Читать книгу Practical Engine Airflow - John Baechtel - Страница 9
ОглавлениеOne 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 largely overcomes more traditional methods of coaxing more air into the engine via manipulation of the flow path dimensions through ram tuning, head porting, and valve work.
For the most part, I concentrate on examining and understanding airflow in the naturally aspirated sense. Moreover, it is a fortunate coincidence that most airflow enhancements are pretty much complementary to any type of supercharging.
Next, you need to quantify some basic properties of air and think about how you can influence or employ them to fill your engine’s cylinders more effectively. As shown in the chart (at right), the basic properties of air are characterized by mass, volume, density, specific volume, pressure, temperature, and viscosity. Their relationships seem complicated for something as seemingly intangible as ordinary air, but they are pretty well defined according to scientific principles.
Oxidizers and Oxygen-Releasing Compounds
Atmospheric oxygen is the most plentiful oxidizer available for IC engines. It offers the considerable advantage of an unlimited supply that does not require onboard storage. There is no weight penalty and the power-to-weight ratio is favorably increased for free.
Although my focus here is on engine airflow and the techniques that can be employed to improve it, you should note that in addition to mechanical supercharging (and turbos) more oxygen can also be supplied to enhance IC engine performance in two primary forms: nitromethane racing fuel and nitrous oxide injection. Both release extra oxygen during the combustion process and thus require more fuel to ensure the proper air/fuel ratio for best power. This is often referred to as chemical supercharging because the added oxygen is artificially introduced via a tube and not directly supplied by atmospheric pressure.
A secondary means of supplying the additional fuel to support the increased oxygen content is required. Kits to support this are plentiful and easy to apply. Numerous other oxygen-releasing compounds are available, but most are too toxic, corrosive, or difficult to store and handle. This makes nitromethane and nitrous oxide the most popular secondary sources of oxygen.
This color-coded chart illustrates common combinations of molecules. For the purpose of this engine discussion, only oxygen and nitrous oxide are useful. In a nitrous oxide–injected engine, the oxygen separates at 565 to 575 degrees F, thus providing additional oxygen that must be supplemented by additional fuel for more power.
Most of this has a much greater effect on tuning issues than airflow, but it’s important to understand the fundamentals. Maximizing airflow is your goal, but some properties of air can impede this to some slight degree.
Mass is generally represented by air density, which is a function of temperature and pressure. Cooler air is heavier and denser; hotter air is lighter and less dense. When discussing these properties it is advantageous to recognize the distinction between mass and weight. Scientifically, mass is defined as the amount of matter contained within an object. It does not change. Weight is the force of gravity acting upon a given mass. It is represented as:
W = mass × g
Where:
W = weight (force)
g = gravity (acceleration)
The mass of an object times the acceleration equals the active force or weight with acceleration or gravity equal to 1 g.
Weight is a force. Gravity causes the upper levels of the atmosphere to press down on the air underneath it, creating pressure relative to the particular elevation point in the vertical air column. Air surrounding an object presses against all sides of the object with equal force. The specific weight of air varies with elevation, temperature, pressure (elevation determines pressure), and the amount of water vapor and/or fuel contained within a given air mass (volume of air). Standard air at sea level weighs 0.0763 ft-lbs3.
Basic Properties of Air
Dry air at 15 degrees C (59 degrees F) and minimal or dry moisture content is the standard (ideal) reference. Dry air contains little or no moisture so there is more room for fuel vapor in a given volume. Cooler air also promotes greater density (oxygen content), which requires the addition of more fuel to burn completely.
These terms are frequently misused because the mass of an object is commonly (incorrectly) referred to as its weight. For example, the volume of an air/fuel mixture in a given cylinder is referred to as the “trapped mass” when the valves are closed. The greater the trapped mass, the higher the theoretical power. That mass has a specific weight and volume according to its density, which varies according to temperature and pressure.
The weight of air depends on where the air is located within the local column of air that extends from sea level to the edge of space. Air on a mountaintop is less dense and lighter because there is less air above it pressing down on it; the temperature is also lower. So you have a temperature and pressure reduction and a lessening of water vapor pressure as well.
