This is a work in progress and may be modified at any time for corrections, additional
information, etc.
~What is detonation - and how to avoid it~
First, we have to define detonation and a related term, which can also lead to detonation,
known as preignition.
Pre-ignition =
Ignition of the fuel/air mixture before the pre-set ignition spark would ignite it. This is
caused by hot spots in the combustion chamber, which results from advanced ignition
timing, a spark plug that is too hot, high compression, low octane fuel, lean air/fuel mix,
insufficient engine cooling, or carbon or sharp spots in the combustion chamber that get
hot enough to ignite the fuel mix. Pre-ignition and detonation are separate events, but preignition frequently leads to detonation - Especially when the two flame fronts collide.
Preignition does NOT necessarily mean "ping". It can occur with no sound if the burn is
still relatively smooth and the flame front does not go super-sonic.
Detonation =
The rapid, uncontrolled burning of the air/fuel mixture. During normal combustion, the
flame front progresses across the cylinder at a controlled subsonic speed with the piston
near TDC. During detonation, the flame front goes supersonic and/or collides from
different directions. The shock wave resulting when the detonation flame front strikes the
top of the piston causes the piston to rattle in the cylinder. The sound of this shock wave
and the sound of the rattling piston is what people commonly refer to as ping. Detonation
radically increases cylinder pressure and temperature, and can quickly burn the end of the
spark plug. Excessive heat is usually what causes engine damage when detonation is
ignored.
After a normal spark ignition, the burning fuel causes a pressure wave that travels
outward from the plug gap. The surface of the pressure wave, called the flame front,
expands outward typically 40 to 50 centimeters per second. The flame front is ragged and
"fractal" as turbulence and swirls in the mixture disturbs the front. This turbulence
increases the surface area of the flame front causing more unburned fuel to be in contact
and thus increasing the flame front velocity and hence the speed at which the fuel burns.
The faster the fuel burns, the faster the pressure rise in the combustion chamber.
Typically a pressure rise of 20-30 psi per degree of crankshaft rotation.
In reality, combustion is never complete. There is an unburned boundary layer of air-fuel
mixture insulating the metal components of the combustion chamber from the
propagating flame front. This boundary layer is in thermal contact with the cool metal
whose surface temperature is well below the ignition temperature of the fuel/air mixture
and does not burn when the flame front passes over it. The boundary layer is roughly the
same temperature as the metal below and acts as an insulating layer preventing direct
contact of the metal to the flame. If the flame front touches the aluminum it melts. Your
EGT out the exhaust is up to 1600 degrees F. while the pouring temperature of aluminum
is approximately 1380 deg. F.
Deposits from unburned or partially burned fuel collect in the boundary layer especially
in isolated pockets causing at times significant build-up of carbon deposits. Typical areas
are between the seat and the cylinder wall and at the juncture of the combustion chamber
and the cylinder wall. Turbulence, by speeding up the flame front, reduces the thickness
of the boundary layer and reduces the buildup of combustion deposits. Turbulence
increases efficiency as liquid fuel doesn't burn
You want "swirl" or turbulence in the combustion chamber. Changing the shape, design
or other characteristics of your head/combustion chamber can lead to loss of turbulence
or swirl.
Swirl ->
* Adds more surface area to the flame front leading to better fuel atomization
* A thinner boundary layer (less unburned fuel)
* More rapid pressure rise
* Less carbon build-up
Detonation engine damage is caused by an instantaneous pressure rise that can exceed
1500 psi. This is more than double the normal peak combustion pressure, and will blow
head gaskets, break piston ring lands and hammer the rod bearings. Another form of
damage seen is that the tops of the pistons will be eroded and can even melt. The rod
bearings can be hammered thin and/or out of shape, leading to rod bearing failure and
crankshaft damage.
Preceding each flame front is its own sonic pressure wave. Colliding sonic pressure
waves concentrate on the irregular shapes present (edges of pistons, valves, even the
spark plug) or at the edges of the piston dome where reflecting pressure waves from the
piston or combustion chamber walls can constructively recombine to cause localized high
pressures and temperatures.
Detonation may be caused by the fuel pre-igniting from a hot spot in the combustion
chamber. If the fuel is ignited by means other than the spark plug, the two flame front
pressure waves can collide, causing detonation. One reason why some engine
manufacturers point the spark plug toward the exhaust valve is to propagate the flame
front across the hot exhaust valve early before the pressure (and temperature) in the
combustion chamber has risen very far. Later in the combustion cycle when temperatures
of the fuel become hotter and more likely to be ignited from a hot surface, only unburned
gasses remain in the area of the hot exhaust valve.
During detonation the almost instantaneous ignition of the fuel/air mixture causes such a
rapid pressure rise that shock waves pound against the insides of the combustion
chamber and piston. These shock waves produce the knocking sound in your engine.
Shock waves that are strong enough to mechanically "ping" the walls of the combustion
chamber are strong enough to sweep away any unburned boundary layer of fuel/air
mixture near the metal surfaces of the combustion chamber.
