The Compression Ratio: Plain And Fancy

THE COMPRESSION RATIO: PLAIN AND FANCY

Wherein, "Corrected" Compression Ratio Is Exposed For What It Is, A Marketing Gimmick.

COMPRESSION RATIO IS one of the basic parameters of engine design and performance. It interacts with fuel type and octane, with combustion temperatures and pressures, with maximum power and the spread of power at different rpm, with the speed and smoothness of burning, with port and ignition timing, with combustion chamber geometry, and with the chemical composition of the exhaust—a major consideration in these days of concern about air pollution. Still, compression ratio is only a guide, only one factor among many. Precisely because all these factors interact, they must all be considered together. Understandably, compression ratio is not a sacred jewel of information to be studied and admired in splendid isolation from other engine parameters.

Ideally, a higher compression ratio gives more power and greater thermal efficiency, but other factors intervene to limit this effect. Excessive compression may cause detonation (knocking, pinging), or the combustion chamber may become too crowded, impeding mixture flow and actually reducing the power output. For a given engine, the best gasoline is the one with an octane rating just high enough to prevent knocking; higher octane gas does not improve power and, according to some studies, may even slightly reduce it.

Compression ratio is determined by geometry, as are stroke, bore, displacement, carburetor venturi diameter and piston crown area. But power—the sole object of the game—depends in a complicated way on dynamics, chemistry, materials and aerothermodynamics, and only indirectly on geometry. Obviously the relation between geometry and power is at best approximate. Even so, geometric ideas are useful for organizing observations, for describing normal design practice or variants therefrom, for correlating engine design to engine performance, and for predicting the likely effects of modifications. Most importantly, a wisely selected geometric quantity is easy to measure and easy to understand.

In an attempt to improve the correlation between compression ratio and other engine parameters, the Japanese manufacturers of two-stroke motorcycles have for some years been using a new quantity which they call the "effective compression ratio."* Compression ratio and effective compression ratio are very different things, though under certain conditions it is possible to relate them, one to the other, as will be shown. Both ratios may be computed for the same engine, at least if the engine is a conventional Japanese two-stroke. A motor with a compression ratio of 11 may have an effective compression ratio of 7, so you should make sure you know exactly what a listed number means. Manufacturers' literature and magazine road tests are not always clear about which is intended. At the present time you can expect that data on Japanese two-strokes will give effective compression ratio,

*What is here called "effective" compression ratio is the same thing as "corrected" or occasionally—for a maximum of confusion—''actual" compression ratio. Calling it "actual" compression ratio is like calling the ",actual" height of a tree the length of its shadow.

even if it's not identified as such, while compression ratio will be specified for all other kinds of engines. It looks like this situation is going to continue, so it's worth understanding the relation between "compression ratio" and "effective compression ratio."

COMPRESSION RATIO DEFINED

Compression is the thermodynamic process of squeezing a fluid to increase its density, which is mass-per-unit volume, If enough is known about the fluid and the process of compres sion, and if certain assumptions, approximations and idealiza tions can be justified, it is possible to calculate the thermody namic conditions, or "properties," after compression, given those at the outset. For example, finding the final pressure from the initial temperature and pressure.

J.G. KROL

One common assumption is that the amount, or mass, of gas is constant throughout the compression. Then we can say:

The ratio of volumes is defined to be the compression ratio R. Sometimes we can't do this, as in a gas turbine engine. Compression takes place, sure enough, but with a continuous flow of air. So jet engine designers talk in terms of pressure ratios (which they can measure), not in terms of compression ratios (which they can't even define). The pressure ratio and compression ratio are equal only in special situations: "isothermal" compression of an "ideal" gas.

In defining compression ratio R for an internal combustion engine, the traditional way assumes that the pertinent ratio of volumes is:

This definition applies to any internal combustion engine that does have identifiable maximum and minimum working volumes, which means it applies to everything but continuousflow engines like jets and rockets.

