Super Charging Science

Supercharging: that mystical means of cheating atmospheric pressure as it applies to both fuel efficiency and outright power

There are several aspects of the basic internal combustion engine. Ways in which air entered and exited an engine operating under the influence of atmospheric pressure. Further more it was the “force” of atmospheric pressure that caused air and fuel to pass into an engine’s cylinders. And that “suction” in an engine was the absence of atmospheric pressure—or if we didn’t, we meant to.
Now we take a visit to the study of what happens to an engine when atmospheric pressure is replaced with some sort of mechanism that pumps or forces air (or air/fuel mixtures) into an engine at pressures higher than that available from normal aspiration (a fancy way of saying engine operation under the influence of atmospheric pressure). And we’ll include some of the mechanical ways such “pressurization” of an engine can be accomplished. First, let’s talk about the results of supercharging, regardless of method employed.

Increased cylinder pressure: Strip away all the theory and verbal footwork and this is the net objective. Pack more air and fuel into an engine’s cylinders, burn a high percentage of this mixture, and cylinder pressure will increase. Properly evacuate each cylinder at the conclusion of the combustion process and cylinder pressure will continue to be increased for subsequent firings of all cylinders. Fail to remove adequate amounts of combustion residue at the end of each combustion cycle and diluted air/fuel mixtures will not produce high levels of combustion efficiency. But that’s something previously discussed.
In terms of forcing air and fuel into an engine, mixture density will be increased. That is, air and fuel particles will be more rapidly oxidized (“burned” during combustion) if they are packed closer together at the time of and during combustion. This is a direct benefit of supercharging.
A satellite effect of such increases in mixture density is the fact that cylinder pressure (and precombustion compression heat of air and fuel) will be higher earlier in the compression cycle. For example, let’s assume that we first have a normally aspirated engine running at a given rpm and load. At a crankshaft angle of 90 degrees before top dead center on the compression stroke, there will be some amount of cylinder pressure. In part, this measured pressure will be a function of how much air and fuel passed into the cylinder during the intake stroke.
Now let’s consider the same engine, running at the same rpm and load, operating under the influence of supercharging. By virtue of increased air/fuel mixture in the cylinders during the intake stroke (the supercharger forces more air and fuel into the engine than atmospheric pressure), precombustion cylinder pressure will be increased. Once there is ignition and continued piston movement toward top dead center, effective cylinder pressure will be higher yet. Also, the effects of higher mixture density and, consequently, faster burning rates will further boost the pressure force on the pistons. Never mind the time you spun the rear tires on wet pavement in second gear; we’re now getting into the business of making honest-to-dollar-a-gallon-gasoline horsepower. And under the proper circumstances, there can be fuel economy benefits. But we’ll get into that in a few minutes.
So supercharging can increase effective cylinder pressure, which translates into more usable torque and overall power, but such increases in horsepower mean boosts in combustion heat. And since up to 40% of an internal combustion engine’s heat (power) is lost out the exhaust system, it would be nice if there was a way this lost energy could be retained for usable power. Turbo-supercharging is one accepted method that does this.
But regardless of exactly how we are planning to increase the amount of air/fuel mixture in an engine’s cylinders throughout a range of rpm, the key to increased power is volumetric efficiency. This is a term introduced in a previous Shop Series. It is a measure (or comparison) of how much mixture is passed into the engine vs. how much it could theoretically hold if there were no frictional losses or other flow restrictions. Stated another way, it’s the ratio of inducted mixture to measured piston displacement. Complicated, huh? Not really. But this is important, so consider it this way. At any given rpm, an engine is “receiving” some amount of air and fuel. Since it cannot receive as much as it could if there were no flow losses (to any one of a number of causes), it cannot be 100% “volumetric efficient” at all engine speeds. Superchargers help overcome this problem. They increase the amount of induction pressure so that during some span of engine rpm, the engine receives more mixture volume than it would under the conditions of atmospheric pressure.
For us dummies, this means that volumetric efficiency exceeds 100% and the engine produces power increases accordingly. What you should remember is relatively simple: The more air and fuel there are in a given cylinder at the time of combustion, the higher the cylinder pressure will be. And that’s power. Do something to the engine that ensures the combustion of a high percentage of available mixture and—that’s potentially good fuel economy. Now let’s talk about some of the terminology that relates to a fundamental understanding of any basic supercharging method.
First, if we consider the relationship between atmospheric pressure and the effects of atmospheric pressure and pressure provided by some form of supercharger on an engine, we can derive a term that indicates what the blower is doing relative to volumetric efficiency. For example, if we call “delivery pressure” the results of the supercharger and “inlet pressure” the effects of atmospheric pressure, a so-called “pressure ratio” is established between the two. Let’s say the engine is being operated at sea level (14.7 psi) with a boost pressure of 6 psi.

