Pre-Ignition vs Detonation: Causes & Symptoms

You’ll often see words like detonation, knock, and pre-ignition being used interchangeably or at least in the same context.

While many confuse them to be the same, the reality is that they are two different concepts belonging to an umbrella term called abnormal combustion.

So, how is detonation different from pre-ignition? What about knocking? Is misfire the same thing as detonation?

Join us as we take a deep dive into the two main types of abnormal combustion and discuss their symptoms, causes, and differences.

We’ll also touch on how to identify detonation and pre-ignition while offering a few suggestions to prevent these issues.

What Is Pre-ignition?

As the name suggests, pre-ignition happens when something other than your spark plug abnormally ignites your air/fuel mixture. In other words, it happens before the ignition event.

There are many things that can prematurely set off an ignition event. It could be a cracked spark plug tip, a burned exhaust valve, or rouge carbon deposits that turn into hotspots.

Pre ignition illustration

Another form of pre-ignition, called stochastic pre-ignition or low-speed pre-ignition (LSPI), is described as a pre-ignition event happening in turbocharged gasoline engines with direct injection, often under low-speed high-load conditions.

Internal combustion engines are designed to conduct the spark event right as the piston compresses the A/F mixture and approaches TDC or Top Dead Centre.

Ideally, the ignition should be followed by expansion inside the cylinder in a quick yet controlled manner.

To ensure that happens, automakers fine-tune the ignition timing based on multiple factors like the engine design, power requirements, octane rating, A/F ratio, and so on.

Air fuel mixture

Without going into too much detail, pre-ignition is an undesirable event. Manufacturers go to great lengths to prevent it from happening.

It’s worth mentioning that pre-ignition often occurs without warning, usually during the compression stroke.

If pre-ignition occurs, engine failure is almost certain. The uncontrolled combustion creates too much pressure far too early in the cycle, putting a lot of stress on the internals.

It’s one thing to compress a relatively cold mixture, but continuing to compress burning/expanding gases takes a massive toll on the piston. Ergo, reduced engine life.

Pre-ignition Symptoms

It’s almost impossible to predict pre-ignition. There aren’t usually any warning signs. Determining the exact moment is difficult unless you have access and the know-how to operate an oscilloscope. It’s a pretty useful tool for ignition diagnosis.

Molten spark plug electrode
Signs of detonation on a spark plug

Pre-ignition symptoms may include loss of power and increased exhaust gas temperature.

But as you can tell, these symptoms are not exclusive to pre-ignition. You can, however, confirm pre-ignition from a few obvious signs. These include:

  • Sudden/unexpected engine failure
  • A hole melted through the piston, especially in the middle.
  • Melted spark plugs with the central electrode surrounded by splattered/fused porcelain.
  • High exhaust gas and cylinder head temperatures.

Ultimately, the only solution to avoid pre-ignition is to prevent it from occurring. Install spark plugs with the OEM recommended heat range.

Fouled spark plugs due to pre-ignition

Make sure the installation is done properly to avoid too much clearance or loose fitment. It helps to know the symptoms of bad spark plugs.

Keeping an eye on the cooling system is also advised to avoid pre-ignition. Additionally, stick to OEM-recommended oil viscosity and grade.

Otherwise, the oil can seep through the cylinder and piston ring, potentially creating a hotspot. Plus, ensure the oil levels are sufficient.

What Causes Detonation in an Engine?

Detonation is the spontaneous burning (more like an explosion) of the remaining intake charge right after the spark event initiates normal combustion.

Engine detonation illustration

The spark plug, upon firing, creates a flame front that’s designed to burn the A/F mixture progressively.

But, if the fuel is of a lower octane rating than recommended, the charge could detonate at some point in the normal combustion burn, creating an audible ping or knock sound.

The unburned end gas is unable to withstand the heat and pressure exerted, causing it to spontaneously combust.

Detonation occurs when the fuel/air mixture is excessively lean. Other reasons include bad fuel quality, advanced timing, and improper cooling.

Too much spark advance allows the mixture to burn sooner, causing an abnormal yet very sharp pressure spike.

Cam timing

The remaining charge may not have sufficient octane rating to withstand this combination of increased heat and pressure. The gas combusts, sending pressure waves across the engine, which we hear as a knock.

Fortunately, knocking/detonation is not as destructive as pre-ignition. However, prolonged exposure to high levels of knocking can cause engine damage.

These include damaged piston rings, rods, rod bearings, and deformed piston and cylinder walls. As a safety measure, OEMs install knock sensors to keep detonation in check.

Pre-ignition vs Detonation vs Knocking

We’ve established that knocking and detonation are basically the same thing. However, detonation and pre-ignition, although often interchangeably used, are technically different concepts of abnormal combustion.

As mentioned above, pre-ignition occurs before the spark event, whereas detonation/knocking happens after the spark plug ignites the intake charge.

As discussed, pre-ignition can cause serious, often irreversible, engine damage. Detonation, on the other hand, although an undesirable event, is not as catastrophic as pre-ignition.

Damaged pistons due to pre-ignition

Most engines can withstand light to moderate levels of detonation. Some engines are even designed to sustain heavy detonation without causing any serious damage.

Another thing worth mentioning is that knock can be detected via knock sensors or by observing the pressure waves inside the cylinder. Pre-ignition is fairly difficult to predict.

Detonation often includes symptoms like an audible knocking or pinging sound. Additionally, the exhaust gas temperature (EGT) falls, while the cylinder head temperature (CHT) rises in the case of detonation.

What Is the Difference Between Misfire And Detonation?

Engine misfire

Engine misfire is usually categorized as incomplete or partial combustion in one or more cylinders. Detonation, however, is a type of abnormal/uncontrolled combustion that occurs inside an engine.

While detonation is characterized by a noticeable knocking or pinging sound, misfires are usually indicated by loss of power, stalling, or an excessively shaky idle. That said, a misfire can create a popping or banging sound, depending on how severe it is.

Detonation is almost exclusively caused by improper engine timing or bad-quality low-octane fuel. Misfires, on the other hand, can arise from a failing ignition system, faulty air/fuel delivery, ECU failure, or malfunctioning MAF sensors.

How to Avoid Detonation

Detonation is almost always caused by low-octane fuel or bad engine timing. Of course, engine cooling and A/F ratio do play their part.

But the latter may not be a huge concern in modern fuel-injected engines unless you start fiddling with the ECU. In this case, it’s best to get a tune from a reputable shop.

If all the internals are in working order, and you still experience detonation, try using high-octane fuel or octane boosters. It’s one way to avoid detonation.

Fuel octane ratings

An octane rating is described as the fuel’s ability to resist detonation. The higher the octane rating, the more resistance it has to detonation.

Older engines are more susceptible to detonation, given their age and material wear and tear. Modern engines are far better at combating detonation via low-octane fuel, thanks to a multitude of sensors and more advanced engineering.

Summing Up

Both detonation and pre-ignition are undesirable events in the combustion cycle.

A highly-stressed low-capacity, forced-induction engine is more likely to experience detonation or pre-ignition than a fairly conservative low-compression, high-capacity design.

Ideally, you don’t want either event occurring inside your engine. But as engines age or are poorly maintained, they open a window of opportunity for such abnormal combustion to take place.

While pre-ignition may be more destructive than detonation, you can avoid the two through routine maintenance, following the OEM recommendations. Also, use the right fuel. For the average user, that’s all that we can recommend.

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