For engines with reciprocating pistons, the minimum volume is that above the piston at top dead center and is called the clearance volume C. The maximum volume adds to C the cylinder displacement D and occurs with the piston at bottom dead center. Therefore:

Displacement is defined as the stroke S times the cross-sectional area of the cylinder which, for round cylinders, is 0.785398 times bore times bore. For other engine designs, like a Wankel, R depends on displacement and clearance volume in exactly the same way, though, of course, we don't have the same interpretations about tdc, etc., and D and C have to be calculated in the appropriate way.

EFFECTIVE COMPRESSION RATIO

The effective compression ratio is defined for two-stroke piston engines with exhaust and bypass ports on the cylinder wall and exhaust duration greater than bypass duration. As used by Japanese motorcycle manufacturers, it is:

This is the same as the definition of R except that "effective displacement" DE replaces "displacement" D. Effective dis placement is defined as the "effective stroke" SE times (as

before) the cross-sectional area of the cylinder. Whereas stroke S is the piston travel from bdc to tdc, the effective stroke SE is the piston travel from the point where the exhaust port closes up to tdc. In other words:

The reasoning behind this definition is that compression can't really begin until the exhaust port closes, since, until that happens, the mixture can flow out the exhaust. We assume here that the exhaust is the last port (or valve) to close. It should be clear that the relation between R and RE depends somehow on the relation between S and SE.

RELATION BETWEEN R AND RE

The two definitions can be combined by solving each for clearance volume, then equating the two expressions for C. This gives the basic relation between compression ratio R and ff~t~tiv~ cnmnr~ccinn r~th-~ RF~

Suppose you know compression ratio and the ratio of effective stroke to stroke. Then the effective compression ratio is given by:

For example, a European motorcycle has a listed compression ratio of 11 and SE/S = 4Omm/7Omm. Its effective compression ratio is:

On the other hand, suppose you start with the effective compression ratio. Then the compression ratio is:

For example, a Japanese motorcycle has a listed compression ratio-actually an effective compression ratio-of 6.8 and SE/S = 38mm/6Omm. Its compression ratio is:

Since effective stroke is always less than stroke, it follows that effective compression ratio is always less than compression ratio. As you can see from the examples, which are realistic, the difference in the sizes of the numbers can be considerable.

For typical high-performance two-strokes a 1 percent increase in R (holding effective stroke constant) gives a 0.9 percent increase in RE. Another way of saying this is that a one-point increase in compression ratio gives a 0.6-point increase in effective compression ratio. If you leave the compression ratio alone, but increase the exhaust duration, you will reduce the effective stroke. Typically, a 1 percent decrease in effective stroke gives a 0.8 percent decrease in effective compression ratio. These figures are only intended as a guide to the sensitivity of one quantity to another.

EXHAUST DURATION/ROD LENGTH

Equations (1a), (1b) show that you must know SE/S in order to relate compression ratio and effective compression ratio. SE/S varies from less than 0.5 for a highly tuned racer to as much as 0.7 for a mildly tuned trials bike. Since the effective stroke is rarely specified, you have to take off the cylinder head to measure it.

If you know or can measure or can estimate the exhaust duration in degrees, you can find the effective stroke ratio from:

where L is the center-to-center length of the connecting rod and E is the exhaust duration in degrees. All measurements of length are in the same unit, whether inches or millimeters.

Don't know the con-rod length? Don't fret. In the first place, the ratio of rod length to stroke does not vary much among different engines. Assume L/S = 1.75 and you won't be far off. In the second place, the first two terms tend to cancel out the value of L/S so that SE/S depends much more on exhaust duration E than on L/S. Typically, a 1 percent error in estimating rod length gives only 0.16 percent error in SE/S, i.e., only one-sixth as much. If you know the con-rod length, by all means use it, but if you don't, the error is negligible in assuming L/S = 1.75.

Equation (2) is rather formidable but it can be solved easily by means of the graph. Read over from the exhaust duration to the appropriate L/S curve, then drop down to find SE/S. The dashed lines show that an engine with E = 155 deg. and L/S = 2 has an effective stroke that is 67 percent of the full stroke.

