Hello and welcome to this article where I will be discussing a crucial concept in the field of combustion science, "Autoignition".
In this article, I will cover various aspects of autoignition, including the definition, autoignition temperature of different substances such as paper, diesel, ethanol, hydrogen, avgas, and more.
I will also discuss the difference between flash point, fire point, and autoignition temperature, and how pressure affects autoignition temperature.
By the end of this article, you will have a comprehensive understanding of autoignition and its implications. So, let's get started!
What is Autoignition?
The formal definition is:
Spontaneous ignition of some or all of the fuel-air mixture in the combustion chamber of an internal combustion engine. Also known as spontaneous combustion.
Autoignition is when a substance starts to burn on its own, without any help from an outside source. The heat inside the substance causes it to catch fire on its own.
What is the Autoignition Temperature?
The autoignition temperature is the lowest temperature at which a substance will start to burn in a normal atmosphere. This temperature is affected by things like pressure, oxygen concentration, and the amount of fuel to air. It is important to remember that the temperature at which a substance will start to burn on its own depends on the type and make-up of the substance.
Autoignition in Internal Combustion Engines
Internal combustion engines can self-ignite due to the heat of compression alone or when this heat is combined with fuel injection. Self-ignition is also called spontaneous combustion, and it can happen in the engine's combustion chamber.
When working with flammable materials, the autoignition temperature is a key factor in figuring out the risk of a fire or explosion. It is used to figure out how likely it is that a substance will catch fire on its own in a normal atmosphere, without a spark or flame from outside.
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Autoignition Temperature of Different Substances
Wood:
The temperature at which wood starts to burn on its own is about 482°F (250°C). At 700°F (371°C), it can catch fire right away, but it takes a few minutes between 450°F (232°C) and 500°F (260°C).
At the minimum heat flux needed to start a direct flame, the surface temperature is between 300 and 365°C (572 and 689°F).
Avgas (aviation gasoline):
The temperature at which it starts to burn on its own is 280°C (536°F). Avgas is a type of gasoline made for airplane engines.
It has a flash point of -43°C (-45°F). It is like kerosene, which also has a temperature of 280°C (536°F) at which it can start to burn on its own.
http://large.stanford.edu/courses/2014/ph240/ukropina2
Ethanol:
The autoignition temperature of ethanol is 365°C (689°F), which is lower than that of diethyl ether (160°C or 320°F) and higher than that of gasoline (247-280°C or 477-536°F).
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6266291
Hydrogen:
The temperature at which hydrogen will start to burn on its own is 535°C (995°F).
https://en.wikipedia.org/wiki/Autoignition_temperature
Ethylene oxide:
The temperature at which ethylene oxide starts to burn on its own is 429°C (804.2°F). But if there is rust, it could be as low as 140°C (284°F).
At temperatures above 51.3°F (10.7°C), ethylene oxide is a colorless gas that can catch fire on its own.
https://www.petrochemistry.eu/wp-content/uploads/2018/01/Guidelines_EO_2013_UK_v6-final.pdf
Diesel:
Autoignition point: approximately 210°C.
Steel:
Autoignition point: approximately 1315°C (2399°F).
Paper:
Autoignition point: Approximately 218 - 248°C.
Once a paper fire starts, the heat at the center can reach up to 815°C.
Other gases:
- Methane: 580 °C
- Propane: 493 °C
- Ethylene: 425 °C
- Acetylene: 305 °C
- Naphtha: 290 °C
- Carbon disulfide: 102 °C
How to Tell the Differences Between Autoignition, Flash Point, and Fire Point
In summary, flash point, fire point, and autoignition temperature are all related to how flammable a substance is, but each term refers to a different aspect of how the substance acts when exposed to heat and fire.
Autoignition
Autoignition is the lowest temperature at which a flammable material will start to burn on its own, without a spark or other source of ignition from the outside.
This happens when the material's temperature reaches its autoignition temperature, turning it into a flammable gas that starts to burn on its own.
Flash Point
Flash point, on the other hand, is the lowest temperature at which a liquid that can catch fire gives off enough vapor to catch fire when it comes in contact with a source of ignition.
For each flammable liquid, the amount of vapor in the air that is needed to keep a fire going is different, and the flash point temperature shows the lowest temperature at which a flammable liquid will catch fire when it is near a source of ignition.
Flash Point vs Autoignition Point:
Flash point and autoignition point are two important ways to measure how easily a substance will catch fire.
Both terms refer to the temperature at which something can start to burn, but they talk about different parts of the process.
The flash point is the temperature at which a substance can catch fire from an outside source, and the autoignition point is the temperature at which it will catch fire on its own.
Fire Point
The fire point is the temperature at which a flammable liquid's vapor keeps burning after it has been set on fire.
It is higher than the flash point temperature and shows the lowest temperature at which a substance will keep burning after it has caught fire.
Expeltec made a really nice infographic to visually explain some differences:
https://expeltec.com/5-terminology/flash-point-auto-ignition-temperature/
Autoignition vs Other Processes
Spontaneous combustion is different from autoignition because it does not need an outside source of energy to start.
Autoignition temperature is the lowest temperature at which a substance will catch fire in a normal atmosphere without a spark from outside.
Autoignition and autoexposure are also different processes. Autoexposure is when a fire spreads from one floor to another in a building with more than one story, and autoignition is the lowest temperature at which a flammable material will start to burn on its own.