Pressure, temperature, and water vapor content are important components of engine performance from a tuning standpoint. Higher pressure (density) improves performance; higher temperature lessens performance. And water vapor is the joker that spoils everything by displacing oxygen. Your charge as an engine builder is to achieve optimal airflow despite the influence of these components, which are generally identified as weather conditions.
Density is the mass (weight, in a sense) per unit of volume, or the specific amount of matter contained within a specified volume (density = mass ÷ volume). Density increases with pressure; high density increases the mass-per-unit volume. Higher density equals greater trapped mass and, thus, more power from the increased amount of oxygen, provided you can supply the correct amount of fuel to encourage effective combustion.
Water with a density of 1 is the common reference, or 1.000 g/cm3 by definition.
Air density is a huge factor in operating high-performance engines. Denser air contains more oxygen molecules unless it is saturated with water vapor. Density is primarily influenced by elevation, temperature, and humidity, but not pressure. A given volume of colder air contains more air molecules than the same volume of warmer air, thus more oxygen is available to burn more fuel. Your car feels peppier on a cool, dry day as opposed to a blisteringly hot day with lots of humidity displacing what few oxygen molecules are available.
To a lesser degree, high humidity also affects air density by displacing oxygen molecules. Air saturated with water vapor is less dense than dry air and because it contains water vapor and fewer oxygen molecules, it doesn’t support greater power output. Cool, dry air is the most desirable. It is denser, thus providing ample room for additional oxygen molecules.
If you look at a cubic foot of air at any given location, you are required to contemplate its density based on its temperature, pressure, and amount of water vapor. When any of these variables changes, the density varies. So a hotter cube of air is less dense than a cooler cube. Oxygen content changes with these variables. Vapor pressure is also a factor by displacing some amount of oxygen molecules within the given volume.
Dealing with these atmospheric variables makes it difficult to accurately judge air density. In the absence of an “oxygen meter” you have to rely on density standards for tuning purposes. Air density gauges that display density as a percentage based on the measured air temperature and barometric pressure are essentially “ballpark” devices. Racers have learned how to work with them to improve performance, but they can’t measure or factor in humidity to account for how the vapor pressure influences oxygen content independently of pressure and temperature.
Basic Properties of Air
The MSA uses 29.92 inches Hg for standard sea-level pressure. As elevation rises, pressure decreases in almost linear fashion. As shown here, it drops to 24 inches Hg at 6,000 feet. (Photo Courtesy Patrick Hale)
The motorsports industry relies on a standard temperature of 60 degrees F and 29.92 inches Hg (mercury) barometric pressure with dry air (no humidity). These conditions correspond to the standard atmospheric pressure of 14.696 psi, commonly expressed as 14.7. (You may also see 14.68 psi, which is calculated by using the aviation standard temperature of 59 degrees F.) This is zero density altitude without the influence of vapor pressure. Any drop in pressure or increase in temperature or vapor pressure raises the density altitude with a corresponding reduction in indicated air density.
All of this affects the oxygen content of the air, which is a big factor in fuel combustion. And as previously mentioned, engines perform their best under conditions of high barometric pressure, low temperature, and minimal water-vapor pressure (low humidity).
Important Terms
The following terms are useful in understanding the various qualities attributed to air. They are variable and influenced by changes in each of them.
Dry Bulb Temperature
Regular “air temperature” measured with a thermometer.
Dew Point
The temperature at which water vapor condenses and separates from air.
Pressure
Air applies pressure to any surface it touches. Pressure is measured as force per specified area. Gravity pulls air toward the earth because air has weight. That causes pressure, resulting in denser, heavier air near the surface exerting a normal sea-level air pressure of 14.7 psi (pounds per square inch, or 1,013 mbar) on everything around it.
The pressure of air moving within a runner varies according to the port velocity, port cross section, and any restrictions where it may encounter abrupt variations in area or direction.
Relative Humidity
This is the percentage of water vapor in the air relative to the maximum amount the air could hold under those temperature and pressure conditions. Relative humidity is 100 percent at the dew point.
We generally discuss relative humidity as it relates to human comfort, with the average person feeling most comfortable at about 50-percent relative humidity. Perspiration increases above that point; lower percentages are often too dry for many people, causing headaches and dry throat, skin, and nasal passages.
