Whenever you see something burning, you are observing combustion. While it may be tempting to think that all burning is the same, in reality there are several different types of combustion. All combustion requires fuel, a heat source and oxygen.
TL;DR (Too Long; Didn't Read)
Combustion is the act of burning, in which fuel, heat and oxygen release energy. There are several types of combustion, such as internal combustion, diesel combustion, low temperature combustion and other novel forms.
What Is the Definition of Combustion?
Burning is combustion. But what does that mean? The definition of combustion is more specifically a chemical reaction in which energy is released when fuel and the oxygen in air are mixed and exposed to heat. There are many kinds of fuels: wood, natural gas, gasoline, diesel, ethanol and biofuels, among others. Altering the fuel, oxygen or heat availability can control combustion.
You can observe combustion as a flame. On Earth, a flame resembles a teardrop because when it burns, air expands and gravity pulls colder air to the flame’s base. The hot air rises up, and that is the flame you see. In the microgravity of space, such as on the International Space Station, combustion can still occur. However, it will not look like a flame on Earth because there is no upward flow of hot air in microgravity. This creates a strange, round, slow kind of flame, but it uses less oxygen and can burn longer than flames on Earth. Scientists continue to learn more about how combustion works in space.
Products of Combustion
Combustion produces heat, and other kinds of products result depending on the type of fuel used in combustion. For example, when methane is used as a fuel, such as with natural gas, and is oxidized, this yields the main products of carbon dioxide and water. A combustion reaction produces pollutants as well. These kinds of combustion products include nitrogen oxide, carbon monoxide and soot, which is mostly carbon. Exhaust is another word for the gaseous products of combustion, while soot is basically a solid form of exhaust.
One interesting fact is that heat, while a product of combustion, is also needed to initiate combustion. The heat produced will continue the combustion, and you can see this in action in a wood-burning fireplace.
Types of Combustion
Internal combustion is a common type of combustion. This type of combustion happens inside an engine, which is why that type of engine is called an "internal combustion engine." Most of the vehicles you will see on roads are powered by internal combustion.
Diesel combustion involves spraying jets of fuel into a compression heating system that ignites and makes a flame.
Clean diesel combustion occurs in a similar manner to regular diesel combustion. The main difference is there is more fuel and air mixing prior to ignition. Less soot results from this efficient combustion, as well as less nitrogen oxide pollution.
Homogenous charge compression ignition (HCCI) is an advanced form of combustion that works at a lower temperature. Before it is compressed, fuel is vaporized and mixed with air before being sprayed and compression-heated. This type of combustion is considered highly efficient because it does not yield soot.
Low-temperature combustion (LTC) is a lower temperature flameless combustion. A diluted fuel-air mixture is compressed until it autoignites. Diluting the fuel mix means less fuel is needed, making this form of combustion more efficient than diesel combustion. Another benefit to LTC is the control of autoignition timing and the heat-release rate. Having a lower temperature also means the engine will not lose as much energy to its surroundings. The engine can work longer, and will not knock as in spark ignition. This can reduce engine noise and damage.
Dilute gasoline combustion involves the use of premixed fuel and air that the engine dilutes. Fuel is injected right into the cylinder of the engine, close to a spark plug, when the plug sparks. This is a very efficient type of combustion that relies on the amount of fuel used to control a load.
What Are the Types of Engines?
The internal combustion engine is the most common one you will interact with on a daily basis. This is the most prevalent kind of engine in motor vehicles. It is also called a piston engine. Combustion occurs inside the engine, and the gases made by the combustion move the pistons, which turn the crankshaft and propel the vehicle.
There are two subtypes of internal combustion engines, spark ignition gasoline and compression ignition diesel. Spark ignition engines mix fuel and air and force them into the engine’s fixed cylinder. This mixture gets compressed and is ignited by a spark. The combustion gases that are produced then push the pistons.
Rudolph Diesel invented the compression ignition engine. Today, these engines are called diesel engines. They are another kind of internal combustion engine. Diesel engines induce only air. They do not need an ignition system. Liquid fuel is then sprayed into this compressed air, which heats up and leads to ignition. Diesel engines are commonly found in large trucks, construction equipment, ships and buses. This is because diesel engines produce more torque, which aids in moving heavy loads such as with freight trucks. Diesel engines are far more common in cars in Europe rather than in America and are more fuel-efficient than regular gasoline engines. One of their disadvantages is that they are often noisy.
Other types of engines include jet engines and rocket engines. Jet engines suck in air with a fan, and that air is compressed by a compressor that spins at high speed. Fuel is sprayed onto the compressed air, and a spark ignites it. The expanding gases exit the engine, and the aircraft is propelled forward with great thrust.
Rocket engines were initially used in rocket-powered experimental aircraft. These were used to break the sound barrier and to push the limits of speed. Fuel and an oxidizing agent are combined and ignited in a combustion chamber. The resulting explosion pushes hot air out the nozzle and provides thrust to the vehicle. While this process is similar to a jet engine, the difference lies in the fluid used. Jets use air as their working fluid, whereas rockets use combustion exhaust gases. There are two kinds of rocket engines, liquid rockets and solid rockets. Liquid rockets use liquid propellants that are kept apart until they are pumped into a combustion chamber and ignited. The propellants of a solid rocket are combined into a solid cylinder. These will not burn until given a heat source.
Engineers work hard to make engines more efficient. New methods of generating lower temperature combustion combined with better engines offer great promise. A more efficient engine will not emit so many pollutants like nitrogen oxide and particulates into the Earth’s atmosphere. Efficient combustion means improved fuel economy for vehicles as well, saving drivers money at the pump! When combustion is optimized, the environment and the customer both gain benefits.
About 75 percent of the power people use comes from combustion. The next time you witness a fire of any kind, or watch a vehicle or airplane, consider the combustion definition and see if you can guess what type of combustion is happening.
- Office of Energy Efficiency & Renewable Energy: Internal Combustion Engine Basics
- Office of Energy Efficiency & Renewable Energy: Advanced Combustion Strategies
- Smithsonian Magazine: In Space, Flames Behave in Ways Nobody Thought Possible
- NASA Glen Research Center: Combustion
- Lund University: What Is Combustion?
- Minnesota State University: Engaged in Thermodynamics: Diesel Engine
- NASA Glen Research Center: How Does a Jet Engine Work?
- NASA Glen Research Center: Rocket Propulsion