The CFD Model of the Combustion Process in a Radial Engine

<div class="section abstract"><div class="htmlview paragraph">This article is to investigate the inlet and exit pipe effect on a rotary engine performance. A 1-dimensional, three-cylinder reciprocating engine model was adopted to simulate the operation of a rotary engine with three separate chambers. The chamber volume variation in this model was identical to a real rotary engine. The test data of the real rotary engine were used as a benchmark test for this model. Various parameters are then studied, including pipe length, pipe diameters, and pipe shape with convergent/divergent angles. In the performance analysis, the results showed that the averaged performance data (BSFC, brake work, brake torque, pressure distribution) was within 5 % in tolerance. The results of pipe length variation showed that in a range of short inlet pipe brought higher power (8.4 %). On the contrary, the exhaust pipe had a better work output over a certain length (10%). With a shorter inlet pipe and a longer exhaust pipe, the work output makes about 14.3% higher. The results of pipe diameter variation showed that in a certain range, enlarging pipe diameter gained better work output (14%). The results of pipe shape appeared that convergent inlet pipe at a certain range had a better work output up to 11% more. Divergent exhaust pipe made slight gain in work. Under a specific rotational speed, well-tuning the air pipes in length, shape and diameter would enhance the engine work output 20% more.</div></div>

<div class="htmlview paragraph">Controlled Auto-Ignition (CAI) combustion, also known as HCCI or PCCI, has recently emerged as a viable alternative combustion process to the conventional spark ignition (SI) or compression ignition (CI) process for internal combustion (IC) engines, owing to its potential for high efficiency and extremely low emissions. One of the most effective and practical means of achieving CAI combustion in an engine is to retain or recycle the burnt gases. In order to understand better the effects of recycled burnt gases on CAI combustion, detailed analytical and experimental studies have been carried out. The analytical studies were performed using an engine simulation model with detailed chemical kinetics.</div> <div class="htmlview paragraph">The five effects of the recycled burned gases studied include:</div> <div class="htmlview paragraph"> <ol class="list nostyle"> <li class="list-item"><span class="li-label">(1.)</span><div class="htmlview paragraph">Charge heating effect: higher intake charge temperature due to hot burned gases;</div></li> <li class="list-item"><span class="li-label">(2.)</span><div class="htmlview paragraph">Dilution effect: the reduction of oxygen due to the presence of the burned gases;</div></li> <li class="list-item"><span class="li-label">(3.)</span><div class="htmlview paragraph">Heat capacity effect: due to higher specific heat capacity of burned gases;</div></li> <li class="list-item"><span class="li-label">(4.)</span><div class="htmlview paragraph">Chemical effect: chemical reactions involving CO<sub>2</sub> and H<sub>2</sub>O in the burned gases</div></li> <li class="list-item"><span class="li-label">(5.)</span><div class="htmlview paragraph">Stratification effect: recycled burned gases are stratified from the fuel/air mixture.</div></li> </ol> </div> <div class="htmlview paragraph">And the main conclusions are:</div> <div class="htmlview paragraph"> <ol class="list nostyle"> <li class="list-item"><span class="li-label">(1.)</span><div class="htmlview paragraph">The charge heating effect is mainly responsible for the advanced AI timing due to hot burned gases. It increases the heat release rate and shortens the combustion duration.</div></li> <li class="list-item"><span class="li-label">(2.)</span><div class="htmlview paragraph">The dilution effect does not affect AI timing, but it extends combustion duration. Its slows down the heat release rate only when large amount of burned gases is present.</div></li> <li class="list-item"><span class="li-label">(3.)</span><div class="htmlview paragraph">The heat capacity effect is mainly responsible for the reduction in the heat release rate, and it has a similar effect to extend the combustion duration as the dilution effect.</div></li> <li class="list-item"><span class="li-label">(4.)</span><div class="htmlview paragraph">The chemical effect does not affect AI timing and heat release rate. It reduces the combustion duration slightly at high concentrations of burned gases.</div></li> <li class="list-item"><span class="li-label">(5.)</span><div class="htmlview paragraph">The stratified burned gases facilitate the CAI combustion due to the presence of higher temperature region at the boundary between of the hot burned gases and combustible charge.</div></li> </ol> </div>

