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.

By definition, each complete movement of an engine’s piston upward or downward is called a piston “stroke.” If the design of a particular engine requires four strokes of the piston to complete the total combustion cycle, the engine is a 4-stroke-cycle engine. Should it be capable of completing the total combustion cycle in only two strokes of the piston, it’s a 2-stroke cycle. Exactly how either 4- or 2-stroke engines are designed to accomplish all this varies somewhat, but the accompanying illustrations show pretty well the fundamental differences between the two engine types.
In application, there are some power-producing characteristics that typify each of the two engines. For example, 2-stroke-cycle engines normally operate in higher rpm ranges than 4-stroke-cycle engines of comparable piston displacement. They may produce comparable amounts of torque, but they do not normally have as broad (or flat) a torque curve as a similar-sized 4-stroke engine does.
One reason for this is the fact that in 2-stroke-cycle engines there is less time for the expanding air/fuel mixture (during combustion) to act on the piston, as compared to a 4-stroke-cycle design. This is because exhaust cycle timing is relatively short, causing some loss in effective cylinder pressure which could be used to advantage in net torque output. In a 2-stroke-cycle engine, it is the arrangement of the inlet and exhaust ports (relative to the piston’s action in the cylinder) that accounts for intake and exhaust event variations, much like a change in camshaft will produce for 4-stroke-cycle engines.
Fundamentally, then, the basic 4-stroke-cycle engine has intake, compression, power (combustion gas expansion) and exhaust cycles, all of which require four strokes of the piston. Theoretically, the intake valve would open exactly at top dead center (TDC) position of the piston as it begins to descend.

At or near bottom dead center (BDC) piston position, the inlet valve would close and the entire upward stroke of the piston would be used for compressing the inlet air/fuel mixture. Then, exactly at TDC piston position (the end of the compression stroke), ignition would occur and the piston would be forced by the pressure of burning air and fuel. Upon reaching BDC (the end of the power stroke), the piston would move upward to its TDC pos-tion, exhausting combustion residue and readying the cylinder for beginning the next intake stroke.
That’s in theory. In fact, there are many variations of this pattern. Typically, such variations come as a result of camshaft and ignition system design whereby it may be desirable to begin either ignition or any one of the four intake and exhaust valve opening points other than at TDC or BDC during the 4-stroke cycle. Reasons for this will be discussed a little later.
The classical 2-stroke-cycle engine also involves the compression of air/ fuel mixtures by an upward sweep of the piston. Near TDC, ignition occurs, and the sudden increase in cylinder pressure brought about by combustion drives the piston toward BDC. Near BDC, an exhaust valve opens (or an exhaust port is exposed), enabling blow-down of the cylinder to occur. This means cylinder pressure (at the end of the blow-down “cycle”) will be at or near atmospheric pressure. By this time, the piston has moved far enough to expose the intake port (tail-end exhaust flow is still taking place), resulting in a degree of cylinder scavenging and pressure drop across the cylinder to create intake air/fuel charge flow (see illustrations).
Some 2-stroke-cycle engines are classified as “loop-scavenge” designs, meaning incoming air/fuel mixtures pass into the engine’s crankcase cavity before being allowed to enter the combustion chamber portion of the cylinder (see illustration). Actually, the same chain of events takes place as described with the basic 2-stroke-cycle engine, but in this case the arrangement of intake and exhaust porting provides a mixture flow path routed through the crankcase instead of directly from the mixing valve (carburetor) into the combustion portion of the cylinder.

Such an arrangement of porting is common in both model airplane and outboard boat engines. Normally, exhaust ports are placed higher in the cylinder than the intake port. This way the exhaust port is exposed ahead of the intake port, resulting in the exhaust blow-down period already mentioned. By the time the intake port is exposed, fresh air/ fuel mixture has been compressed in the crankcase by the downward sweep of the piston, causing mixture flow into the combustion portion of the cylinder (volume above the piston). Such engines are commonly fitted with pistons that incorporate a dome shape (or deflector) which aids the loop-scavenge principle by creating mixture flow upward in the cylinder. The purity of each air/fuel mixture, at the time of combustion, is a function of how efficiently the cylinder is blown down prior to combustion. (You might want to refer to the illustrations again. We had to.)
Internal combustion engines that depend upon an ignition spark to initiate combustion are called “spark ignition” engines.

