Ignition Timing: every bit applied to the flicker ignition engines ( petrol engines ) is a procedure of puting the clip at which the flicker stopper should fire in the burning chamber during the compaction with regard to the Piston place and the crankshaft angular speed. The flicker stopper should fire before TDC and the fire should end after TDC.
Puting the appropriate ignition timing is really important as it decides the clip available for burning of the air-fuel mixture. Hence, the ignition clocking affects many variables including fuel economic system and engine power end product. Earlier engines that use mechanical flicker distributers rely on the inactiveness of revolving weights and springs and multiplex vacuity in order to put the ignition clocking throughout the RPM scope of the engine ; whereas the latest engines consists of an ECU ( engine control unit ) which uses a computing machine to command the ignition clocking throughout the engine ‘s RPM scope.
Factors act uponing ignition timing:
Type of ignition system used.
Load of the engine: with more burden ( larger restrict gap ) necessitating less progress ( as the mixture burns faster ) .
Components used in the ignition system.
Settings of the ignition system constituents.
Temperature of the engine ; lower temperature allows for more progress.
The ignition timing to some extent besides depends on the octane figure of the fuel, and the air-fuel ratio as this determines the velocity with which the fuel Burnss.
Normally, any major engine alterations or ascents will necessitate a alteration to the ignition clocking scenes of the engine.
Clocking Progress: refers to the figure of grades before top dead Centre ( bTDC ) that the flicker will light the air-fuel mixture in the burning chamber during the compaction shot. In contrast to that, clocking idiot refers to the altering in ignition timing, so that the fuel ignition takes topographic point subsequently than the maker ‘s specified clip. As an illustration, if the set ignition clip was 12 & A ; deg ; bTDC, so when the fuel ignition starts subsequently than 12 & A ; deg ; bTDC, it is known as ignition idiot ; likewise when the air-fuel mixture is ignited at an angle greater than 12 & A ; deg ; bTDC, it would be known as ignition progress.
Clocking progress is necessary because it takes clip for the burning of the air-fuel mixture to finish. Igniting the mixture before the Piston ends its compaction shot would maximise the bound to which the mixture burns wholly, and therefore aid to construct up maximal force per unit area shortly after the Piston reaches the TDC. This would guarantee maximal power end product by maximising the force with which the Piston is pushed down, by maximising the force per unit area every bit shortly as the Piston starts traveling down when the power shot is initiated. Ideally, the mixture should be wholly burnt by 20 & A ; deg ; aTDC ( after TDC ) .
If the ignition occurs at a place that is excessively advanced comparative to the Piston place, the quickly spread outing air-fuel mixture can really force against the Piston still traveling up, doing explosion and lost power ; whereas if the ignition is excessively retarded relation to the Piston place, the maximal cylinder force per unit area will happen after the Piston has already travelled excessively far down the cylinder. This would ensue in lost power accompanied by high emanations and unburnt fuel.
Why is Ignition clocking progress required?
The ignition clocking demands to be progressively advanced ( comparative to the TDC ) as the engine velocity additions, so that the air-fuel mixture has the right sum of clip to fire wholly. As the engine velocity additions, the clip available to fire the mixture decreases while the firing itself returns at the same velocity ; this requires the combustion to get down earlier to finish in clip. The right timing progress for a given engine velocity will let for maximal cylinder force per unit area to be achieved at the correct crankshaft angular place.
Combustion in SI Engines:
The burning procedure in SI engines consists of three major parts:
Ignition and fire development,
Flame extension, and
Consumption of the first 5-10 % of the air-fuel mixture is by and large considered as the fire development. During the fire development period, the flicker stopper fires and the burning procedure starts, but really small force per unit area rise is observed ( graph-1 ) . Almost all the utile work is produced in an engine rhythm during the flame extension period of the burning procedure. During this period 80-90 % of the air-fuel mass is burnt ; the cylinder force per unit area is greatly increased which provides the force to bring forth work in the enlargement shot. The concluding 5-10 % of the air-fuel mass which burns is classified as fire expiration. During this clip, force per unit area beads and burning is eventually terminated.
