P
OZNANU
NIVERSITY OFT
ECHNOLOGY FACULTY OF MACHINES AND TRANSPORTATIONDOCTORAL DISSERTATION
OPTIMIZATION OF COMBUSTION PROCESS
IN INDUSTRIAL GAS ENGINES
M.SC.JAKUB ŁUKASZ ROJEWSKI
SUPERVISOR:
PROF.TOMASZ IGNACY DOBSKI
AUXILIARY SUPERVISOR: PH.D.RAFAŁ ŚLEFARSKI
Poznań 2014
Jakub Łukasz Rojewski is a scholarship holder within the project “Scholarship support for PH.D. students specializing in majors strategic for Wielkopolska’s development”, Sub-measure 8.2.2 Human Capital Operational Programme, co-financed by European
Union under the European Social Fund.
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all who contributed to this dissertation, especially to Professor Tomasz Dobski, my colleagues at Gas Technology Laboratory for their tips and comments.
To my wife Aleksandra and parents for their patience
Table of Contents
3
Abstract. ... 5
Nomenclature ... 6
1 Genesis ... 8
2 Scope of thesis ... 10
3 Gas engines ... 11
3.1 Cycles of internal combustion engines ... 11
3.1.1 Two-stroke engine: ... 12
3.1.2 Four – stroke engine ... 14
3.1.3 Atkinson cycle ... 16
3.1.4 Miller cycle ... 18
3.2 The supply of fuel gas/mixture: ... 21
3.2.1 Central system of preparing the mixture and two-stage charging: ... 21
3.2.2 Woodward TecJet™:... 22
3.2.3 Woodward SogavTM: ... 23
3.2.4 Venturi mixers: ... 24
3.2.5 MAN ME-GI + ME-LGI: ... 25
3.2.6 Wärtsilä DF: ... 27
3.3 Ignition systems ... 28
3.3.1 Spark plug ... 28
3.3.2 Prechamber ... 30
3.3.3 MAN PGI (Performance Gas Injection): ... 30
3.3.4 Micro-pilot ... 32
3.3.5 Laser ignition ... 32
3.3.6 Summary: ... 33
3.4 The future of two-stroke engines ... 34
4 Combustion process ... 37
4.1 Adiabatic temperature of combustion process ... 37
4.2 Temperature of self-ignition ... 38
4.3 Laminar velocity of flame. ... 41
4.4 Minimum energy of ignition ... 44
4.5 Emissions of toxic compounds ... 46
4.5.1 Nitrogen oxides ... 46
4.5.2 Carbon monoxide ... 48
4.5.3 Hydrocarbons ... 49
5 The research part ... 50
4
5.1 The crank-mechanism of gas engine-compressor Cooper-Bessemer
GMVH-12 ... 50
5.2 Inlet and exhaust ports ... 56
5.2.1 The left row of cylinders: ... 60
5.2.2 The right row of the cylinders ... 68
5.3 Fuel supply system ... 75
5.4 Measurement of pressure in the cylinders Cooper-Bessemer GMVH-12 engine ... 77
5.4.1 Measuring equipment ... 77
5.4.2 Pressure measurement ... 80
5.4.3 The analysis of exhaust gases ... 87
6 Computational model ... 92
6.1 The model of real gas ... 92
6.2 Model of specific heat at constant volume ... 95
6.3 Model of isentropic exponent ... 96
6.4 The model of dynamic viscosity ... 97
6.5 One-dimensional flow ... 99
6.6 Combustion process ... 111
7 Comparison of simulation results with measurements ... 114
8 Summary ... 119
9 Bibliography ... 120
10 List of figures ... 123
11 List of tables ... 128
Abstract
5
Abstract
In the thesis appeared a number of attempts to solve the problems the companies using in Polish pipeline system the gas engine-compressor Cooper-Bessemer GMVH-12 have been coping with for years. Undeniably, there is a need to modernize outdated (nearly forty years old) machines. Despite their age, these machines play the extremely important role in the natural gas pipeline system, and thus they are vital for the entire energy economy of the country. Failures to upgrade these machines are due to many factors. The most important include an almost complete lack of technical documentation, lack of knowledge and skilled personnel to handle these machines and the economic pressures forcing the continuous operation of machinery. All technical changes, repairs, upgrades require stopping the machine. Stop means huge costs that users are unwilling to pay without knowledge about the final result of the investment.
Making a simulation of a specific solution using the simulation model in real time, and determine the resulting benefits of reduced emissions of NOx, CxHy and reduce fuel consumption, clearly shows the potential effects.
The advantage of the simulation model is a wide possibility of configuration and selection of the most optimal one for the engine system.
In this study, for flow calculations there was used finite difference method of flux correction, proven in industrial practice by employees of Brown Boveri. For this purpose, there was provided a correctness of gas models with respect to the reality by the use of data from GERG-2004.
As part of the study there were carried out measurements and calculations of the atypical crank-piston system and the load exchange in the cylinder. The outcome are numerous mathematical relations, graphs and diagrams. Models designed on their basis were used in the simulation. Their construction allows the introduction of modules enabling the analysis of the crank-piston system with regard to resulting forces and moments.
