Prediction Of Spark Ignition Engine Testing Engineering Essay

Introduction

Internal combustion engines date back to the 1800’s. Since then, they have improved considerably as the knowledge of the engine process has evolved. The engine process is seen as a complex one and so, calculating the various engine parameters is a complicated task. There have been a number of computer programmes with the aim of estimating these parameters; OpenWAM is relatively new simulation software that intends to do this. OpenWAM, or open wave action model, is a free, open-source code that has been developed to solve the thermo- and fluid dynamics of compressible flow through the different components of an engine. The aim of this project to use OpenWAM to generate a full set of engine performance and fuel economy prediction estimates and to then compare these with experimental results. The engine in question is that of a BMW-Mini Direct Injection Spark Ignition Engine. It will be tested over a range of different operating loads and speeds. A successful interpretation of this software, and the results, could optimise the operation of the internal combustion engine.

This Interim Report details a synopsis of the literary review done to date. It includes the main principles of an internal combustion engine, including the intake and exhaust system as well as the in-cylinder process. The details of OpenWam software are mentioned and its applications. The aims of this project are also described.

Literary Review

Basic Principles

Internal combustion engines have one main purpose, that is; the production of mechanical energy from the chemical energy contained in the fuel. The basic principles behind any reciprocating engine are the same.

The cycle has four stages; intake, compression, expansion, exhaust. The intake stroke begins with the piston at the top of the cylinder (TDC) and the inlet valve open. As the piston moves down a vacuum is created and air-fuel mixture is drawn into the cylinder. When the piston reaches the bottom (BDC) the inlet valve is closed and the compression stroke begins. This involves the piston moving up and compressing the air-fuel mixture. This is then ignited in the expansion stroke. As the air-fuel mixture is heated it expands, pushing the piston down, to bottom centre (BC). The outlet valve is then opened and the exhaust gases are removed to the atmosphere. The piston moves up to TDC as the exhaust stroke finishes the cycle [1].

Figur-1 Basic Combustion Cycle

The engine used in this project is a four-cylinder engine. Most engines used for automobiles have four cylinders. The number of cylinders is an important consideration for the overall performance of an engine. Each of the cylinders, contain a piston that is connected to the crankshaft. The movement of the piston rotates the crankshaft. The crankshaft is what turns the wheels. The more pistons powering the crankshaft means more power can be generated in less time.

The engine used in this project is a Direct Injection Spark Ignition Engine. This means that the fuel is injected directly into the cylinder. With regular engines, the fuel and air is mixed before entering the cylinder. This will be discussed in further detail in preceding sections.

Basic Components

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Figure-2 Engine Cutaway

Intake

The engine intake process governs many important aspects of the flow within the cylinder. The efficiency of combustion and the production of pollutants are strongly dependent on the flow of air during the intake stroke.

Fluid Flow during Intake Process

The gas flowing into the cylinder, through the intake valve, behaves as a conical jet. The axial and radial velocity components, of the jet, are up to ten times that of mean piston speed. High speeds of the fluid lead to turbulence. Turbulence is generated due to the large velocity gradient formed when the jet separates from the valve. Turbulent flow undergoes irregular mixing and the speed of the fluid is constantly changing magnitude and direction. By increasing the rate of momentum, heat and mass transfer of the fluid, turbulent flow encourages mixing within the cylinder. It leads to the formation of vortices. Vortices are large-scale rotating flow patterns that are unsteady and react with each other. These vortices are important governing factors of the overall behaviour of the flow. They remain until the end of the intake stroke, where they become unstable and break up.

Turbulence is essential to the effective operation of an SI engine. It is a goal of any engine to maximise the turbulent effect, however this is complicated by the fact that flow patterns change during the engine cycle. They are largely dependent on the design of the intake system and are quite sensitive to small variations in flow. They are largely dependent on the design of the intake system and are quite sensitive to small variations in flow. This can lead to substantial cycle-to-cycle variations. The turbulent flow of air within the combustion chamber is almost exclusively generated during the intake stroke [3]

Volumetric Efficiency Pg.209

Volumetric efficiency is defined is the measure of success with which air is inducted into an engine. It is defined as the ratio of the volume flow rate of air into the intake system, to the rate at which the volume is displaced by the engine. More simply, it is the actual rate at which air enters the cylinder, over a given period in time, to the theoretical rate at which it should enter, over the same time period [6].