TYPICAL 4-STROKE CYCLE
Here’s a drawing you’ve likely seen many times on schoolroom walls. But what the other illustrations may not have indicated is the fact that these are “theoretical” piston and valve positions. What happens in the real world is that intake and exaust valves do not open and/or close at exactly top dead center piston positions. Results of this are discussed elsewhere in the story. You might also want to consider that four strokes are required to complete the combustion cycle of an engine of this type. Thus, it’s a 4-stroke-cyc/e engine—not a 4-cycle engine.

TYPICAL 2-STROKE CYCLE
The trick here is that much of the exhaust flow portion of the process utilizes combustion pressure exposed to an exhaust passage late in the process of oxidizing or burning an air/fuel mixture. Whether the exhaust passage is provided by a valve (as shown) or exposed by piston movement, the expanding force of combustion gas is used to initiate flow out of the engine’s cylinder(s). Note also that during the intake/scavenge stroke, a low cylinder pressure condition created by exhaust gas flow helps induce intake flow into the engine—thus the term “scavenge.” Other than these conditions, the 2-stroke-cycle engine can be compared to 4-stroke-cycle engines.

And by way of historical note, the first such successful spark ignition, 4-stroke-cycle engine was designed in 1876 by a German engineer, Dr. A. N. Otto; and it was for him that the 4-stroke-cycle engine is occasionally called the Otto-cycle engine. (This little tad of information won’t be among the quiz questions at the end of the story.)
So-called “compression ignition” engines depend upon high air/fuel mixture temperatures that are the result of compressed intake air and a shot of fuel that is injected just about the time the compression stroke is completed. The following combustion, not initiated by a spark, is a result of high cylinder pressures and fuel that will ignite as a direct result of such pressures. These are called diesel engines, so named after Rudolf Diesel, who developed the concept in the early 1890s.
The common method for sizing the internal combustion engine involves its “swept volume” (or piston displacement). As you might expect, this relates to the amount of air that a given engine’s pistons will sweep out or displace from the cylinders in moving from BDC to TDC positions. The cylinder volume that lies above a piston at the TDC position is called “clearance volume,” and the difference between the total volume above a piston at BDC and the clearance volume is the actual “piston displacement” for the cylinder in question. If you multiply the piston displacement of one cylinder by the total number of cylinders, you’ll have the overall piston displacement.

Using the clearance volume value and the total cylinder volume above a piston while it’s at its BDC position, a piece of data called the mechanical compression ratio (static compression ratio or, simply, compression ratio) can be determined. For example, if the total cylinder volume is compressed eight times into the clearance volume (total volume is eight times larger than the clearance volume), then the engine has a compression ratio of 8:1. Just remember that clearance volume is defined at that volume above the piston at its TDC position and will be comprised of actual cylinder head combustion chamber volume (including spark plug location in the chamber) and any effects on volume by piston-dome configuration and cylinder head gasket thickness.
In reality, compression ratio can affect the amount of cylinder pressure developed in a given engine, even though the timing of both intake and exhaust events (even in the 2-stroke-cycle engine) has particular influence on net combustion pressure relative to rpm. For example, an engine with a measured compression ratio of 10:1 using a “stock” camshaft will produce far more cylinder pressure below 4000 rpm than the same engine using a cam of increased intake and exhaust valve timing. The increased valve timing allows more combustion pressure to be lost (due to earlier and later intake/exhaust valve openings and closings). Conversely, an engine of 8:1 compression ratio using a camshaft of shorter-than-stock valve timing can increase effective cylinder pressure in the lower engine speeds, making an engine perform as if a higher mechanical compression ratio existed.
But regardless of how specific valve timing is arranged, there are certain temperature and pressure conditions within an engine’s cylinders that exemplify what you can expect from conventional internal combustion engines, just about regardless of design. And although there are certainly exceptions to these data (especially in the case of non-normally aspirated engines), you can get a fundamental understanding of what is going on during the combustion process by the examination of such numbers.
For example, typical spark ignition takes place just before TDC of the piston’s compression stroke. Resulting combustion is a progressive process (much like setting a burning match to the corner of a piece of paper) that spreads throughout all regions of the combustion space. This process usually continues during the next 40 or so degrees of crankshaft rotation.