On the basis of available data, were developed gas models, which is a working factor of inlet, exhaust or fuel system. It consists of fifteen pure gases, which can form, any mixtures. It can therefore be used to simulate another gas supply, e.g. nitrogenated.
SciLab program allows to combine smaller models into larger systems forming, among others, crank-piston system, cylinder and exhaust manifold. It is thus possible to model the whole engine with all operating systems. Collected data allowed a full simulation of combustion process in Cantera program. This free software allows a complete simulation of combustion including kinetics of reaction. Final results in the form of graphs comparing the measured values and determined by simulation confirm their convergence.
The thesis confirms the justification of use of geometrically simplified simulation for use in industry.
6
Nomenclature
General symbols:
surface area, bottom dead center,
heat capacity at constant volume, heat capacity at constant pressure, internalenergy,
energy,
emissivity of the medium,
electric efficiency of power generation, acceleration due to gravity,
mass fraction of gas i in the mixture, minimum ignition energy,
enthalpy,
reaction rate coefficient, thermal conductivity, adiabatic exponent,
second coefficient of viscosity, excess air ratio,
mass,
dynamic viscosity,
number of moles of j presents in the system, shear force,
time step,
lenght of a control volume, chemical reaction rate, pressure,
rate of heat evolution, heat,
radius,
individual gas constant,
Nomenclature
7 density,
distance between electrodes, laminar flame speed, Stefan-Boltzmann constant, time,
wall thickness, temperature, top dead center, speed,
specific volume, volume,
work.
Cylinders in Cooper-Bessemer GMVH-12:
8
1 Genesis
In Polish pipeline system of high pressure of twenty gas engine-compressors of Cooper- Bessemer type GMV are used. This is a construction connecting two-stroke gas engine with piston compressor by means of common crankshaft (Figure 1.1).
Figure 1.1 Cooper-Bessemer Motosprężarka GMVH-12 (a - view of the complete engine and compressor, b - cross section showing how to connect the motor and compressor).
The first of them were installed in Poland in the seventies. The oldest machines have worked more than 100 thousand hours. Elements such as pistons, piston rings and bearings were changed several times, but these are the elements of routine wear and tear.
The report of INGAA [1] clearly shows that the costs of operating these machines are lower than of compressors driven by gas turbines and electric motors. The cost of the revitalization of the motor is several times lower than the purchase price of a new engine. In the United States till now there have been working about 2600 two-stroke gas-engine compressors [2] [3] manufactured by companies like Cooper-Besser, Clark or Ingersol-Rand. Most of them were manufactured soon after World War II. Due to their simple construction and age the engines have quite a huge potential as for their modernizing. [4].This is not an easy process because the machines work continually and this does not allow for a complicated and time consuming examination. The construction connecting the compressor and the engine by means of common crankshaft causes that during the work of engine the compressor is also operating, pumping at least 30000 Nm3 of compressed gas under the pressure of 54 bars per hour (specifications for GMVH-12).
Companies that produced these machines do not exist any longer so there are also no factory research positions. It is only possible to test new solutions for engines operating in the pipeline system. This is not easy because of the commitments of pipeline operators for the proper mass flow in gas pipeline and the continuity of supply. It is therefore not possible to verify and test new solutions on operating systems, without disrupting the system. Pipeline
1 Genesis
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operators expect proven and reliable solutions, as they responsibilities are having a substantial impact on the safety and efficiency of the energy systems of the country.
The only rational solution is simulation of the whole engine together with heat exchange and combustion process. Considering geometrical complexity of the whole engine and calculating potentials of modern computers 3-D simulation of the whole unit does not seem to be rational. This is not real to get a complete for such calculations in the form of full engine geometry and boundary conditions like fuel gas pressure on valves, the temperature of cylinder walls etc. It is, however, possible to get the simulation of engine, consisting of one-dimensional gas flow with parameters close to reality and zero or one-dimensional combustion process in the cylinder. Such a calculation model allows to simulate the processes occurring in the engine smoothly and relatively quickly. However, the results obtained will have values close to reality.
Commercial soft dedicated for this type of simulation (GT Power, AVL Fire, Ricardo Vectis, etc.) cannot make calculations for two-stroke engines with a specific crank-piston system. This is very expensive not only because of purchase price but also because it requires the employment of highly qualified staff to operate it. Companies dealing with maintenance of gas engine-compressors are specialized in technical issues. However they will be potentially interested in the results of such simulation.
While using of open source type like Scilab and Cantera it is possible to create a tool necessary for optimization of work of two stroke gas engine-compressors working in the pipeline system.
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2 Scope of thesis
The previous chapter deals with the issue of inability to run a complex optimization of work of Cooper-Bessemer GMVH-12 gas engines operating in Polish pipeline system. The aim of the thesis is setting up a simulation model that would be based on open-source, generally available software that would allow the simulation of machine operations, implementation of changes in fuel system, intake and exhaust and control systems. The outcome of these simulations will be the basis for optimization of the work of engines in terms of NOX and CXHY
emission and fuel consumption. Simulation model is supposed to provide pipeline system operators or engine users with specific, quickly available results. Simulation model will enable optimization of possibilities of the above parameters with no operational interruptions of transmission units.