Specific Volume
The specific volume of a substance is the ratio of its air volume to its mass. As described in Boyles Law, the given volume of a gas varies depending on its temperature and pressure. For any given temperature, the gas occupies a specific volume. Remember that (mathematically) specific volume is the inverse of density (specific volume = 1 ÷ density).
Temperature
Temperature is usually expressed in degrees Fahrenheit unless conversion to Centigrade, Celsius, or Kelvin is specified. A gas (air) at any given temperature or pressure occupies a specific volume according to its mass, temperature, and pressure.
An absolute temperature scale (Rankine) is used for engineering and thermodynamic calculations. The Rankine scale has the same number of increments as the Fahrenheit scale, but the absolute zero point is equal to –459.67 degrees F (absolute zero is recognized as the coldest temperature in the universe). Outside the fields of meteorology and petroleum engineering, Rankine is used rarely and you will likely not encounter it for the purpose of understanding and working with engine airflow.
Viscosity
Air also has fluid qualities that affect its movement through a passage or port. Air viscosity is a measure of friction and the resistance to efficient flow. It is based on differing velocities near the center of the port and near the port walls (drag).
Because air is compressible, it can gain or lose pressure depending on airspeed and flow area, and the drag varies depending on viscosity. Shear, or stress, determines air pressure depending on viscosity, which varies depending on fuel content, droplet size, and the severity of area and direction change.
Because air is compressible, it can also emulate basic spring characteristics when subjected to sudden starts, stops, and abrupt direction changes along with various shear properties depending on how heavily it is laden with fuel or water vapor.
Water Vapor
Water vapor is the most variable component of the engine’s intake air, and it exerts considerable influence on the combustion process. It constantly changes according to temperature, time of day, weather conditions, and the proximity of water sources, such as lakes, rivers, or clouds. Although water vapor provides a cooling effect it also displaces fuel molecules, thus requiring a leaner mixture to obtain optimal fuel ratios. This is primarily noted as the weather effect for tuning purposes, but it is also experienced when using water injection for charge cooling.
The presence of water vapor actually decreases density and essentially makes the air lighter. That’s because the molecular weight of water is less than the combined molecular weight of the oxygen and nitrogen that make up the bulk of the atmosphere. This is difficult to grasp at first until you recognize that you’re not talking about liquid water, but rather water vapor, which itself is a gas that is lighter than the combined weight of oxygen and nitrogen.
Although temperature and pressure exert the greatest influence on air density, humidity has a lesser but potentially harmful effect. In addition to being lighter, it occupies space that could otherwise be occupied by fuel, thus reducing the oxygen content and the energy content (fuel) available to produce power.
Wet Bulb Temperature
This is equal to dry bulb temperature when air reaches its saturation temperature (dew point), or the lowest temperature water can reach via evaporative cooling. The difference between wet bulb and dry bulb temperatures is a measure of the humidity.
Engine performance is fully dependent on the airflow through the engine. Because there are so many variables and factors that influence airflow and air quality, correction factors were conceived to help engineers make more accurate comparisons. They use the correction factors during product development and dyno testing, whether on an engine dyno or in a running vehicle on a chassis dyno.
The following three laws relate air density to pressure and temperature. They combine to define gas behavior based on a hypothetical ideal gas. They are named for the three men who postulated them and they apply perfectly to the gas you know as air.
• Avogadro’s Law states that at constant pressure and temperature, the volume of a gas is directly proportional to the amount of gas. (Volume is relative to temperature and pressure.)
• Boyle’s Law states that the volume of a gas is inversely proportional to its pressure. (Volume increases as pressure decreases.)
• Charles’ Law says that the volume of a gas is directly proportional to its temperature. (The gas expands with temperature.)
This equation represents the ideal gas law:
PV = nRT
Where:
P = Absolute air pressure
V = The given volume of air
n = The actual amount of air measured in moles
R = The Ideal Gas constant
T = The absolute air temperature (this is where degrees Rankine fits in: you don’t use degrees F)
The volume, pressure, and temperature of air are easier to measure than its specific mass. Your primary concern is the behavior of air (a gas) as it undergoes changes in temperature and pressure, which are the parameters that affect engine power output. You want to understand the behavior of air as it moves through the engine negotiating restrictions (area and velocity changes) and reacts to the influence of fuel droplets present in the airstream. The ideal gas laws allow you to predict air movement based on known constants.
