Fuel Injection Variable

Introduction: -

Fuel injection, in an internal-combustion engine, introduction of fuel into the cylinders by means of a pump rather than by the suction created by the movement of the pistons. Diesel engines do not use spark plugs to ignite the fuel that is sprayed, or injected, directly into the cylinders, instead relying on the heat created by compressing air in the cylinders to ignite the fuel. In engines with spark ignition, fuel-injection pumps are often used instead of conventional carburetors. Fuel injection into a chamber upstream from the cylinders distributes the fuel more evenly to the individual cylinders than does a carburetor system; more power can be developed and undesirable emissions are reduced. In engines with continuous combustion, such as gas turbines and liquid-fueled rockets, which have no pistons to create a pumping action, fuel-injection systems are necessary.

Fig. Injector System

Traditionally, the fuel/air mixture is controlled by the carburettor , an instrument that is by no means perfect.

Its major disadvantage is that a single carburettor supplying a four- cylinder engine cannot give each cylinder precisely the same fuel/air mixture because some of the cylinders are further away from the carburettor than others.

One solution is to fit twin-carburettors, but these are difficult to tune correctly. Instead, many cars are now being fitted with fuel-injected engines where the fuel is delivered in precise bursts. Engines so equipped are usually more efficient and more powerful than carburetted ones, and they can also be more economical, as well as having less poisonous emissions .

Diesel fuel injection

Fig. CI Engine 

The fuel injection system in petrol engined cars is always indirect, petrol being injected into the inlet manifold or inlet port rather than directly into the combustion chambers . This ensures that the fuel is well mixed with the air before it enters the chamber.

Many diesel engines , however, use direct injection in which the diesel is injected directly into the cylinder filled with compressed air. Others use indirect injection in which the diesel fuel is injected into the specially shaped pre-combustion chamber which has a narrow passage connecting it to the cylinder head .

Only air is drawn into the cylinder. It is heated so much by compression that atomized fuel injected at the end of the compression stroke self-ignites.

Injectors

Fig. Fuel Injector

The injectors through which the fuel is sprayed are screwed, nozzle-first, into either the inlet manifold or the cylinder head and are angled so that the spray of fuel is fired towards the inlet valve .

The injectors are one of two types, depending on the injection system. The first system uses continuous injection where the fuel is squirted into the inlet port all the time the engine is running. The injector simply acts as a spray nozzle to break up the fuel into a fine spray - it doesn't actually control the fuel flow. The amount of fuel sprayed is increased or decreased by a mechanical or electrical control unit - in other words, it is just like turning a tap on and off.

The other popular system is timed injection (pulsed injection) where the fuel is delivered in bursts to coincide with the induction stroke of the cylinder. As with continuous injection, timed injection can also be controlled either mechanically or electronically.

The earliest systems were mechanically controlled. They are often called petrol injection (PI for short) and the fuel flow is controlled by a mechanical regulator assembly. These systems suffer from the drawbacks of being mechanically complex and having poor response to backing off the throttle.

Mechanical systems have now been largely superseded by electronic fuel injection (known as EFi for short). This is thanks to the increasing reliability and decreasing costs of electronic control systems.

Fig. A Complete CI Engine System

                          

Working:-

• Start of injection (SOI) or injection timing is the time at which injection of fuel into the combustion chamber begins. It is usually expressed in crank angle degrees (CAD) relative to TDC of the compression stroke. In some cases, it is important to differentiate between the indicated SOI and actual SOI. SOI is often indicated by an easily measured parameter such as the time that an electronic trigger is sent to the injector or a signal from a needle lift sensor that indicates when the injector needle valve starts to open. The point in the cycle where this occurs is the indicated SOI. Due to the mechanical response of the injector, there can be a delay between the indicated SOI and the actual SOI when fuel exits the injector nozzle into the combustion chamber. The difference between the actual SOI and indicated SOI is the injector lag.

• Start of delivery. In some fuel systems, fuel injection is coordinated with the generation of high pressure. In such systems, the start of delivery is the time when the high pressure pump starts to deliver fuel to the injector. The difference between start of delivery and SOI is affected by the length of time it takes for a pressure wave to travel between the pump and injector and is influenced by the length of line between the high pressure pump and the injector and by the speed of sound in the fuel. The difference between the start of delivery and SOI can be referred to as injection delay.

• End of injection (EOI) is the time in the cycle when fuel injection stops.