TYPICAL LOOP-SCAVENGED 2-CYCLE
In cases where 2-stroke-cycle engines store air/fuel mixture in the crankcase just prior to entry into the cylinder, a so-called “loop-scavenge” principle is involved. Here, fresh air/fuel mixtures flow into the crankcase while the piston is moving upward on its compression stroke. The less-than-atmospheric-pressure condition left in the crankcase allows air/fuel mixture flow into the engine (normally including an amount of oil mixed with the fuel from which bearing lubrication is provided). This design is common to both model airplane and 2-stroke-cycle outboard motorboat engines. Some form of piston-dome “deflector” is typically used to improve cylinder scavenging during the exhaust “blow-down” period.

And regardless of the relationship between crank stroke and connecting rod length, combustion or flame travel near and at TDC “sees” very little change in combustion space or volume above the piston. Under such conditions, cylinder pressure rise is both rapid and high, as compared to pressure increases combined with radical changes in combustion space (or volume above the piston). For example, combustion gas expansion could be rapid, but if combined with a rapid increase in combustion space, net pressure on a piston would be minimized.
Assuming a cylinder contents temperature of about 450° F. (just before ignition but at the end of compression), the changing of chemical or air/fuel mixture energy into mechanical energy (force on the piston) can elevate cylinder contents to a level of 4000° F. at peak combustion temperature. Pre-combustion cylinder pressure on the order of 140-160 psi (just prior to ignition) can rise to the 680-710 psi range during peak cylinder pressure conditions in a normally aspirated engine.

The point of all this is that we want to emphasize how quickly and proportionately higher both temperature and pressure increases during compression vs. combustion. For it is this relationship that governs many aspects of fuel requirements relative to octane, preignition and detonation problems.
In general, then, the internal combustion engine is required to produce some amount of power at an acceptable level of efficiency. By way of definition, “work” consists of transferring mass from one location (or condition) to another. Since 778 ft.-lbs. of work performed equals one B.T.U. (British Thermal Unit), energy absorbed or provided by an engine (or system) can be described in terms of torque (ft.-lbs.) or B.T.U.
If we consider the rate at which an engine does work (torque vs. time), the term “power” comes into conversation. And if an engine can do work at the rate of 550 ft.-lbs. per second (or 33,000 ft.-lbs. per minute), the amount of power being produced is equal to one horsepower.

And, finally, we come to a term that relates to how well a given engine is performing the job of producing horsepower: efficiency. Let’s define “efficiency” as a comparison of an engine’s actual power output to what it might be under ideal conditions. Further, since we can’t really expect an engine to produce “ideal” power, efficiency numbers are typically less than 1.0.
In the case of internal combustion engines, two “efficiency” factors are worth remembering: (1) thermal efficiency and (2) volumetric efficiency. If we were to compare the amount of work produced by an engine to the amount of work it could have produced (based on the chemical energy potential of the fuel it received), a thermal efficiency factor could be obtained. Arithmetically, this means dividing produced work by potential chemical energy available through fuel delivered. In the real world, it means “how well has a given engine converted the supplied fuel into usable horsepower?”

PISTON DISPLACEMENT VS. COMPRESSION VOLUME
At bottom dead center (BDC) of the piston stroke, some amount of air space or volume exists between the top of a piston and the remaining surfaces above it. This includes the combustion chamber and cylinder bore wall. By volume calculation, this represents all space indicated by (B) in the illustration. This is piston displacement. All volume space above the piston at its top dead center (TDC) position is considered compression volume, since it is the volume into which the piston displacement will be compressed when the piston is at TDC. A comparison of all the volume above a piston at BDC vs. the volume above it at TDC represents the number of times the total cylinder volume has been compressed into the compression volume. This is the numerical ratio you know of as compression ratio—as in 8.5:1. Ten years ago, these numbers were larger. But so were supercars.

