Classification of Fuels | Types of Fuels

Classification of fuels

1. Solid fuels

Natural solid fuels are readily available in nature. Example wood, peat, lignite, bituminous coal, anthracite coal etc.


Wood is used in places where waste wood is available in large quantity. Peat is not widely used because it has lower heating value. Lignite is an intermediate product between peat and bituminous coal. It has more moisture content and lower heating value. Bituminous coal contains very little moisture and high carbon. This is mostly used for power generation in thermal power plants. Anthracite coal is the final stage of coal formation. It is used for domestic purposes because of its smokeless combustion. It has higher heating value. It is also used for thermal power plants.

Artificial solid fuels are prepared fuels. Examples: Charcoal, coke, briquetted coal, pulverized coal etc.


Charcoal is prepared by the dry distillation of wood. It is the best fuel since it has no sulphur content. But it is costly Coke is obtained by destructive distillation of soft coals in closed retorts. Briquetted coal is obtained by briquetting the dried coal by applying pressure alone. Pulverised coal is obtained by pulverized the low grade fuels.

2. Liquid Fuels

Liquid fuels are obtained from natural petroleum. They are used in internal combustion engines and also for steam raising purposes. Gasoline, paraffin, diesel oil, crude oil are the examples of liquid fuels.


Advantages of liquid fuels

1. They have a higher calorific value.

2. They require less space than solid fuels.

3. Consumption is easily controlled.

3. Gaseous fuels

Fixed composition gases are acetylene, methane, ethylene and ethane. Composite industrial gases are blast furnace gas, coke over gas, producer gas, illuminating gas, blue water gas, carbureted water gas etc.


Blast furnace gas is a by-product in the production of pig iron in the blast furnace. It is used for power generation or steam raising in boilers or for preheating the blast furnace. Coke oven gas is obtained by destructive distillation of coal in closed retorts. It is used for industrial heating and power generation. Producer gas is obtained from the incomplete combustion of coke or charcoal. It has a lower calorific value. Its major constituent is nitrogen, as inert gas. Illuminating gas is also prepared as coke even gas. It is also used for industrial heating and power generation. Blue water gas is produced in a generator by passing steam through red hot coke. Carburetted water gas is obtained by blowing steam through incandescent bed of fuel containing carbon. By adding unsaturated hydrocarbons, this gas is used for domestic purposes.

Advantages of gaseous fuels

1. Physical handling of gaseous fuel is not required. It may be easily piped into the furnace.

2. It is free from ashes and other foreign matters.

3. It burns completely.

4. It produces less smoke with a low percentage of excess air.

5. In high temperature work, it is economical than coal or oil.

Idling Devices of Automobile | Idling Devices in Carburetor | Anti-Dieseling Device

Idling devices in carburetor

During idling, the throttle plate is almost in the closed position. Then the mass rate of air flow through the venture is small. Hence, the vacuum or depression produced at the venture is also small. With this small depression, no fuel can issue from the fuel orifice. Hence, an idling device is incorporated in the carburetor unit.


The engine idling device can be seen in the picture. This device utilizes the large vacuum that prevails at the edge of the throttle plate for effecting fuel supply, when the throttle plate is almost in the closed position.

The required idling mixture strength can be obtained by adjusting the idling air screw. The quantity of the mixture supplied to the engine is controlled by the throttle plate setting. This setting decides the extent of closure of the inlet passage by the throttle plate.

The idling device operates at maximum capacity when the throttle plate is almost in the closed position. The effectiveness of the idling device gradually diminishes, as the throttle plate is being opened.

When the throttle plate is wide open, the depression felt at the idling get is extremely small. This small depression is not capable of raising the fuel through the large height in the idling jet upto the discharge point. Now the maximum depression shifts to the venture throat. As such the main orifice starts supplying the fuel.

Anti-dieseling device

A spark ignition engine sometime continues to run for a very small period, even after the ignition is switched off. This phenomena is called dieseling or after running. This causes wastage of fuel and pollution.

01 - IDLING DEVICES - Anti-dieseling device

Some modern cars have the anti-dieseling system as shown in the picture. This system has a solenoid valve operated idling circuit. When the ignition switch is turned on, current flows in the coil of the solenoid valve and thereby generates a force. This force pulls a needle valve and opens the passage for slow speed mixture. When the ignition switch is turned off, the magnetic force disappears. Then the needle valve goes to the original position immediately by the action of the spring in the solenoid valve. By this way, the slow speed mixture passage is cut off. Hence, the engine stops and the fuel wastage are also eliminated.

Hot idling compensator: Some modern cars have this system in the carburetor unit. Under certain extremely hot operating conditions there is a tendency for the idling mixture to become too rich. This causes idling instability. The hot idling compensator system incorporates a bimetallic value which admits air directly into the manifold in correct quantity when needed. Thus the mixture richness is adjusted and stable idling is ensured.