<div class="section abstract"><div class="htmlview paragraph">To explore the production and oxidation characteristics of soot in the flame of diesel jet under the condition equivalent to the direct injection diesel engine condition, the effect of three different important parameters (including injection pressure, injection duration, and oxygen concentration) are experimentally examined. For these purposes, a small CVCC (constant volume combustion chamber) with the volume of 60cc equivalent to the volume of combustion chamber of automotive diesel engine is used.</div><div class="htmlview paragraph">To obtain the experimental data of soot production and oxidation, in experiments, the ambient condition of temperature, pressure and oxygen concentration before injection timing are prepared by the combustion of lean hydrogen mixture (with help of 8 spark plugs) at a high temperature and pressure condition around 1000K and 4.5MPa. The common rail type injector with 8 injection holes for modern diesel engine is attached to this vessel. Injection pressures are set up at 80, 100, and 120 MPa. The effects of three important factors are analyzed including: (1) different injection pressures with constant injection mass; (2) different injection pressures with constant injection duration; (3) oxygen concentrations of 21% and 17% in volume. The optical window is attached on one side of this vessel and the full image of the spray flames can be visualized and recorded through this window by a high-speed color video camera. From the visualized color image of diesel flame, the flame temperature and the amount of soot inside the flame can be quantified by the two color method. This method is based on the theoretical principle that the light emission from the soot cloud is a thermal radiation of gray body which is a function of soot temperature and the KL factor, an index of the amount of soot.</div><div class="htmlview paragraph">The obtained results show that the injection duration and the injection rate are the dominant factors for heat release rate (HRR). On the contrary, within the experimental condition, the heat release rate is affected by both of the injection pressure and the oxygen concentration of ambient mainly at late combustion phase. The injection pressure and the injection duration give small effect on the level of flame temperature; however, the ambient oxygen concentration gives a large effect on the flame temperature.</div><div class="htmlview paragraph">Concerning the soot production and oxidation, all of the considered parameters have large effect. The interesting result is that the effect of injection pressure and ambient oxygen concentration on the maximum value of total KL factor is not so large. However, the soot production rate becomes lower as the decrease of injection pressure and as the decrease of ambient oxygen concentration, and results in the delayed arrival of the maximum total KL factor. Comparing with the HRR curve, it is revealed that the soot oxidation reaction is occurred under the very low HRR combustion in the case of low injection pressure and oxygen concentration condition.</div></div>

<div class="section abstract"> <div class="htmlview paragraph">This SAE Standard applies only to spark plugs used for ground vehicles and stationary engines.</div> <div class="htmlview paragraph">This document is intended to serve as a guide to dimensions common to the majority of current production spark plugs and future applications. It is not the intent of this document to prohibit the manufacture of spark plugs having dimensions differing from those presented. Many applications exist which require specialized or nonstandard spark plugs. It is recommended that this document be used in spark plug design and engine applications wherever possible. Whenever design situations arise that prevent the use of one of these standard spark plugs, a spark plug manufacturer should be contacted for guidance.</div> <div class="htmlview paragraph"><span class="xref">Figures 1</span> to <span class="xref">13</span> and <span class="xref">Tables 1A</span> to <span class="xref">6</span> show typical configurations of unshielded and shielded spark plug designs, their dimensional characteristics, installation, threaded hole, and spark plug thread sizes.</div> </div>