Knowing SE/S you can use the R-lines and the right-hand scale of the graph to relate R and RE. To find RE, given R = 10, go up the dashed line from SE/S - 0.67 to the R = 10 line, then go over to an effective compression ratio of 7.05; or you can use Equation (1a). To find R given SE/S and RE, locate the intersection of the vertical through SE/S and the horizontal through RE. Since this won't often fall exactly on one of the R-lines, use the hashmarks around the edges to interpolate. Each hashmark represents 0.2 points of R. Connect them with a straightedge. Or you can simply use Equation (1b).

The difference between R and RE depends on the ratio of effective stroke to stroke which, in turn, depends on exhaust duration and con-rod length. If all engines had about the same

exhaust duration and compression ratio, you could use a rule of thumb like add 3 points to RE to get R." Actually the difference between R and RE ranges from less than two points for very mildly tuned engines up to more than eight points for full banzai engines. When you have nothing else to go on, you can begin by estimating exhaust duration based on your general impression of the motor's state of tune:

160 deg. — very mild 170 deg. — mild 180 deg. — brisk 190 deg. — hot 200 deg. — wild

Then with L/S = 1.75 look up the value of SE/S, and you're in business.

SUZUKI/MAICO COMPARISON

The July 1971 Cycle gives measured effective strokes for Maico and Suzuki motocrossers, For the Maico 400 the figures are:

SE = 1.828 in S = 3.27 in R = 12

Calculate SE/S = 0.559 and read up from this value to R = 12; go over to an effective compression ratio of RE = 7.2. While you're at it, read up to the L/S = 1.75 curve and over to an exhaust duration of 183 deg.

The numbers given for the Suzuki 400 are:

SE = 1.614 in S = 2.95 in R = 6.5

The vertical through SE/S = 0.548 intersects the horizontal through RE = 6.5 at a compression ratio of R = 11.0; also, going up to L/S = 1.75 indicates an exhaust duration of 185 deg.

The Maico has a much higher stroke/bore ratio than the Suzuki, but these two engines are otherwise surprisingly similar. Their exhaust durations, as close as I can estimate them, are the same. The Maico compression ratio is only slightly higher (12 vs. 11), and its effective compression ratio is also just slightly higher (7.2 vs. 6.5). This is a completely different story than told by comparing the Maico's Usted 12 (compression ratio) to the Suzuki's listed 6.5 (effective compression ratio).

YAMAHA OR BULTACO?

Another example is provided by a recent travel article by a chap who was correctly concerned about the erratic quality of gasoline in Baja California. One of the reasons he chose a Yamaha DT-1 (RE = 6.8) over a Bultaco Matador (R = 10) was that he expected the former to tolerate lower octane gas because of its lower "compression ratio." Evidently he did not realize that he was comparing unlike quantities. To get a true comparison we can assume exhaust durations in the mid-160s for both of these mildly tuned engines. Then you can see from the graph that there is not a grapeskin of difference between them if compression ratio is compared to compression ratio (or if effective compression ratio is compared to effective compression ratio). The manufacturers' specifications have to be put on common ground before a sensible comparison can be made.

HODAKA HOP-UP OR HOP-DOWN

A third example will be of interest to do-it-yourself engine tuners. To advance the exhaust and bypass timing on motors without removable cylinder sleeves, the simplest approach is to make an aluminum plate to slip under the cylinder casting. A 1/8 in. spacer lifts a Hodaka cylinder about 3mm, extending exhaust duration from a nominal 164 deg., 46 min. to 179

deg. Milling 3mm off the top of the cylinder returns the compression ratio to its original value of 10. Effective stroke, however, has been reduced from 64 percent to only 58 percent. Therefore, the effective compression ratio has been reduced from 7.4 to a mere 6.2, which seems more like souping down than souping up. To restore the original RE = 7.4 you'd have to mill the top of the cylinder 4mm, not 3; in which case, the new compression ratio would be 12. A 3-mm cut restores the original compression ratio, a 4-mm cut restores the original effective compression ratio . . . but you can't have it both ways.