Without a boundary layer protecting the aluminum piston, the surfaces are exposed to the
combustion flame which melts through the piston. As we mentioned earlier, melting
occurs on the edge of the piston next to the cylinder wall where pressure waves reflecting
off the wall combine and amplify the pressure at specific locations. Sharp bends in the
metal such as at the edge of the piston and along valve cut-outs in high compression
pistons are difficult for boundary conditions to provide protection and are usually first
damaged by detonation.
In more severe or prolonged detonation, local temperatures melt the aluminum causing
"termite holes" in the piston. With any engine type, burning of holes into or through the
piston firsts require the destruction of the boundary layer and a burning off interceding
carbon deposits. Evidence of boundary layer destruction by detonation is best early
identified by the complete removal of carbon deposits in areas that would normally
contain carbon. This removal often is localized and close to the outside edge of the piston
dome.
Detonation damage is not limited to burning of holes in pistons. The rapid increase in
pressures can over-load the rod bearings causing lubrication failure and quite possibly
bearing failure. There could be other more hidden damage caused by detonation such as
over stressing of the crankshaft.
Detonation can be caused by:
1. Lean fuel mixture
2. Fuel octane too low
3. Improper ignition timing
4. Lugging
5. Carbon deposits (which raise compression pressures or leads to hot spots causing
preignition)
6. Excessive milling of heads or block, which will increase compression ratio.
Pre-ignition can be caused by:
1. Carbon deposits that remain incandescent
2. Spark plugs too hot a heat range
3. Spark plugs not firmly seated against gasket
4. Detonation or the condition leading to it
5. Sharp edges in combustion chamber
6. Valves operating at higher than normal temperature because of excessive guide
clearance or improper seal with valve seats.
7. Overheating
8. Ignition cross-firing. Induced voltage in spark plug wires that run parallel to each
other for long distances
Building to reduce detonation:
A good (and fairly easy) first step is to set the quench distance to .035". This is the
compressed thickness of the head gasket plus the deck height, (the distance your piston is
down in the bore). If your piston height, (not dome height), is above the block deck,
subtract the overage from the gasket thickness to get a true assembled quench distance. If
your head gasket compresses to .035" when the head is torqued on and the piston reaches
exact deck height, you would have .035" quench. If the piston top moves to .005" above
deck, you'd have a quench or squish of .030", if it stopped .005" short of deck height, the
quench would be .040".
The quench area is the flat part of the piston that would contact a similar flat area on the
cylinder head if you had .000" assembled quench height (as in no head gasket). In a
running engine, the .035" quench decreases to a close collision between the piston and
cylinder head due to rod stretch, etc.. The shock wave from the close collision drives high
velocity air through the combustion chamber. This tends to cool hot spots, average the
chamber temperature, reduce detonation and increase power.
If you are building an engine with steel rods, tight bearings, tight pistons, modest RPM
and automatic transmission, a .035" quench is the minimum practical to run without
engine damage (due to pistons contacting the head). The closer the piston comes to the
cylinder head at operating speed, the more turbulence is generated. Turbulence is the
main means of reducing detonation. Note that the operating quench height varies as RPM
and temperature change. If you run aluminum rods, loose pistons (which can rock or "tip"
and hit the head), and over 6000 RPM, a static clearance of .055" could be required. A
running quench height in excess of .060" will forfeit the benefits of the quench head
design and can cause severe detonation. So do not attempt to lower your compression by
using the wrong pistons - those that stop short of coming all the way up in the bore or the
incorrect combination of stroke and piston design.
The suggested .035" static quench height is recommended as a good usable dimension
for stock rod engines up to 6500 RPM. Since it is the close collision between the piston
and the cylinder head that reduces the chances of detonation, never add a shim or head
gasket to lower compression on a quench head engine. If you have 10:1 with a proper
quench and then add an extra .040" gasket to give 9.5:1 and .080" quench, you will create
more ping at 9.5:1 than you had at 10:1. Ways to lower the compression include larger cc
combustion chamber heads, or to use a dish piston. Dish (reverse combustion chamber),
pistons are designed for maximum quench, also called squish area. Having part of the
combustion chamber in the piston improves the shape of the chamber and flame travel.
High performance motors will see some detonation, which can lead to preignition.
Detonation occurs at five to ten degrees after top-dead-center. Preignition occurs before
top-dead-center - and before the spark plug fires. Detonation damages your engine with
impact loads and excessive heat.
Camshaft choice can also reduce the chances for detonation. An intake valve that closes
later as the piston travels upward on the compression stroke releases a bit of the mix,
reducing actual dynamic compression and thus heat and pressures. Longer exhaust
strokes can lead to cooler combustion chamber temperatures. A well built racing engine
with a "more radical cam" can actually be less prone to detonation than a mild street
engine due to lower dynamic compression the cooling effects of valve overlap and
duration.