The need to develop such a model results from the lack of a reliable and economical software that could be employed for such optimization. The concern is not only the shortage of the software but also the lack of knowledge about machines which are to be modernized.
Technical documentation is very poor. Missing engine specifications needed to implement to the simulation model should be obtained from specific objects. The only feasible option of carrying out such an operation requires planned overhaul of the engine.
Rudimentary technical documentation on these engines calls for the need to develop, virtually from the ground up, the dependencies of the crank-piston systems. Relevant dependencies described in literature refer only to traditional systems.
The simulation model will require the development of crank-piston models, models of gases in the air and models of natural gas and exhaust gases as there are no ready-made libraries containing these models, and they were not available in Scilab.
The knowledge of the principles ruling the dynamics of gases has been known for years but its implementation in the Scilab will be a complete novelity. The use of flow simulation with proper accuracy and the use of the results in Cantera for the purpose of calculation of the combustion process will provide a broad spectrum of options as for GMVH optimization.
Depending on your needs you will be able to optimize the shape of the cam of fuel gas supply system, fuel valve springs, ignition timing and boost pressure.
It should be noted that during the forty year period of operation no attention has been paid to completing technical documentation of the engines nor to collecting information about the use of these machines and selecting options of operational parameters. This negligence cannot be made up. In spite of similarities, each machine is different not only in terms of its shape and surface area of inlet and exhaust ports but also in terms of technical condition and wear of particular elements. However the application of simulation model will help take advantage of still existing potential of these machines bringing profits to energy economy of the country.
3 Gas engines
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3 Gas engines
3.1 Cycles of internal combustion engines
As the theme of the thesis is connected with piston engine, its simplified geometry and relevant terminology will be presented prior to the operating cycles. Figures 3.1a and 3.1b show the typical scheme of crank piston internal combustion engine. The crankshaft rotary motion evokes the reciprocating motion of the piston.
Figure 3.1 Scheme showing engine piston in the position called:
a - TDC (Top Dead Center) volume of compression chamber Vcch marked pink, b - BDC (Bottom Dead Center) total volume of the cylinder Vc marked pink.
The notions referring to the piston end positions are very important:
Top Dead Center (Figure 3.1a) – TDC. According to the geometry shown in Figure 3.1, it is a position for which the piston is farthest from the crankshaft, the volume of the compression is the smallest then and is called the volume of the compression chamber and means
Bottom Dead Center (Figure 3.1b) – BDC. This is a position in which the piston is closest to the crankshaft, the volume of the cylinder is the greatest then and it is called the total volume of the cylinder, this is the sum of the compression chamber volume and stroke volume - resulting from the reciprocating movement of the piston.
The thesis describes selected cycles which, according to the author, are most relevant to understand descriptions included in further parts.
12 3.1.1 Two-stroke engine:
In case of two-stroke engine with uniflow scavenging, where controling of the load flow is carried out by inlet ports and exhaust valves we can distinguish the following cycles:
scavenging (Figure 3.2):
Figure 3.2 Two-stroke engine with uniflow scavenging - scavenging.
The process takes place when exhaust valves are open and at the same time inlet ports are uncovered. The content of the cylinder is scavenged by air which is supplied at the pressure higher than the atmospheric one (turbocharger or compressor).
intake (Figure 3.3):
Figure 3.3 Two-stroke engine with uniflow scavenging- intake.
With the closure of the exhaust valve of the cylinder there starts filling with fresh load, i.e. depending on the way of the supply either with air-fuel mixture or with the air itself.
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compression (Figure 3.4):
Figure 3.4 Two-stroke engine with uniflow scavenging - compression.
It starts with the closure of intake ports and finishes when the piston reaches TDC, depending on the construction, during compression there may be fuel injection or/and ignition a few degrees before TDC.
work (Figure 3.5):
Figure 3.5 Two-stroke engine with uniflow scavenging – work.
Work begins at the moment of crossing TDC by the piston and it continues to open the exhaust valves. It is a cycle in which the pressure acting on the piston results in a force by the piston crank system on the shaft.
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exhaust (Figure 3.6):
Figure 3.6 Two-stroke engine with uniflow scavenging - exhaust.
It starts with opening of exhaust valves when hot exhaust gases are sucked into exhaust system. The process is intensified during scavenging which starts with opening of inlet ports.
This method of charge exchange is used in the latest two-stroke diesel engines, e.g.
propulsion of tankers and container ships (MAN, Wärtsilä, Hyundai) and locomotives (EMD Electromotive).
3.1.2 Four – stroke engine
In most of the currently produced wheeled vehicles four-stroke engines with spark ignition or auto-ignition are used. In the next part of the thesis there will be described different parts of the cycle of a typical four-stroke engine with spark-ignition:
intake (Figure 3.7):
Figure 3.7 Four-stroke engine – intake.
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This is a process in which the cylinder is filled and which starts with the opening of intake valves and finishes with their closure. With some approximation, this cycle lasts from TDC to BDC. In fact, the intake valves are opening during exhaust and are closing at the beginning of the compression stroke.
compression (Figure 3.8):
Figure 3.8 Four-stroke engine – compression.