The burning procedure ideally consists of an exothermal sub-sonic fire patterned advance through a premixes about homogeneous air-fuel mixture. The spread of the fire forepart is greatly enhanced by the induced turbulency and whirl within the cylinder.
Ignition and Flame Development:
The procedure of burning is initiated by an electric discharge across the electrodes of a flicker stopper anyplace between 10 & A ; deg ; to 30 & A ; deg ; bTDC, depending on the geometry of the burning chamber. The high-temperature plasma discharge between the electrodes ignites the air-fuel mixture in the immediate locality, and the fire spreads outwards from here.
Graph. The addition in force per unit area rise is really slow after ignition during the fire development period. This consequences in a slow force per unit area force addition on the Piston and a smooth engine rhythm. Maximal force per unit area occurs 5 & A ; deg ; to 10 & A ; deg ; aTDC.
The burning starts really easy due to the high heat losingss to the comparatively cold flicker stopper and the gas mixture. The fire can by and large be detected at about 6 & A ; deg ; of grouch rotary motion after the flicker stopper fire.
The applied possible across the flicker stopper is normally 25,000-40,000 V. overall flicker discharge lasts about 0.001 2nd with an mean temperature of about 6000 K. The discharge of the flicker stopper delivers about 30 to 50 mJ of energy, most of which is lost by heat transportation.
The few normally used methods used to bring forth the high electromotive force potency, which is required to do the electrical discharge across the flicker stopper electrodes, are:
Most cars use a 12-volt electrical system, including a 12-volt battery. This electromotive force is multiplied many times by the spiral that supplies the really high potency delivered to the flicker stopper.
Some systems use a capacitance to dispatch across the flicker stopper electrodes at the proper clip.
Most little engines and some larger 1s use a magneto driven off the engine crankshaft to bring forth the needed flicker stopper electromotive force.
Some engines have a separate high-voltage coevals system for each flicker stopper, while the others have a individual system with a distributer that shifts from one cylinder to the following.
The Spark Plug:
The spread between the electrodes on a modern flicker stopper is approximately 0.7 to 1.7 millimeter. smaller spreads are acceptable if there is a rich air-fuel mixture or if the force per unit area is high ( i.e. high recess force per unit area by turbocharging or a high compaction ratio ) . Normal temperature of flicker stopper electrodes between fires should be about 650 & A ; deg ; to 700 & A ; deg ; C. A temperature above 950 & A ; deg ; C risks the possibility of surface ignition, and a temperature below 350 & A ; deg ; C tends to advance surface fouling over drawn-out clip.
For older engines with worn Piston rings that burn an surplus of oil, hotter stoppers are recommended to avoid fouling. Hotter stoppers have a greater heat conductivity opposition than colder stopper. Modern flicker stoppers have a greater life span than the old 1s. Some of the high quality flicker stoppers with platinum-tipped electrodes are made to last 160,000 km or more. Harley Davidson uses gold-tipped flicker stoppers. One ground this is desirable is the trouble in replacing flicker stoppers in some modern engines due to the complexness and concentration of engine and increased sum of engine equipment.
Figure. An NGK flicker stopper
Spark stopper fire:
When a flicker stopper fires, the plasma discharge ignites the air-fuel mixture between and near the electrodes. This creates a spherical fire forepart that propagates outward into the burning chamber. At first, the fire front moves really easy because of its original size ; it does non bring forth adequate energy to rapidly heat the surrounding gases and therefore propagates really easy. As a consequence of this, the cylinder force per unit area is non raised rapidly and really small compaction warming is experienced. Once the first 5-10 % of the air-fuel mass is burned, the fire speed reaches higher values with matching rise in force per unit area, the fire extension part.
It is desirable to hold a rich air-fuel mixture around the electrodes of the flicker stopper at ignition, as it ignited easy and more readily, has a faster fire velocity and initiates the burning procedure good. Spark stopper are by and large located near the consumption valves to guarantee a richer mixture, particularly when get downing a cold engine.
