Supercharging in Engine Performance

Forced Induction Systems

• Forced induction systems increase the inlet air density, thus increasing the mass of air induced during the suction stroke of an engine. This is called supercharging.
• Naturally aspirated systems rely on the pressure difference between the cylinder and atmosphere during the suction stroke.
• Supercharging increases engine power output by supplying air at a higher pressure than created during suction.
• This is achieved using a pressure boosting device called a supercharger.

How to Increase Engine Power

• Engine power output depends on three factors:
  • Amount of air inducted per unit time.
  • Degree of utilization of this air.
  • Thermal efficiency of the engine.
• Increasing inlet air density can increase air inducted per unit time. This can be done by increasing engine speed or increasing intake air density.
• Higher engine speeds lead to increased inertia loads, requiring a more robust engine design. Friction and bearing loads also increase with engine speed, diminishing volumetric efficiency.

Increasing Engine Power Via Supercharging

• Increasing the compression ratio of the engine increases its thermal efficiency, yet increases the maximum cylinder pressure.
• The rate of increase in break mean effective pressure is greater when using a supercharger compared to increasing the compression ratio for equivalent increases in cylinder pressure.
• Maximum temperature increases are lower when using supercharging, leading to reduced thermal loads.

Objectives of Supercharging

• Supercharging primarily increases the amount of air inducted per unit time for combustion. This allows for a greater amount of fuel to be burned, resulting in higher engine power output.
• Other objectives may include:
  • Increasing power output for a given weight and bulk.
  • Compensating for altitude-induced power loss.
  • Increasing the power output of an existing engine.

How Supercharging Works

• Supercharging increases the intake pressure, affecting the engine's pressure-volume diagram. Visually, this can be seen with a comparison between the pressure-volume diagram of a naturally aspirated engine and that of a supercharged engine, showcasing different pressure curves.

Important Differences in Supercharging

• The primary differences between supercharged and naturally aspirated engines are:
  • Supercharged engines have increased pressure values throughout the combustion cycle.
  • The pumping loop of the supercharged engine is positive, unlike naturally aspirated engines with a negative pumping loop.

Supercharging Power Calculations

• Power required to drive supercharger is determined by steady flow energy equation.
• This equation assumes no heat loss (Q=0).
• The output formula shows useful power calculations from input quantities (temperatures, pressures, volumes, and specific heat).

Supercharging of Spark-Ignition Engines

• Supercharging is not common for spark-ignition engines, outside of racing and flight applications.
• This is due to a greater tendency for detonation related to pressure increase, and the subsequent impact on fuel-air ratio.
• Supercharging reduces ignition delay and increases flame speed, increasing detonation and pre-ignition tendencies in spark-ignition engines. Lower compression is typically used in supercharged engines.
• The increased heat losses and higher specific heat values result in reduced thermal efficiency.
• Fuel consumption usually increases in supercharged gasoline engines.
• Rich mixtures are often used in supercharged gasoline engines to mitigate detonation.

Supercharging Limits

• Limits of supercharging in spark-ignition and compression-ignition engines.
• Knock, thermal loading, and mechanical stress are critical limits that restrict supercharger operation in gasoline engines.
• Thermal loads and mechanical stress often set the limits on supercharging in compression-ignition engines.

Methods of Supercharging

• Different types of supercharging methods exist with unique flow diagrams. The designs can be of the mechanical or electrical variety, and have an impact on part load efficiency.

Effect of Supercharging on Engine Performance

• Supercharging increases power output due to increased air intake per cycle, improved mechanical efficiency, improved scavenging, and reduced exhaust gas temperatures; thereby leading to enhanced combustion and volumetric efficiency.
• These benefits are common in both gasoline and diesel engines.
• Diesel supercharging, however, is less prone to knock.

Fuel Consumption

• Supercharger power consumption varies with different implementations.
• Part load efficiency is sometimes reduced, impacting fuel economy.
• Richer mixtures can be used to mitigate knock, but this causes higher specific fuel consumption.
• Supercharging generally increases fuel consumption in gasoline engines.
• Specific fuel consumption is generally reduced in diesel engines using lower fuel-air ratios due to better combustion and increased mechanical efficiency.