<div class="htmlview paragraph">In a free piston engine the piston is neither connected to a crankshaft mechanism nor to any other kinematic system. Instead the piston movement is determined by the free forces that act upon it. This difference between the kinematic principle of the crankshaft engine and the free piston principle has a significant influence on the combustion process.</div> <div class="htmlview paragraph">In this paper the combustion process in a free piston engine is described on the basis of experiments. The experimental data were obtained from measurements on the free piston engine that has been developed by the Dutch company Innas.</div> <div class="htmlview paragraph">This article discusses the influence of the free piston principle on cold start, ignition delay, heat release, heat transfer, indicated efficiency and emissions.</div> <div class="htmlview paragraph">In the optimum point the engine has an indicated efficiency of 51%, a NOx emission of 6 gr/kWhi and a soot emission corresponding to a Bosch Filter Number of less than 0.5. The combustion process of the free piston engine is furthermore characterised by a nearly constant volume combustion process.</div>

The paper presents the results of the experimental research on the combustion process inside the cylinder of a large piston radial aircraft engine with dual spark plug ignition. The tests were carried out on the ASz-62IR-16X radial engine equipped with a double, electronically controlled ignition system with two spark plugs in each cylinder. The constant engine speed tests allowed for comparing the combustion processes with ignition carried out with different ignition advance angle offset between both spark plugs. In the test, the ignition advance angle for one spark plug was changed while leaving the ignition advance angle for the other spark plug. The tests performed for two engine operating points were to change the angle of the front spark plug and the rear spark plug. The analyses were based on the recorded pressure signal from one of the engine cylinders. It has been shown that the ignition advance angle offset significantly affects the combustion process. The obtained results indicate the leading role of the rear spark plug in initiating the combustion process. When ignition is carried out with a steady rear spark plug advance angle, the combustion process practically does not change when the ignition advance angle of the front spark plug changes. However, changing the rear spark plug advance angle at a constant front spark plug ignition advance angle significantly changes the combustion process.

<div class="section abstract"><div class="htmlview paragraph">Strict emission regulations and the need of higher efficiency of future dual fuel engines require an optimized combustion process. For getting a better understanding of the in-cylinder combustion process optical investigations represent a powerful tool. For medium speed dual fuel engines, optical investigations are pretty rare respectively not available. Especially the avoiding of knock events within the combustion process is a key development topic to realize high engine load and high engine efficiency.</div><div class="htmlview paragraph">For the investigations a fully flexible dual fuel test engine was used. The engine is operated with a natural gas / air cylinder charge which is ignited by a small micro pilot diesel injection within the gas mode.</div><div class="htmlview paragraph">Beside standard development measurement techniques like cylinder pressure measurement and exhaust gas emission measurement, optical measurement techniques were used to gain information about the in-cylinder combustion process of the pilot fuel combustion as well as the combustion of the natural gas / air cylinder charge. It is desirable to realize an entire visualization of the combustion within the combustion chamber under real engine conditions at full load. This means the need for a minimal invasive measurement technique with small modifications of the combustion chamber design and surface temperatures.</div><div class="htmlview paragraph">Due to these requirements a tomographic, fiber based measurement technique was used synchronal to a high speed visualization of the in-cylinder OH* chemiluminescence. The fiber based measurement technique uses 128 optical fibers in which every single fiber measures the luminescence of a special observation volume. The raw data of the single fibers is used to calculate, based on a tomographic algorithm, the in-cylinder light emission intensity distribution to visualize the combustion in a horizontal cutting plane over the whole bore diameter. To complete the information of the combustion by the third dimension, a synchronized, intensified high speed camera was used to record the OH* chemiluminescence. Both measurement techniques in combination give a far-reaching image of the in-cylinder combustion process.</div><div class="htmlview paragraph">To increase the understanding of the in-cylinder processes and to optimize the combustion process, typical combustion relevant parameters like pilot injection nozzle, pilot injection timing, pilot injection pressure and excess air ratio were varied. In this publication the basic combustion mechanisms are described for two different injection timings at full load. In addition the isolated pilot fuel combustion in pure air for an injection timing which gives a transient point between partly premixed and strongly premixed combustion is shown.</div><div class="htmlview paragraph">The measurement results show an ignition process which is strongly influenced by the reaction kinetic mechanisms. Especially for early injection timings a long ignition delay was measured which result in a well premixed pilot fuel within the natural gas / air mixture. Moreover, the flame propagation speed was calculated and a reaction kinetically controlled post flame combustion process was observed.</div><div class="htmlview paragraph">In addition a different mechanism for in-cylinder pressure oscillation, which usually is detected as engine knock, was shown which differs from the generally known mechanism in spark plug ignited passenger car engines.</div></div>