Latest developments in flicker plug/ignition system engineering:
The attempts to develop better ignition system continue. Spark plugs with several electrodes and two or more coincident flickers are now available. They give a more consistent ignition and quicker fire development. One of the modern systems still under development gives a go oning discharge after the initial discharge ; this extra flicker will rush up burning and give a more complete burning as the air-fuel mixture whirls through the burning chamber. Development work has been done to make a flicker stopper with variable electrode spread size. This would let flexibleness in ignition for different operating conditions. At least one car maker is experimenting with engines that use a point on top of the Piston as one of the flicker electrodes. Using this system, spark ignition can be initiated across the spreads of 1.5 to 8 millimeter with a reported lowering of fuel ingestion and emanations.
Induced turbulency and whirl causes the fire extension velocity to increase by 10 times than if there were a laminal fire forepart traveling through a stationary gas mixture. These gestures besides cause the fire forepart to spread out spherically from the flicker stopper in stationary air and is greatly deformed and spread. As the gas mixture Burnss, the temperature and force per unit area rise to high values.
Figure. A typical fire extension form.
The burnt gases behind the fire forepart are hotter than the unburnt gases before the fire forepart, with all the gases at about the same force per unit area. This decreases the denseness of the burnt gases and expands them to busy a greater per centum of the entire burning chamber volume. Compaction of the unburnt gases raises their temperature by compressive warming. In add-on, radiation warming emitted from the fire reaction zone, which is at a temperature on the order of 3000 K, farther heats the gases in the burning chamber, unburnt and burned, raising the force per unit area further. Heat transportation by conductivity and convection are minor as compared to radiation, due to really short existent clip involved in each rhythm.
The environment inside the burning chamber is such that the progressive addition in temperature and force per unit area in taking topographic point, doing the reaction clip to diminish and flare front velocity to increase. The temperature of the burnt gases is non unvarying. It is higher near the flicker stopper where the burning had initiated. Ideally, the air-fuel mixture should be around two-thirds burnt at TDC and about wholly burned at about 15 & A ; deg ; aTDC. This causes the maximal force per unit area and temperature of the rhythm to happen someplace between 5 & A ; deg ; and 10 & A ; deg ; aTDC.
A lesser force per unit area rise rate gives lower thermic efficiency and danger of knock. The burning procedure is hence a via media between the highest thermic efficiency possible and a smooth engine rhythm with some loss of efficiency.
Burn angle, Ignition and Ignition progress:
The typical burn angle, the angle through which the crankshaft turns during burning, is about 25 & A ; deg ; for most engines. If burning is to be completed at 15 & A ; deg ; aTDC so ignition should happen at approximately 20 & A ; deg ; bTDC. If ignition is excessively early, the cylinder force per unit area will increase to unwanted degrees before TDC, and utile work would be wasted in compaction shot. If ignition is late, peak force per unit area will non happen early plenty, and work will be lost at the start of power shot due to take down force per unit area.
Graph. Average flame velocity in the burning chamber. Thin air-fuel mixtures have slower fire velocities, with maximal velocity happening when somewhat rich mixture at an equality ratio near 1.2
Actual ignition timing is typically anyplace from 10 & A ; deg ; to 30 & A ; deg ; bTDC, depending on the fuel used, engine geometry, and engine velocity. For any given engine, the burning occurs faster at higher engine velocity. Real clip for burning is hence less, but existent clip for engine rhythm is besides less, and the burn angle is merely somewhat changed.
This little alteration is corrected by progressing the flicker as the engine velocity in increased. This initiates burning somewhat earlier in the rhythm, peak temperature and force per unit area staying at about 5 & A ; deg ; to 10 & A ; deg ; aTDC. At portion accelerator, ignition timing is advanced to counterbalance for the ensuing slower fire velocity.
Graph. Burn angle as a map of engine velocity.
