The first concept to grasp is that a normal fuel combustion explosion takes time to burn the fuel. Ignition occurs at the spark plug and a flame front spreads across the combustion area. Low octane fuel explodes easier and burns faster. High octane fuel resists ignition due to high compression, and takes a little longer to burn. Perhaps it is easier to see octane as resistance to detonation. Ignition starts the combustion process a few degrees Before Top Dead Center (BTDC) at an idle. The faster the engine spins, the earlier the ignition must start, typically 30 to 45 degrees BTDC at high rpms. Higher compression engines need less ignition spark advance, as the fuel burns faster.
The net effect of an ignition timing curve is to achieve Peak Cylinder Pressure (PCP) somewhere close to 10 degrees after the piston goes over the top and is on it’s way down. Peak cylinder pressure is generally between 600 psi and 1000 psi for a spark ignition gasoline engine, and about twice that for a diesel engine. Higher peak cylinder pressures correspond to more power and better efficiency. This century’s turbocharged and supercharged engines can control peak cylinder pressure by varying the boost pressure. Last century’s turbocharged engines were just Blow & Go, slow to spool up, and no electronic control. Normally aspirated engines have peak cylinder pressure curves that are basically set by the engine design, with a small amount of control through variable valve timing technology.
Knock sensor technology evolved in the 1980’s. All modern engines are equipped with knock sensors. Inside is a piezoelectric quartz crystal that will generate a voltage pulse in response to the vibration of a combustion event. Each knock sensor is tuned to a specific engine to eliminate other engine noises, and is sensitive to vibrations at around 15 kilohertz (15 kHz).
When the peak cylinder pressure comes too early, the voltage corresponding to the vibration rises. At some calculated point, the Engine Control Unit responds by reducing the spark advance to stop the knocking. Charged engines may reduce the charge overpressure. Some engines might increase the Exhaust Gas Recirculation (EGR) volume, as exhaust gases have less oxygen and thereby cushion and cool the explosion, reducing knocking and NOx production.
The most efficient engines run right at the edge of a hazy grey area of knocking, where the most fuel energy turns to power instead of heat. Engineers concentrate on enhancing swirl, variable cam timing and stratified fuel charge to manage engines that have high compression of 11:1 to over 12:1 yet still run on high octane pump fuel. Last century, engines with compression this high required much higher octane fuel to avoid knocking.
Some engines run a lower range of knock sensor voltages, typically .1 to .2 Volts at idle, around a half a volt when cruising, and up to 1 volt at full acceleration. Other engines may have a range that starts around .5 to .7 volts at idle or higher. In the engine computer’s diagnostic measuring value blocks, the voltage of the knock sensor of each cylinder will be displayed and all cylinders should be within around 20% of each other. Voltage variances may indicate ignition, mixture or compression issues. An engine with half the cylinder knock voltages way higher than the other half may have the camshafts timed incorrectly yielding lower compression on one cylinder bank.
The engine computer uses both the height of the knock sensor voltage as well as the timing of that voltage pulse to control ignition timing and regulate mixture to each cylinder individually. One can surmise that uneven knock sensor voltages can also result in poor fuel mileage.
Knocking is more commonly known as pinging. But it differs somewhat from detonation. Knocking / pinging is caused by the peak cylinder pressure event coming too early because the flame front from the spark plug initiated too soon. Detonation is a bit variable, since the cause is basically heat and pressure. ALL of the fuel can explode simultaneously as a result of high heat and/or high compression, a bit like diesel combustion. Detonation may initially begin after the spark starts firing the compressed mixture, caused by heat and colliding pressure waves. A detonation explosion can be started at some hot spot, such as the sharp edge of an accidental dent, or the tip of some component heated up by the initial heat produced during initial detonation.
In the case of either knocking / pinging and detonation, the noise is generated by a combination of factors. The majority of the noise is from the explosion itself, think dynamite going off during a firecracker’s explosion. Uncontrolled flame fronts are colliding. The extremely high pressure spikes generate a loud and sharp noise. The noise and effect is similar to nailing in a diesel motor when injectors leak and the diesel combustion starts too early. Detonation is potentially more damaging than pinging, as it likely cannot be controlled by the ECU changing the ignition timing. Dropping boost, enriching the fuel mixture and/or adding more EGR may well stop detonation.
Some noise is generated by the piston banging against the cylinder wall. In all engines, the wrist pin of the piston is offset slightly towards one side of the piston, to the right when viewed from the pulley end of a clockwise rotating engine. This reduces overall piston friction, gives a straighter connecting rod angle to the crankshaft during peak cylinder pressure, and helps the piston change sides of the cylinder before peak cylinder pressure hits. The rod is slightly offset sideways along the wrist pin from the center of the piston to help control piston rotation. When peak cylinder pressure comes too early, the piston changes sides of the cylinder bore violently, generating a noise as well as wearing rapidly. Way more energy from the fuel is expended as heat and vibration, so mileage will diminish, although not necessarily perceptibly.
The peak cylinder pressures exerted during detonation are severe, but cannot be accurately generalized. Too many variables exist including the piston position where the detonation starts, the nature of the fuel mixture, and whether a secondary hot spot is participating in and/or causing or continuing the detonation events.
Engine Damage From Detonation
The first and most obvious indicator of detonation damage is visible on the porcelain of the spark plug. Tiny specks of metal blown off the top of the piston and/or cylinder head by the heat and shock waves show up as little dots called “pepper”. The ground electrode may loose metal from the sharp edges of the tips.
Engine disassembly and analysis may show increased ring gap in the cylinder where detonation occurs. In a normal running engine, there is a layer of molecules of the intake mixture right at the cylinder walls and head that remain cooler due to thermal transfer from the gases to the metal. This boundary layer remains slightly cooler than the main body of molecules during combustion and helps shield the metal parts from heat damage. There is also a tiny layer of engine oil on the cylinder walls above the moving piston for lubrication. Both these layers get blown off by the extreme heat and pressure of detonation, allowing the rings and cylinder walls to wear more than usual.
In the racing world, detonation is a fairly commonly examined and controlled problem, as racing engine peak cylinder pressures can exceed 3000 psi. In real life, detonation is a problem experienced by supercharged and turbocharged motors running too low an octane gasoline in very hot weather and/or during long hill climbing at high speeds. Low quality fuel is different from low octane fuel. Using low quality fuel will lead to high carbon deposits, fuel vapor system varnish and PCV system grit. Those who choose low octane fuel can usually drive mildly with no consequences. When driving briskly, the ECU has to keep retarding timing, reduce boost if turbocharged or supercharged, and/or add additional EGR during high power events. The result is reduced mileage. Using low octane fuel can lead to engine damage if detonation occurs.