• Injected fuel quantity is the amount of fuel delivered to an engine cylinder per power stroke. It is often expressed in mm³/stroke or mg/stroke

• Injection duration is the period of time during which fuel enters the combustion chamber from the injector. It is the difference between EOI and SOI and is related to injection quantity

Advantages: -

·         Optimised air-fuel mixture and atomisation allows for cleaner, more efficient combustion

·         Sharper throttle response

·         Better fuel efficiency and marginally more power than carbureted systems

·         They are typically maintenance free and does not break down

Disadvantages:-

·         Substantially more expensive than carburetors

·         Cannot be repaired with simple tools, have to be replaced, which is expensive.

·         Cannot be customised, unless you go for custom ECU maps, which again is expensive

     EXHAUST GAS RECIRCULATION

INTRODUCTION

• Major problem faced by today’s world is environmental pollution.Of these vehicular traffic is a major contributor . Exhaust gases from vehicles includes CO,CO2,HC,NOx Of these NOx is particularly very harmful.
• These are one of the chief constituents of smog, which have an adverse effect on ecological systems.They also contribute to the formation of acid rain. NOx also cause breathing illness in human beings. Exhaust Gas Recirculation is an efficient method to reduce NOx emissions from the engine.

                                                                 Fig.Schematic Diagram of An EGR

• It works by recirculating a quantity of exhaust gas back to the engine cylinders. Intermixing the recirculated gas with incoming air reduces the amount of available O2 to the combustion And lowers the peak temperature of combustion.
• Recirculation is usually achieved by piping a route from the exhaust manifold to the intake manifold.A control valve within the circuit regulates and times the gas flow.

BASIC PARTS OF EGR

There are 3 basic parts of EGR

1. EGR Valve

2. EGR Cooler

3. EGR Transfer Pipe

EGR OPERATING CONDITIONS

There are three operating conditions for EGR flow.

1. High EGR flow

2. Low EGR flow

3. No EGR flow

EGR OPERATION

• The purpose of the EGR system is to precisely regulate the flow under different operating conditions.
• By integrating the fuel and spark control with the EGR metering system, engine performance and the fuel economy can be enhanced
• For this an ECM (Electronic Control Machine) is used to regulate the EGR flow. When EGR is required ECM opens the EGR valve.
• The ECM is capable of neutralizing the negative aspects of EGR by programming additional spark advance and decreased fuel injection duration during periods EGR flow.

ADVANTAGES

• It reduce Nox and save the environment.
• It decreases the engine temperature.
• Improve Engine life through reduced cylinder temperature.

DISADVANTES

• As EGR reduce O2 so it is difficult to combust the fuel.
• Continues reduction of O2 reduce the peak power required for the engine.
• EGR valves can’t responds all the time and it will take time to flow the EGR gases.
• As the amount of recirculated gas is less so for multicylinder engine the EGR gas is not reach in proper ratio.

CONCLUSION

• Using Exhaust Gas Recirculation Technique in engines, the emissions are vary much controlled due to lesser amounts of NOx entering the atmosphere.
• Exhaust Gas Recirculation is a very simple method. It has proven to be very useful and it is being modified further to attain better standards.
• This method is very reliable in terms of fuel consumption
• EGR is the most effective method for reducing the nitrous oxide emissions from the engine exhaust.

CATALYTIC ELECTRONIC INJECTION SYSTEM

Introduction

Immediate solution of road transport sector is implementation of BS-VI for reducing emissions. Existing diesel and petrol engines can be modified with respect to combustion and exhaust after treatment to satisfy strict BS-VI emission norms. In diesel engines, diesel particulate filter (DPF) is used for the reduction of PM and particulate numbers, while selective catalytic reduction (SCR) system is used to reduce NOx emissions. For a gasoline or diesel engine of lower cubic capacity, lean NOx trap (LNT) is sufficient to reduce NOx levels. Complex after-treatment calibration is required for efficient functioning of DPF and SCR.  Basic function of after-treatment system is to reduce engine-out emissions of THC, CO, NOx, and PM. Efficiency of after-treatment system is a critical parameter for sufficient reduction of emissions. Engine-out HC and CO are reduced by DOC, which also converts NO component of NOx into NO2. NO2 reacts with soluble organic fraction (SOF) and soot in DPF to reduce them to CO2. Engine-out PM is thus burnt inside DPF. Only regulatory emission out of DPF is NOx. NOx is converted into nitrogen through the use of SCR. Some part of NH3 remains unreacted with NOx and is emitted out of SCR. Traces of NH3 emitted through engine tailpipe is restricted to 15 ppm in EU-VI norms. This remaining NH3 is controlled and neutralized by the ammonia slip catalyst (ASC). Figure 1 shows the complex layout of a typical exhaust gas after-treatment system with its subcomponents and sensors.

