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 mixture 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.
REVERSION PRESSURE EXCURSION
The relationships among the induction, exhaust and cylinder cavities in an internal combustion engine are both interesting and influential in the ability of a given engine to make power. These four illustrations indicate how each area is pressure-related during the so-called “reversion period.” A “horizontal V symbol” with the open end toward the right (reading from left to right) means the quantity on the left is less than the quantity on the right. Reading from right to left, it means the right-hand quantity is greater than the left-hand quantity. For example, in figure a, cylinder pressure (PJ is greater than exhaust system pressure (P2). For this reason, there is exhaust gas flow out of the Pl area. But since P2 is shown to be greater than P31 there will be some amount of exhaust gas flow back into the induction system (P3 area) when the intake valve first opens. This is shown in figure b. Keep in mind that the direction of intake or exhaust flow is toward the region of lowest pressure. In figure c, as the exhaust valve is closing and the intake valve is into the induction stroke of the engine (note piston movement direction), some amount of exhaust gas will be drawn into the combustion space, simply because P, is the lowest pressure area in the system. It is this “tail end” of the intake/exhaust valve overlap period (when both valves are off their respective seats with the intake opening and exhaust closing) where changes in camshaft design can greatly affect engine efficiency. Detrimental effects of the overlap period are related to the amount of time available for their influence. This means that the lower the engine rpm, the greater the possibility for “ideal” intake/exhaust cycle progress. In figure d, you can see that downward motion of the piston (and a closed exhaust valve) is sufficient to produce P3 and Px pressure differences that relate to the main part of the intake cycle. If intake and exhaust valves opened and closed exactly at top and bottom dead center positions of a piston, there would be more time for reversion pressure conditions to be reduced. But since a certain amount of “Kentucky windage” is necessary to adequately fill and evacuate the cylinders of a given engine, variations in “ideal” flowing pressure conditions are unavoidable.
1. Exhaust gas contamination of fresh air/fuel charges results from residual combustion material either passing into the intake system (flow located as shown) or residing in the combustion chamber at the time of ignition. Low-lift intake flow characteristics toward the carburetor govern much of the extent to which this condition can exist. 2. On the exhaust side of the combustion chamber, “clean” surfaces such as the area shown on the valve indicate raw fuel passage during late exhaust cycle periods. For engines operating at relatively low rpm, such a condition can reduce the amount of volumetric efficiency and net engine output. Reduction in low-lift flow capability (on the exhaust side) can diminish such efficiency losses. 3. On the intake side of the combustion chamber, specific color patterns can spell out the extent of combustion efficiency problems. Here you can see that the intake valve is opening too soon, exposing the exhaust valve side of the valve head to combustion flame (note “clean” side of the valve head situated on the intake manifold side of the combustion chamber).
Combustion surface to combustion volume relationships
Don’t let this get too sticky. What it relates to is how much air space (everything above the piston during combustion) exists vs. how much surface area (cylinder wall, piston dome and combustion chamber in the cylinder head) is available over which to spread the combustion flame. For example, consider the rotary engine. If too much combustion surface is provided (or exposed) to the combustion flame during combustion, there is a chance that premature “flameout” or quench will take place, resulting in lost power and efficiency. This is not meant to be critical of the rotary engine, but it gives a little food for thought on why a second spark plug and an additional ignition system were added to this engine design in an effort to reduce unwanted un-burned hydrocarbons (unburned fuel) during exhaust emissions tests.
The overall point here is fundamental: Maintain high cylinder pressure during combustion (hopefully promoting smooth air/fuel oxidation or flame travel) and avoid sudden combustion of the mixture, which can result in lost power and damaged parts. There are three other factors that can be included as having particular influence on efficient combustion: piston dome shape, connecting rod length (relative to crank stroke length) and “blow-down” efficiency. Of these three, rod length probably has less direct effect on combustion, but the effects of rod length change can be characterized as follows.
Let’s assume we have an engine of given rod length. During one revolution of the crankshaft, this length of connecting rod (relative to crank stroke) will cause piston motion to have certain characteristics. This means it will have both velocity and acceleration motion in the cylinder that relate to this particular rod/ stroke combination. Taken one step further, it means that the rate at which air and fuel will be compressed prior to ignition and then additionally compressed during combustion (both before and after top dead center piston position) can be affected by rod length.
For example, we know that combustion begins before TDC piston position, while the piston is moving upward. Once there is ignition, the burning air and fuel mixture continues to be compressed by the upward-moving piston. After TDC, the rate at which the burning mixture can expand is associated with the speed with which the piston moves away from TDC.
Increases in connecting rod length tend to allow more piston “residence” time around top and bottom dead center positions. This causes slower piston movement during mixture compression before ignition and as the piston moves away from TDC in the power stroke. Slower piston movement during early stages of the power stroke means higher pressure on the piston (combustion space or volume is not increasing as quickly as with a shorter rod).