REVIEW QUESTIONS: True or False
1. The combustion process, whereby air and fuel are “burned,” is a process in which all of the air/fuel mixture combusts at the same time.
2. Four-stroke-cycle engines operate at a higher rpm level, in terms of maximum efficiency, than 2-stroke-cycle engines.
3. Generally, 2-stroke-cycle engines have broader torque output ranges than 4-stroke-cycle engines.
4. So-called “blow-down” efficiency relates to how well a given cylinder is evacuated of combustion residue and, as a result, how efficiently the next inlet charge will enter the engine.
5. There are only a few types of 4-stroke-cycle engines that qualify as “loop-scavenge” engines.
6. Otto-cycle engines are 2-stroke-cycle engines.
7. Diesel engines are also called compression ignition engines.
8. The cylinder volume that lies above a piston at its bottom dead center (BDC) position is also called the clearance volume.
9. Mechanical compression ratio is a mathematical comparison of swept volume and total cylinder volume.
10. Effective cylinder pressure is affected by camshaft design.
11. Short-rod engines cause pistons to “reside” at or near top dead center longer than long-rod engines.
12. Compared to cylinder pressures measured just before combustion, a pressure rise of more than 2000 psi is not uncommon in conventional internal combustion engines.
13. One B.T.U. (British Thermal Unit) is equal to 778 ft.-lbs. of work.
14. Horsepower can be defined as the rate at which an engine does work (torque vs. time).
15. Thermal efficiency is the amount of work an engine does as compared to the work it could have done under ideal conditions.
16. Stoichiometric is another name for a metric system baby carrier.
17. Specific fuel consumption is a means of determining how many pounds of fuel were required to produce an amount of horsepower.
18. There’s a chance you’ve already cheated on the answers to these questions.
19. You probably think we did too.

Volumetric efficiency is an entirely different subject. It relates to how well an engine provides itself with fresh air/fuel mixtures over a range of operating rpm. For example, if a given cylinder was left exposed to atmospheric pressure, there would eventually be no pressure difference between cylinder and atmospheric pressure. The cylinder would be as full as atmospheric pressure could make it. But once the engine was running, several factors might prevent the cylinder from receiving the same amount of incoming mixture as it would have if left exposed only to atmospheric pressure. The difference between “static cylinder filling” and “operational cylinder filling” is volumetric efficiency, a term you should keep in mind for future reference.
An engine that shows a volumetric efficiency maximum of 90% means that at the rpm for which the 90% was obtained, .9 of what the engine could have achieved in inlet flow was being produced. And as you might expect, high levels of volumetric efficiency throughout an engine’s operational range are very desirable features. Which brings us to two remaining terms: (1) air capacity and (2) specific fuel consumption. You’ll become more familiar with these terms than you may be with yogurt. And, for a fact, neither needs to be refrigerated.
Air capacity: Regardless of how much fuel is provided to an engine’s cylinders, the amount of work that can be produced depends on how much oxygen is present with this fuel at the time of combustion. The oxygen present in a pound of air will normally “combust” efficiently .067-pound of gasoline. In terms of air/ fuel ratio, this computes to about 14.925 air-to-fuel. Most internal combustion engine textbooks define a ratio of 14.7:1 as a “stoichiometric air/fuel ratio.” This is a big word for “chemically correct” air/fuel ratio but is a good number for steady-state engine operation where maximum fuel economy is concerned.
If we were to define “chemical energy per pound of fuel,” there would be a yardstick measurement for determining how much horsepower could be expected from complete combustion of a given amount of fuel. And even though the difference between what laboratory measurement conditions show compared to what you’d find in a real-life engine does vary, there is some value of its use in comparing theoretical engine performance. This little tidbit is included for those of you who may be students of combustion engineering.
And, finally, let’s touch on the term “specific fuel consumption.” Technically, this means how many pounds of fuel were required to produce each horsepower. Stated another way, it indicates a relationship between fuel supplied and power produced. As measured horsepower (brake horsepower) increases and fuel flow decreases, specific fuel consumption (or brake specific fuel consumption, B.S.F.C.) also decreases. And the fact of the matter is that there are many engine dynamometer “operators” who pay more attention to B.S.F.C. numbers relative to rpm than they do to actual horsepower produced. When the B.S.F.C. “curve” becomes relatively flat throughout an engine’s rpm range, in-car performance is going to be good. And that’s something you can write home about.

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.

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.
For engines operating below 4500 rpm, the relationship between low rpm and low velocity inlet mixture flow is critical, since air and fuel can separate on their way into the cylinders. [Flow velocities of typically stock, 350-cubic-inch-displacement engines will be on the order of 240-310 ft./sec. (maximum) in this range of rpm.] This results in variation in air/fuel ratios among an engine’s cylinders and, consequently, lost power and efficiency. Exhaust emissions can even increase (with air/fuel separation), since raw fuel can be passed right through the engine if lean mixture misfire results from separated air and fuel.

