Never mind all the fancy words. what it all boils down to is how efficiently air and fuel are converted into usable power
The conversion of chemical energy into mechanical energy is an interesting process. Typically, there is the requirement that some amount of fuel be mixed with some corresponding amount of air. Then the two are subjected to an increase in pressure (compression). And then, depending upon the nature of the fuel being used, some form o? ignition spark (or ignition by way of compression) begins what is commonly known as combustion. A combustion engineer might call it an “oxidation” process. In the months that follow, many aspects of this process will be discussed, including factors that affect how well combustion takes place and a variety of conditions that govern how well an engine makes power. This month, we’ll examine some fundamental ways in which internal combustion engines accomplish the process of internal combustion. But first, let’s get into basic engine types.
To set the stage for combustion, we must first get air and fuel into an engine. Once this is accomplished, air/fuel charges must be compressed, ignited (or allowed to begin combustion) and given the opportunity to expand while exerting force on a downward moving piston. Once this is accomplished, exhaust residue must be passed out of the engine prior to the entry of a fresh charge of air and fuel. Regardless of how long this takes, or what is mechanically required to accomplish it, these are the steps.
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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.
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Originally called “mixing valves,” today’s carburetion and injection systems are still required to produce properly mixed air and fuel
In plain language, the combination of fuel with oxygen (during the conditions of combustion) is called oxidation. We could also call it, simply, combustion. But regardless of how this is accomplished, air and fuel must be combined in the proper ratio if either maximum fuel economy or power is to result. Experimentally, you could determine that gasoline (for example) will not “burn” by itself. Some amount of oxygen (or air) is necessary.
What all this means is that internal combustion engines require some means of mixing air and fuel in proper proportions to produce good combustion efficiency. And depending upon the fuel requirements of a given engine, carburetor “calibration”
should be adjusted to fulfill the engine’s demands. Suppose we consider the following as an example of what a fundamental carburetor must accomplish, based upon typical air flow into a normally aspirated (non-supercharged) engine.
First, let’s assume that our sample engine is being operated at sea level. This will establish an atmospheric pressure condition of about 14.7 pounds per square inch (psi). Since the downward movement of our engine’s pistons causes pressure above a given piston to be less than atmospheric pressure, air (and fuel) will be forced into the engine. The greater the differential between atmospheric pressure and that above the piston, the greater will be the velocity
of incoming mixtures. (more…)
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