WHAT'S IT ALL ABOUT?

Now that you are on your guard to the existence of two different kinds of compression numbers, and can convert one to the other as necessary, you are probably wondering which is "better," R or RE. Why doesn't everybody use the same definition and put an end to the confusion? Let's dope out the two contenders.

In this corner is compression ratio R, the reigning champion for more than a century. R is easy to understand and reasonably easy to measure. It is purely geometric in nature and never pretended to be anything else. Its definition can be applied to two-strokes, four-strokes and diesels; to naturally aspirated and to supercharged engines; to configurations with poppet, sleeve or rotary valves. Compression ratio makes sense for conventional pistons, opposed pistons, split-pistons ( twingles ) or rotary pistons (Wankels, etc.). In fact, it makes sense for any kind of engine that has an identifiable maximum and minimum working volume, being but the ratio of the two. Its one weakness is that you must interpret it in the context of other engine variables like fuel octane, ignition timing and, notably, port timing.

And in the other corner we have effective compression ratio RE, the promising young challenger from the Orient, in top condition from millions of miles of roadwork aggregated by hordes of Japanese two-stroke motorcycles plying the freeways and byways of the world. What RE gives up in generality and simplicity it gains back in the area of its specialization for conventional two-stroke motorcycles. By making a correction for port timing it more nearly approximates the thermodynamic ideal of compressing a fixed quantity of gas. Mixture can escape out the exhaust until the port closes, so compression can t effectively begin until this event has occurred. This is the basic argument in favor of RE.

Now let's pull a dirty trick in the infighting and hope the referee doesn't spot it. Let's wholeheartedly accept the basic argument in favor of RE, apply it wherever we can, and see where we are led. If we are led to penetrating insights, to useful analytical or practical results, or to unexpected understandings, we ought to put our money on the challenger, RE. But if we are led to impasses, paradoxes and absurdities . . . well, we'd better stick with the champ.

If we accept the ideal of dealing with a constant mass of mixture, we'll have to do something about leakage past the rings. Phil Irving, designer of the legendary Vincent V-Twin, argues that leakage is a big enough factor that a short-stroke multi-cylinder engine inevitably has less low-speed power than a long-stroke single of equal displacement. The reason is, the former has greater area of ring/cylinder seal through which leakage can occur. If we insist on constant mass and want to continue with a geometric parameter, we'll have to correct RE for the circumference of the cylinder and for the engine rpm—which governs the length of time over which leakage can take place. Got any ideas?

Combustion releases tremendous heat and puts an end to simple isentropic, or even adiabatic, compression models. This is recognized not only in practice, but even in the most abstract and idealized thermodynamic analyses. Surely, then, we should count the end of compression at the point of ignition-some 3 to 4mm before tdc. Now the effective compression ratio will change with spark timing. This is, it seems to me, entirely consistent with the reasoning behind RE, which is more nearly to approximate the thermodynamically ideal compression process.

YOU GOTTA HAVE FAITH

Next, if we really believe in RE, we must reevaluate four-stroke engines so their compression ratio is figured from the piston position at the instant the intake valve closes. Granted, some funny numbers will result. A 1500-horsepower supercharged Chrysler fuel dragster engine could wind up with a rated compression ratio of 2! But you've gotta have faith.

Let s go on. Since DE is to RE as D is to R we should, if we choose RE instead of R, also choose DE instead of D. Instead of classifying the Suzuki 400 on the basis of its 396-cc displacement, we should rate it according to its 217-cc effective displacement-putting it well below the 250cc class limit. Go argue that with the AMA rulesmakers!

How about this one? Over the years Sears has sold many thousands of Puch-built "twingles," plenty of which, I'm sure, are still vibrating their way along at a heady 40 mph. The difficulty is that, on a split-piston engine, the exhaust opens before the bypass—as usual—but it also closes before the bypass. This was a hot design idea in the days before-Schnurle porting and expansion chambers. Now what do we do? Does compression "effectively" begin when the exhaust closes? Or does it effectively" begin when the last port closes, in this case the bypass? You tell me.