A. Typical updraft carburetor showing how venturi causes pressure reduction (compared to atmospheric) leading to flow of fuel from fuel bowl through fuel discharge nozzle. Efficiency of fuel atomization is a function of nozzle design, location in venturi and rate of air flow through venturi. B. At low engine speeds where throttle blade opening is slight and total air flow through the carburetor is low, the addition of an “idle” circuit provides air/fuel enrichment until engine speeds increase to the point where additional fuel delivery cannot be supplied by a “fixed-size” idle circuit. C. Worth remembering is the fact that an engine does not “suck” fuel into its cylinders. The passage of air through the carburetor causes a difference in pressure between atmospheric and carburetor throat conditions. This amounts to lower pressure in the carburetor than that available from the atmosphere (P! is less than P2 as indicated), causing fuel to flow from the bowl area into the carburetor throat(s). In a sense, atmospheric pressure forces fuel into an engine at a rate dependent upon how low the pressure is inside the carburetor.

Aside from the problems (associated with air/fuel separation) relating to the design of intake manifolding, the carburetor is responsible for much of the initial mixing of air and fuel. So to better understand elements of the basic carburetor, let’s now discuss an example of its elements.
In its simplest form, this could be nothing more than an updraft design (mixture flow upward into the engine) consisting of a venturi, fuel chamber and sized fuel exit (or jet), as shown in figure A. Some sort of air throttling valve is located just above the venturi to govern the amount of both air and fuel reaching the cylinders. Unfortunately, this design is not efficient at low air flow rates (engine rpm), resulting in inadequate control of air/fuel mixtures and lost performance and economy.

The addition of a low-load (idling or low rpm) circuit, shown in figure B, provides fuel during conditions not satisfied by the basic, single-circuit, fixed-jet design. Here, fuel is discharged into the air stream just below the throttle valve (plate) when the throttle is in the closed or near-closed position. Typically, idle circuits tend to be richest (fuel/air mixtures) at low engine speed. As rpm and mass air flow increase, a “saturation” point is reached beyond which additional fuel delivery cannot be provided.

This makes the idle circuit appear to be lean (fuel-to-air), giving way to the need for an additional circuit: the main fuel supply. Here, sufficient fuel should be provided so that the main circuit operates at an almost constant air/fuel ratio throughout its effective range. Actual circuit overlap between the idle and main circuits is accomplished in the following manner: Opening of the throttle from its closed position can increase the amount of idle fuel discharge passageway exposed. As air flow through the carburetor increases, sufficient pressure drop across the carburetor (depression) activates the main circuit, causing fuel to flow through the main jet.
And although this pretty well covers the absolute fundamentals of fuel supply circuits, there is one omission.

AIR FLOW RATE (CFM)
D. Comparison of idle fuel delivery circuit vs. main fuel delivery shows how increases in air flow through the carburetor effectively cause the idle circuit to become “leaner” while the main circuit stabilizes over a wider range of air flow. Even though the idle circuit acts like a lean mixture circuit at higher air flow rates, it continues to contribute some amount of fuel throughout the rpm range. E. Flow velocity at d2 is greater than at d1( since cross-sectional area of flow path is less at d2 than dx. This results in lower pressure conditions in the region of d, and a greater differential between pressure in this area (d,) and that of atmospheric pressure acting across the flow path (carburetor). The greater the differential between venturi and atmospheric pressures, the higher the flowing velocity through the carburetor.

When the throttle is opened quickly, and intake manifold vacuum drops sharply, there is a brief period of time during which the pressure drop across the carburetor (depression) provides what might be called a hesitation in fuel delivery “signal.” Stated another way, you could say that there is an interruption in the air flow signals that cause fuel to flow from the carburetor. To cover up this spot in engine operation, a mechanical means is often used to provide fuel until either the idle or main circuits (or both) can begin delivering sufficient fuel to sustain proper engine operation. Since such sudden throttle openings are normally associated with acceleration of the vehicle, this circuit is appropriately called the accelerator pump circuit. (Ain’t science sumpthin’?) Its function is to provide a “shot” of fuel mechanically, irrespective of how the engine is being run. In fact, you can even manually operate an accelerator pump while the engine is running, since it has no relationship to manifold vacuum or net flow.
So what we have here, in stark basics, is a venturi passageway into one end of which a throttle blade is installed. From a reservoir of fuel, air passing through the venturi causes venturi bore pressure to be less than atmospheric pressure, resulting in some amount of fuel being forced into the air stream.

