These elements are inseparable and although the physics and thermodynamics that govern them are extraordinarily complex you only have to grasp a portion of it to gain a working knowledge of how you can influence and manipulate engine airflow to suit your power demands. To that end, I define and examine the engine airflow paths and discuss the appropriate processes that need to occur in each of them. But first I discuss some of the basic properties of air and fuel and how they affect your decisions about how you mix and process them into power.
Despite all of the modern technology and power adders, a single 4-barrel carburetor with conservatively sized intake passages in a light car still makes a remarkably fast hot rod.
Although turbocharging is quickly becoming the go-to power adder, traditional supercharging is still the darling of the hot rod set.
One of the goals of this book is to provide racers and engine builders with basic engine airflow information and some simple ways of looking at it without drowning them in mathematical theory and equations. My intent is to build a foundation from which to make intelligent airflow decisions based on known facts and fundamentals. The most important thing is to understand the characteristics of air movement through an engine and the various forces that influence it.
First and foremost, air is its own master. You can trick it and manipulate it to some degree, but it pretty much does what it wants: seeking to fill any void of equal or lesser pressure via the path of least resistance. You can predict its behavior according to known principles and values, but you remain subject to its whims. For most applications, air is a uniform gas of constant composition with average values comprised of all its constituents. The relationship of these components at any given time is the “state” of the gas and is defined by the Ideal Gas Law, which says that any change in air density is directly related to a deviation in temperature and pressure. The state of the gas also comprises its chemical composition, mass, volume, density, temperature, and pressure. Most testing and design work is performed to these industry standards (actual aerospace standards), or average values.
In the absence of a supercharging device, the earth’s atmosphere conveniently fills an engine’s cylinders for free, with the annoying exception of various penalties for airflow restrictions that interfere on the way to the valve. Restrictions prevent atmospheric pressure from completely filling the cylinders, so engine builders must take steps to assist it. Improving the volume and quality of airflow through the engine is a challenging task. It cannot be fully appreciated or easily accomplished without a reasonable understanding of the physical properties of air and the different fuels you mix with it.
The whole atmosphere is available to power engines, but only about 14.7 psi of air pressure is accessible at any given sea-level location. This illustration shows altitude (in kilometers) and temperature variations within the earth’s atmosphere. As a familiar point of reference, a dragstrip is almost exactly .4 kilometer long.
A veritable ocean of air surrounds us. We move through it and breathe it comfortably as if it were not there, but it has significant and measurable properties such as mass, density, pressure, temperature, specific volume, and viscosity. The values, or dimensions, of these properties are variable and change in a predictable fashion as each one varies according to another. In its simplest form air is defined as a gaseous mixture containing 78-percent nitrogen and 21-percent oxygen with traces of carbon dioxide, argon, water vapor, and other components in minute amounts.
A 1-inch-square column of air reaching from sea level to the edge of space distributes 14.7 psi of pressure on whatever it touches. Wherever a lower pressure is presented (as in a cylinder), the atmosphere attempts to fill it. Pressure and air density decrease with altitude, as does engine performance.
With the exception of water vapor, these percentages remain relatively fixed. However, volume, density, and viscosity vary with temperature and pressure, and these characteristics affect the movement of air through the engine. Contaminants such as dust and smog may also contribute to air quality in varying degrees depending on the amount of pre-induction filtering, leakage, and reversion in the intake tract.
The component of air that interests us most is its oxygen content, the part that supports combustion when ignited with various fuels in appropriate ratios. Fuel, whether gasoline, alcohol, or some exotic blend, is the fundamental source of power in an IC engine, but it does not burn without oxygen. Because air is only 21-percent oxygen, only one-fifth of its total content is used to burn the fuel. Therefore, only 21 percent of the total airflow is used to achieve proper combustion within the cylinders. That is why airflow is the key to engine power.
Superchargers and turbochargers increase the air density in engines beyond that provided naturally by the atmosphere. Packing the cylinders with a denser air/fuel charge extracts more power from the engines.
The green portion of this pie chart depicts the amount of atmospheric oxygen available to power the engine. The airflow through the engine must be dramatically increased because only 21 percent of it is oxygen available for combustion with the fuel.
Nitrogen is inert and contributes nothing to the power equation. However, it does take up space along with the rest of the trace constituents, leaving less room for power-producing fuel. They are pretty much along for the ride with minimal effect on power, but often have quite specific effects on emissions.
Adding more oxygen lets you burn more fuel and make more power if you can effectively contain it within the cylinders. The primary means of adding more oxygen is by adding more air via supercharging. This increases the oxygen content by packing the cylinders with a denser mixture of air and fuel.
Another way to add more oxygen is to use oxygen-releasing compounds such as nitromethane and nitrous oxide (see sidebar “Oxidizers and Oxygen-Releasing Compounds” on page 31). This requires additional fuel to accommodate the extra oxygen. It is a key path to power, one that