Jawaharlal Nehru Technological University Hyderabad B.Tech IV AUTOMOBILE ENGINEERING Previous Question Paper (December - 2020 - R16)

Engine charging, or forced induction, is the process of increasing the density of the air-fuel mixture entering the engine cylinders beyond what is naturally aspirated. Its primary importance lies in increasing the volumetric efficiency of the engine, which directly results in higher power output and torque for a given engine displacement. By forcing more air into the combustion chamber, more fuel can be burned, leading to greater thermal energy release and improved power-to-weight ratios. This allows smaller engines to perform like larger ones, improving overall vehicle performance and fuel efficiency.

The fundamental difference between turbo-charging and super-charging lies in the power source used to drive the air compressor:

In summary, while both systems increase air density, super-charging provides immediate power delivery at the cost of some mechanical efficiency, whereas turbo-charging offers superior overall efficiency by recycling exhaust energy, albeit with potential delays in power delivery.

The design of a combustion chamber is critical in controlling the combustion process, which directly influences the formation of pollutants like hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). A well-designed combustion chamber optimizes the air-fuel mixture preparation, flame propagation, and heat transfer to minimize these emissions.

Key design factors that contribute to reduced emissions include:

• Surface-to-Volume Ratio: Minimizing the surface-to-volume ratio reduces the quench layer effect. A large surface area relative to volume allows the flame to be extinguished near the cold cylinder walls, leaving unburned fuel that results in HC emissions. A compact, hemispherical, or pent-roof design reduces this area.
• Turbulence and Swirl: Enhanced turbulence and air motion (swirl and tumble) promote rapid and uniform mixing of air and fuel. This ensures complete combustion, reducing CO and HC emissions by preventing localized rich or lean zones.
• Flame Path Length: A compact design with a centrally located spark plug minimizes the flame travel distance. A shorter flame path ensures that the combustion process is completed rapidly before the exhaust valve opens, preventing the discharge of partially burned gases.
• Temperature Control: NOx formation is highly dependent on peak combustion temperatures. Combustion chamber geometry can be optimized to manage heat release rates. By avoiding 'hot spots' and controlling the peak pressure and temperature through controlled combustion rates, the formation of thermal NOx is significantly suppressed.
• Squish and Quench Areas: Strategic use of squish areas forces the mixture toward the center of the chamber during the compression stroke. This promotes high-velocity turbulence near the end of compression, which accelerates flame speed and ensures more complete combustion of the end gases.

In summary, by promoting rapid, complete, and uniform combustion while minimizing wall-quenching effects, a high-efficiency combustion chamber design effectively lowers the concentration of harmful pollutants in the exhaust stream.