DIESEL OXIDATION CATALYST (DOC) TECHNOLOGY

Fig 1. Diesel Oxidation Catalyst (Doc) Technology

As the name signifies, DOC assists in oxidation of HC, CO, and NOx. In a two-way catalytic converter, HC and CO get oxidized with the excess oxygen present in the exhaust gas to form water vapor (H2O) and CO2, while three-way catalytic converter also promotes oxidation of NOx into N2. To promote this oxidation, DOC consists of precious metals as catalyst. This catalyst usually contains platinum (Pt) and palladium (Pd). The catalyst is covered in a wash-coat material of alumina Al2O3 or silica Si2O3. The layer of wash-coat and catalyst is spread on DOC substrate. This substrate can be either ceramic or metallic in honeycomb structure. DOC efficiency is dependent on catalyst quantity, substrate size, cells per square inch (CPSI), and wall thickness of substrates. Depending on engine raw emissions, catalyst loading can typically vary from 5 to 50 g/ft3. Higher the precious metal loading, higher will be the HC and CO conversion rates. Oxidation of HC and CO is also dependent on exhaust gas temperature. Figure 1 shows typical conversion rate with respect to DOC inlet temperature. For a typical DOC, reaction starts after a minimum temperature called light-off temperature. Light-off temperature varies according to precious metal loading and increases according to DOC poisoning and aging. Along with HC and CO oxidation, DOC assists in oxidation of NO to NO2 and some part of SOF burns in DOC. The presence of SOx generates negative impact on DOC performance due to its oxidation into sulfuric acid, which poisons the catalyst.

Fig 2. Diesel Oxidation Catalyst (Doc) Technology

Fig 3. Conversion efficiency w.r.t. exhaust gas inlet temperature 

Selective Catalytic Reduction (SCR) System

Fig 4. Typical SCR system layout

Selective catalytic reduction (SCR) system operates by chemically reducing the NOx (NO and NO2) to nitrogen (N2). This reaction is initiated through the injection of reductant in the exhaust gas stream. This reductant is ammonia NH3, which is generated in the exhaust stream through reaction of aqueous urea called AdBlue and also known as diesel exhaust fluid (DEF). Sufficient working temperature is required for SCR system to be functional, otherwise ammonia reacts to form un-intendent chemical components such as iso-cyanic acids, ammonium nitrate, or ammonium sulfate, which poison the SCR catalysts. SCR substrate is coated with Cu-zeolite or combination of base metals such as vanadium, tungsten, and titanium oxides. These catalysts assist in hydrolysis, thermolysis, selective reduction, and oxidation reactions. NH3 is formed from hydrolysis and thermolysis of urea. If sufficient temperature is not reached, urea residues are deposited in SCR causing reduction in efficiency. Base metal oxides-based SCR can function in low-temperature range, but Cu-zeolite-based SCR requires sufficient exhaust gas temperature. In some cases, SCR coating is integrated into DPF called as SDPF. SDPF lowers the exhaust system volume and temperature loss till SCR is avoided. Temperature benefits in SDPF results in higher conversion efficiency than SCR alone. Figure 4 shows a closed-loop SCR system. A NOx sensor is installed downstream and upstream of SCR and a NH3 sensor after the SCR. NOx emissions are continuously monitored, and the signal is transmitted to the SCR dosing control unit called DCU. Based on NOx, NH3, and exhaust gas temperature sensors, DCU decides the urea injection quantity. DCU has embedded NH3 storage strategies and continuously predicts the remaining ammonia stored in SCR. Urea is stored in a tank and heated to avoid decomposition. Based on command from DCU, injector functions to release aqueous urea in the exhaust stream. Coolant is supplied to injector for releasing heat during the operation. A mixer is present before the SCR to ensure homogeneous mixing of urea in the exhaust stream. With a closed-loop system, efficiency of SCR exceeds 90%. To enhance the efficiency of SCR at low temperature, NO2 levels are increased in the exhaust stream. Following are the complex reactions occurring in a SCR system. Explosive ammonium nitrate and ammonium sulfate can form at low temperature. Generally SCR calibration is performed by maintaining NH3/NOx ratio of 1 at SCR outlet to avoid excess NH3 slip.

Benefits:

• Over 80% reduction in CO & HC emissions

• About 30% reduction in particulates

• Widely used on light duty diesel vehicles