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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 5 to 40 crankshaft degrees prior to top dead center (TDC), depending on 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 allowing 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 to turbulence rapidly stretching 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. In normal combustion, this flame front moves throughout the fuel/air mixture at a rate characteristic for the fuel/air 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 increasing pressure can give the piston a hard push when its speed and mechanical advantage on the crank shaft gives the best recovery of force from the expanding gases.
Detonation
 When unburned fuel/air mixture beyond the boundary of the flame front is subjected to a combination of heat and pressure for a certain duration (beyond the delay period of the fuel used), detonation may occur. Detonation is characterized by an 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 may rise sharply beyond its design limits. 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. Severe knocking can lead to catastrophic failure in the form of physical holes punched through the piston or head, either of which depressurizes the affected cylinder and introduces large metal fragments, fuel, and combustion products into the oil system.
Detonation can be prevented by the use of a fuel with high octane rating, which increases the combustion temperature of the fuel and reduces the proclivity to detonate; enriching the fuel/air ratio, which adds extra fuel to the mixture and increases the cooling effect when the fuel vaporizes in the cylinder; reducing peak cylinder pressure by increasing the engine revolutions (e.g., shifting to a lower gear); decreasing the manifold pressure by reducing the throttle opening; or reducing the load on the engine. Because pressure and temperature are strongly linked, knock can also be attenuated by controlling peak combustion chamber temperatures at the engineering level by compression ratio reduction, exhaust gas recirculation, appropriate calibration of the engine's ignition timing schedule, and careful design of the engine's combustion chambers and cooling system. As an aftermarket solution, a water injection system can be employed to reduce combustion chamber peak temperatures and thus suppress detonation.
Knocking is unavoidable to a greater or lesser extent in diesel 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 electronic common rail injection have very low levels of knock. Engines using indirect injection generally have lower levels of knock than direct injection engine, due to the greater dispersal of oxygen in the combustion chamber and lower injection pressures providing a more complete mixing of fuel and air.
Pre-ignition
Pre-ignition (or preignition) in a spark-ignition engine is a technically different phenomenon from engine knocking, and describes the event wherein the air/fuel mixture in the cylinder ignites before the spark plug fires. Pre-ignition is initiated by an ignition source other than the spark, such as hot spots in the combustion chamber, a spark plug that runs too hot for the application, or carbonaceous deposits in the combustion chamber heated to incandescence by previous engine combustion events.
The phenomenon is also referred to as after-run, or run-on when it causes the engine to carry on running after the ignition is shut off, or sometimes dieseling, in reference to the fact that a heated diesel engine may, by design, run without an external ignition trigger so long as a suitable fuel/air mixture is supplied to the cylinders. This effect is more readily achieved on carbureted gasoline engines, as the fuel supply to the carburetor is typically regulated by a mechanical float valve and fuel delivery can feasibly continue until fuel line pressure has been relieved, provided the fuel can be somehow drawn past the throttle plate. The occurrence is rare in modern engines with throttle-body or electronic fuel injection, as the injectors will not be permitted to continue delivering fuel after the engine is shut off, and any occurrence may indicate the presence of a leaking (failed) injector.
Preignition and engine knock both sharply increase combustion chamber temperatures. Consequently, either effect increases the likelihood of the other effect occurring, and both can produce similar effects from the operator's perspective, such as rough engine operation or loss of performance due to operational intervention by a powertrain-management computer. For reasons like these, a person not familiarized with the distinction might describe one by the name of the other. Given proper combustion chamber design, preignition can generally be eliminated by proper spark plug selection, proper fuel/air mixture adjustment, and periodic cleaning of the combustion chambers.
Avoidance
There are several no/low cost ways to solve an engine’s detonation problems.
The easiest way to avoid detonation is to use the highest octane fuel available in your area. Sure, high octane fuel might cost a few cents more per gallon than the low-grade stuff but the long-term saving is potentially thousands of dollars. Using premium unleaded may also release slightly more power from your car’s engine.
If your car’s engine continues to detonate on the highest grade fuel that’s available, it may be worth adding octane booster to each full tank.