<div class="htmlview paragraph">To examine the effects of exhaust gas recirculation (EGR) on the exhaust emissions and on the cycle-by-cycle variation of combustion process in a methanol engine, the authors have tried to measure the emissions of total hydrocarbon, carbon monoxides, nitric oxides and formaldehyde and to record the cylinder pressure development histories of 1000 cycles duration. Moreover, the standard deviation of peak pressure Pmax, the heat release rate and the averaged pressure history are analyzed.</div> <div class="htmlview paragraph">The results are compared with those of gasoline fueled operation.</div> <div class="htmlview paragraph">The results are summarized as follows:</div> <div class="htmlview paragraph"> <ol class="list nostyle"> <li class="list-item"> <span class="li-label">(1)</span><div class="htmlview paragraph">In the case of a methanol-fueled engine operated under exhaust gas recirculation (EGR), similar to that already carried out in a gasoline-fueled engine, the concentration of NOx, in the exhaust gas can be decreased but those of formaldehyde (HCHO) and unburned methanol are increased.</div> </li> <li class="list-item"> <span class="li-label">(2)</span> <div class="htmlview paragraph">The effect of EGR, such as 20% of it, on Nox reduction is the same as those of hydrous methanol of 50% water content.</div> </li> <li class="list-item"> <span class="li-label">(3)</span> <div class="htmlview paragraph">In the case of EGR applied to an EFI engine, its output and thermal efficiency are decreased within the lean mixture region (λ>1.0) but these recover and the concentration of hydrocarbon is decreased by raising the fuel temperature.</div> </li> <li class="list-item"> <span class="li-label">(4)</span> <div class="htmlview paragraph">When the optimum conditions of the EGR and fuel temperature are selected, it is possible to improve the exhaust gas characteristics concerning NOx, THC, etc., while the decreased in engine output and thermal efficiency is reduced to the minimum.</div> </li> </ol> </div>

<div class="section abstract"><div class="htmlview paragraph">Numerical calculations were performed to investigate the mixture formation, ignition, and combustion processes in a diesel spray. The spray was formed by injecting n-heptane into a constant volume vessel under high-temperature and high-pressure conditions. The fuel droplets were described by a discrete droplet model (DDM). Numerical calculations for the flow and turbulent diffusion processes were performed on the basis of large eddy simulation (LES) to describe the processes of local non-homogeneous mixture formation and heat release. The oxidation processes in the mixture were calculated by Schreiber's five-step mechanism for n-heptane. Calculations were performed for sprays formed by single-stage injection and pilot/main two-stage injection. The flame structure in a diesel spray and its temporal change were discussed using a flame index proposed by Yamashita et al. The results show that combustion of a premixture release heat around the boundary between the unburnt mixture in the upstream region and burnt mixture in the downstream region, and diffusion flame is formed in the surrounding region of the burnt mixture. The different flame structures are formed according to the surrounding gas conditions. In addition, in the pilot/main two-stage injections, the ignition delay of the main spray becomes very short and the region of premixture combustion locates upstream, which could result in larger soot emissions.</div></div>