Measures of Fuel Resistance to Autoignition
Octane Rating
In short, the octane rating is a measure of how well a fuel can resist detonation and preignition, and it also shows how well it can resist starting on its own.
The octane rating is a standard way to measure how hard fuels like gasoline and diesel are to start burning on their own in spark-ignition engines.
This rating is based on the pressure at which the fuel will auto-ignite (start burning on its own) in a controlled environment.
The octane rating is written as either the research octane number (RON) or the motor octane number (MON).
The higher the octane rating, the more resistant the fuel is to self-ignition, and the less likely it is that a S.I. Engine will knock or ping during compression.
Autoignition and its Characteristics
In crank angle degrees, the autoignition delay is the amount of time between the start of fuel injection (SOI) and the start of combustion (SOC) (CADs).
It is also called ignition delay time (IDT), and it shows how long it takes for a mixture of fuel and air to react at a certain temperature and pressure.
Autoignition Temperature
Ethanol can start to burn at a temperature of 685°F (363°C). It is the temperature at which a mixture of fuel and air will light on its own, without any help from an outside source.
One way to figure out the autoignition temperature is to measure the rise in temperature between the end of the compression stroke and the beginning of the autoignition event.
Another way is to find the autoignition temperatures of 1-propanol and 2-propanol.
Autoignition and Radiation Heat Transfer
This happens when a flammable liquid is heated above its ignition temperature and the vapors that are released self-ignite when exposed to radiation like infrared radiation from hot objects or fire plumes.
There are two different kinds of ignition: piloted ignition and autoignition. The flammable material needs an outside source of heat, like a spark or flame, to start a piloted ignition.
Autoignition, on the other hand, does not need an outside source of heat. Instead, it uses the material's own energy to start a fire.
Autoignition Temperature
A substance's autoignition temperature is the lowest temperature at which it starts to burn. It is the temperature at which something can start to burn on its own.
Understanding the Importance of Autoignition Temperature
The autoignition temperature is a key concept in the science of combustion. It means the lowest temperature at which a substance will self-ignite and catch fire without a spark from outside.
The temperature at which a material will start to burn on its own can change depending on things like what else is in the area.
When it comes to fuel mixtures that can catch fire, the autoignition temperature is the lowest temperature at which the fuel will catch fire on its own without a spark from outside.
Understanding a substance's autoignition temperature is important for fire safety and for safely using and storing things that can catch fire.
Hydrogen Gas and Autoignition
When hydrogen gas is put into an engine, it changes the way autoignition works in a unique way. The way it burns is affected by the fact that its auto-ignition temperature and thermal conductivity are higher than those of fuels like methane.
Hydrogen's Effect on Autoignition
When hydrogen is mixed with other fuels, the higher temperature at which it ignites on its own speeds up the rate of combustion and raises the pressure in the cylinder.
But using hydrogen can also cause problems like safety issues, premature ignition, backfire, and less power.
When certain loads are put on engines with spark ignition, uncontrolled autoignition can cause the engine to knock.
In engines that can use both diesel fuel and hydrogen, putting hydrogen into the intake manifold can cause diesel fuel to start burning on its own.
Autoignition Temperature of Air
Air is not flammable, so it does not have a temperature at which it starts to burn on its own. Fuels and chemicals like butane, coke, hydrogen, and petroleum often have their autoignition temperatures given.
Its autoignition temperature is the lowest temperature at which a material starts to self-heat, which can lead to fire.
Autoignition Pressure
The pressure at which hydrogen will start to burn is between 3.5 and 7 Mpa (35-70 bar). Spontaneous ignition is more likely when the initial release pressure is high and the leading shock is strong.
The pressure needed to start a fire depends on how much oxygen and helium are in the air.
Effect of Pressure on Autoignition
As pressure goes up, the temperature at which a mixture of gases or vapors can start to burn on its own goes down.
This is because higher pressures speed up the reaction, which lowers the temperature at which the reaction starts.
Hydrogen Onboard Storage
Most onboard hydrogen storage systems work at pressures of up to 700 bar. Even higher pressures are used at refueling stations for these kinds of storage.
Internally ignited gases can lose heat through a shock discharge that causes the flame to go out before the whole volume is on fire. This can stop the gas from igniting itself.
Autoignition Testing of Crude Oil
Autoignition testing is done on crude oil to find out the lowest temperature at which it will spontaneously catch fire in normal room temperature.
The American Society for Testing and Materials (ASTM) has set standards for testing crude oil's ability to self-ignite.
ASTM E659: Temperatures at which a substance starts to burn on its own
ASTM E659 is the standard test used to see if crude oil will catch fire on its own. This test method shows how to find the hot-flame and cool-flame autoignition temperatures of a liquid chemical in air at atmospheric pressure in a uniformly heated vessel.
The results of this test show what the lowest temperature is at which crude oil will normally catch fire.
Comparing ASTM E659 and ASTM D2155
ASTM E659 is a more up-to-date way to test crude oil for autoignition than ASTM D2155. Even though the tools for each method are different, the results are the same.
ASTM D93A: Flash Point Determination
ASTM D 93A is a way to figure out the flash point of petroleum and products made from petroleum. But it does not cover tests for self-ignition.
The flash point is the temperature at which a petroleum-based product gives off enough vapor for a small flame to start a fire and keep it going.
The flash point temperature is not the same as the autoignition temperature. The autoignition temperature is the temperature at which a substance will start to burn on its own without a spark from outside.
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