Volumetric Efficiency

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Never mind all the fancy words. what it all boils down to is how efficiently air and fuel are converted into usable power.

More pressure on the piston, and for a longer period of time. If you had to reduce overall engine efficiency into a short sentence, it would probably include much of this thinking. And the event that creates “pressure on the piston” is related to combustion: a process of converting air and a fuel from an air/fuel mixture into a high-temperature, high-pressure molecular interaction called combustion.
In fact, the “burning” of air and fuel is much like what follows touching a burning match to the corner of a piece of paper. The flame begins at one point and moves throughout the remainder of the paper. If, for some reason, this “burning process” were to be accelerated very rapidly, combustion would take place spontaneously. Such sudden combustion would cause a sharp rise in cylinder pressure, resulting in either parts damage or reduced engine performance (assuming that everything stays glued together). So with this random collection of “facts” affecting how an engine produces power, let’s dip into some simplified theory on what takes place during, and the conditions that affect, the production of horsepower.
Air/fuel mixture density. Since we are attempting to create a rapid and, efficient chemical reaction between air and fuel, the tighter they are compacted the more rapidly such a reaction can take place. And since low engine rpm is normally associated with relatively low intake air/fuel flow velocities (thus mixture density), there is a good chance that mixture density is going to be reduced accordingly. What you’d like to achieve is a tightly packed mixture of air and fuel at whatever engine rpm (or range of rpm) you plan to use.
For example, if you were to examine the fundamental characteristics of an OEM (Original Equipment Manufacturer) engine with respect to where it produced maximum brake torque, you would see that there was a certain level of rpm required to produce this output. And it would be flow-velocity related. If, for some reason, an induction path was provided in which relatively low flow velocities were experienced in the rpm range where predominant engine operation

was centered, there would be the possibility of (1) air/fuel mixture separation and (2) resulting decreases in the charge density of such mixtures at time of arrival in a given cylinder.
The point here is that the rate at which combustion flame (air/fuel mixture oxidation) can move through a given combustion chamber is related to how compactly air and fuel exist in the combustion space at the time of ignition. Since this process involves chemical reaction between molecules of air and fuel, and since proximity of such molecules is important to the speed of reaction (all else being equal), it isn’t a bad idea to pack them as tightly as possible before the action starts.

But there are other factors to consider. And at the risk of oversimplification, we’ll now discuss some of the more important ones.
For example, how well a given sample of air and fuel are mixed (homogeneous air/fuel mixtures) can affect both combustion flame speed and efficiency. Let’s assume that we ignite a mixture of air and fuel not representative of a well-mixed (homogeneous) charge of combustibles. If, at the time of ignition, the air/fuel mixture passing the spark plug happened to be more lean than rich, additional spark energy intensity would be required to “ignite” the mixture. Actually, ignition spark voltage requirements increase with the density of the air/fuel mixture lying between the spark plug’s electrodes. It is partially for this reason that so-called “lean burn” low exhaust emissions engines require more ignition spark energy (high energy ignition systems) than engines of more mixture density (richer) conditions.
Right about here, there’s another “situation” that bears some thought. Late-model cars seem to have a driveability condition, especially during cruise or constant throttle operation, called “surge” or “lean mixture misfire.” Just because the contents of a given cylinder get an ignition spark from the spark plug does not automatically mean correct combustion will occur. During part- or wide-open throttle operation, intake manifold pressures can be relatively high (near atmospheric), resulting in increased density of the air/fuel mix-

ture at the time of ignition. This helps initiation of the combustion process.
But when throttle opening is decreased (resulting in lower manifold pressure or “higher” vacuum gauge readings), there is a corresponding reduction in the density of air/fuel mixtures at the time of combustion-resulting in irregular mixture firing and an engine that surges or stumbles. Introduction of exhaust gas (exhaust gas recirculation or EGR) into the cylinder provides a material already the by-product of normal combustion and not capable of secondary combustion. This tends to (1) dilute fresh air/fuel mixtures and (2) decrease the amount of actual cylinder pressure available to aid engine output. The fact that EGR reduces combustion heat (thereby decreasing the output of oxides of nitrogen, or NOx) is an indication of why many EGR-equipped vehicles show lower fuel economy than comparable non-EGR cars.
Preignition. It was previously mentioned that uneven “burning” of air/ fuel mixtures can cause problems with both performance and parts life. If for some reason there is a “hot spot” somewhere in the combustion chamber, air/fuel mixtures can ignite as if spark ignition was provided by a spark plug. But since such hot spots are not necessarily located near a given spark plug nor initiated at the same time of spark ignition, normal combustion will likely take place too quickly. This can result in cylinder pressures that are abnormally high, a result of spontaneous combustion of any remaining air/fuel mixtures suddenly brought to temperature levels associated with normal combustion. The resulting sharp increase in cylinder pressure is usually associated with engine “rattle” or “ping” during vehicle acceleration. Reductions in overall engine performance and possible parts damage may result if such conditions are allowed to exist.
Actually, surface temperatures of around 2000° F. are normally required to produce preignition, but such factors as carbon deposits within the combustion area, spark plug heat range, and sharp-edged material on piston domes and combustion chambers can affect the opportunity and extent of preignition.
More on this subject later.