Engine Energy Science

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.

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.