Correction factors are established to support accurate comparisons of data recorded under different conditions such as location, elevation, time of day, and basic weather data. Although useful in testing, they require careful application and very accurate data to ensure meaningful comparisons. They also invite abuse and the potential for misleading information based on improper input or, in rare cases, attempts to fool the sensors. Although most dyno testing is comparative, both observed and corrected data can serve your needs.
For that matter, observed numbers are often more instructive as long as you maintain consistent comparisons. Observed numbers tell you what is really going on, right then and there. The important thing is to choose a standard and stick with it during all your testing. There are two basic dyno correction factors: the SAE (Society of Automotive Engineers) and the STP (Standard Temperature and Pressure). SAE numbers are used by OEMs for all their testing and they are generally the standard for most chassis dynos as well. These are automotive standards.
Most dyno shops and “magazine” testers stress STP numbers because they are roughly 4 percent higher than SAE numbers. STP numbers are recognized as the “motorsports” standard and are described as such by motorsports authority and author Patrick Hale in his book, Motorsports Standard Atmosphere and Weather Correction Methods.
Hale is the founder of Racing Systems Analysis (RSA) and the author of the popular Quarter and Quarter Jr. dragstrip computer simulation software and the Engine Pro engine simulation program. He eventually sold off the RSA software business and now operates Drag Racing Pro (DRPro), a motorsports consulting firm. Hale’s book defined the Motorsports Standard Atmosphere and how it applies to weather for tuning purposes.
Engine dynos collect only raw uncorrected data. Whatever correction factor is applied is merely a percentage calculation based on contributing factors input by the operator or recorded by sensors. Your dyno printout can be set to list observed (raw or as-recorded), SAE, and STP numbers plus the actual numerical multiplier so you can determine the “validity” of the correction factor for yourself. Serious tuners use uncorrected numbers and raw recorded data and recognize that the larger the correction factor, the more likely the numbers are to skew incorrectly.
Correction Factor SAE J607
This factor is common to the performance industry, particularly for engine dyno testing. It corrects observed data to standard temperature and pressure (STP), or 60 degrees F at a barometric pressure of 29.92 inches Hg and dry air (zero humidity). It also subtracts corrected vapor pressure from observed barometric pressure to correct for water vapor in the air. Numbers according to this correction factor used to be referred to as STP corrections, but are now commonly referred to as Hale’s Motorsports Standard Atmosphere (MSA) corrections.
SAE J607 = (29.92 – corrected barometric pressure) 1.2 × [(observed inlet Fahrenheit temperature + 460) ÷ (520)0.6]
Where:
29.92 = standard barometric pressure
460 = Rankine conversion
The resulting factor is multiplied by observed torque and horsepower to obtain figures corrected to the common standard. This most often results in a higher reading than the observed numbers. Note that temperature in this formula is converted to degrees Rankine (by adding 460 to the Fahrenheit number), and that it yields power numbers approximately 4 percent higher than SAE 1349.
Correction Factor SAE 1349
This is the auto industry standard and is the normal correction for chassis dyno work, although most engine and chassis dyno software automatically calculates both. The software converts raw data to 77 degrees F air temperature, 29.31 barometric pressure (990 millibars), dry air, and includes a factor for an agreed standard of 84.7-percent mechanical efficiency. Numbers according to this correction factor are commonly referred to as SAE corrections.
SAE 1349 = 1.18 × [(29.31 ÷ Pd) × {(Tc + 460) ÷ 537)0.5} – 0.18
Where:
29.31 = barometric pressure
Pd = pressure of dry air in hPa (990 hPa = 99kPa)
Tc = air temperature in degrees Celsius
460 = Rankine conversion
Why Use Corrected Numbers?
Uncorrected test figures are your benchmark; they permit accurate analysis of fuel and air usage, and BSFC numbers. If you’re seeking or selling dyno numbers, you need to know the correction factor. If you’re an engine builder validating your combination, uncorrected numbers tell you everything you need to know. The chances that your engine will ever run with the same conditions as the correction factor are slim to none, so what’s the point?