Clocking accommodation in Modern engines:
Modern engines automatically adjust ignition timing with electronic controls. These non merely utilize engine velocity to put the timing but besides sense and do all right accommodation for knock and wrong exhaust emanations. Earlier engines used a mechanical timing accommodation that consisted of a spring-loaded ignition distributer that changed with engine velocity due to centrifugal forces. Ignition clocking on many little engines is set at an mean place with no accommodation possible.
Graph. Average burning chamber fire velocity as a map of engine velocity for a typical SI engine.
90 – 95 % of the air-fuel mass has been combusted by 15 & A ; deg ; to 20 & A ; deg ; aTDC and the fire forepart has reached the utmost corners of the burning chamber. The last 5 – 10 % of the mass has been compressed into a few per centum of the burning chamber volume by the spread outing combustion gases behind the fire forepart. Although at this point the Piston has already moved off from TDC, the burning chamber volume has merely increased on the order of 10 – 20 % from the really little clearance volume. This means that the last mass of air and fuel will respond in a really little volume in the corner of the burning chamber and along the chamber walls, at a decreased rate.
Near the walls, turbulency and mass gesture of the gas mixture have dampened out and there is a dead boundary bed. The big mass of metal cylinder walls besides act as a heat sink and carry on off much of the energy being released in the reaction fire. Both these mechanisms cut down the rate of reaction and fire velocity, and the fire is eventually terminated as it easy dies out.
Although really small extra work is delivered by the Piston during the fire expiration, it still is a desirable happening. Because the rise in cylinder force per unit area tapers off easy towards zero during this fire expiration, the forces transmitted to the Piston besides taper off easy ensuing in smooth engine operation.
During the flame expiration period, self-ignition will sometimes happen in the terminal gas and engine knock will happen. The temperature of the unburnt gases in forepart of the fire forepart continues to lift during the burning procedure, making a upper limit in the last terminal gas. The maximal temperature is frequently above self-ignition temperature. Because the fire front moves easy at this clip, the gases are frequently non consumed during ignition hold clip, and self-ignition occurs.
The ensuing knock is normally non obnoxious or even noticeable. This is because there is so small unburnt air-fuel left at this clip that self-ignition can merely do really little force per unit area pulsations. Maximal power is obtained from an engine when it operates with really little self-ignition and knock at the terminal of the burning procedure. This occurs when maximal force per unit area and temperature exist in the burning chamber and knock gives a little force per unit area encouragement at the terminal of burning.
Abnormal burning is referred to a burning procedure in which a fire forepart may be started by hot burning chamber surfaces either anterior to or after spark ignition, or a procedure in which some portion or all of the charge may be consumed at highly high rates.
Figure. Phenomenon of unnatural burning
The two of import unnatural burning phenomena of major concern are:
They are of major concern, because:
When terrible, they can do major engine harm ; and
Even if non terrible, they are regarded as an obnoxious beginning of noise by the engine or vehicle operator.
Knock: is the name given to the noise which is transmitted through the engine construction when basically self-generated ignition of a part of the terminal gas. This is when the fuel, air, residuary gas, mixture in front of the propagating fire occurs.
When this procedure takes topographic point, there is an highly rapid release of much of the chemical energy in the terminal gas, doing really high local force per unit areas and the extension of force per unit area moving ridges of significant amplitude across the burning chamber.
Surface Ignition: is ignition of the fuel-air mixture by a hot topographic point on the burning chamber walls such as an overheated valve or flicker stopper, or glowing burning chamber sedimentation: i.e. by any other means other than the normal flicker discharge.
It can happen before the happening of the flicker ( pre-ignition ) or after ( post-ignition ) . Following the surface ignition, a disruptive fire develops at each surface-ignition location and starts to propagate across the chamber in an correspondent mode to what occurs with normal spark ignition.
Types of Abnormal Combustion in SI Engines:
A knock which is perennial and quotable in footings of audibleness. It is governable by the flicker progress ; progressing the flicker increases the knock strength and retarding the flicker reduces the strength.
