Engine knocking

Inspark-ignition internal combustion engines,knocking(alsoknock,detonation,spark knock,pingingorpinking) occurs when combustion of some of theair/fuel mixturein the cylinder does not result from propagation of the flame front ignited by thespark plug,but when one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The fuel–air charge is meant to be ignited by the spark plug only, and at a precise point in the piston's stroke. Knock occurs when the peak of the combustion process no longer occurs at the optimum moment for thefour-stroke cycle.The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive.

Knocking should not be confused withpre-ignition—they are two separate events. However, pre-ignition can be followed by knocking.

The phenomenon of detonation was described in November 1914 in a letter from Lodge Brothers (spark plug manufacturers, and sons of SirOliver Lodge) settling a discussion regarding the cause of "knocking" or "pinging" in motorcycles. In the letter they stated that an early ignition can give rise to the gas detonating instead of the usual expansion, and the sound that is produced by the detonation is the same as if the metal parts had been tapped with a hammer.It was further investigated and described byHarry Ricardoduring experiments carried out between 1916 and 1919 to discover the reason for failures inaircraft engines.

Normal combustion

Under ideal conditions the common internal combustion engine burns the fuel/air mixture in the cylinder in an orderly and controlled fashion. The combustion is started by the spark plug some 10 to 40 crankshaft degrees prior totop dead center(TDC), depending on many factors including engine speed and load. This ignition advance allows time for the combustion process to develop peak pressure at the ideal time for maximum recovery of work from the expanding gases.

The spark across the spark plug's electrodes forms a small kernel of flame approximately the size of the spark plug gap. As it grows in size, its heat output increases, which allows it to grow at an accelerating rate, expanding rapidly through the combustion chamber. This growth is due to the travel of the flame front through the combustible fuel–air mix itself, and due toRayleigh–Taylor instability(resulting from the hot, low-density combustion gasses expanding into the relatively cold and dense unburnt fuel–air mix) which rapidly stretches the burning zone into a complex of fingers of burning gas that have a much greater surface area than a simple spherical ball of flame would have (this latter process is enhanced and accelerated by any pre-existing turbulence in the fuel–air mixture). In normal combustion, this flame front moves throughout the fuel/air mixture at a rate characteristic for the particular mixture. Pressure rises smoothly to a peak, as nearly all the available fuel is consumed, then pressure falls as the piston descends. Maximum cylinder pressure is achieved a few crankshaft degrees after the piston passes TDC, so that the force applied on the piston (from the increasing pressure applied to the top surface of the piston) can give its hardest push precisely when the piston's speed and mechanical advantage on the crank shaft gives the best recovery of force from the expanding gases, thus maximizing torque transferred to the crankshaft.

Abnormal combustion

Main article:Cool flame

When unburned fuel–air mixture beyond the boundary of theflame frontis subjected to a combination of heat and pressure for a certain duration (beyond the delay period of the fuel used),detonationmay occur. Detonation is characterized by an almost instantaneous, explosive ignition of at least one pocket of fuel/air mixture outside of the flame front. A local shockwave is created around each pocket, and the cylinder pressure will rise sharply – and possibly beyond its design limits – causing damage. (Detonation is actually more efficient than deflagration, but is usually avoided due to its damaging effects on engine components.)

If detonation is allowed to persist under extreme conditions or over many engine cycles, engine parts can be damaged or destroyed. The simplest deleterious effects are typically particle wear caused by moderate knocking, which may further ensue through the engine's oil system and cause wear on other parts before being trapped by the oil filter. Such wear gives the appearance of erosion, abrasion, or a "sandblasted" look, similar to the damage caused by hydrauliccavitation.Severe knocking can lead to catastrophic failure in the form of physical holes melted and pushed through thepistonorcylinder head(i.e. rupture of thecombustion chamber), either of which depressurizes the affected cylinder and introduces large metal fragments, fuel, and combustion products into the oil system.Hypereutectic pistonsare known to break easily from such shock waves.

Detonation can be prevented by any or all of the following techniques:

• retarding ignition timing
• the use of a fuel with highoctane rating,which increases the combustion temperature of the fuel and reduces the proclivity to detonate
• enriching theair–fuel ratiowhich alters the chemical reactions during combustion, reduces the combustion temperature and increases the margin to detonation
• reducing peak cylinder pressure
• decreasing themanifold pressureby reducing the throttle opening or boost pressure
• reducing the load on the engine

Because pressure and temperature are strongly linked, knock can also be attenuated by controlling peak combustion chamber temperatures bycompression ratioreduction,exhaust gas recirculation,appropriate calibration of the engine'signition timingschedule, and careful design of the engine's combustion chambers and cooling system as well as controlling the initial air intake temperature.

The addition oftetraethyl lead(TEL), a soluble organolead compound added to gasoline, was common until it was discontinued for reasons of toxic pollution. Lead dust added to the intake charge will also reduce knock with various hydrocarbon fuels.Manganesecompounds are also used to reduce knock with petrol fuel.

Knock is less common in cold climates. As an aftermarket solution, awater injectionsystem can be employed to reduce combustion chamber peak temperatures and thus suppress detonation. Steam (water vapor) will suppress knock even though no added cooling is supplied.

Turbulence, as stated, has a very important effect on knock. Engines with good turbulence tend to knock less than engines with poor turbulence. Turbulence occurs not only while the engine is inhaling but also when the mixture is compressed and burned. Many pistons are designed to use"squish" turbulenceto violently mix the air and fuel together as they are ignited and burned, which reduces knock greatly by speeding up burning and cooling the unburnt mixture. One example of this is all modern side valve orflathead engines.A considerable portion of the head space is made to come in close proximity to the piston crown, making for much turbulence near TDC. In the early days of side valve heads this was not done and a much lower compression ratio had to be used for any given fuel. Also such engines were sensitive to ignition advance and had less power.

