Detonation and Pre-Ignition

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Piston damage caused by detonationWithout a doubt the single biggest barrier to high performance tuning is detonation.

Detonation (also called knock, knocking or spark knock, pinking or pinging) in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but 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 time in the piston's stroke cycle. The peak of the combustion process no longer occurs at the optimum moment for the four-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. It should not be confused with pre-ignition (or preignition), as they are two separate events.

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 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

Engine block damage caused by detonationWhen unburned fuel/air mixture beyond the boundary of the flame front is subjected to a combination of heat, 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 take the form of:

  • The fuel/air mixture being lit-off too early in the engine cycle by over advanced ignition timing
  • Fuel with too low an octane rating causes spontanous ignition of the fuel/air charge before the spark plug would normally initiate the combustion process
  • Once the spark plug ignites the fuel air charge and begins to burn, a second unplanned ignition of the fuel/air charge occurs simultaneously in another area of the combustion chamber

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. In turbo-charged cars, sensible boost pressures and controlling intake charge temperatures also has a significant effect on controlling 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.

More piston damage caused by detonationProlonged or heavy detonation in petrol engines can be very damaging, so if you hear knocking or pinging when accelerating or lugging your engine, you probably have a detonation problem. Despite all the evidence to the contrary, some very curious attitudes towards detonation in petrol engines still persist, and are hopefully cleared up below:

  1. There is no such thing as minor or non-damaging detonation
  2. An engine that detonates only occasionally is not correctly setup
  3. Forged pistons and racing connecting rods are not detonation proof
  4. Each detonation event on a highly modified engine making large amounts of power will be proportionally stronger and far more damaging compared to a detonation event on the same engine in un-modified standard trim
  5. Sophisticated closed-loop knock control is not a crutch on which to support a badly modified and tuned engine
  6. A knock sensor is only a glorified microphone, and in the case of some highly modified engines, noisy engine pats and high RPMs can make the ECUs job of listening for knock similar to trying make out a flute at a heavy metal concert.
Detecting Detonation

Checking for detonation during the tuning process is not an easy task. Loud exhausts and noisy dyno rollers can easily drown out the tell-tale sounds of detonation. Professional engine tuners get around this issue by using acoustic probes clipped to the engine block, fed back through an amplifier and then into a set of headphones. Probably the best form of knock detection, it is also the hardest to master, as the operator has to be familiar with the way in which knock can sound subtly different depending on the type of engine being tuned.

There are other commercially available electronic knock detection sensors available. Typically they use a Bosch style knock sensor that is attached to the engine block. Sensor output is then sent back into a head unit inside the vehicles cabin. Once correctly calibrated and adjusted, knock events are displayed visually by means of different intensity LED lights. It is important to note though, these units cannot be depended upon to detect detonation with 100% accuracy.

Pre-ignition

Another condition that is sometimes confused with detonation is "pre-ignition." This occurs when a point within the combustion chamber becomes so hot that it becomes a source of ignition and causes the fuel to ignite before the spark plug fires. This, in turn, may contribute to or cause a detonation problem.

Instead of the fuel igniting at the right instant to give the crankshaft a smooth kick in the right direction, the fuel ignites prematurely (early) causing a momentarily backlash as the piston tries to turn the crank in the wrong direction. This can be very damaging because of the stresses it creates. It can also localize heat to such an extent that it can partially melt or burn a hole through the top of a piston!

Pre-ignition can also make itself known when a hot engine is shut off. The engine may continue to run even though the ignition has been turned off because the combustion chamber is hot enough for spontaneous ignition. The engine may continue to run-on or "diesel" and chug erratically for several minutes.

To prevent this from happening, some engines have a "fuel cutoff solenoid" on the carburetor to stop the flow of fuel to the engine once the ignition is turned off. Others use an "idle stop solenoid" that closes the throttle completely to shut of the engine's air supply. If either of these devices is misadjusted or inoperative, run-on can be a problem. Engines with electronic fuel injection don't have this problem because the injectors stop spraying fuel as soon as the ignition is turned off.

Causes of pre-ignition

Carbon deposits form a heat barrier and can be a contributing factor to preignition. Other causes include: An overheated spark plug (too hot a heat range for the application). Glowing carbon deposits on a hot exhaust valve (which may mean the valve is running too hot because of poor seating, a weak valve spring or insufficient valve lash).

  • A sharp edge in the combustion chamber or on top of a piston (rounding sharp edges with a grinder can eliminate this cause).
  • Sharp edges on valves that were reground improperly (not enough margin left on the edges).
  • A lean fuel mixture.
  • Low coolant level, slipping fan clutch, inoperative electric cooling fan or other cooling system problem that causes the engine to run hotter than normal.

A very comprehensive report of detonation and pre-ignition was published in Contact Magazine and can be read here.