PRE-IGNITION = Squeeze the fuel molecules so tightly (high compression) that they ignite on their own (before the spark plug fires.
DETONATION = Ignition of fuel and expanding gases is early and fighting piston travel. They slam into each other making a pinging noise.
This is not quite accurate.
As Jerry mentions, detonation is caused by the heat of combustion raising the combustion chamber temperature above the auto-ignition point of the fuel, which causes unburned pocket(s) of air/fuel mixture to self ignite. The pressure waves from these flame fronts collide, causing a rapid increase in cylinder pressure. These pressure spikes actually cause the cylinder walls to resonate, resulting in the audible “pinging” sound heard from the engine.
Peak pressures during detonation occur after TDC, since the pressure/temperature rise that initiates it is a result of the combustion event. Severity of detonation can range from mild (annoying) to severe (engine damage).
One way to control detonation is to use fuel with higher octane. The octane rating of fuel is directly related to the fuel’s resistance to self-ignite. The higher the octane rating, the higher the resistance to self-ignite, which means higher resistance to detonation. Anyone old enough to remember black and white TVs or 0.30/gallon gas, knows that the go-to octane enhancer used to be tetraethyl lead. Lead was phased out of fuels in the 1970’s to support the use of catalytic converters, and for environmental concerns. Ethanol is frequently used in modern gasolines to increase octane rating.
Running a 5% Diesel fuel mix can raise the octane rating of the fuel, sometimes as much as 5 points.
Jerry, you mention that you use diesel fuel to increase octane. Since diesel engines are designed to ignite the air/fuel mixture using the heat of compression, diesel fuels have extremely low octane ratings. Their self ignition temps are low by design. I’m struggling to understand the science behind how adding diesel fuel can increase octane rating of gasoline. Please explain.
Some of the factors related to detonation are:
- Combustion chamber temp/press
- Spark advance (which affects combustion chamber temp/press)
- Compression ratio (ditto)
- Air/fuel mixture (ditto)
- Inlet air temperature (ditto)
- Engine load (ditto)
Modern engine management systems monitor for detonation using a detonation (aka “knock”) sensor. When detonation is detected, the engine controller can take corrective action to mitigate detonation, such as retarding spark advance, reducing throttle, or changing the air/fuel mixture. As was also previously mentioned, the time honored demonstration of this was to tap on the side of the block with a hammer while watching spark advance - either with a timing light or scan tool. The hammer hit causes the block to resonate at a frequency close enough to detonation to cause the engine controller to react.
In contrast, preignition, is caused by an ignition source other than the spark. Sources include: hot spots in the combustion chamber due to carbon build up, a spark plug that is too hot due to incorrect heat range selection, sharp edges on the cylinder head, valves, etc,
The bad thing about preignition is that as the name implies, it occurs well before the piston has reached TDC, causing very rapid pressure and temperature increases in the combustion chamber. It often occurs so quickly, that the engine damage is done before the driver is even aware that it is happening.
BTW, preignition is not just something to be worried about with our engines. A relatively new phenomenon called “Stochastic Preignition” is currently a source of concern among automakers designing small displacement, direct inject, turbocharged engines.
Stochastic DetonationAs a quick sidebar to detonation, in the summer of 1982, I was at a party talking to a very attractive young brunette, who had just got her 1981 Olds Cutlass back from the dealer for the umpteenth time. The complaint was that the engine often made a “rattling noise” when she stepped on the gas. Each time she got it back, it seemed like the car ran worse. The dealer finally advised her to start running premium fuel, because of the “high performance” Olds V8 under the hood — which was beyond stupid, because as anyone who worked on the cars of that era knows, the compression ratio of that Olds 307 was lower than low back then.
I told her that I would look at it, and if I fixed it, she could buy me dinner.
I quick check of the ThermAC system (the thermostatic air control that preheats incoming air to the carburetor when the engine is cold) showed that the vacuum control valve was faulty, causing the system to provide heated air to the carb at all times. This super heated air, entering the combustion chamber, was enough to induce detonation under certain engine loads. I replaced the valve, and the system worked fine. A check of the ignition timing, showed that in an effort to fix the problem, the nobs at that dealer had uber-retarded the spark causing the engine to lose power and fuel economy. I reset it to factory specs and the car ran great.
I got my dinner. As of last May, we have been married 37 years.
Best Regards…
p.s. A lot of the above information was gleaned from this source. It an older textbook, but it has a lot of interesting information related to internal combustion engines and related topics. Plus, it is a freebie download.
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