This constitutes fuel delivery until air flow (venturi depression) is sufficient to cause activation of the main fuel system, typically through a combination of venturi and venturi booster depression (see figure C). Relocating all this fancy theory into the real world * of downdraft carburetors, we find that there are some practical similarities between the classroom model and what you’ll likely find under the hood of your basic grocery getter.
Simply related, the fundamental downdraft carburetor also has (1) a venturi, (2) throttle blade or control, (3) fuel chamber whose level is controlled by a float and some sort of needle valve and (4) a main metering system (the idle and accelerator pump circuit will be included shortly). Noting Figure F, you can see that as fuel is pumped (or flowed by gravity) into the fuel chamber, a fuel level will be reached at which the needle valve will prevent additional fuel entry. As fuel is passed out of the chamber, this needle valve fluctuates to maintain a relatively constant level of fuel in the bowl.
Air flow through the venturi (at a rate depending upon throttle opening and engine rpm) reduces pressure at the fuel discharge nozzle, causing atmospheric pressure acting on fuel in the bowl (fuel bowls are typically vented to atmospheric pressure) to push fuel into the air stream.

The addition of idle and acceleration pump fuel circuits allow this model down-draft carburetor to satisfy most engine operating conditions. Most stories on basic carburetion will carry you to this point. What is often omitted is the fact that there are other, frequently used, methods of controlling main circuit fuel delivery.
Perhaps the most common of these is the so-called “metering rod” method. Such a design is particularly common to the Rochester Quadrajet and Carter AFB and TQ series of carburetors. Since main metering systems are typically designed to provide air/fuel mixture ratios on the order of 14.7-16.0:1 (at full throttle and for maximum fuel economy), it becomes necessary to design a means whereby optimum power and fuel economy over a wide range of part-throttle operation can be achieved with air/fuel ratios outside this range (see figure G).
If you’ve become confused at this point, consider how simple it really is. Engine speed and load both affect intake manifold vacuum. If you’ve ever driven a vehicle equipped with a vacuum gauge (and if you haven’t, you should), you know that more throttle opening usually means less vacuum. To the engine, this means less fuel delivery “signal,” because flow velocity through the carburetor depends upon both throttle opening and engine speed.

DIRECTION OF FUEL FLOW
F. As air flow increases through a carburetor, pressure drop (pressure reduction) in the venturi near the main fuel discharge nozzle increases in relation to the square of the flow. Through mechanical linkage, this causes the fuel metering rod to be lifted out of the fuel metering jet, increasing the amount of fuel flow (more enrichment) at a rate according to the shape of the “steps” on the end of the metering rod. G. Fuel flow into a conventional carburetor is controlled by a needle-seat arrangement as shown. As fuel rises in the fuel bowl, the float pivots, causing the needle shutoff level to force the needle into its seat. Leaky needle/seat combinations can allow excessive fuel entry into the carburetor and oven-rich air/fuel mixture conditions.

