Heating Value And Incomplete Combustion: Impact On Fuel Efficiency

The heating value of a fuel is the amount of heat released during its combustion. Incomplete combustion occurs when there is not enough oxygen to allow the fuel to react completely to produce carbon dioxide and water. This results in the production of additional byproducts, such as carbon monoxide, unburned hydrocarbons, and soot. The heating value of a fuel can be calculated with the results of an ultimate analysis of the fuel. The calorific value is the total energy released as heat when a substance undergoes complete combustion with oxygen under standard conditions.

Insufficient oxygen supply

The combustion of a substance, also known as its calorific value, is the amount of heat released when a given amount of the substance undergoes combustion with oxygen under standard conditions. The heating value of a fuel is a measure of its energy density and is expressed in energy per specified amount (e.g. kilograms).

When there is an inadequate supply of oxygen, not all fuel molecules can oxidize fully, resulting in incomplete combustion. This can lead to the production of carbon monoxide and unburnt hydrocarbons. Incomplete combustion occurs when there is not enough oxygen to allow the fuel to react completely to produce carbon dioxide and water. Instead, carbon and carbon monoxide are produced, along with water.

To improve combustion and promote complete combustion, it is crucial to ensure a sufficient oxygen supply. The oxygen supply must be carefully controlled and optimized for the type of fuel being used. This is particularly important in scenarios with limited ventilation, poorly designed combustion chambers, or when there is an excessive amount of fuel in proportion to the available oxygen.

Furthermore, the temperature within the combustion chamber also plays a significant role in achieving complete combustion. An appropriate temperature ensures that the energy released from the reacting fuel is sufficient to maintain a self-sustaining reaction. If the temperature is too low, the fuel may not achieve complete combustion, leading to a buildup of byproducts such as soot or carbon monoxide.

Poor mixing of fuel and oxygen

The quality of the combustion process depends on the proper mixing of fuel and oxygen. When fuel and oxygen are adequately mixed, efficient and complete combustion can occur. However, when mixing is poor, it can lead to incomplete combustion, resulting in the production of harmful byproducts such as carbon monoxide, unburned hydrocarbons, and soot. Incomplete combustion can also reduce the efficiency of the combustion process and lead to energy losses.

To promote complete combustion and minimize the formation of harmful byproducts, it is essential to ensure proper mixing of fuel and oxygen. This can be achieved through various methods, including improving fuel atomization and optimizing the design of the combustion chamber to ensure even distribution of the air-fuel mixture. Additionally, maintaining appropriate combustion temperatures and sufficient reaction time are also crucial factors in achieving complete combustion.

The combustion process is a complex phenomenon involving chemical reactions, heat transfer, and mass transfer. The quality of combustion depends on various factors, including the local conditions within the combustion chamber. These conditions include temperature, oxygen content, and residence time of the fuel molecules within the combustion zone. By optimizing these conditions, the likelihood of complete combustion can be increased.

Furthermore, the design of the combustion chamber plays a crucial role in achieving complete combustion. The chamber should be designed to ensure adequate oxygen flow and minimize heat loss to the surroundings. A well-designed combustion chamber can enhance the efficiency of the combustion process and reduce the formation of harmful byproducts.

Low combustion temperature

Low-temperature combustion (LTC) is an advanced combustion concept for internal combustion (IC) engines that has gained global attention due to its ability to simultaneously reduce both oxides of nitrogen (NOx) and particulate matter (PM), in addition to lowering specific fuel consumption. LTC differs significantly from conventional spark ignition (SI) combustion and compression ignition (CI) diffusion combustion concepts. LTC technology offers the advantage of reducing NOx and PM emissions while maintaining higher thermal efficiency.

The LTC mode engine operates at a lower combustion temperature than conventional diesel mode engines. This is achieved by either using a high rate of exhaust gas recirculation (EGR) or an excess air ratio (λ) much higher than 1. Controlling ignition timing and heat release rate (HRR) are the primary challenges that need to be addressed before LTC technology can be implemented in automotive engines commercially.

