficiency and non-excessive soot emission was investigated. Two EGR rates, 48% and 38% with two fuel rates were determined for 99.5 vol% ethanol in comparison with one gasoline fuel and Swedish diesel fuel (MK1). Engine was operated at 1250 rpm and 1600 bar injection pressure with single injection. Results revealed that efficiency was reduced and soot emission increased from lean PPC to stoichiometric PPC operation.
Technical Paper

A PDF-Based Model for Full Cycle Simulation of Direct Injected Engines

2008-06-23
2008-01-1606
In one-dimensional engine simulation programs the simulation of engine performance is mostly done by parameter fitting in order to match simulations with experimental data. The extensive fitting procedure is especially needed for emissions formation - CO, HC, NO, soot - simulations. An alternative to this approach is, to calculate the emissions based on detailed kinetic models. This however demands that the in-cylinder combustion-flow interaction can be modeled accurately, and that the CPU time needed for the model is still acceptable. PDF based stochastic reactor models offer one possible solution. They usually introduce only one (time dependent) parameter - the mixing time - to model the influence of flow on the chemistry. They offer the prediction of the heat release, together with all emission formation, if the optimum mixing time is given.
Technical Paper

Investigation of Partially Premixed Combustion Characteristics in Low Load Range with Regards to Fuel Octane Number in a Light-Duty Diesel Engine

2012-04-16
2012-01-0684
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.
Technical Paper

A Fast Tool for Predictive IC Engine In-Cylinder Modelling with Detailed Chemistry

2012-04-16
2012-01-1074
This paper reports on a fast predictive combustion tool employing detailed chemistry. The model is a stochastic reactor based, discretised probability density function model, without spatial resolution. Employing detailed chemistry has the potential of predicting emissions, but generally results in very high CPU costs. Here it is shown that CPU times of a couple of minutes per cycle can be reached when applying detailed chemistry, and CPU times below 10 seconds per cycle can be reached when using reduced chemistry while still catching in-cylinder in-homogeneities. This makes the tool usable for efficient engine performance mapping and optimisation. To meet CPU time requirements, automatically load balancing parallelisation was included in the model. This allowed for an almost linear CPU speed-up with number of cores available.
Technical Paper

The Usefulness of Negative Valve Overlap for Gasoline Partially Premixed Combustion, PPC

2012-09-10
2012-01-1578
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.
Technical Paper

Waste Heat Recovery from Multiple Heat Sources in a HD Truck Diesel Engine Using a Rankine Cycle - A Theoretical Evaluation

2012-09-10
2012-01-1602
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.
Technical Paper

Diesel-PPC engine: Predictive Full Cycle Modeling with Reduced and Detailed Chemistry

2011-08-30
2011-01-1781
Partially Premixed Combustion (PPC) engines have demonstrated a potential for high efficiency and low emissions operation. To be able to study the combustion in detail but also to perform parametric studies on the potential of the PPC concept a one dimensional (1D) engine simulation tool was used with 1; a prescribed burn rate 2; predictive combustion tool with reduced chemical model and 3; predictive combustion tool with detailed chemical models. Results indicate that fast executing reduced chemistry work reasonably well in predicting PPC performance and that n-decane is possibly a suitable diesel substitute in PPC modeling while n-heptane is not.
Technical Paper

Using Oxygenated Gasoline Surrogate Compositions to Map RON and MON

2014-04-01
2014-01-1303
Gasoline fuels are complex mixtures which consist of more than 200 different hydrocarbon species. In order to decrease the chemical and physical complexity, oxygenated surrogate components were used to enhance the fundamental understanding of partially premixed combustion (PPC). The ignition quality of a fuel is measured by octane number. There are two methods to measure the octane number: research octane number (RON) and motor octane number (MON). In this paper, RON and MON were measured for a matrix of n-heptane, isooctane, toluene, and ethanol (TERF) blends spanning a wide range of octane number between 60.6 and 97. First, regression models were created to derive RON and MON for TERF blends. The models were validated using the standard octane test for 17 TERF blends. Second, three different TERF blends with an ignition delay (ID) of 8 degrees for a specific operating condition were determined using a regression model.
Technical Paper

