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Cars • Maintenance Technician (MT) Engine Compustion Basic Training Working Document Participant Document A repeat order for this document or supplementary delivery can not be offered These documents are determined to be used in training programmes only and are not touched by current modification Printed in Germany ã 2004 Copyright AutomotiveTraining GmbH This work including all its details is protected by copyright Reproduction, copying, revision, translation, distribution, micro filming and data storing and/or use in electronic systems (including data banks and online services) of this publication, wholly or in part, is only allowed with our previous written permission and with source credit 1511 1495 02 - st edition 05.04 1,0 74 Index Page Title Foreword Combustible mixture Mixture formation in gasoline engines 20 Ignition system at gasoline engines 34 Exhaust-gas treatment on gasoline engines 40 Mixture formation in diesel engines 54 Exhaust-gas treatment on diesel engines 58 Supercharging 66 Power and torque 68 Exhaust system 05/04 Basic Training for Maintenance Technician • Module Engine-Combustion Index I 05/04 Basic Training for Maintenance Technician • Module Engine-Combustion Index II Foreword Main objective Basics motor combustion The Maintenance Technician should be able to carry out inspections and simple repairs competent and conscientiously with the provided instruments No matter if the combustion takes place according to the diesel or the gasoline principle there has to take place mixture formation, compression, ignition and combustion This learning unit introduces into the combustion process and its involved systems It shows alternatives of the power raise of combustion engines and clarifies questions to the formation of exhaust gases and its treatment The Training modules should be carried out practically in conjunction with the workshop information system WIS Alongside the basics of the automobile technology background information from the WIS is also being included The objective of these modules is the intermediation of basic physical knowledge The MT should know terms and units, which he or she uses on a daily basis in the field of work on the vehicle These subjects are being treated separately according to the combustion principles of gasoline and diesel engines Target group of these Modules are ”Trainees” who have not absolved a technical apprenticeship and for previous professionals The participants can name and explain all components for the fuel-mixture generation and exhaust gas treatment They can obtain and use information for the disassembly and assembly of an engine 05/04 Basic Training for Maintenance Technician • Module Engine-combustion Foreword Combustible mixture For a complete combustion of the fuel a certain amount of oxygen resp air is required The blend of fuel and air is called mixture Now, first the fuel Fuel Fuels and their placing in the vehicle have to fulfil certain requirements regarding the suitability of combustion and the transport: • • Simple and reliable formation of a combustible fuel-air mixture which ignites very easily and fast and combusts fast and without any residues Low weight and little space for the energy unit and the fuel container Besides, there are more fuels on the basis of regenerative (reusable) energies The following so-called alternative fuels are partly in use resp partly in discussion: • Methane, produced partly of animal by-products, • Methanol, produced of wood-biomass, • Ethanol, produced of sugar-cane biomass, • Fail-safe transport • Vegetable oil, produced of rape-biomass, • Immediate readiness for operation • Hydrogen, produced of nuclear energy or solar energy • Comfortable and possibly safe placing and transport in the vehicle Types of fuels There are solid, liquid and gaseous fuels Nowadays, however, only liquid and limited gaseous fuels are significant The best way to fulfil the abovementioned requirements is by using liquid fuels 05/04 Common liquid fuels are gasoline and diesel They consist of different hydrocarbons of the same origin (crude oil) Hydrocarbons are compounds of carbon (C) and hydrogen (H2) The manufacturing process starts with the distillation of the crude oil in its components By further procedures the components are being refined to gasoline and diesel Basic Training for Maintenance Technician • Module Engine-Combustion Combustible mixture The use of alternative fuels requires conversions on the engine, vehicle and the infra structure of the fuel distribution In the near future their use is only expected in certain areas In medium-term, methanol seams to be the most promising alternative whereas the chances for hydrogen technology might be given only for long-term Characters of fuels The main character why fuels are being used for combustion at all is that they have saved chemically bonded energy The chemical energy the combustion engine converts into mechanical energy (force) The chemical energy of the fuel is being declared with the so-called specific calorific value in joule per kilogram (J/kg) Besides, fuels own certain characters concerning the readiness to ignite themselves even under high pressure and temperatures This readiness is called ignition quality The dimensional figure for the ignition quality is the octane number (gasoline engine) and the cetane number (diesel engine) The higher the octane number the more knocking resistant and therefore high-graded is the fuel On petrol stations, the octane number is declared by the letters ROZ ROZ means Research Octane Number and equals to the international standardisation Diesel fuel needs to have a high ignition quality in contrast to gasoline fuel As a reminder: The diesel principle is based on self-ignition The standard