<div class="section abstract"><div class="htmlview paragraph">The spark ignition (SI) - controlled auto-ignition (CAI) hybrid combustion, also known as spark-assisted compression ignition (SACI), is achieved by utilizing the temperature and pressure rise from the early flame propagation induced by the spark-ignition to trigger the auto-ignition of the remaining unburned mixture. This hybrid combustion concept can be used to effectively extend the operating range of gasoline CAI combustion and achieve smooth transitions between SI and CAI combustion mode in gasoline engines. However, the significant cycle-to-cycle variation (CCV) of the SI-CAI hybrid combustion hinders the practical application of the hybrid combustion. In order to understand the cause of its high CCVs, the SI-CAI hybrid combustion process in a gasoline engine was studied in this study by the large eddy simulations (LES). The turbulence is modelled by the sub-grid k model. The spark ignition and subsequent flame propagation were modelled by the ECFM-3Z LES model. A tabulated database of the gasoline auto-ignition chemistry was coupled with the CFD simulations to depict the subsequent auto-ignition process of the unburned mixture after the initiation of flame propagation. The LES simulation was validated and applied to analyze the hybrid combustion process in a single cylinder engine at 1500 rpm and 5.43 bar IMEP, which was characterized with a coefficient of variation (COV) of 11.81% in IMEP. The LES simulations of 15 consecutive cycles were performed and analyzed to evaluate the potential of LES simulations to predict the CCV of SI-CAI hybrid combustion. The analysis of the LES simulation results indicates that the average thermal and compositional parameters are not the main reason for the cycle-to-cycle variations of the SI-CAI hybrid combustion. The temperature and residual gas fraction (RGF) in the spark zone is also very stable among different cycles. In comparison, the average velocity in the whole cylinder reduces from 7.8 m/s in the strong combustion cycle (Cycle 11) to 6.4 m/s in the weakest combustion cycle (Cycle 14) with 21.9% reduction, and the average velocity in the spark zone reduces from 6.3 m/s to 3.8 m/s with 60.3% reduction. Therefore, the variations of the in-cylinder flow velocity, especially around the spark plug, could be the main reason for the large variations of the hybrid combustion observed in the experiments.</div></div>

<div class="section abstract"><div class="htmlview paragraph">Wankel rotary engines (REs) are often used for unmanned aerial vehicle (UAV) applications due to their excellent power-to-weight ratio and their smooth operation. Existing RE propulsion units are mainly designed to run on high-volatility fuels like aviation gasoline or regular gasoline. However, specific applications require a jet fuel or even multi-fuel capability. Due to their geometry, the low compression ratio (CR) of REs prevents the implementation of compression ignition (CI) combustion processes. While publications of modified spark-ignition engines that are able to run on low-volatile fuels are already few in number, publications of heavy-fuel spark-ignited (SI) REs can hardly be found at all.</div><div class="htmlview paragraph">The purpose of this paper is as follows: The operation of a SI RE operated on kerosene is discussed. Accordingly, a thermodynamic analysis is carried out at warmed-up operation with kerosene. It is shown that sufficient performance and power output can be achieved on kerosene for full-load behavior. Furthermore, cold-start tests are carried out to investigate the limits of kerosene operation. Therefore, a low-temperature test bench is developed that allows the investigation of engine starts down to −30°C. The challenges of mixture formation for heavy fuels in port fuel injection systems for REs at cold-start conditions are investigated and discussed. It is demonstrated that cold starting a SI RE down to −25°C is possible with correct measures.</div></div>