Spark plug choice can play an important part in avoiding detonation. If an engine is detonating when using the standard heat-range spark plugs, you might want to try a set of plugs 1 or 2 ranges colder. Colder spark plugs have a shorter center electrode, which serves to more effectively transfer heat away from the plug. Note that spark plug brand can also make a difference to detonation. The only downside of colder plugs is potential spark-plug fouling. (Read more about this here. )
A relatively easy and cheap mechanical modification that helps avoid detonation is the fitment of a cold air intake. Many modern vehicles draw hot induction air from inside the engine bay. The likelihood of detonation is reduced by relocating the air intake pick-up to an area isolated from engine bay heat. Inside the wheel arch is a popular choice.
Air-fuel ratio and ignition timing are also critical factors in avoiding detonation.
Air-fuel ratios can be cheaply adjusted in any EFI car that uses a voltage type airflow meter or MAP sensor load input. By increasing the output of the load sensor, you will achieve a richer air-fuel ratio. Ignition timing may also be slightly retarded. Note, however, light-load closed-loop running will not be affected with just these modifications.
Another way to increase fuelling is to feed the ECU a false coolant temperature reading. Use an adjustable potentiometer on the coolant temperature circuit to fool the ECU into thinking the coolant is colder than it really is. This will almost exclusively richen the air-fuel ratio. Note, however, this enrichment might occur only at relatively low revs and/or load.
And what about the all-important ignition timing?
A zero cost – but crude - way to prevent detonation on some cars is to simply retard the distributor or crank angle sensor slightly. This has the effect of retarding ignition timing a few degrees across the board. The downside is reduced engine performance at all revs.
Another way to retard ignition timing is to alter the output of the engine’s intake air temperature sensor using a potentiometer. By fooling the ECU into thinking intake air temperature is hotter than it really is, it will give retarded ignition timing.
The actions described above are perfectly adequate approaches to eliminating detonation. However, if none of these work – or you simply don’t want to go down these tracks – you can turn to these higher cost alternatives...
For rpm and load specific changes to air-fuel ratio and ignition timing you’ll need an aftermarket interceptor, an ECU remap or you can start from scratch with an off-the-shelf programmable ECU. The costs involved may run to thousands of dollars but, on the upside, you’ll have the tuning flexibility to make the most of the vehicle’s existing mods. These approaches also allow the engine to be retuned to suit later set-ups.
Turbo and supercharged cars – in particular those that are modified – rely heavily on an intercooler to eliminate detonation. If money isn’t a problem, you can purchase a large-capacity aftermarket air-to-air intercooler from anywhere from $400 to $2000. These gigantic intercoolers offer a huge thermal mass, large frontal cooling area and sophisticated internal finning to reduce charge-temperatures to near ambient. These are an all-out solution to charge-air cooling.
Note, however, the heat exchange performance of your vehicle’s intercooler can be improved in a couple of ways - there’s often no need to buy an aftermarket replacement.
The most popular intercooler enhancement is the addition of a water spray. A water spray removes heat from the charge-air through the evaporation of water on the core. An intercooler water spray performs best when there’s always cooling airflow through the core (eg induced by a fan) and when a water atomising nozzle is employed. Keep in mind that most drag strips will NOT allow you to use such a system as it tends to drip water onto the track, and that can be a danger to the car running behind you.
If detonation problems are evident only in traffic conditions, your solution might be the inclusion of an electric fan to draw cooling air through the intercooler core when the vehicle is stationary. This will remove the intake air temperature spikes that occur in heavy traffic.
An interesting approach that can reduce the likelihood of detonation on some engines is the use of intake manifold insulators. Manifold insulators are intended for use with alloy intake systems, which are prone to heat-soak as the result of under-hood heat. These spacers – usually made from phenolic material – are sandwiched between the manifold and engine and serve to reduce the amount of heat conducted to the alloy manifold. This can reduce intake air temperatures by several degrees.
Another approach to intake system heat insulation is to use a ceramic coating on relevant parts of the engine. Ceramic coatings can also be used on piston crowns. Of course, this necessitates an engine tear-down to access the parts.
Water injection is arguably the most effective solution for detonation.
Water injection eliminates detonation in a number of ways – it substantially cools the induction air and, when the water turns to steam upon reaching the combustion chamber, it acts as an anti-detonant and cleans the inside of the engine. This cleaning action helps eliminate detonation-inducing combustion ‘hot spots’. Water injection is also usually combined with methanol which increases octane substantially, effectively solving two problems at once.
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