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Where; ηv – volumetric efficiency

Ma – mass flow rate of air

Ρa, 0 – density of air

Vd – displaced cylinder volume

N – crankshaft rotational speed

Volumetric efficiency is used to measure the overall effectiveness of an engine. It is mainly affected by the density of air entering the cylinder, the design of the intake and exhaust manifolds, and the timing of the intake and exhaust valves. The high temperatures within the combustion chamber have a limiting effect on the mass flow rate of air into the system, thus reducing volumetric efficiency. To counteract this, air with higher density, i.e. lower temperature and higher pressure, is preferred. This increases the amount of air entering the system, improving the volumetric efficiency. The intake and exhaust manifold and valve timing have similar limiting effects. The amount of air entering the cylinder is also largely dependent on these parameters. These parameters constrain the maximum possible engine power. This is why the volumetric efficiency of an engine is very important.

Frictional Losses Pg.212

Losses due to friction have a major impact on the engines performance. During the intake stroke, losses due to friction, in every part of the intake system, mean the in-cylinder pressure (pc) is less than the atmospheric pressure (patm). The difference between these two values is dependent on the square of the speed. The total friction loss is the combined losses from each of the components in the intake system; air-filter, inlet manifold, inlet valve and inlet port. Each component adds a loss of a few percent, on average, pc can be 10-20% lower than atmospheric [1].

RAM effect

During the intake process the RAM effect needs to be considered when calculating an engine’s performance. It occurs when the open valve phase is extended beyond that of the intake stroke to improve charging the cylinder and make best use of the inertia of the gases in the intake system. As the piston reaches TDC during the intake stroke, the inlet valve does not close immediately. Instead it remains open, as the compression stroke begins. This allows any extra air to be added to the cylinder. The momentum of the air during the intake stroke carries it into the cylinder even after the piston has reached the bottom of the cylinder. At high speeds, the intake valve can remain open for longer to optimize the RAM effect. The inlet valve isn’t closed until a crank angle of approximately 40-60o after BDC to take advantage of this. However for engines running at lower speeds, the momentum is not high enough, this can cause the air already in the cylinder to be forced out. Adapting the inlet valve open phase can have a major impact on the engine’s performance [2].

Overlap

When considering both the RAM effect and the blowdown phase (discussed later), it is clear to see that there is a period of overlap, when both the inlet and outlet valves are open. If the pressure inlet to outlet ratio is less than one then backflow occurs. This involves a rush of exhaust gases out through the exhaust manifold that aids the intake of air into the cylinder during the intake stroke. This works best at higher speeds, when its main advantage of overlap is the improvement in volumetric efficiency.

As with any fluid flowing through a system of intricate pipes, cylinders, valves, there are friction, pressure and inertial forces present. The importance of these forces is dependent on the both the velocity of the fluid and the geometry of the system. These forces along with the effects of changing engine design affect the volumetric efficiency.

In-cylinder (NB Pg.372,)

Gas motion within the engine cylinder is one of the major factors that control the combustion process. Both the bulk motion of the gas and the turbulence characteristics of the fluid are important.

The in-cylinder combustion process can be divided into four distinct phases;

Spark-Ignition

Early Flame Development

Flame Propogation

Flame termination

Spark Ignition Pg 585

Close to the end of the compression stroke, the discharge between the spark plug electrodes by the ignition system starts the combustion process. The spark develops a self-sustainable and propagating flame. The function of the ignition system is to initiate the flame propagation process, to repeat this for each cycle, over the full range of load and speed of the engine, at the appropriate time.

Spark-timing is an important consideration during the engine process. It can have a number of affects on the efficiency, formation of pollutants and other parameters of the engine. Advancing the timing, so that combustion occurs earlier in the cycle, increases the peak cylinder pressure (compression stroke work transfer, which is work form piston to gases in the cylinder, also increases). This is because more fuel is burned before TDC and the peak pressure moves closer to TDC where the cylinder volume is smaller. Delaying the timing means the peak pressure occurs later in the cycle and is also decreased in magnitude. This is because more of the fuel is burnt after TDC. Higher peak cylinder pressure result in higher peak burned gas temperatures, and therefore higher NOx formation results.