System Simulations to Evaluate the Potential Efficiency of Humid Air Motors

2013-10-14
2013-01-2646
In the quest for efficiency improvement in heavy duty truck engines, waste heat recovery could play a valuable role. The evaporative cycle is a waste heat recovery technology aimed at improving efficiency and decreasing emissions. A humid air motor (HAM) uses the waste heat from the exhaust of the engine to humidify the inlet air; this humid air, with higher specific heat, reduces NOx emission to a greater extent [1] [2]. Despite this benefit of emission reduction, the increase or decrease in efficiency of the humid air motor compared to the conventional engine is not discussed in the literature [3] [4] [5]. In this paper, an attempt is made to study the efficiency of the HAM using system model simulations of a 13-liter heavy duty Volvo engine with a humidifier. The commercial software GT-SUITE is used to build the system model and to perform the simulations. The efficiency improvement of the HAM comes from the expansion of the vapor mass flow produced as a result of humidification.
Technical Paper

Loss Analysis of a HD-PPC Engine with Two-Stage Turbocharging Operating in the European Stationary Cycle

2013-10-14
2013-01-2700
Partially Premixed Combustion (PPC) has demonstrated substantially higher efficiency compared to conventional diesel combustion (CDC) and gasoline engines (SI). By combining experiments and modeling the presented work investigates the underlying reasons for the improved efficiency, and quantifies the loss terms. The results indicate that it is possible to operate a HD-PPC engine with a production two-stage boost system over the European Stationary Cycle while likely meeting Euro VI and US10 emissions with a peak brake efficiency above 48%. A majority of the ESC can be operated with brake efficiency above 44%. The loss analysis reveals that low in-cylinder heat transfer losses are the most important reason for the high efficiencies of PPC. In-cylinder heat losses are basically halved in PPC compared to CDC, as a consequence of substantially reduced combustion temperature gradients, especially close to the combustion chamber walls.
Technical Paper

Effects of EGR and Intake Pressure on PPC of Conventional Diesel, Gasoline and Ethanol in a Heavy Duty Diesel Engine

2013-10-14
2013-01-2702
Partially Premixed Combustion (PPC) has the potential of simultaneously providing high engine efficiency and low emissions. Previous research has shown that with proper combination of Exhaust-Gas Recirculation (EGR) and Air-Fuel equivalence ratio, it is possible to reduce engine-out emissions while still keeping the engine efficiency high. In this paper, the effect of changes in intake pressure (boost) and EGR fraction on PPC engine performance (e.g. ignition delay, burn duration, maximum pressure rise rate) and emissions (carbon monoxide (CO), unburned hydrocarbon (UHC), soot and NOX) was investigated in a single-cylinder, heavy-duty diesel engine. Swedish diesel fuel (MK1), RON 69 gasoline fuel and 99.5 vol% ethanol were tested. Fixed fueling rate and single injection strategy were employed.
Technical Paper

Experimental Comparison of Heat Losses in Stepped-Bowl and Re-Entrant Combustion Chambers in a Light Duty Diesel Engine

2016-04-05
2016-01-0732
Heat loss is one of the greatest energy losses in engines. More than half of the heat is lost to cooling media and exhaust losses, and they thus dominate the internal combustion engine energy balance. Complex processes affect heat loss to the cylinder walls, including gas motion, spray-wall interaction and turbulence levels. The aim of this work was to experimentally compare the heat transfer characteristics of a stepped-bowl piston geometry to a conventional re-entrant diesel bowl studied previously and here used as the baseline geometry. The stepped-bowl geometry features a low surface-to-volume ratio compared to the baseline bowl, which is considered beneficial for low heat losses. Speed, load, injection pressure, swirl level, EGR rate and air/fuel ratio (λ) were varied in a multi-cylinder light duty engine operated in conventional diesel combustion (CDC) mode.
Technical Paper

Transition from HCCI to PPC: the Sensitivity of Combustion Phasing to the Intake Temperature and the Injection Timing with and without EGR

2016-04-05
2016-01-0767
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.
Technical Paper

Potential Levels of Soot, NOx, HC and CO for Methanol Combustion

2016-04-05
2016-01-0887
Methanol is today considered a viable green fuel for combustion engines because of its low soot emissions and the possibility of it being produced in a CO2-neutral manner. Methanol as a fuel for combustion engines have attracted interest throughout history and much research was conducted during the oil crisis in the seventies. In the beginning of the eighties the oil prices began to decrease and interest in methanol declined. This paper presents the emission potential of methanol. T-Φ maps were constructed using a 0-D reactor with constant pressure, temperature and equivalence ratio to show the emission characteristics of methanol. These maps were compared with equivalent maps for diesel fuel. The maps were then complemented with engine simulations using a stochastic reactor model (SRM), which predicts end-gas emissions. The SRM was validated using experimental results from a truck engine running in Partially Premixed Combustion (PPC) mode at medium loads.
Technical Paper