Magazine testers use corrected numbers because they are higher and editorially expedient. Engine builders glean most of what they need to know from the raw data, particularly with regard to BSFC, VE, air consumption, fuel flow, air/fuel ratios, airflow, brake mean effective pressure (BMEP), and the shape of the torque and power curves.
If you think about it, the engine makes what it makes and the data recorded tells how and why it performed the way it did. The numbers change wherever you go. Correcting them to some pie-in-the sky number serves only your ego and can lead to considerable confusion.
For example, if you were in San Diego the engine would make more power. Well, yeah! If the air were colder it would make more power and on and on. If you’re testing and tuning in Denver, the numbers are different than if you go to the beach. But it doesn’t matter if you only race in Denver.
Bonneville racers routinely see big numbers on the dyno and then go to the salt flats and experience density altitude numbers anywhere from 4,000 to 8,000 feet and the corresponding loss of power they have come to expect. Either way the tune-up is different.
The correction factor’s real value is not in predicting power based on an arbitrary correction; rather, it provides the direction for tuning adjustment to compensate for actual weather conditions. Either way, your engine is still going to make less power in Denver, but standardized corrections can help you optimize its performance for existing conditions. That doesn’t impact actual engine airflow from a physical standpoint to any huge degree. But it does influence the weight and content of air, which has some effect on air movement through the inlet tract.
The MSA and the Air Density Index
The motorsports industry has universally adopted the MSA as described by Patrick Hale. That reference is for 60 degreees F, 29.92 inches Hg (sea level), and dry air (the absence of water vapor). These conditions constitute an Air Density Index of 100 percent. Any elevation above sea level or temperature above 60 degrees F has an Air Density Index of less than 100 percent.
At the MSA standard, the actual density of air calculates to 0.07633 lbm/ft3. Based on this, a pound of air at sea level has a volume of 13.1 cubic feet. Confusion is introduced with regard to atmospheric pressure, which decreases with increasing elevation, ranging from 29.92 inches Hg at sea level to 23 inches Hg at 7,100 feet. Hale’s description is clear: “Whenever the ‘actual’ ambient pressure equals the corresponding ‘standard atmosphere’ pressure for the ‘actual’ elevation, the ‘corrected’ barometric pressure will always be 29.92 inches Hg.”
Density altitude is a great tool for making sure your plane is able to fly in given weather conditions. And it does a great job of helping you jet a carburetor, but it’s not really ideal for motorsports corrections from an engine-tuning standpoint. Hale points out that engine power is not directly related to density altitude. He also states flat out that density altitude is not even necessary for proper engine tuning. Instead he recommends the HP Correction Factor, citing the importance of the type of fuel being burned and whether or not the engine is naturally aspirated.
According to Hale, engine torque and, thus, power varies with the inverse square root of absolute temperature. He points out that tuners have incorrectly assumed that the VE percentage of a given engine is not affected by changing weather conditions. This assumption suggests that a given inlet path flows the same regardless of pressure, temperature, or water vapor content; and that, of course, relates to your airflow concerns because you select and adjust airflow components based on calculations to achieve maximum VE. The VE percentage increases with rising temperature because the speed of sound increases with the square root of absolute temperature. Hale reminds us that peak mach numbers within the intake runners remain constant for any given RPM. This affects the timing of pressure waves within the inlet tract.
The Air Density Index based on the MSA equals 100 percent at sea level with 60-degree dry air and 29.92 inches Hg pressure. It also decreases almost linearly (hold a straightedge next to the line), dropping to about 84 percent at 6,000 feet. (Photo Courtesy Patrick Hale)
Naturally aspirated engines take considerably more finesse to pack the cylinders with air. Each application requires its own optimized package of intake and exhaust components to maximize efficiency.
Large Roots-style superchargers have always been popular in hot rodding and racing circles. Every hot rodder keenly understands the imposing look of a big blower.
For a detailed explanation of how this works refer to Hale’s handbook, Motorsports Standard Atmosphere and Weather Correction Methods, available at DragRacingPro.com. It is the most comprehensive description available on weather corrections for motorsports. It is primarily a tuning guide for drag racing, but it discusses how atmospheric conditions affect engine performance. It provides a solid foundation for tuning and makes you aware of the behavior of air from a tuning standpoint.