<div class="section abstract"><div class="htmlview paragraph">In this paper, the results of an extensive experimental analysis regarding a twin-cylinder spark-ignition turbocharged engine are employed to build up an advanced 1D model, which includes the effects of cycle-by-cycle variations (CCVs) on the combustion process. Objective of the activity is to numerically estimate the CCV impact primarily on fuel consumption and knock behavior.</div><div class="htmlview paragraph">To this aim, the engine is experimentally characterized in terms of average performance parameters and CCVs at high and low load operation. In particular, both a spark advance and an air-to-fuel ratio (α) sweep are actuated. Acquired pressure signals are processed to estimate the rate of heat release and the main combustion events. Moreover, the Coefficient of Variation of IMEP (CoV<sub>IMEP</sub>) and of in-cylinder peak pressure (CoV<sub>pmax</sub>) are evaluated to quantify the cyclic dispersion and identify its dependency on peak pressure position.</div><div class="htmlview paragraph">In parallel, the experimentally tested engine is fully schematized in a 1D framework. The 1D model, developed in the GT-Power™ environment, makes use of user defined sub-models for the description of combustion, turbulence and knock phenomena. 1D analyses are carried out for various engine speeds, load levels, α ratios, and spark timings, without changing any tuning constant. In a first stage, the model is validated in terms of overall engine performance parameters, and ensemble-averaged pressure traces inside the cylinder, and within the intake and exhaust ducts, as well. A more detailed comparison is also performed with reference to the average rate of heat release in different operating conditions.</div><div class="htmlview paragraph">In a subsequent step, the effects of CCVs are introduced in the model in terms of Gaussian distributed modifications of the burning rate, predicted with reference to the ensemble-average operation. Consistently with the experimental data, applied burning rate are modified to simulate a train of pressure cycles statistically equivalent to the measured ones. The influence of the CCVs on the instantaneous peak position, air flow rate, and Indicated Specific Fuel Consumption (ISFC) is consequently investigated on a cycle-by-cycle basis, and compared to the average operation.</div><div class="htmlview paragraph">Numerical analyses show that CCVs cause a reduced ISFC penalty, which can be considered significant only in case of delayed combustions and increased CoVs. Knock limited spark advance is also identified with and without CCV, highlighting some additional fuel economy penalties.</div></div>

<div class="htmlview paragraph">This SI engine research investigation extends a previous study [<span class="xref">14</span>] concerning one kind of non-conventional firing device: the Pulsed Jet Combustion (PJC) igniter.</div> <div class="htmlview paragraph">The PJC device was compared to a conventional spark plug during operation in a 4-valve single cylinder engine at 2000 rpm and a variety of air/fuel ratios and loads. Additionally, skip-firing was used to vary the residual gas rate. The mass fraction burned intervals were calculated from the pressure trace for each cycle via a heat release analysis that accounted for cycle-to-cycle variations in the trapped mass of fuel. Statistical analyses were performed for 100 cycles of operation for each test condition.</div> <div class="htmlview paragraph">Similar results were found for the PJC device as for the spark plug with zero residual (five skipped cycles). For both igniters, the cycle-to-cycle variability increased with increasing residual, but the variability was less pronounced for the PJC device. It is concluded that the residual within the vicinity of the spark plug has a strong influence on cyclic variability. Therefore, it appears that the major advantage of the PJC device is that it enflames several flame kernels.</div> <div class="htmlview paragraph">However, the exhaust HC emissions are much higher for the PJC device than for the spark plug. This factor remains a critical issue in development of the PJC device for use in conventional engines.</div>

Statistical study on engine knock oscillation and heat release using multiple spark plugs and pressure sensors

<div class="htmlview paragraph"><b>THE word “ceramic” is derived from the Greek word “keramos” meaning “burned stuff,” thus indicating its inorganic character, Mr. Riddle explains. Although the word is commonly associated with the substance known as clay, in modern usage it includes a large number of minerals and rocks and a correspondingly large number of mineral products, such as the many clay products, glass in its many forms, enamels, and the cements. The chief actors on the stage of ceramic insulators, he indicates, are the silicates, especially the silicates of aluminum.</b></div> <div class="htmlview paragraph"><b>Taking up clays first in his discussion of ceramics, he shows that clays differ enormously in their physical properties chiefly because of differences in mineral structure, particle size, and the impurities associated with them because of their geological history. Quartz, feldspar, porcelain and spark-plug porcelain also are treated in his presentation. He shows that spark-plug glaze has a surprising effect on the physical properties of the insulator.</b></div> <div class="htmlview paragraph"><b>Mr. Riddle then lists and discusses some of the properties which are involved in the performance of spark plugs: density, porosity, and refractoriness; mechanical strength; thermal expansion; electrical resistance and other electrical properties; thermal conductivity; resistance to heat shock; and resistance to chemical agencies.</b></div>