Compression-also known as the compression stroke. The process of compression in the cylinder begins at the moment when all the valves are closed, namely, a few degrees of crank angle after BDC and finishes at TDC, wherein a few degrees before TDC ignition of the burn takes place. In the case of spark ignition engines the mixture of fuel and air is compressed, while the diesel engines have air compression and fuel is injected into the cylinder a few or a dozen degrees before TDC.
work (Figure 3.9):
Figure 3.9 Four-stroke engine-work of expansion.
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Stroke work is the expansion of the exhaust gases formed during combustion of air-fuel mixture, wherein the increase in pressure results in a force acting on the piston and thus causes its movement in the direction of BDC.
exhaust (Figure 3.10):
Figure 3.10 Four stroke engine – exhaust.
The outlet stroke or exhaust stroke. It is the process of emptying the cylinder from the exhaust gases which begins with the opening of the exhaust valve, which is a few degrees before BDC, even during the working stroke, and ends a few degrees after TDC, already during the intake stroke.
It is commonly accepted that the cycle of four-stroke engine with spark ignition is called Otto cycle, and with auto-ignition it is called Diesel cycle. In both cases, the names are derived from the names of the constructors of the first operating units.
3.1.3 Atkinson cycle
Patented in 1882 by James Atkinson engine cycle run allowed for performance of all work strokes characteristic for a four-stroke engine during a single rotation of the crankshaft.
This was possible thanks to a special arrangement of the piston crank system (Figure 3.11):
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Figure 3.11 Scheme of James Atkinson's piston machine contained in patent No. 3,522 [60].
The system was created to omit a patent describing a four-stroke engine. Ideal Atkinson cycle consists of the following steps (Figure 3.12):
Figure 3.12 Ideal Atkinson cycle presented in the graph P=f(V) [63]
1-2 - Reversible adiabatic compression, 2-3 - Constant volume heat addition, 3-4 - Reversible adiabatic expansion, 4-1 - Constant pressure heat rejection.
A characteristic feature of the cycle is that the process of expansion (work) (3-4) is longer than the compression process (1-2), which provides a higher efficiency than in the Otto cycle.
Currently, Atkinson cycle engine is used in hybrid cars, wherein the piston-crank system is identical to Otto cycle engines and Diesel engines, and the effect of the prolonged expansion process/shortened compression is obtained by appropriate control of the intake valves.
Shortening of the process of compression is achieved by prolonged opening of the intake valves - in the initial phase of compression valves are open.
18 3.1.4 Miller cycle
Industrial gas engines use cycles described and patented by Ralph Miller in the fifties of the twentieth century. Initially [5] the inventor proposed the use of early intake valve closing (EIVC) in order to reduce the work required to compress the charge by cooling the cylinder before compression stroke. Further it was proposed [6] to increase the boost pressure to compensate the reduced amount of charge supplied to the cylinder. Miller described his solutions in numerous patents.
The idea of this solution is the same as in the Atkinson cycle, while there is another way of reducing the compression stroke - in both cases it is done by means of the intake valves, but once it is by the prolonging of the opening time and once by the shortening. Additionally, higher boost pressure is applied in Miller cycle.
Currently, this approach is used mainly because of the possibility of reducing the maximum combustion temperature and thereby limiting NOX emissions. The advantages of using this approach are presented by leading engine manufacturers e.g. GE Jenbacher [7] and Wärtsilä [8] and ABB Turbo Systems turbochargers [9]. This solution is used in both gas and diesel engines. [10].
Typical Miller cycle used in gas engines consists of the following cycles [11]:
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intake (Figure 3.13):
Figure 3.13 Figure Miller cycle:
a – the intake stroke, the intake valve is open,
b – the intake stroke, the intake valve closes before the piston reaches BDC.
It starts with the opening of intake valves and finishes with their closure, however contrary to traditional Otto cycles the valves are closing before the piston reaches BDC (Figure 3.13a).
Thanks to this we have reduction of effective compression thus expansion cycle is longer than the compression.
compression (Figure 3.14):
Figure 3.14 Miller cycle - compression.
The compression process starts after the piston exceeds BDC, but only on the level where intake valves are closed. The compression finishes with the ignition of the mixture, a few degrees before TDC.
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work (Figure 3.15):
Figure 3.15 Miller cycle- expansion.
Expansion of exhaust gases.
exhaust (Figure 3.16):
Figure 3.16 Miller circuit – exhaust.
Emptying the cylinder from exhaust gases, preparation for filling the cylinder with a new load.
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3.2 The supply of fuel gas/mixture:
3.2.1 Central system of preparing the mixture and two-stage charging:
Commonly used Miller cycle allows for getting a higher efficiency and meeting standards of NOX emission. Increasing the boost pressure involves the necessity of expanding the system of air/mixture preparation. Currently the most advanced achievement in this area is a two-stage charging with both interstage and final cooler (Figure 3.17).
Figure 3.17 Scheme illustrating a two-stage system with cooling after each stage used in the engine GE Jenbacher J624 [12].
GE Jenbacher company was the first one to present J624 engine equipped with such system. The efficiency generating electrical energy amounted to el=46,5% [12]. Later construction, namely Jenbacher J920 has el=48,7% [13] with efficiency of charging system on the level of 75%. The system is so complex that it was constructed as a separate, auxiliary module attached to the engine (Figure 3.18).