4A. This is the first of two examples involving combustion flame travel in a small-block Chevrolet racing engine. Note that there is a lack of combustion residue in the deck area of the cylinder head, suggesting that flame travel did not involve this portion of the combustion chamber. 4B. Here you can see how little combustion process involved the intake side of the pistons. If the clearance between piston dome and cylinder head material is too tight in this area, flame “quench” or interference can result in lost combustion process pressure—and lost pressure. 5A. These cylinder heads show an increase in both combustion residue intensity and “reach” into the deck area. Note also the relatively uniform contrast of carbon color of both valve heads and combustion chamber. This is another indication of smooth, progressive flame travel during combustion. 5B. Note the increased area of combustion color in the intake valve region. This suggests improved combustion flame travel and increased horsepower. It also points out the importance of piston-dome shaping relative to combustion chambers (and/or modifications). Keep in mind that the “combustion chamber” is a combination of piston dome, cylinder wall and cylinder head chamber. Improper matching of any of these to another can result in lost combustion efficiency—and horsepressure.
It might be worth mentioning that increases in rod length tend to reduce induction system flow velocities (during early stages of inlet flow), thereby causing more rpm to achieve flow rates comparable to engines of less rod length. Actually, we’ll get into this subject in a forthcoming Shop Series on induction systems.
The other two factors (piston dome shape and cylinder blow-down) are more combustion efficiency-related. In particular, it is the evacuation (scavenge or blow-down) of a given cylinder that can really affect power. You might think of it this way: Following combustion, there is exhaust gas inside the cylinder. Compared to fresh air and fuel, this gas will not burn (or oxidize) a second time. So we’d like to get all of it out of the cylinder. But if for some reason it does not, there will be some amount of dilution of the fresh air/fuel charge (sort of a built-in EGR). This results in less available heat and pressure from the next combustion cycle. And that’s less power.
Fundamentally, as the fresh air and fuel (hydrocarbon “chains”) begin their chemical reaction, the presence of exhaust gas either prevents complete combustion or reduces the total amount of combustion, since some of the space in the cylinder is occupied by a noncombustible gas.
Factors affecting efficient blow-down include exhaust system efficiency, intake/exhaust valve timing, induction system design and combustion space configuration. Since we know that cylinder pressure (at the time of intake valve opening) is typically higher than intake manifold pressure, some amount of exhaust gas is going to pass back into the intake manifold (the old “reversion” game). Once this pressure differential favors flow into the cylinder, there is a chance this amount of exhaust gas will move back into the cylinder. But it’s a very slim chance. More likely it will go into another cylinder already receiving air/fuel mixture from the manifold. So another cylinder will have its combustion affected by exhaust gas.
In the real world,,all this means is that we need to pay attention to how best we can reduce the influence and amount of residue an engine experiences in the rpm range of anticipated operation. If it’s a race engine that will be operated from 6000 to 7500 rpm, then the problem is less difficult than if the span was from off-idle to 5000.
Obviously, the more time we give to the reversion problem the more influence it will have on engine performance. At the higher engine speeds, there is less time for this dilution to get back into the induction system. But it also means there is less time available to get it out of the exhaust side of the engine too. While it’s doubtful there are absolute rules of thumb on how all this is accomplished, we’ll keep touching on various possibilities as the series continues.
Piston dome shape. Try to keep in mind that all surfaces above the piston comprise the combustion surface. Remember also that combustion begins before TDC and must move throughout this space until completion. Therefore, flame is traveling across the piston’s top near and during TDC. Think of it all in ultra-slow motion. As the flame moves across the piston dome surface, it must do so uniformly. Dome material that comes in close proximity to the combustion chamber (in the cylinder head) can affect air/fuel mixture density. And remember what was said about density and combustion rate; it slows down as density decreases.
So both the presence of combustion color and the variation in color are indications of what is going on in the cylinder. You’ll note in a couple of the included photographs that there is less combustion residue near the “deck” surface on the piston than out near the center of the chamber. This is because there is almost no combustion space between the dome and head in this area. In contemporary racing engines where average piston speeds are in the area of 5000 ft./min., it doesn’t take much of an imagination to see how little time there is to adequately oxidize a complete air/fuel mixture.
The provision of combustion surfaces that promote efficient combustion, therefore, is an absolute necessity. It also applies to an engine for which 3500 rpm is a maximum and fuel economy is the goal. That might be something to think about too.
At the outset of this segment of the series, we suggested that “piston pressure” was the object of efficient combustion. And while there are numerous factors that influence the efficiency of this process, we’ve tried to spread out the more common ones for your examination here.
REVIEW QUESTIONS: True or False
1. Oxidation means adding oxygen to an air/fuel mixture.
2. As air/fuel mixture density decreases, the molecules of air and fuel are more spread out, resulting in more rapid combustion.
3. Low engine speed helps keep air/fuel mixtures homogeneous (well mixed).
4. As air/fuel mixtures are made “lean,” less ignition spark intensity is required to initiate combustion.
5. Carburetor throttle opening has little or no effect on the density of air/fuel mixtures.
6. The presence of exhaust gas in a cylinder at the time of combustion tends to increase combustion pressure.
7. Preignition means air/fuel mixture combustion that was begun by something other than the spark plug.
8. The combustion-surface-to-volume relationship does not include any exposed cylinder wall area.
9. Engines with “long” connecting rods cause the compression of fresh air/fuel mixtures to be more rapid near TDC than engines of shorter rod length.
10. “Residence” time means how long you park the vehicle in front of your house.
11. Good “blow-down” efficiency means an engine is capable of effectively exhausting a high percentage of combustion residue during the exhaust cycle.
12. The more back-pressure a given engine has, the more difficult it is to achieve good blow-down efficiency.
13. Color variation on a piston top is an indication of air/fuel mixture density during combustion.
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