The faster air moves through the carburetor, the greater the difference between ven-turi pressure and atmospheric pressure. To the carburetor, this translates into a stronger fuel delivery signal—and more fuel flow.
It was pointed out earlier that our basic discussion of carburetor functions assumed an atmospheric pressure of 14.7 psi (normally associated with sea-level operation). Carburetors correctly calibrated for sea-level operation tend to provide fuel/air mixtures that are excessively rich when operated at higher-than-sea-level conditions. This is caused by reductions in air density relative to increases in altitude. Also related to this condition is a need for additional oxygen supply at higher elevations. In fact, any engine modification that increases the amount of available combustion oxygen (increased induction system flow velocity, shorter intake/exhaust valve timing, etc.) tends to improve high altitude engine efficiency.
Another point seldom mentioned in the analysis of basic carburetion (and you thought this was going to be the usual blackboard, wall-chart treatment stuff) is the ability of a given carburetor to provide good fuel atomization. Consequently, the ability of a given carburetor design to atomize fuel into small droplets is critical, especially if the engine is to be operated in a below-4000-rpm range. Of course one solution to such low-rpm fuel atomization problems is the use of small-diameter or few-in-number carburetor throats. This tends to increase air flow rates through a given carburetor, resulting in improved fuel atomization (more efficient “mechanical shearing” of liquid fuel into the air stream) and increased engine efficiency. Little wonder that contemporary engines designed for improved fuel economy and reduced exhaust emissions are fitted with two-throat carburetion. Since it has been established that certain intake flow velocities are associated with an engine’s ability to produce torque, smaller carburetion is a natural fallout of reduced operating rpm. What turbo-charging does to all this will be discussed in a Shop Series to follow.
Students of chemistry (and even those of you who might throw rocks at such folks) might think of the combustion process vs. fuel particle size situation as follows: Small droplets burn quickly. Larger droplets burn more slowly, resulting in fuel not being consumed in combustion but passed through the engine. In plain terminology, this means both lost power and fuel economy. The rule of thumb here is: The smaller the fuel particles, the greater the probability each will be consumed in the combustion process. And the better you’ll spin the “tars.”

Two points remain. The first is to keep in mind that the overall objective of any carburetor is the supply of fuel in proper proportion to the air requirement of an engine. Maximum power requires certain air/fuel ratios while optimum fuel economy typically requires less fuel for the same amount of air.
The second point concerns a measurement relative to power produced vs. fuel consumed. By comparing (under dynamometer test conditions) brake horsepower vs. fuel flow to the engine, a computed value known as brake specific fuel consumption (BSFC) can be used to evaluate engine performance. This is a direct means of determining how well each pound of fuel is converted into usable power (mathematical units indicate BSFC = lbs./bhp-hr., where bhp is brake horsepower and hr. is hours of engine operation or fuel flow).

As an analogy, if you consumed 6.0 ounces of orange juice and jogged 1.3 miles, a “BSFC” of 0.216 would result, based on the analysis of comparable engine performance. If, for some reason, this 6.0 ounces of o.j. produced less than 1.3 miles of travel, a lower BSFC value would suggest a reduction in “jog power.” Optimized, the lower the BSFC (without an attending loss in power), the more efficient the engine. And when fuel economy is the objective, the lower the BSFC numbers (given acceptable driveability), the better the engine efficiency.
Relative to carburetor efficiency, delivery of proper air/fuel ratios over the operational range of a given engine normally produces optimum power and economy. And a basic understanding of how each circuit of a carburetor contributes to “correct” air/fuel ratios is fundamental to efficient engine operation. Unfortunately, carburetion is only the first step in providing combustion-efficient air/ fuel mixtures. Regardless of how efficient a given carburetor may be, you might be surprised at what takes place once air/fuel mixtures leave the “mixing valve” and head toward the combustion chamber. Well, maybe you won’t. But we were.

REVIEW QUESTIONS: True or False
1. By itself, gasoline is “burnable.”
2. Carburetor calibration means providing “correct” air/fuel mixtures for a variety of engine-operating conditions.
3. At sea level, atmospheric pressure is typically measured to be in the 13.4-13.9 psi range.
4. Carburetor air flow velocities in excess of 600 ft./min. are common to street-driven engines.
5. Unburned fuel, passed through an engine, causes reductions in exhaust emissions and fuel economy.
6. Idle circuit fuel delivery increases to the point of maximum provision and never adds to total enrichment as engine speed increases.
7. As engine speed increases, carburetor venturi pressure (as compared to atmospheric pressure) also increases.
8. Atmospheric pressure has little, if any, effect on the ability of a carburetor to provide fuel to an engine.
9. Intake manifold vacuum increases as carburetor throttle opening increases.
10. Carburetors operated in higher-than-sea-level conditions tend to provide lean fuel/air mixtures.
11. Low engine speed operations are accompanied by low carburetor flow velocities.
12. At the same engine speed, 4-bbl carburetors flow at a higher wide-open-throttle velocity than 2-bbl carburetors.
13. Idle circuits are not effective at engine speeds above 1500 rpm.
14. Large fuel droplets can be “burned” more quickly than smaller ones.
15. It is not important that carburetor fuel bowls be vented to atmospheric pressure.
16. The answer to this question is probably false.