Forced Induction

23-01 Understand why forced induction is used on internal combustion engines (ICEs).

A method of using a mechanical device to compress and pressurize a greater volume of air into the cylinders, overcoming the limitations of natural aspiration. A naturally aspirated engine is limited in the amount of air that can be inhaled, relying solely on atmospheric pressure. The purpose of forced induction is to increase the density of the air charge before it is transported to the cylinders. This improves an engine’s efficiency and performance by increasing volumetric efficiency. Increasing volumetric efficiency requires adding fuel to the increased air-fuel charge while still maintaining the intended air-fuel ratio, increasing engine performance. Forced induction also overcomes the atmospheric pressure loss accompanied by an increase in altitude. While atmospheric pressure drops as altitude increases, boost pressure remains unchanged. Boost pressure is the same at sea level as it is at high elevation. Once thought of as only a means to increase performance, forced induction is now increasingly being used as a means for smaller displacement engines to meet increasingly stringent CO2 emissions and fuel economy standards without sacrificing performance (FIGURE 23-1).

FIGURE 23-1 A forced induction engine has a compressing component that increases the pressure in the intake tract so that when it is introduced to the cylinder, the greatest amount of air-fuel mixture can be used.

Air Density

Air charge density represents the amount of air and fuel that are present in the cylinder. As density increases (more air charge), the power produced when the charge is ignited results in an increase in engine output, reduced emissions, and improved fuel economy. In its simplest terms, low-density air is thinner and reduces volumetric efficiency (FIGURE 23-2).

FIGURE 23-2 As the altitude changes, the density of the air changes, and that requires the engine to change how it operates.

Air density decreases with both altitude and temperature. High altitudes naturally produce a thinner, lower air density. Hot air, which is also thinner, reduces air density and volumetric efficiency. By increasing the air density from the pressure created, more air enters the combustion chamber while the intake valve is open. The more air and the denser the charge that enters the engine, the more useful and the more efficiently power can be produced.

Engine Airflow/Volumetric Efficiency

Most intake systems are normally aspirated, which limits their efficiency and power output. This means that the only pressure forcing air into the engine is atmospheric, in conjunction with the vacuum created when the piston descends on the intake stroke. The engine’s air intake is limited because the atmospheric force pushing air into the engine is only 14.7 psi (101.4 kPa) at sea level and drops as altitude increases. One way to improve engine output is to increase the amount of air-fuel mixture burned in the cylinder (increasing volumetric efficiency).

An engine is basically an air pump, that based on design has a predicted volume of air it can flow. Volumetric efficiency represents a measurement of how full of air an engine’s cylinder is: its ability to breathe. A comparison of the theoretical maximum amount of air that can be drawn in versus the actual amount of airflow that is drawn into the engine is expressed as a percentage. A volumetric efficiency of 100% indicates that the combustion chamber is filled completely to its designed maximum. Volumetric efficiency changes with rpm and is also significantly affected by intake valve opening and manifold design.

Normally aspirated engines have lower overall volumetric efficiency, most notably at higher rpm, due to lack of time for airflow to enter the combustion chamber. Air speed for cylinder fill at higher rpm is approximately 50 feet (15 m) per second. Increasing the speed of the airflow increases cylinder filling and volumetric efficiency. The volumetric efficiency of a naturally aspirated engine can never exceed 100%, except at rest (static). In theory, a normally aspirated cylinder that is given time to fill slowly could also potentially achieve 100% volumetric efficiency (FIGURE 23-3). To overcome the limitations of atmospheric pressure and increase volumetric efficiency, forced induction can be used to push more air into the cylinders. An engine using forced induction can achieve a volumetric efficiency ranging from 100% to 120%, on average. In comparison, a normally aspirated engine typically operates at 75% to 85% efficiency; performance race engines have an operating range of 90% to 95% if they’re normally aspirated.

FIGURE 23-3 Including a supercharger on an engine increases the engine’s efficiency at burning fuel and generating power.

Forced induction uses one of two air pump designs, either a turbocharger or supercharger, to increase the air pressure, volume (density), and temperature of air entering the engine. The increased mass of air that is compressed in each cylinder during every intake and compression stroke contains more oxygen for combustion than a normally aspirated engine. The combination of increased air volume and density allows more fuel to be burned, increasing its power output by maximizing fuel usage. The increase in air temperature from friction created during boost decreases air density, negating some of the benefits of forced induction. This increase in air charge is referred to as boost and is measured by using one of three types of units:

Forced induction overcomes many inherent engine design irregularities and restrictions by forcing air into the engine at a much higher rate by pressurizing the air above atmospheric pressure. Boost also aids in overcoming a major setback of normally aspirated engines that operate at a reduced atmospheric pressure from an increase in altitude. Forced induction also carries numerous benefits, the most noticeable being engine downsizing for increased fuel economy and CO2 emissions reductions while not adversely affecting engine drivability.

The Boost and Compression Relationship

The higher the compression ratio, the more torque an engine produces. Forced induction increases the effective compression ratio of an engine by adding more air into the cylinder. As more air (boost) is added, the effective compression ratio continues to increase. Although the mechanical compression ratio remains unchanged, air and fuel entering the cylinder are already at a higher pressure (from the boost), which raises overall cylinder pressure. Boost pressures of 6 to 8 psi force approximately 50% more air into the engine (FIGURE 23-4).

FIGURE 23-4 The relationship between the increased boost pressure and the increased cylinder pressure is relative to the amount of boost.