Figure 3.18 TCA module (Turbocharger Auxiliary Module) of GE Jenbacher J920 engine consisting of turbocharger module, intercoolers, gas trains, oil and water heat exchangers, blow-by
system and electrical connections [13].
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The engine with TCA module and the generator is quite a bulky construction (Figure 3.19) with dimensions: 16,6 m x 2,9 m (TCA - 6,4 m) x 3,4 m (length x width x height) and mass of 177 tons.
Figure 3.19 GE Jenbacher J920 gas engine consisting of (from the left) generator, engine and TCA module [13].
3.2.2 Woodward TecJet™:
TecJet™ is a family of electronic gas metering valves controlling gas supply to the engine in the form of a single point injection. Current offer includes valves for engines with power from 50 to 3000 kW. Importantly, they are suitable both for high-methane gas-fuelled engines and those that have a low calorific value (such as landfill gases). It is mounted in front of a turbocharger [14]. Below, there is a drawing (Figure 3.20) of the largest valve offered by Woodward, Inc. TecJet 110.
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Figure 3.20 Woodward TecJetTM 110 shown in projections [14].
3.2.3 Woodward SogavTM:
Figure 3.21 Woodward SOGAVTM for gas metering in intake manifold of gas engines [15].
SOGAV (Solenoid-Operated Gas Admission Valves) (Figure 3.21) it is a family for dispensing gas in the intake manifolds of the gas engine [15] of solenoid valves for gas injection in the intake manifold of the engine (Figure 3.22).
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Figure 3.22 Scheme showing placing the valve in the intake system of the engine and the process of controlling the opening and closing of the valve [15].
The opening of the valve can be freely controlled – in the field when the intake valves are open. The process of mixing of the gas and air takes place in the intake manifold of the engine.
3.2.4 Venturi mixers:
Figure 3.23 Venturi mixer manufactured by Woodward Inc.: a – picture showing the whole element, b - the cross-section of the element [61].
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Venturi tubes are also used for the preparation of fuel-air mixture, typically they are used to measure the volumetric flow, but here their primary objection is to form a homogeneous mixture. These constructions are very simple (Figure 3.23) used in both engines, fueled by gas only or dual fuel (Figure 3.24) There are practically no moving parts in them, so the risk of failure is minimal. The principle of operation is used in mixers installed in the engines of motor vehicles and industrial gas engines with power up to 4 MW.
Figure 3.24 Venturi mixer GTI manufactured by Altronic for dual fuel engines - mounting scheme in the intake system of the engine [59].
3.2.5 MAN ME-GI + ME-LGI:
MAN Diesel & Turbo introduced into its offer the dual fuel low-speed, two-stroke engines operating in Diesel cycle [16]. They can be used as stationary units and as main propulsion of a ship transporting the gas. The minimum amount of diesel oil required during operation is 5% with natural gas to complete the rest. Both low and high calorific gases can be used but the latter require bigger amount of diesel oil due to certainty of ignition of fuel-air mixture which affects operational stability of the unit. Gas is injected directly into a cylinder under high pressure (Figure 3.25).
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Figure 3.25 The cross-section of high pressure gas injector used in engines of MAN B&W ME-GI Dual Fuel with the description of most important elements [17].
The gas pressure in the system amounts to 300 bar for LNG and 550 bar in the case of LPG. These are very high pressures, especially considering that these engines can be used as main drive of the ships. All pipes the gas flows through have double walls to ensure an adequate level of safety. The main objective of such technology is a significant reduction in emissions of CO2, NOX, SOX and solid particles compared to the same units powered by HFO (Heavy Fuel Oil) [17] and of course the cost of the fuel itself. This combination, despite complicated gas system, may be in the future a source of drive of large ships.
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27 3.2.6 Wärtsilä DF:
As a second, beside MAN B&W manufacturer of two-stroke marine engines (Figure 3.26).
Figure 3.26 Wärtsilä two-stroke diesel engine with DF technology [62].
It also offers the technology of supply with fuel gas, with minimal amount of liquid fuel.
Unlike the competition, the fuel system is a low-pressure one (10 bar), so it is less demanding in terms of all kinds of security measures and the engine runs in Otto cycle (Figure 3.27).
The air-fuel mixture is prepared outside the cylinder (premix). Liquid fuel is on the level of only 1% of the total amount-the fuel is injected into the pre-chamber.
Figure 3.27 Scheme presenting Otto cycle in a two-stroke engine of Wärtsilä DF [62].
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3.3 Ignition systems
In the case of gas engines, initiation of ignition is needed from an external source. In this section there will be presented both traditional solutions and the latest developments.
3.3.1 Spark plug
Spark plug is the primary source of ignition in gas engines, whereby it is used as the only source of ignition in engines fuelled by mixtures, where air excess ratio is =1. The ignition of homogeneous and lean mixtures only with this method would be very unstable. Because of very high demands as for the parameters and working time, progress and technical innovations are still being introduced in this area.
Federal Mogul Corporation has introduced on the market spark plug with three times longer lifetime than those manufactured by the competition [18]. It is featured by the electrode which is different from those (Figure 3.28) applied in traditional solutions - it is so unique that the company patented the innovation.
