Detonation internal combustion engine

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1 Detonation internal combustion engine

Detonation internal combustion engine

Detonation also commonly referred to as Knock, ping or pinking is a form of abnormal combustion in the internal combustion engine.

Normal Combustion

Under ideal conditions the common piston internal combustion engine burns its fuel air mix in the cylinder in a orderly and controlled fashion. The combustion is started by the spark plug some 15 - 40 crankshaft degrees prior to TDC (top dead center) the point of maximum compression. 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. This point is typically 14-18 crankshaft degrees ATDC (after top dead center).

The spark plug produces an electrical spark that jumps a small gap from its center electrode to its ground electrode. This spark, if the fuel air mix is within the flamable range for the fuel begins combustion. The initial phase forms a small kernel of flame approximately the size of the spark plug gap. For the first few milliseconds of the combustion process, this flame kernel is struggling to survive, producing only slightly more heat than is necessary to continue the combustion process. As it grows in size its heat output increases allowing it to grow even faster.

After this early slow burn phase passes, the flame kernel grows much faster expanding rapidly across 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 fuel air that have a much greater surface area than a simple spherical ball of flame would have. This greatly accelerates the combustion process.

In normal combustion this flame front moves throughout the fuel air mix at a predictable rate. Pressure rises smoothly to a peak and then falling as combustion completes burning nearly all the available fuel. In normal combustion this produces a rapid increase in cylinder pressure as the piston passes TDC and begins to move down the cylinder. As mentioned above in a properly tuned engine the 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.

The detonation process

Detonation is an abnormal combustion process where some of the fuel air mix burns in an out of control manner at extremely high speeds. This creates a violent jump in cylinder pressure when the piston is nearly stationary near TDC and cannot effectively transfer the load to the crankshaft.

As engine load increases, if the temperatures and pressures are too high within the combustion chamber, a complex series of pre-combustion reactions begin to break down unburned fuel before the flame front gets to them for normal combustion to take place. If the breakdown proceeds far enough, there are areas in the combustion chamber that will contain very unstable high temperature mixtures of unburned or partially burned fuel and air. If a critical temperature and pressure is reached these highly combustible gasses ignite explosively rather than in the normal progressive process.

The actual detonation process is still, some 50-100 years after is was first examined, not entirely understood. What is clear, is that this highly unstable pocket of gas can undergo a very rapid supersonic combustion or detonation, which can push cylinder pressures to very high levels, and cause massive shock to the mechanical parts of the engine.

Studies done by NACA (predecessor agency to NASA) on aircraft engines following WWI and during the WWII time period, tried to understand the process. They were performed using ultra high speed cameras with frame rates of 40,000 to 200,000 frames per second. In those studies the detonation process frequently went to completion in only a few film frames indicating very high combustion speeds. Photographic evidence indicated the detonation process occurs in a time interval on the order of (5 x 10 ^-5 seconds) and computed speeds of 6500 - 6800 ft/sec. [1] [2] [3]

As a result of this very rapid release of energy, cylinder pressures briefly spike to several times normal, causing a characteristic sound which gives engine detonation its common names. When it occurs under low rpm high load conditions it produces a brutal knocking rattle sound (hence the common name). As engine rpm increases the perceived pitch of the sound would increase often described as sounding like rocks shaken in a tin can. At high rpm it could progress to a sharp ping or tink sound that leads to the other common names of "ping" in the U.S. and "Pink" or "Pinking" in the UK and Europe. The sound was caused by the ringing of the entire engine due to the sudden hammer blow of high pressure to the piston and cylinder head, caused by the detonation. These photographic studies demonstrated that auto-ignition of the unburned or partially burned gasses often occurred along with detonation but were not pre-requisite conditions for detonation, as knock events were observed where no auto-ignition preceded the detonation.

Detonation can range from a low order form that an engine can endure for hours to a intense form that can destroy pistons and crack cylinder walls in a matter of seconds. [4]

The ultimate limit for power production in an internal combustion spark ignition engine is nearly always determined by the maximum cylinder pressure you can develop without producing destructive detonation. This came to be known as "knock limited performance". Nearly all high performance internal combustion piston engines are limited in their maximum power output by the octane and knock sensitivity of the fuel they use.

Research to understand engine detonation or Knock resulted in extensive research in high octane fuels. The octane rating system was set up to allow different fuels to be ranked according to how sensitive they were to detonation. From this, was developed a standard method of octane ratings based on mixtures of two specific pure hydrocarbon fuels that would give the same sensitivity to detonation as the test fuel. This allowed fuels to be rated up to 100-120 octane. During WWII this index was extended with Performance Numbers for aircraft fuels, by computing out an approximate effective octane number for a fuel based on how much TEL ( Tetra-ethyl lead ) needed to be added to the fuels.

Up until the 1970's TEL was a common octane increasing additive added to both automotive and aircraft gasoline. Due to environmental concerns, it has been replaced by MMT a magnesium compound ( Methylcyclopentadienyl Manganese Tricarbonyl ) or by changes in fuel blending which add very knock resistant chemicals to the fuel. High octane unleaded fuel, frequently contains high percentages of aromatic compounds like Toluene to enhance the fuel octane. Some other chemicals like ethanol and MTBE (methyl tertiary-butyl ether) have also been used in many locations both to reduce emissions of CO, HC or NOx compounds in the exhaust gas, and to enhance fuel octane. MTBE is currently being phased out in many areas due to its potential to cause pollution of ground water.

The most effective methods found to control knock were:

To raise the octane level of the fuel.
Speed up the combustion process by using multiple spark plugs in each cylinder.
Reduce combustion time by increasing engine rpm.
Improved combustion chambers that came to be known as "fast burn" chambers.
Lowering the intake air temperature of the fuel air mix, and the use of water or water alcohol injection to cool the combustion gasses were also effective.

Cooling the combustion chamber with water injection came to be known as ADI in WWII aircraft standing for "Anti Detonation Injection". In the modern performance automotive community it is commonly known as Water injection or simply WI.

In recent years as more high performance turbocharged and supercharged automobile engines have become more common there has been increase interest in compound fuel systems in the form of Alcohol injection where a water alcohol or straight alcohol is injected into the engines intake air stream under high load, this both cools the combustion process but adds additional high octane fuel to the fuel air mix raising the effective octane to levels high enough to push knock limited performance to new limits.

References

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Categories: Engine technology

Detonation internal combustion engine

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