reference number for the ignition quality of diesel is the cetane number Beyond it, diesel fuel has got one more character Diesel is getting viscid at low temperatures and therefore becomes useless From about – 24°C onwards the inspissation starts The table below shows the octane and cetane numbers for common fuels Gasoline fuel (petrol) should own a low ignition quality in order to prevent the rest of the mixture from igniting automatically As a reminder: The gasoline engine ignites controlled by external ignition In this way the octane number characterises the safety of petrol against the appearance of undesired self-ignition Undesired self-ignitions are also called knocking combustion The knocking of the engine is feared because it can come to destruction on the engine The knocking noise mostly appears at low rotational speed and full load (accelerating knocking) or at high rotational speed (high-speed knocking) 05/04 Basic Training for Maintenance Technician • Module Engine-Combustion Combustible mixture Fuels Octane-/cetane numbers Petrol 91 Premium gasoline 95 Premium high grade gasoline 98-100 Diesel 50 Air demand The air demand is the amount of air, which is needed for the complete combustion of a fuel It is also called stoichiometric air demand Concerning the motor combustion in general one differs more or less from the stoichiometric mixture ratio The ratio of the actual air mass mL to the stoichiometric air mass is called air ratio λ different mixture conditions are being differed in the motor operation: • A mixture with air deficiency has got a λ < and is called a “rich” mixture • A mixture with a stoichiometric mixture ratio has got a λ = (ratio: fuel to air = to 14,8) and is called stoichiometric mixture • A mixture with excess air has got a λ > and is called “lean” mixture Nowadays, gasoline engines are almost exclusively equipped with a regulated catalytic converter Therefore it is necessary to operate them with nearly a stoichiometric mixture Diesel engines are always being operated with excess air to avoid too much soot production 05/04 Basic Training for Maintenance Technician • Module Engine-Combustion Combustible mixture ! 05/04 Basic Training for Maintenance Technician • Module Engine-Combustion Combustible mixture Mixture formation in gasoline engines The treatment of gasoline fuels is being proceeded outside the cylinder with the help of a carburettor (here not dealt with because not the state of technology anymore) by injection into the intake manifold or by direct injection into the combustion chamber The pictures show the intake-manifold injection and the direct injection The following tasks must be fulfilled concerning the mixture formation and the air-fuel-mixture metering: • Formation of a gaseous fuel-air mixture of delicately distributed fuel • Exact dose of the fuel for the desired air ratio • Fuel metering of the mixture amount by throttle devices to adjust the power 2 AT_07.3_0006 Nozzle Spark plug Intake manifold Exhaust channel AT_07.3_0007 05/04 Basic Training for Maintenance Technician • Module Engine-Combustion Mixture formation in gasoline engines Tasks: Which statement is correct? a The diesel engine has got an inner mixture formation b The diesel engine has got an outer mixture formation What is meant by ignition lag? The space between the injection start and a b c the highest combustion pressure the beginning of the self-ignition the complete combustion of the fuel Which fresh gas filling contains the diesel engine during the intake? a b c d diesel-air mixture oxygen-fuel mixture pure air pure oxygen Which pollutants are being generated during the combustion in the diesel engine? a b c d carbon dioxide, oxygen, soot particulate non-combusted hydrocarbons, soot particulate, nitrogen soot particulate, nitrogen oxides, non-combusted hydrocarbons nitrogen oxides, carbon dioxide, lead Which control causes a measuring of the fuel quantity at the in-line injection pump? a b c d 05/04 edge control by the pump plunger pressure valve control in the inflow passage solenoid valve in the injection line edge control by a control-sleeve ring on the pump plunger Basic Training for Maintenance Technician • Module Engine-Combustion Exhaust-gas treatment on diesel engines 56 Is the air ratio of the diesel engine higher or lower than at the gasoline engine? Describe the advantages and disadvantages of the combustion-chamber procedure and the direct injector! How does the fuel-delivery control on the distributor injection pump work? 05/04 Basic Training for Maintenance Technician • Module Engine-Combustion Exhaust-gas treatment on diesel engines Why produce direct injecting diesel engines more noise than indirectinjection engines? 57 Supercharging The purpose of supercharging is to achieve an increased performance and torque of the combustion engine without an increased rotational speed At diesel engines the reduction of noise can be another aim There are different possibilities to increase the performance of engines: The supercharging procedures differ by the way how the charger is being driven in: • efficiency of the exhaust-gas energy for the compression of the charge, the increase of the rotational speed, • mechanic charge, a better filling of the combustion chamber with fuel mixture (charging) • compound procedure • the enlargement of the capacity, • • A larger capacity means more material, weight and space is needed An increase of the rotational speed goes along with the higher mechanic load and the shorter lifetime Both measures influence the economy in a negative way At supercharged engines the air in an air compressor (the so-called charger) is being compressed and pushed in a cylinder The density increase of the intake air enlarges the charge of the cylinder in order that more fuel per