Figure 3.28 Spark plug Federal-Mogul Corporation Champion® Bridge Iridium [18] – picture of the spark plug and the magnification showing the electrode.
MWM GmbH, being currently the owner of Caterpillar Inc.,uses spark plugs with integrated pre-chamber. Contrary to pre-chambers which will be described in next parts of the thesis in this case fuel gas is not supplied to the chamber. It gets inside through the same bores the exhaust leaves the chamber (Figure 3.29).
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Figure 3.29 Spark plug with pre-chamber used in gas engines MWM GmbH [52]:
a - the interior of the chamber and the electrode, b-compartment cover and flame propagation bores.
Such shape of the chamber enables power reduction of the plug thus prolonging its lifetime. According to MWM Gmbh such solution enables to increase the rate of flame propagation (Figure 3.30) and reduce emission of NOX. Like solution described above, this innovation is also protected by the patent.
Figure 3.30 Simulation of the process of burning made by MWM GmbH presenting the propagation of the flame front [52].
30 3.3.2 Prechamber
Figure 3.31 Cross section of the pre-chamber installed in the GE Waukesha AT engines (1 - pre- chamber, 2 - spark plug, 3 - supply of fuel gas, 4 - holes connecting the pre-chamber with
the cylinder of the engine).
Pre-chambers are used in gas engines fuelled with lean mixture. These are additional chambers (Figure 3.31) mounted in the place of spark plugs/injector in which there is additional spark plug or micro-pilot and gas supply. Lean mixture constitutes the problem of ignition certainty. This can be solved by pre-chamber. Pre-chamber (Figure 3.31-1) is supplied with pure gas (Figure 3.31-3) and lean mixture from the cylinder is supplied through the bores (Figure 3.31-4). The value of air excess ratio in the chamber amounts to so there is no problem with ignition. Hot exhaust resulting from burning in the chamber are pushed inside the cylinder (due to the difference of pressure in the chamber and the cylinder) where it starts ignition of lean mixture. This allows the use of engine fuelled only with gas what in case of pipeline system is the safest solution.
3.3.3 MAN PGI (Performance Gas Injection):
Technology presented by MAN B&W in 2008, allows for the elimination of the spark plug and replace it with a glow plug, known from diesel engines. According to MAN it is possible to use this solution for engines with Otto cycles. The unit is characterized by a medium effective pressure and NOX emission (lower than 250 mg/Nm3) typical for diesel engines. Figure 3.32 is a graphic presentation of the level of advancement of PGI engines in comparison to traditional gas engines.
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Figure 3.32 Schematic presentation of the development of engines type PGI versus traditional engines with spark plugs [53].
The construction of the head with PGI system is similar to the construction of traditional gas engines where pre-chamber is also found. The only difference is using glow plug in pre- chamber in place of spark plug (Figure 3.33).
The biggest advantage of PGI was the simplicity of construction. However later it turned out that it was not a perfect solution. Currently there is no production of engines with these systems. MAN Diesel & Turbo returned to traditional spark plug.
Figure 3.33 Cross section of the head and the cylinder of engine equipped with PGI system [53].
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Figure 3.34 The head of the engine equipped with Micro-Pilot [64].
3.3.4 Micro-pilot
Technology patented by AVL and Niigava TLO inc called Micro-pilot is based on common-rail i.e. solution known from diesel engines. Gas is the main fuel here but ignition is caused by autoignition of diesel fuel injected to the pre-chamber. The remainder is analogous as in the pre-chamber spark plug. Figure 3.34 shows a cross section through the head equipped with Micro-pilot system. The engine with this solution is actually a dual fuel not a gas engine.
The fact that minimal amount of diesel fuel is necessary for ignition requires a constant supply of both kinds of fuel to the engine to allow its proper run. Moreover the fuel system is very complex due to existence of a system of supplying the oil under high pressure and a separate gas-powered system. In the solutions of competition there is used auto-ignition of diesel fuel which is injected directly into the cylinder.
3.3.5 Laser ignition
Constructors of engines for many years have been engaged in the problem of laser ignition. Many manufacturers have prototype engines fitted with this ignition-they include Ford, AVL in collaboration with the Carinthian Tech Research AG and Jenbacher - unfortunately, no one has introduced such an engine for mass production. There is the list of the many advantages of such a solution [19].
smaller size in comparison to the spark plug,
faster ignition,
possible ignition of very lean mixtures (even λ>2,6), practically no emission of NOX
(according to Jenbacher),
no wear of electrodes,
lower fuel consumption,
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possibility of precise setting of the angle of ignition-no delays and inaccuracies of spark plug,
higher efficiency of the engine – the increase of average pressure induced in gas engine from 1,7 MPa to 2,2 MPa.
Unfortunately a lot is said about disadvantages of such solution - like the amount of energy required to supply the laser. Currently engineers managed to reduce the dimensions of the lasers, which are used to start ignition. Initially large industrial units were used, which not only due to their size, but also the power consumption are not acceptable in commercial use.
Another concern is the durability of the device which in the case of a car for example, should be at least the same as when using a traditional plug. The industrial engines require reliability on the level of several thousand hours of continuous operation.
