100% VE AIRFLOW (scfm) = DISPLACEMENT (ci) x RPM / 3456(Equation 3)(For curious minds, '3456' is the product of 1728, the number of cubic inches in a cubic foot, and 2, the number of revolutions it takes for a 4-stroke engine to fill and empty all its cylinders.)Using that equation to evaluate a 540 cubic-inch engine operating 2700 RPM reveals that, at 100% VE, the engine will flow 422 SCFM.We have already shown (see Equations 1 and 2 in ) how to calculate the fuel flow required for a given amount of power produced. Once you know the required fuel flow, you can calculate the mass airflow required for that amount of fuel, then by using that calculated airflow along with the engine displacement, the targeted operating RPM, and the achievable VE values, you can quickly determine the reasonableness of your expectations. Here's how.Once you know the required fuel flow, you can determine the required airflow. It is generally accepted (and demonstrable) that a given engine (of reasonable design) will achieve its best power on a mixture strength of approximately 12.6 parts of air to one part of fuel (gasoline) by weight. (Other fuels have different best-power-mixture values. Methanol, for example, is somewhere around 5.0 to 1.)Using that generally-applicable best-power air-to-fuel ratio (12.6), you can calculate the airflow required. REQUIRED AIRFLOW (scfm) = 2.745 x HP x BSFC(Equation 7)So, by using a reasonable estimated BSFC and a reasonable best-power air to fuel ratio, you can use Equation 7 to estimate the airflow required for a given amount of horsepower, and with Equation 3, you can calculate the airflow of your engine at a given RPM if it was operating at 100% VE.If you divide AIRFLOW REQUIRED by AIRFLOW AT 100% VE, you get the VE that would be required for a given power output.In order to produce an equation that calculates REQUIRED VE, we divide Equation 7 by Equation 3, which produces Equation 8.
REQUIRED VE = ( 9487 x HP x BSFC ) / (DISPLACEMENT x RPM)(Equation 8)(Again, for those curious about the mysteries of 8th grade algebra, '9487' is the product of the 3456 from Equation 3 and the 2.745 from Equation 6.)Equation 8 enables you to evaluate the reasonableness of any claimed engine power level by knowing four values:. Required HP,.
The angle between the effective airflow and the relative airflow is known as the induced alpha. This increase in drag caused solely by the need to maintain lift is induced drag (sometimes called vortex drag). Induced drag is an undesirable by-product of lift. The stronger the vortices - the greater the induced drag. Is it the angle of downwash in relation to relative airflow that. Is this correct, or can someone provide an understandable explanation?
An assumed reasonable BSFC (a reasonable value for estimating purposes is 0.46).tHere is an example of how useful that relationship can be. Suppose you decide that a certain 2.2 liter (134 cubic inches) engine would make a great aircraft powerplant. You decide that 300 HP is a nice number, and 5200 RPM produces an acceptable mean piston speed (explained ). How reasonable is your goal?The required VE for that engine will be:Required VE = (9487 x 300 x.46 ) / (134 x 5200 ) = 1.879 (188%)Clearly that's not going to happen with a normally aspirated engine. Supercharging of some form will be required, and you can use the 188% required VE figure to calculate the approximate minimum Manifold Absolute Pressure (MAP) needed.In this example, the engine airflow must be increased to 188% of the assumed 100% VE value. Airflow is proportional to the square root of the pressure differential, so to double the airflow requires 4 times the pressure differential. Therefore, the approximate MAP required for a 1.88 increase in airflow will be (1.88 squared) x 29.92, or 106' MAP (75.8 inches of 'boost') for that power level.Here's another example.
Suppose you want 300 HP from a 540 cubic inch engine at 2700 RPM, and assume a BSFC of 0.46. Plugging the known values into equation 7 produces:Required VE = (9487 x 300 x.46 ) / (540 x 2700) = 0.898 (90%)That is a very reasonable, real-world number. (If you recognized those figures as being for the 300-HP Lycoming IO-540 discussed above, well done.) Manifold Absolute Pressure (MAP)We mentioned this term (MAP) in the preceding discussion, and it is used regularly in discussing engine performance, but just in case it is unfamiliar, here is a clarification.First, the term Absolute Pressure means the pressure above a zero reference (a perfect vacuum).
Ambient atmospheric pressure at sea level on a 'standard day' is approximately 14.696 psi absolute (or 29.92 inches of mercury, 'HG, explained below).Manifold Absolute Pressure, then, is just what it says: The absolute pressure which exists in the inlet manifold, usually measured in the plenum (if one exists). The MAP in an engine which is not running is equal to atmospheric pressure. If, on a 'standard day', an engine is idling at a measured manifold 'vacuum' of 14 inches, the MAP is actually 15.92 'HG (29.92 - 14 = 15.92).The term 'inches of mercury', as used to express a pressure, can be a bit confusing.
One common unit of measurement for MAP, barometric pressures, and other precise pressure measurements is 'inches of mercury'. Mercury is a heavy metal that is in the liquid state under conditions of standard temperature and pressure. Mercury is commonly used in manometers and barometers (a special application of a manometer) because of its high density and its liquidity. Recalling from high school chemistry, 'HG' is the chemical symbol for the element Mercury, derived from the Greek word HYDRAR GERIUM, literally silver water.In a mercury-filled barometer, the vertical distance between the two manisci, at sea-level, standard conditions, is 29.92 inches, hence the term inches of mercury, 'HG, or for the lazy, just inches. DISCLAIMER: EPIĀ Inc. And the contributors and reviewers of the material presented on this website have confidence that every effort has been made to ensure the accuracy and completeness of the information available, but we cannot be responsible for any errors or omissions.
Your use of the website and any of the available information indicates your understanding and acceptance of these terms. Is not liable to any party for any direct, indirect, special, or consequential damages resulting from use of any information on this website.