Knocking is more or less unavoidable indiesel engines,where fuel is injected into highly compressed air towards the end of the compression stroke. There is a short lag between the fuel being injected and combustion starting.By this time there is already a quantity of fuel in the combustion chamber which will ignite first in areas of greater oxygen density prior to the combustion of the complete charge. This sudden increase in pressure and temperature causes the distinctive diesel 'knock' or 'clatter', some of which must be allowed for in the engine design.

Careful design of the injector pump, fuel injector, combustion chamber, piston crown and cylinder head can reduce knocking greatly, and modern engines using electroniccommon railinjection have very low levels of knock. Engines usingindirect injectiongenerally have lower levels of knock thandirect injectionengines, due to the greater dispersal of oxygen in the combustion chamber and lower injection pressures providing a more complete mixing of fuel and air. Diesels actually do not suffer exactly the same "knock" as gasoline engines since the cause is known to be only the very fast rate of pressure rise, not unstable combustion. Diesel fuels are actually very prone to knock in gasoline engines but in the diesel engine there is no time for knock to occur because the fuel is only oxidized during the expansion cycle. In the gasoline engine the fuel is slowly oxidizing all the time while it is being compressed before the spark. This allows for changes to occur in the structure/makeup of the molecules before the very critical period of high temperature/pressure.

Knock detection

Due to the large variation in fuel quality, atmospheric pressure and ambient temperature as well as the possibility of a malfunction, every modern combustion engine contains mechanisms to detect and prevent knocking.

A control loop is permanently monitoring the signal of one or moreknock sensors(commonlypiezoelectric sensorwhich are able to translate vibrations into an electric signal). If the characteristic pressure peak of a knocking combustion is detected the ignition timing is retarded by steps of a few degrees. If the signal normalizes indicating a controlled combustion the ignition timing is advanced again in the same fashion keeping the engine at its best possible operating point - the so-called ″knock limit″. Modern knock control-loop systems are able to adjust ignition timings for every cylinder individually. Depending on the specific engine the boost pressure is regulated simultaneously. This way performance is kept at its optimum while mostly eliminating the risk of engine damage caused by knock (e.g. when running on low octane fuel).An early example of this is inturbochargedSaab H engines,where a system calledAutomatic Performance Controlwas used to reduce boost pressure if it caused the engine to knock.

Knock prediction

Since the avoidance of knocking combustion is so important to development engineers, a variety of simulation technologies have been developed which can identify engine design or operating conditions in which knock might be expected to occur. This then enables engineers to design ways to mitigate knocking combustion whilst maintaining a highthermal efficiency.

Since the onset of knock is sensitive to the in-cylinder pressure, temperature and autoignition chemistry associated with the local mixture compositions within the combustion chamber, simulations which account for all of these aspectshave thus proven most effective in determining knock operating limits and enabling engineers to determine the most appropriate operating strategy.

Knock control

The objective of knock control strategies is to attempt to optimize the trade-off between protecting the engine from damaging knock events and maximizing the engine's output torque. Knock events are an independent random process.It is impossible to design knock controllers in a deterministic platform. A single time history simulation or experiment of knock control methods are not able to provide a repeatable measurement of controller's performance because of the random nature of arriving knock events. Therefore, the desired trade-off must be done in astochasticframework which could provide a suitable environment for designing and evaluating different knock control strategies performances with rigorous statistical properties.

Further reading

• di Gaeta, Alessandro; Giglio, Veniero; Police, Giuseppe; Rispoli, Natale (2013). "Modeling of in-cylinder pressure oscillations under knocking conditions: A general approach based on the damped wave equation".Fuel.104:230–243.doi:10.1016/j.fuel.2012.07.066.
• Giglio, Veniero; Police, Giuseppe; Rispoli, Natale; Iorio, Biagio; di Gaeta, Alessandro (2011). "Experimental Evaluation of Reduced Kinetic Models for the Simulation of Knock in SI Engines".SAE Technical Paper Series.Vol. 1.doi:10.4271/2011-24-0033.
• di Gaeta, Alessandro; Giglio, Veniero; Police, Giuseppe; Reale, Fabrizio; Rispoli, Natale (2010). "Modeling Pressure Oscillations under Knocking Conditions: A Partial Differential Wave Equation Approach".SAE Technical Paper Series.Vol. 1.doi:10.4271/2010-01-2185.
• Predictive combustion simulations for “downsized” direct injection spark-ignition engines: solutions for pre-ignition ( “mega-knock” ), misfire, extinction, flame propagation and conventional “knock”,cmcl innovations, accessed June 2010.
• Engine Basics: Detonation and Pre-Ignition,Allen W. Cline, accessed June 2007.
• Giglio, Veniero; Police, Giuseppe; Rispoli, Natale; di Gaeta, Alessandro; Cecere, Michele; Della Ragione, Livia (2009). "Experimental Investigation on the Use of Ion Current on SI Engines for Knock Detection".SAE Technical Paper Series.Vol. 1.doi:10.4271/2009-01-2745.
• Taylor, Charles Fayette (1985).The Internal-combustion Engine in Theory and Practice: Combustion, fuels, materials, design.MIT Press.ISBN9780262700276.

External links

• NACA - Combustion and knock in a spark-ignition engine
• NACA - Ionization in the knock zone of an internal combustion engine
• NACA - Interdependence of various types of autoignition and knock