AVL prototype of laser ignition (Figure 3.35) which is only slightly larger than the injector in a diesel engine. So far, the device has not found serial application usage.
Figure 3.35 A prototype of the device used for the production of laser ignition AVL [54].
3.3.6 Summary:
Presented technologies are not the only ones used in practice. This is just an overview of these solutions, which can be a compared with technologies applied to the engine which is the subject of this study, i.e., Cooper-Bessemer GMVH-12. Some of the solutions described refer to natural gas engines, but as they use diesel fuel for ignition, in fact they are dual-fuel engines. In the case of an engine running in the transmission system much more convenient and more reliable solution in terms of fuel supply, is engine powered only by gas. It should be emphasized that there is a different character of the engine driving the power generator, and different character of the engine driving the compressor. The engines for the latter application are of heavy duty type and are manufactured by GE Waukesha and Caterpillar. Both companies use only spark ignition and pre-combustion chamber. In the oil & gas industry, where these machines are used, reliability and long service intervals are the most important features.
Therefore, the simplicity of construction is valued more than efficiency. Miller cycle and two stage boost have not been used in driving the compressors so far.
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3.4 The future of two-stroke engines
Currently, only two companies produce two-stroke gas engines. JSC Russian and Ukrainian RUMO BELMZ (Borislav experimental foundry-mechanical factory). These machines are constructed on the basis of Cooper-Bessemer gas-engine compressor GMV. No company producing gas engines develops the technology for two-stroke gas engine (except marine two-stroke engines supplied by gas and oil - then called dual fuel) both as individual units and engines integrated with the compressors.
EMD Electromotive Company (now owned by Caterpillar) produces two-stroke diesel engines with a power of less than 4 MW. They are mainly used for drive of locomotives and power generators. These engines meet U.S. emission standards TIER of toxic compounds. EMD plans to introduce a dual-fuel version of the engine to drive the locomotive. The fuel system of this engine will be developed by Westport Innovations Inc., which has been involved in adapting diesel engines to be supplied by gas fuels.
The Companies Eco Motors and Achates Power run advanced developments of introduction into serial production two-stroke engines with counter pistons. So far presented results indicate that these units will be characterized by higher efficiency and lower fuel consumption than the currently manufactured engines.
Achates Power declares 21% lower fuel consumption as an advantage of its engines similar exhaust gas emission, reduced price, lower weight and simpler construction compared to conventional engines. A simpler design and lower weight are due to the lack of heads, timing gear, and the lack of extensive cooling system. No heads results smaller cooling losses and therefore higher thermal efficiency of the unit.
The idea of using the opposed piston engines is not new. The first tests were carried out by Junkers Flugzeug-und-Motorenwerke AG already in 1913 [20]. The unit, which for more than fifty years was successfully used in aircraft, was, introduced in the thirties of twentieth century, Jumo 205 (Figure 3.36).
3 Gas engines
35
Figure 3.36 Cross section of aircraft engine Jumo 2005 [57].
It was a six cylinder, twelve piston diesel engine with a displacement of 16.43 dm3, equipped with a mechanical direct injection and mechanical compressor. The unit weighing 595 kg was characterized by a power of 647 kW at 2800 r / min (647 kW at 2,800 rpm). These values are impressive even in modern times.
Figure 3.37 Motor section Deltic engine production Napier & Son Limited [58].
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On the licence of Junkers there were constructed engines like Deltic produced by Napier & Son Limited (Figure 3.37), successfully used for locomotive drive [21].
After years of oblivion the idea of a two-stroke engine with counter-pistons revived thanks just to Achates Power and Eco-Motors. With the huge investment, developing of the construction could be started from the scratch. But it should not be expected that these engines will be widely used to drive the trucks, not to mention cars.
4 Combustion process
37
4 Combustion process
4.1 Adiabatic temperature of combustion process
This is a maximum temperature of complete and total combustion reached in insulated system (adiabatic) at constant pressure. The basic assumption is an equal value of enthalpy of reagents and products [22].
The sum of heat (or heats) of reagent formation is the enthalpy of reagents [22]:
(1)
where:
– number of moles of presents in the system.
Enthalpy of products is in turn the sum of enthalpies of formation of all elements and the increase of enthalpy which is the result of heating the product from initial to final stage.
(2)
where
– adiabatic temperature of flame,
– heat capacity at constant pressure of presents in the system.
From which we get equation allowing to determine the adiabatic value of temperature of combustion:
(3)
The value of adiabatic temperature of flame can be affected by change of air excess ratio (Figure 4.1). It has a significant meaning in the case of turbines and gas engines which operate at ratio much higher than unity.
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Figure 4.1 The graph of adiabatic relation of flame temperature to air excess ratio for different values of the pressure and the temperature of substrates-calculations made in Cantera for free
outflow of gas from the nozzle, for fuel gas composition: 100% CH4.