Review and Benchmarking of Alternative Fuels in Conventional and Advanced Engine Concepts with Emphasis on Efficiency, CO2, and Regulated Emissions

2016-04-05
2016-01-0882
Alternative fuels have been proposed as a means for future energy-secure and environmentally sustainable transportation. This review and benchmarking show that several of the alternative fuels (e.g. methanol, ethanol, higher alcohols, RME, HVO, DME, and biogas/CNG) work well with several different engine concepts such as conventional SI, DICI, and dual fuel, and with the emerging concepts HCCI, RCCI, and PPC. Energy consumption is in most cases similar to that of diesel or gasoline, with the exception of methanol and ethanol that use less energy, especially in SI engines. Tailpipe emissions of CO2 with respect to engine work output (tank-to-output shaft) can be reduced by more than 15% compared to a highly efficient gasoline SI engine, and are the lowest with CNG / lean-burn SI and with alcohols in several engine concepts. Alternative fuels are considered safe and in most cases are associated with reduced risk with respect to cancer and other health and environmental issues.
Technical Paper

Gasoline Surrogate Fuels for Partially Premixed Combustion, of Toluene Ethanol Reference Fuels

2013-10-14
2013-01-2540
Partially premixed combustion (PPC) is intended to improve fuel efficiency and minimize the engine-out emissions. PPC is known to have the potential to reduce emissions of nitrogen oxides (NOx) and soot, but often at the expense of increased emissions of unburned hydrocarbons (HC) and carbon monoxide (CO). PPC has demonstrated remarkable fuel flexibility and can be operated with a large variety of liquid fuels, ranging from low-octane, high-cetane diesel fuels to high-octane gasolines and alcohols. Several research groups have demonstrated that naphtha fuels provide a beneficial compromise between functional load range and low emissions. To increase the understanding of the influence of individual fuel components typically found in commercial fuels, such as alkenes, aromatics and alcohols, a systematic experimental study of 15 surrogate fuel mixtures of n-heptane, isooctane, toluene and ethanol was performed in a light-duty PPC engine using a design of experiment methodology.
Technical Paper

Emission Formation Study of HCCI Combustion with Gasoline Surrogate Fuels

2013-10-14
2013-01-2626
HCCI combustion can be enabled by many types of liquid and gaseous fuels. When considering what fuels will be most suitable, the emissions also have to be taken into account. This study focuses on the emissions formation originating from different fuel components. A systematic study of over 40 different gasoline surrogate fuels was made. All fuels were studied in a CFR engine running in HCCI operation. Many of the fuels were blended to achieve similar RON's and MON's as gasoline fuels, and the components (n-heptane, iso-octane, toluene, and ethanol) were chosen to represent the most important in gasoline; nparaffins, iso-paraffins, aromatics and oxygenates. The inlet air temperature was varied from 50°C to 150°C to study the effects on the emissions. The compression ratio was adjusted for each operating point to achieve combustion 3 degrees after TDC. The engine was run at an engine speed of 600 rpm, with ambient intake air pressure and with an equivalence ratio of 0.33.
Technical Paper

Multi Cylinder Partially Premixed Combustion Performance Using Commercial Light-Duty Engine Hardware

2014-10-13
2014-01-2680
This work investigates the performance potential of an engine running with partially premixed combustion (PPC) using commercial diesel engine hardware. The engine was a 2.01 SAAB (GM) VGT turbocharged diesel engine and three different fuels were run - RON 70 gasoline, RON 95 Gasoline and MK1 diesel. With the standard hardware an operating range for PPC from idle at 1000 rpm up to a peak load of 1000 kPa IMEPnet at 3000 rpm while maintaining a peak pressure rise rate (PPRR) below 7 bar/CAD was possible with either RON 70 gasoline and MK1 diesel. Relaxing the PPRR requirements, a peak load of 1800 kPa was possible, limited by the standard boosting system. With RON 95 gasoline it was not possible to operate the engine below 400 kPa. Low pressure EGR routing was beneficial for efficiency and combined with a split injection strategy using the maximum possible injection pressure of 1450 bar a peak gross indicated efficiency of above 51% was recorded.
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