Engine Power and Forced Induction Systems

Forced Induction Systems

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

How to Increase Engine Power

• Engine power depends on the amount of air intake per unit time, how efficiently that air is utilized, and the thermal efficiency of the engine.
• Increasing engine speed or increasing the density of intake air can increase the amount of air inducted per unit time.
• Higher engine speeds lead to greater inertia loads, requiring a rigid, robust engine design.
• Higher speeds also increase friction and bearing loads, and decrease volumetric efficiency.
• Increasing the compression ratio increases maximum cylinder pressure. The increase in break mean effective pressure due to a supercharger is greater than that due to increasing compression ratio for a given maximum cylinder pressure.
• Supercharging results in lower rates of maximum temperature increase, which in turn leads to lower thermal loads.

Objectives of Supercharging

• Supercharging increases the amount of air inducted per unit time, enabling the engine to burn more fuel and thus increase power output.
• Supercharging objectives include:
  • Increasing engine power output for a given weight and bulk.
  • Compensating for power loss at high altitudes.
  • Obtaining enhanced power from a pre-existing engine.

How Supercharging Works

• Supercharging increases intake air pressure above atmospheric pressure.
• Pumping loop in supercharged engines is positive instead of negative.
• The increased inducted air at higher density leads to increased output.

Supercharging Power Calculation (Steady Flow Process)

• Power required to run a supercharger is calculated through the steady flow energy equation.
• The equation considers the air's internal energy and pressure changes during the intake and exhaust processes.
• Assuming zero heat loss from the supercharger simplifies the equation (W=(E2+P2V2)-(E1+P1V1)) and is further simplified to W = h2-h1 = cp(T2-T1)
• T2 is the temperature at the end of compression in the supercharger,
• cp is specific heat at constant pressure,
• T1 is the initial intake air temperature and

Supercharging of Spark-Ignition Engines

• Supercharging is typically only used in aircraft and racing engines due to the increased tendency for detonation and pre-ignition.
• Increased intake pressure and temperature decrease ignition delay, and increase flame speed, both increasing the likelihood of detonation.
• Lower compression ratios are used in supercharged spark-ignition engines to help mitigate these issues, leading to decreased thermal efficiency.
• Increased fuel consumption is an outcome.
• Rich mixture control or intercooling can lessen this, but added liquid handling poses a challenge.

Supercharging of Compression-Ignition Engines

• Supercharging improves combustion quality in diesel engines, reducing ignition delay and resulting in smoother combustion.
• Supercharging permits use of poorer quality fuels.
• Increased intake air density contributes to larger volumetric and thermal efficiency increases.
• Supercharging doesn't necessitate intercooling in most instances, except for highly supercharged engines.
• Mechanical and thermal loads increase with supercharging but are generally moderate due to lower compression ratios, valve design, cooling effects, and improved combustion.
• Lower fuel-air ratios may be employed, resulting in lower temperatures over the engine cycle.
• Supercharging leads to improved engine reliability, durability, and exhaust valve life due to smoother combustion and reduced exhaust gas temperatures.

Effect of Supercharging on Engine Performance

• Supercharging increases power output due to:
  • Increased air intake per cycle.
  • Slightly improved mechanical efficiency
  • Reduced exhaust gas temperatures.

Supercharging Limits

• Power output is limited by knock, thermal, or mechanical stresses, depending on the engine type.
• Knock limit is reached first in spark-ignition engines, and thermal/mechanical limits first in compression-ignition engines.
• The degree of supercharging on spark-ignition engines is limited by knock and the temperature of piston/cylinder.
• Supercharging may cause scuffing of piston rings and heavy liner wear if temperatures are too high.

Methods of Supercharging

• Various diagrams or visual aids display schematic representations of supercharger systems.
• Diagrams and/or visuals are used to describe different methods of supercharging like direct-drive or belt-drive systems.

Fuel Consumption

• Supercharger power demand affects fuel consumption.
• Mechcnaically driven supercharger consumes more fuel at part loads.
• Fuel consumption is often higher in supercharged spark-ignition engines due to very rich mixtures required to mitigate knocking issues.