working cycle can be combusted At gasoline engines the supercharging is being limited by the start of the knocking combustion whereas at diesel engines by the strength of the component 05/04 Supercharging procedures Basic Training for Maintenance Technician • Module Engine-Combustion Supercharging Efficiency of the exhaust-gas energy to compress the charge At this procedure the compressor is driven by a turbine wheel without a mechanic connection to the engine This is the so-called exhaust-gas turbocharger The shaft is being installed in a way that the turbine wheel rotates in the exhaust-gas tract and the compressor wheel in the intake tract The two blade wheels are connected torque proof by a shaft and lubricated by the oil circuit of the engine 58 58 Mode of operation If the driver demands a better performance and torque by the accelerator, the throttle valve will open immediately and more fuel will be injected This increases the combustion exhaust and the combustion speed The rotational speed of the turbine wheel increases By the torque proof connection of the turbine side to the compressor side the intake air is being pressed with a higher speed in the direction of the combustion chambers Thereby, the intake pressure in the cylinders rises At each intake procedure now more air can be found in the cylinders than without turbo compression The air-mass meter records this and allows the control unit to inject more gasoline Thereby, the vehicle has got more power at almost the same rotational speed The rise of the charging pressure takes a certain time I.e the increase of the performance takes a few minutes to show up after pressing the accelerator This effect is also called the ”turbo lag” The direct way of the energy transfer from the exhaust-gas to the fresh charge is more efficient than the mechanic charge No additional mechanic energy of the combustion process is being used up for its drive But the kinetic energy of the exhaust cycle is being used, which otherwise escapes unused from the exhaust pipe to the environment 05/04 Basic Training for Maintenance Technician • Module Engine-Combustion Supercharging AT_09_0008 Compressor wheel Turbine wheel Air discharge Exhaust-gas intake Air intake Exhaust-gas discharge Rotor shaft 59 59 Pressure-charging regulation To achieve a possibly well cooperation between the engine and the turbocharger in all operational sectors (full load – part load) a control of the boost pressure is necessary There are two concepts for it: • Waste-gate control This is a valve in the exhaust-gas tract in front of the turbine blade By achieving a defined boost pressure a part of the exhaust-gas is being diverted into the exhaust pipe by bypassing the turbine As a reference dimension the intake pressure lies on the waste-gate valve over a pressure pipe (boost pressure) At a mechanic boost pressure control the boost pressure acts on a membrane in the waste-gate and opens the valve at too high boost pressures At an electronic boost pressure control the motor electronic controls the valve With this control it is guaranteed that already at low rotational speed (only low exhaust pressure) good performance can be realised With it a so-called over-boost can be generated, which helps the engine to get a good pull-through power e.g at up-hill drives The waste-gate valve is being opened when the motor electronic determines an overload of the engine For example by too high oil temperatures, etc 05/04 Basic Training for Maintenance Technician • Module Engine-Combustion Supercharging AT_09_0009 60 60 • Variable turbine geometry (VTG) Mechanic supercharge With this concept the exhaust-gas turbine side is being equipped additionally with a stationary-vane system It changes the air-flow cross-section and the angel of impact on the turbine wheel At this procedure the compressor is connected with the engine and is being driven by it As examples for the mechanic supercharge the compressor and the G-charger will be discussed in the following Concerning the effect a small or a big turbocharger is possible, which varies its boost pressure by the change of the turbine geometry This concept can without waste-gate Compressor The both two-winged rotary pistons of the roots-compressor are running opposite the casing and against each other without any contact The seize of the created the sealing gaps are being determined by the construction, the choice of the material and the manufacturing tolerances The synchronisation of both rotary pistons happens by a pair of toothed wheels running outside the workspace mainly driven by the crankshaft over cog belts AT_09_0002 AT_09_0006 AT_09_0004 05/04 Basic Training for Maintenance Technician • Module Engine-Combustion Supercharging 61 61 G-charger Pressure-wave supercharger (Comprex) The G-charger is also called spiral-type supercharger The displacer element is borne eccentrically and lead in a way that during a rotation of the drive shaft it carries out an oscillating movement with a double eccentricity A rotor mechanically driven by the crankshaft (needs about1% of the motor performance for the drive) rotates with about a - 3,5fold of the motor speed The rotor runs in a cylinder casing Because the procedures are relatively complex we without any closer consideration of it Only one thing: The charger creates pressure waves (with sound velocity) which cause an energy exchange between exhaust-gas and fresh air This exchange takes place in the cells of the rotor An extraordinary course of the torque is being caused by a very good boostpressure even at low rotational speed By the direct pulse transmission there is no delay effect when accelerating as at the exhaust-gas turbocharger Thereby, the work spaces are being opened phase wise for filling, closed for transporting and opened again for