Maximum Brake Torque (MBT) is the use of optimal ignition timing to take advantage of internal combustion engines max power and efficiency. It occurs when the compression stroke work transfer (which is from the piston to the cylinder gases) is increased and the expansion stroke (which is from cylinder gases to the piston) is reduced. The MBT timing occurs when the magnitude of these two opposing trends just offset each other. Altering the timing from MBT lowers the torque [4].

Generally, spark timing is delayed so as to avoid abnormal combustion. Abnormal combustion refers to either knock or surface ignition. Knock is the name given to the noise transmitted through an engine when a spontaneous ignition of a portion of the end-gas occurs. End-gas is the mixture of fuel, air and residual gas ahead of the propagating flames. The spark plug ignites one flame front, however an uncontrolled combustion then occurs and an extremely rapid release of most of the chemical energy in the end-gas leads to the initiation of multiple flame fronts. When these multiple flame fronts collide, the cylinder pressure increases and causes the piston, connecting rods and bearings to resonate [5]. Knock has a direct impact on efficiency because it limits the maximum compression ratio that can be used in any cylinder.

Surface-ignition is another type of abnormal combustion. It occurs when ignition is initiated by a local hot-spot located on the walls of the cylinder.

Direct Injection Spark Ignition

Direct Injection (DI) engines deliver the fuel directly into the combustion chamber. The traditional method pre-mixes air and fuel in the intake manifold and then delivers it to the cylinder. However with DI engines, air enters through the intake manifold, where a specific amount of fuel is sprayed into the cylinder.

Early Flame Development Pg.846-850

During the in-cylinder process of compression and combustion, the increasing cylinder pressure forces some of the gas in the cylinder into the corners or narrow volumes connected to the combustion chamber, e.g. the volumes between the piston, rings and cylinder wall. Most of this gas remains unburned in the primary combustion process as the flame cannot enter these narrow regions.

Spark-Timing Pg 585

“There is always an optimal spark timing for all operating conditions of an engine. MBT is most ideal at WOT however is not desired when the engine is at idle. Although MBT is desired at WOT it is wise to retard timing slightly to prevent knock that may occur and to create a small safety margin. It is possible to calculate the MBT of an engine by taking into account of all the operating conditions of an engine through its sensors. Operating conditions are defined by the engine parameters lambda, engine load, internal exhaust gas recirculation, engine speed, and of course spark advance.”

Magnusson, J. 2007 An Investigation of Maximum Brake Torque Timing based on Ionization Current Feedback

Exhaust ( Pg. 626,570,)

The level of sulfate emissions depends on the fuel sulfur content. Unleaded gasoline contains 150 to 600 ppm by weight sulfur, which is then oxidized during the combustion process to form SOx.

The emission of particles is increased when the engine is cold, i.e. following start-up. The exhaust temperature has a significant effect on pollutant formation. Particles form in the exhaust manifold and either are emitted immediately or deposit on the walls of the exhaust. Many of these are removed when the engine is suddenly accelerated.

The exhaust geometry, specifically the diameter, determines the amount of particles emitted.

“Incomplete combustion due to bulk quenching of the flame in that fraction of the engine cycle where combustion is relatively slow, is a source of hydrocarbons in engines. Such conditions are most likely to occur during transient engine operation when the air/fuel ratio, spark timing, and the fraction of the exhaust recycled for emission control may not be properly matched.”

