Carbon and Fossil Fuels

Hydrogen has the highest calorific value at about 141 MJ/kg, far exceeding coal (24-35 MJ/kg) and wood (15-20 MJ/kg). Rubber's calorific value is around 40 MJ/kg, still significantly below hydrogen. The high energy density makes hydrogen an excellent fuel despite storage challenges.

This is a duplicate of question 21342 with the same error. A primary fuel is used as found in nature. Kerosene is a secondary fuel (refined from crude oil), while wood, coal, and natural gas are primary fuels.

Methane (CH4) is the simplest alkane, having just one carbon atom bonded to four hydrogen atoms. Alkanes are saturated hydrocarbons with only single bonds, and methane has the minimum possible structure, making option A correct.

Internal combustion engines burn fuel inside a combustion chamber within the engine, unlike external combustion engines (like steam engines) where combustion occurs outside. The combustion chamber is where the fuel-air mixture ignites and expands.

The four-stroke cycle sequence is: 1) Intake (fuel-air mixture enters), 2) Compression (mixture is compressed), 3) Power stroke (combustion and expansion), 4) Exhaust (gases expelled). This is the standard Otto cycle used in most gasoline engines.

Combustion is an exothermic chemical process - it releases heat energy. In internal combustion engines, this released energy creates pressure that drives pistons. Endothermic processes absorb heat, isothermic processes maintain constant temperature, and isobaric processes maintain constant pressure.

A four-stroke petrol engine operates on the Otto cycle, which consists of intake, compression, combustion (at constant volume), and exhaust strokes. The Diesel cycle differs by using constant pressure combustion, while the Rankine cycle applies to steam plants and the Carnot cycle is theoretical.

A four-stroke diesel engine operates on the Diesel cycle, which is characterized by constant pressure combustion (heat addition at constant pressure). Unlike the Otto cycle used in petrol engines, diesel engines compress air alone and then inject fuel which ignites spontaneously. Castor and jatropha cycles are not valid thermodynamic cycles.

Carburetors are responsible for atomizing liquid fuel in petrol engines by mixing it with air in the correct ratio before combustion. The carburetor uses venturi effect to break fuel into tiny droplets, creating a combustible mixture. Clutch is for transmission engagement, fuel injectors perform similar function in modern engines but carburetors were the standard in older petrol engines, and engine cylinder is where combustion happens.

Unlike petrol engines that draw in a mixture of fuel and air during the suction stroke, a diesel engine relies on compression ignition and thus inducts only pure air into the combustion chamber during the suction stroke, with fuel injected separately near top dead center.

Diesel engines have higher thermal efficiency than petrol engines due to higher compression ratios and leaner air-fuel mixtures. Diesel engines typically achieve 40-45% thermal efficiency compared to 30-35% for petrol engines. This is why diesel vehicles often have better fuel economy.

Carbon monoxide (CO) is primarily emitted from incomplete combustion in motor vehicles, especially in urban areas with heavy traffic. While industries, ships, and submarines also produce some CO, automobiles are the largest and most significant source of CO emissions in most regions.

Water should NEVER be used on flammable liquid fires (Class B) as it can spread the burning liquid and make the fire worse. CO2, dry powder, and foam are suitable.

Water is highly effective for Class A fires (ordinary combustibles like wood, paper, textiles) as it cools the material and saturates it to prevent re-ignition.