The same difficulty arises with certain large aircraft, tank, truck, locomotive and stationary two-strokes built by such respected names as Napier, General Motors and Junkers. The exhaust closes before the bypass does. This also happens— though not by intent—when you install the liner backwards in an old-fashioned crossflow style two-stroke. They'll usually run, poorly, with the exhaust and bypass timings interchanged. It strikes me that anything we do to RE in order to apply it to such cases has about it a most unsatisfying air of the makeshift.

But the most significant difficulty with effective compression ratio is that its fundamental premise just isn't true . . . not in an era when expansion chambers festoon everything from mighty motocrossers to miniature minibikes. Consider what an expansion chamber does.

As the ports open, the expansion chamber creates a low pressure wave that sucks the burned mixture out of the cylinder and pulls a fresh charge into it. Some of the incoming charge passes clean through the cylinder and flows into the header pipe. As the ports close, the chamber's resonance pushes this extra mixture back into the cylinder. Thus there could well be more mixture in the cylinder when the exhaust closes than when the piston was at bdc. This is exactly contrary to the notion that gas will leak out the exhaust until it closes. Furthermore, at rpm above and below the expansion chamber's design point, the phasing of the pressure pulses will not match the port events. As the exhaust approaches closure, mixture may not merely be leaking out, the pipe may be actively sucking it out. So as the piston rises from bdc to the point of exhaust closure, the amount of mixture in the cylinder may increase, decrease, or even stay the same. This is in the same engine at different rpm. So what is the

justification for effective compression ratio?

"RE" IS A SALES GIMMICK

There is, I'm afraid, no technical justification at all. Being a geometric quantity, RE cannot describe dynamic, chemical and aerothermodynamic effects any more than ordinary compression ratio can. We must still consider those other factors. RE is more complicated and less general than R. If we take effective compression ratio seriously, we are led into a morass of absurdity. So what is the justification for effective compression ratio?

There seems to be only one justification for it: the Japanese two-stroke makers adopted RE purely as a marketing device, which is a nice way of saying, a sales gimmick. Apparently there was some concern that as engine technology advanced, the standard definition for compression ratio was giving numbers high enough to frighten away potential buyers, especially those who lived in countries where high octane gasoline was not available. So the salesmen decided to "help" the buyers by giving them a number less likely to evoke concern with detonation—that is, a smaller number. Engineers quickly devised a definition that would provide smaller numbers. This is wholly in line with the grand tradition of dreaming up ways to correct power so as to get the largest possible number of horsepower. Evidently the plan is working, as witness the rider who bought a "low compression" DT 1 because he feared the "high compression" Matador wouldn't run as well on Mexican back-country gasoline.

And I wonder how many amateur tuners have helped themselves to a holed piston because they figured they could mill about half a yard off the cylinder head of their Japanese two-stroke and still not exceed the compression ratios that work so nicely on a Maico or a Pursang. Of course, you wouldn't make that mistake, would you?

The main objection I see to effective compression ratio is that the moment you start cooking up special definitions for special cases there is no place to stop. If you're willing to accept an "effective" compression ratio for two-stroke motorcycles, why not an "effective-sub-1 " for diesels, an "effectivesub-2" for asymmetric port timing, an "effective-sub-3" for four-strokes? Obviously the ratio of maximum to minimum cylinder volume doesn't tell the whole story. Neither does spark advance or carburetor area or stroke/bore ratio or piston speed or any other single number. Obviously compression ratio is not perfect. Neither are women. But they'll do.

R has to be interpreted in light of the other characteristics of the engine, that's true, but at least it's consistently defined for all engines of all types. As soon as individual manufac turers, or groups of manufacturers, start changing the basic definitions to suit their supposed convenience, you, the buyer, lose whatever small chance you had at making a wise comparison of products. Rotsa ruck. (Öl