Engine Design

The major combustion chamber design objectives which relate to engine performance and emissions are; Pg. 845->

A fast combustion process, with low cycle-by-cycle variability, over the full operating engine range

A high volumetric efficiency at wide open throttle

Minimum heat loss to the combustion chamber walls

A low fuel octane requirement

Faster burn process is more robust and results in the engine being able to operate satisfactorily with much more EGR, or much leaner, without a deterioration in combustion quality. Faster burning chamber designs exhibit much less cycle variation, this permits better control of NOx within the engine. This is achieved in a number of ways. Swirl is used to speed up the combustion process in some spark-ignition engines. Swirl is defined as the organized rotation of the charge about the cylinder axis. Swirl is created by designing the intake system such that the flow enters the cylinder with an initial angular momentum. This is done in two ways, either the flow is discharged into the cylinder tangentially towards the cylinder wall. or intake?

High volumetric efficiency is required to obtain the highest possible power density. Effective vvalve open area, which depends on valve diameter and lift, directly affects volumetic efficiency. Swirl speeds up process and achieves greater combustion stability.

Heat transfer to chamber walls has a major impact on efficiency.

Blowdown – “amount of time/distance/degrees between exhaust port opening and the transfer port opening” Muller. P, 2009, Muller Machine, http://www.muller.net/mullermachine/index.html

The blowdown process is similar to that of the RAM effect.

It is necessary to open the exhaust valve before the piston reaches the bottom of the stroke, as this allows any excess pressure, pressure left over from the last cycle, to be released from cylinder. This ensures there will be no pressure acting against the piston on the compression stroke. Accurate exhaust valve timing is essential. At higher speeds, the valve will have to be opened sooner, whereas for lower speeds if the valve is opened to soon means pressure is lowered and losses are incurred.

AutoWare, 1998, Valve Timing & Performance , http://www.auto-ware.com/combust_bytes/valvetiming.html

The exhaust manifold operates at pressures significantly above atmospheric.

Pollutants (pg.626, 570, CO-Pg. 593, summary Pg.618,)

Theoretically, the combustion process of hydrocarbon fuels, such as petrol, completely oxidizes the fuel and the only by-products are carbon dioxide and water. However, under actual conditions this is rarely, if ever the case. The products of combustion from an internal combustion engine produce pollutants. This is due to the varied composition of the fuel for each cycle. Impurities in the fuel itself mean complete combustion is not possible. Poor control of the air-fuel ratio and variations in the combustions temperature also contribute to the formation of pollutants. The main pollutants formed are sulfur oxides (SOx), nitride oxides (NOx), carbon monoxide (CO) and particulate matter (PM). These pollutants have detrimental health effects. The presence of a catalytic convertor reduces the amount of harmful emissions entering the atmosphere by changing the composition of the pollutants.

One of the most important variables in determining spark-ignition engine emissions is the fuel/air equivalence ration. GRAPH OF EMISSIONS!!

To ensure smooth and reliable operation, SI engines are typically run close to stoichiometric, or slightly fuel-rich. From graph (ABOVE), lean mixtures give lower emissions until the quality becomes poor and back-fire occurs.

In a cold engine, fuel vaporization is slow, the fuel flow is increased to provide an easily combustible fuel-rich mixture in the cylinder. Until the engine warms up and the enrichment is removed, the CO and HC emissions are high

OpenWAM Simulation Software

OpenWAM is a 1-dimensional gas-dynamics engine thermodynamic cycle simulation code. It was developed by the CMT- Motores Térmicos of the Universidad Politécnica de Valencia, Spain.

openWAM

Modeling is an important technique for the optimization of internal combustion engines (ICE). The use of calculation models together with experimental tests is producing unquestionable successes due to the fact that both techniques complement each other.

1D wave action models simplify the engine by means of ducts, where only one dimension is considered, and volumes where mass accumulation is considered and the gas properties are uniform in the entire element. Finally, non dimensional models are used to solve connections between 1D and 0D elements.

Thanks to more than 20 years, more than 10 PhD Thesis and many research projects and publications, CMT-Motores Térmicos has developed an own 1D gas dynamic tool called WAM which gathers an important know-how on air management, compressible flow, turbocharging, chemical species tracking, numerical analysis and many other aspects of engine modeling

http://www.cmt.upv.es/WAM_02_02.aspx

1 dimensional modeling can reproduce the behavior of the engine under transient conditions when the injected fuel and the engine speed change during the simulation. For these applications, a heat transfer model is very important to take into account the heating process that the different parts of the engine undergoes during the transient.

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