Surface Ignition: hot musca volitanss – burning chamber sedimentations:
Surface ignition is ignition of the fuel-air mixture charge by any hot surface other than the flicker discharge prior to the reaching of the normal fire forepart. It may happen before the flicker ignites the charge ( pre-ignition ) or after normal ignition ( post-ignition ) .
Surface ignition can be of two types:
Knocking surface ignition: Knock which has been preceded by surface ignition. It is non governable by flicker progress.
Non-Knocking surface ignition: Surface ignition which does non ensue in knock.
It is the continuance of engine fire after the electrical ignition is shut off.
Runaway surface ignition:
Surface ignition which occurs earlier and earlier in the rhythm. It can take to serious overheating and structural harm to the engine.
Knocking surface ignition characterized by one or more fickle crisp clefts. It is likely the consequence of early surface ignition from sedimentation atoms.
A low-pitched thudding noise accompanied by engine raggedness. It is likely caused by high rates of force per unit area rise associated with early ignition or multiple surface ignitions.
Knock chiefly occurs under wide-open-throttle operating status. It is therefore a direct restraint on engine public presentation. It besides constraints engine efficiency, since by efficaciously restricting the temperature and force per unit area of the end-gas, it limits the engine compaction ratio. The happening and badness of the knock depend on the knock opposition of the fuel and on the anti-knock features of the engine.
Measures to avoid knocking:
The ability of the fuel to defy knock is steps by its octane figure ; higher octane Numberss indicate greater opposition to strike hard. Gasoline octane evaluations can be improved by polishing procedures, such as catalytic snap and reforming, which convert low-octane hydrocarbons to high-octane hydrocarbons.
Besides, antiknock additives such as intoxicants, lead alkyls, or an organomanganese compound can be used. The octane figure demand of an engine depends on how its design and conditions under which it is operated affect the temperature and force per unit area of the end-gas in front of the fire and the clip required to fire the cylinder charge. An engine ‘s inclination to strike hard, as defined by its octane figure is increased by factors that produce higher temperatures and force per unit areas or lengthen the combustion clip.
Octane Requirement: can be defined as the octane evaluation of the fuel required to avoid knock.
Therefore knock is a restraint that depends on both the quality of the available fuels and on the ability of the engine interior decorator to accomplish the coveted normal burning behaviour while keeping the engine ‘s inclination to strike hard at a lower limit. Some major stairss:
The usage of a fuel with higher octane figure.
The add-on of octane-increasing additives in the fuel
Ignition Timing Retardation.
Use of a spark stopper of colder heat scope, in instances, where the flicker stopper dielectric has become a beginning of pre-ignition taking to strike hard.
Decrease of charge temperature e.g. through fuel vaporization inside the cylinder ( GDI )
Anti knock burning chamber design.
Consequences of engine knock:
The engine can be damaged by knock in different ways:
-piston pealing lodging – breakage of the Piston rings – failure of the cylinder caput gasket
-cylinder caput eroding – Piston Crown and top eroding -piston thaw and holing
Examples of component harm due to pre ignition and knock are shown below:
A strobe in an instrument used to do cyclically traveling object appear to be traveling slow or stationary. The rule is used for the survey of revolving, reciprocating, hovering or vibrating objects. Machine parts and vibrating strings are common illustrations.
In its simplest signifier, a revolving phonograph record with evenly-spaced holes is placed in the line of sight between the perceiver and the traveling object. The rotational velocity of the phonograph record is adjusted so that it becomes synchronised with the motion of the ascertained system, which seems to decelerate and halt. The semblance is caused by temporal aliasing, normally known as the stroboscopic consequence.
In electronic versions, the pierced phonograph record is replaced by a lamp capable of breathing brief and rapid flashes of visible radiation. The frequence of the flash is adjusted so that it is an equal to, or a unit fraction below or above the object ‘s cyclic velocity, at which point the object is seen to be either stationary or traveling rearward or frontward, depending on the brassy frequence.
10 & A ; deg ;
17 & A ; deg ;
22 & A ; deg ;