4.2 Temperature of self-ignition
Self-ignition unlike the ignition initiated by an external source such as a spark plug, the micro-pilot or gases from the combustion process initiated elsewhere (pre-chamber) is followed only by contact of gas with the hot cylinder walls. This phenomenon can take place parallel to the normal combustion process initiated by a spark, and run in some distance from the spark plug and the flame front. This is undesirable effect and is called knocking [23]. There is no possibility to influence the course of this process after its occurrence. Engineers are trying to eliminate this undesirable phenomenon. Modern gas engines usually are equipped with knocking detectors. In case of knocking detection, ignition timing is reduced [24].
Self-ignition temperature was defined by the van't Hoff [25], as the temperature at which the rate of heat loss by conduction is equal to the rate of heat generation by chemical reaction.
The rete of heat release as a result of chemical reaction in a chamber with volume can be specified as:
(4)
where:
– enthalpy of reaction, – rate of reaction, – pre-expotential factor,
– energy of activation, – universal gas constant,
4 Combustion process
39 – molar concentration of species ,
– stoichiometric coefficient for species appearing as a reactant, – temperature.
For the second bank the dependency (4) is simplified to the form:
(5)
The rate of heat loss through chamber walls with area of , radius of and temperature of can be put down as:
(6)
where:
– the thermal conductivity of the gas mixture at a temperature equal to the temperature of the walls,
– characteristic thickness of thermal layer close to the wall.
Three cases can be distinguished depending of the temperature of the wall (Figure 4.2):
Figure 4.2 The options of relations between the amount of generated heat and loss of heat of reacting gas mixture in vessel with controlled wall temperature: a - always bigger than ; b - crosses for two temperatures is; c - is tangential to for temperature
.
a – temperature of the walls is high enough for immediate start of chemical reaction of fuel-air mixture and thus generate heat causing gradual acceleration of reaction till it ends,
b – temperature of the walls is too low to generate enough heat by the chemical reaction. The curve crosses with a straight line in two points – for the temperature of i . Point it is a point of balance-reagents get heated to the temperature of , but not higher because of . To reach ignition reagents must be heated to the temperature of . Temperature of the wall is called ignition critical temperature.
c – the curve is tangent to in ignition point .
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Temperature of ignition can be definied in the point of tangency by comparison and , as:
(7)
(8)
Substituting equations on and defining chamber geometry depending on its radius we get:
(9)
and
(10)
where:
– rate constant.
After dividing equation 9 by 10 and making the transformation the following equation is obtained:
(11)
The root of equation:
(12)
After omitting higher order we get the dependence for self ignition
(13)
The graph below (Figure 4.3) illustrates dependence of self ignition temperature from air excess ratio for different temperatures and pressures of substrates.
4 Combustion process
41
Figure 4.3 The graph of self ignition temperature dependencies from excess air ratio for different values of pressure and the temperature of substrates-calculations made in Cantera for free outflow
of gas from the nozzle, for fuel gas composition: 100% CH4.
4.3 Laminar velocity of flame.
The combustion process defined as the basic reaction of a fuel with oxygen always takes place in a laminar layer. Due to the fact that the thickness of the laminar layer is comparable with several median free ways models of laminar flame are commonly used. One of the parameters characterizing the laminar flame is laminar velocity of flame propagation which is defined as the propagation velocity of the flame front in the combustible mixture in a direction perpendicular to its front.
In the literature most common are two methods of defining :
Thermal theory offered by Mallard and le Chatelier [26],
Complex theory by Zel'dovich, Frank-Kamenetsky and Semenon, commonly known as Z-FK-S [27].
In the case of model of Mallard and le Chatelier the flame was divided into two zones (Figure 4.4)
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Figure 4.4 The graph showing the division of flame applied by Mallard and le Chatelier [23].
The first zone is preheated, in which the gases are heated by conduction and reach ignition in ignition area. Area II is the area of chemical reaction where chemical enthalpy is changed into a proper one. There is an increase of temperature from to final temperature .
The balance of energy for the first zone takes the form of:
(14) Left side of equation represents the amount of energy used for heating unburned mixture from temperature to temperature . Right side it is a flow of heat passing to the surrounding.
Mass flow for area unit is defined as.
(15)
where
– density of unburned fuel-area mixture, – laminar velocity of the flame.
The above equations give the following relation:
(16)
Model Z-FK-S is based on the idea of Mallard and le Chatelier. It also assumes the division of flame into two zones. The difference is because of application of species- conservation equation and energy equation. Temperature of ignition is very close to adiabatic temperature of flame what causes substitution of by .
Basic assumptions of this model are as follows:
pressure is constant,
number of moles does not change during reaction,
4 Combustion process
43
and are constant
,
flame is of one dimension and in balance.
Equation presenting laminar velocity of flame has the form:
(17)
where:
– rate of chemical reaction,
– density of unburned components,
– number density of unburned components,
.
Analysis of two dependencies on allows to conclude that its value depends of:
temperature of substrates, which has influence on the velocity of chemical reaction,
pressure which affects the density of reagents.
The presented theories are burdened with large errors relative to the values determined experimentally. It is now widely accepted to use numerical programs that solve the full set of equations describing the laminar flame. One of them is the Cantera used by the author in this work.
Figure 4.5 The graph relation of laminar velocity of the flame to air excess ratio for different values of pressure and temperature of substrates – calculation in Cantera for free outflow of gas
from the nozzle, for fuel gas composition: 100% CH4.