The combustion phasing in LTC engines can be controlled by adjusting the start of injection or intake boost pressure. Increasing the EGR rate or intake boost pressure can delay combustion and maintain a constant combustion phasing. LTC engines have been found to reduce NOx emissions by up to 85% and soot emissions by up to 95% while achieving a thermal efficiency of about 55%.

The use of multiple-injection strategies in LTC engines can help reduce both NOx and soot emissions. By splitting the fuel into smaller quantities injected at different timings, locally undermixed fuel-rich pockets that promote high soot formation can be avoided. Additionally, post-injection can reduce unburned hydrocarbon emissions by enriching and burning the mixture near the nozzle regions.

The LTC concept can be illustrated on a temperature (T) and equivalence ratio (ϕ) map, which strongly influence combustion and emission processes. Soot precursors like C6H6 and C2H2 are predominantly found in low-temperature and fuel-rich conditions, while high NOx emissions occur at high-temperature and near-stoichiometric conditions. LTC aims for a narrow region with low levels of both soot precursors and NOx, typically at a temperature of 1,500–2,200 K and an equivalence ratio below 1.

The application of LTC in gasoline compression ignition (GCI) engines has been studied, demonstrating its potential to achieve cleaner and more efficient combustion. GCI engines use multiple-injection strategies to control combustion phasing and mitigate combustion noise. The autoignition quality of gasoline fuels can be indicated by their research octane number (RON), with higher RON values indicating greater resistance to autoignition and, consequently, more time for fuel-air mixing.

In summary, low combustion temperature is a critical factor in incomplete combustion, and LTC technology offers a promising approach to enhance the performance, emission, and combustion characteristics of IC engines.

Inappropriate reaction time

Combustion is a chemical reaction where a fuel reacts with oxygen to release energy in the form of heat and light. The ideal combustion reaction for hydrocarbons can be simplified to the following equation:

> CxHy + O2 → CO2 + H2O + energy

However, in reality, the combustion process is more complex. For combustion to occur, three components, known as the "fire triangle", must be present: fuel, oxygen, and heat. If any of these components are lacking or not adequately provided, the combustion will be incomplete.

The duration of the combustion reaction, or the reaction time, is a critical factor in determining whether combustion is complete or incomplete. For the reaction to be complete, the fuel and oxygen mixture must be given enough time to react before the combustion products are expelled from the chamber.

In practical applications, such as internal combustion engines, the time available for combustion is often very short. If the reaction time is too short due to high engine speeds or premature expulsion of exhaust gases, the reaction may be prematurely quenched, resulting in incomplete combustion.

To achieve more complete combustion and reduce the emission of harmful pollutants, combustion chamber designs can be modified to allow for more thorough mixing and a longer reaction time. Additionally, engine settings can be adjusted to slow down the combustion process.

Incomplete combustion byproducts

Incomplete combustion occurs when there is not enough oxygen to allow the fuel to react completely with the oxygen to produce carbon dioxide and water. It also happens when the combustion is stopped by a heat sink, such as a solid surface or flame trap.

The main byproducts of incomplete combustion are:

• Carbon monoxide (CO)
• Unburned hydrocarbons
• Soot (particulate carbon)
• Nitrogen oxides (NOx)
• Formaldehyde
• Acetaldehyde
• Benzene
• Acrolein
• Toluene
• O-, m-, p-Xylenes
• Benzo [a]pyrene (BaP, a polycyclic aromatic hydrocarbon (PAH))
• 1,3-Butadiene
• Carbonyls (e.g. aldehydes, primarily formaldehyde)
• Dioxins

Carbon monoxide is a poisonous gas that, when breathed, takes the place of oxygen in the blood, combining with hemoglobin and rendering it unable to transport oxygen. It also produces less heat than carbon dioxide, so complete combustion is preferred.

The formation of these byproducts can be reduced by raising the temperature of combustion, increasing oxygen availability, allowing a longer reaction time, and providing better mixing.

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