exhausting on the hub The intake air is also being compressed and heated Air intake Exhaust-gas discharge Charging cable Casing Cell Fan belt Casing Rotor Exhaust pipe AT_09_0010 AT_09_0011 05/04 Basic Training for Maintenance Technician • Module Engine-Combustion Supercharging 62 62 Intercooling In order to use the compressed air even more effectively the intake air is mostly being cooled down in front of the intake turbine by a so-called intercooler Cool air has got a lower density than hot air, which additionally improves the filling of the cylinders AT_09_0011 05/04 Basic Training for Maintenance Technician • Module Engine-Combustion Supercharging 63 63 Tasks: How can the performance of engines be improved? Name the components! Describe the waste-gate control! AT_09_0008 05/04 Basic Training for Maintenance Technician • Module Engine-Combustion Supercharging 64 64 What is shown in this picture? How is the supercharge of engines limited? AT_09_0002 05/04 Basic Training for Maintenance Technician • Module Engine-Combustion Supercharging 65 65 Power and torque Power is declared in kilo Watt (kW) or in Horse Power (HP) Torque is declared in Newton meter (Nm) Both values are mostly being declared together in a motor diagram Hereby the power and the torque are being inserted by the rotational speed (see below) The rotational speed is being declared in rotation per minute (1/min) 205 Nm at 4000 1/min and 103 kW at 5600 1/min Explanation of the diagram: The power curve is the one, which is similar to a walking-stick Therefore it is also being called walking-stick curve • The power is being read at the left side in kW • The torque line is the remaining one • The torque is being read at the right side in Nm The engine possesses a maximum power of 103 kW at a rotational speed of 5600 1/min The engine has got a torque of 205 Nm at a rotational speed of 4000 1/min Power [kW] Torque [Nm] Supercharged engines can be recognised by the torque curve, which is being displaced to the left and to the top This means that supercharged engines have got a higher torque, which is present in a lower speed range GT-01-00-005-C82 -1 Rotational speed [min ] 05/04 Basic Training for Maintenance Technician • Module Engine-Combustion Power and torque 66 Task: kW Nm In the opposite motor diagram are shown engines A charger engine and an intake engine Inscribe the diagram with the marks for power and torque and declare which torque curve is the one of the charger engine Explain your statement AT_00_0068 05/04 Basic Training for Maintenance Technician • Module Engine-Combustion Power and torque 67 Exhaust system To the exhaust system belong following subsystems: in brackets is the function (in the order of exhaust side to the back of the vehicle at front-engine setup): • Exhaust manifold or simply manifold (gathers exhaust-gases from all cylinders of a cylinder head and bundles these in direction of the catalytic converter), • Lambda sensor (subsystem for exhaust treatment, see above), • Exhaust-gas recirculation system (subsystem for exhaust-gas treatment, see above), • Catalytic converter(s) (subsystem for exhaust treatment, see above), • Middle exhaust pot (sound adsorption), • End exhaust pot or more (sound adsorption) The main function of the exhaust system is the transport of exhaust-gases from the engine to the exhaust outlet underneath the back of the vehicle The outlet is at the back to prevent exhaust-gases from getting into the interior during operation by the interior air-intake The installation of sound absorbers in the way of middle and end pots serves to improve the combustion noise acoustically Sound absorbers are connected one beneath the other and to the catalytic converter and the manifold by tubes These are being connected by exhaust clamps or by flange connections and sealing foils The complete exhaust system is fixed rubber borne with a distance to the body The exhaust systems and the bottom side of the body possess special rings, which are connected by rubber buffers The rubber bearing and the body distances are necessary not to transfer exhaust vibrations and heat to the body and to allow easily length extensions by thermic extension AT_49_0001 05/04 Basic Training for Maintenance Technician • Module Engine-Combustion Exhaust system 68 Repair hints Because combustion products also consist of aggressive acids it is obvious that corrosions of the exhaust are being generated during the operation Some manufacturers equip their vehicles with exhaust system made of stainless steel The conventional exhaust systems are among others made of aluminium-coated steel Especially susceptible for corrosion are the sound absorbers The reason for that is their bathtub shape After the shut down of the engine condensate and acid are being gathered at the bottom of the sound absorber, which accelerate the oxidation procedure Especially susceptible is the exhaust system at repeated ”cold” operation This occurs a frequent short distance drives The exhaust does not heat up completely The gases cannot evaporate after the shut down of the engine By exchanging a sound absorber the clamps or flange connections are being loosened and the sound absorber pulled off Because at the clamp construction the tubes overlap it leads to a contact corrosion This means that the parts can be separated only difficult A disc grinder or something similar has to be applied to slit the overlapping tube The new sound absorber is always being assembled with new clamps / screws and new rubber elements 05/04 Basic Training for Maintenance Technician • Module Engine-Combustion Exhaust system 69 ... table below gives you an overview of the Motronic variants and its installation in the different Mercedes engines: ME-version Engine Types Launch Notes ME-versions Engine Types Launch Notes ME