Higher pressure ratios (numerically) would indicate that at given atmospheric conditions, the supercharger was contributing higher boost pressures. And at the risk of oversimplification, this is a key measurement in examining both performance and selection of a particular supercharger, especially turbochargers.

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A couple of other terms will crop up in a few paragraphs, but it might be helpful to identify the basic types of contemporary superchargers before introducing any more characteristics.
Roots-type superchargers have been associated with automotive engines for many years. These are positive displacement pumps. That is, for each revolution of the pump’s impellers, there will be a volume of air moved that is equal (or nearly equal, depending upon pump efficiency) to the internal air capacity of the pump. A piston engine is a positive displacement pump. If it has a piston displacement of 350 cubic inches, it will displace (intake and exhaust) 350 cubic inches of air when each of its cylinders has been filled and evacuated once. But since such positive displacement pumps (engines) are not 100% volumetric efficient at all rpm, there will be some loss in efficiency somewhere in the rpm range.

Roots-type superchargers have the same characteristics. As you can note in the illustrations, two impellers typically mesh to move air into and out of the blower housing. Any leakage between impellers and housing causes some loss in pumping efficiency, a particular problem with the roots design but not so with the turbocharger. Roots blowers are driven by a mechanical link to the engine (belts, gears, etc.) and therefore absorb some degree of engine power in order to operate the blower. And due to the rather tight fit between impellers and housing, frictional heat is generated, changing both the dimensional fit of moving and static parts and temperature of air (or air and fuel) passing through the supercharger.
Particular benefits of the Roots (or centrifugal) blower include good control of low engine rpm boost pressure. As soon as the engine starts, there’s some degree of boost. And any change in rpm will be accompanied by immediate changes in boost. There’s no “lag” between throttle opening and boost increase.
Exhaust systems don’t require alteration as in the case of turbochargers, and there are no special oiling or pressure relief valves. But the Roots-type have some drawbacks. They’re noisy (if this is a consideration). Considerable heat is generated (heat lost to the engine) and not turned into usable power as in the case of turbochargers. Engine backfire can be a problem not completely solved by some sort of pop-off valve in the blower manifold. And backfires under conditions of high boost (especially with raw fuel in the blower housing) can bring July 4 around a little early.
Turbocharging, although not a panacea, solves many of the problems of a Roots-type blower while offering a few benefits worth considering. The functional design of a turbo-supercharger incorporates a shaft, one end of which carries a vane-type impeller called a “turbine.” The turbine is positioned so that exhaust gas from the engine passes over the vanes, causing the turbine to spin.

A. As an illustration of a “positive displacement pump,” this piston/rod/cylinder assembly shows how a specific volume of air will be displaced from the cylinder for each upward stroke of the piston. If the system is 100% volumetric efficient, a quantity of air equal to the physical cylinder volume (all air space above the piston at its bottom dead center position) will pass into the cylinder for each piston stroke downward. Assuming no leakage past the rings at any time of piston movement, the amount of air moved is solely a function of cylinder bore and piston stroke. B. This is a graphical view of a positive displacement supercharger into which air passes under the influence of atmospheric pressure, is compressed by the action of the impellers, and released on the discharge side with greater density and velocity than at the entry. Of basic Roots design, this method of air compression provides pressure ratios typically in the 1.0-2.0 range, above which blower efficiency drops sharply. This is due in part to the compressibility of air and because further increases in pressure ratio increase the amount of internal leakage (so-called “back leakage”) within the blower—a fancy way of saying the blower has “blow-by” between rotors and housing.

On the other end of this shaft is another vaned impeller that is positioned in the induction system so that a positive pressure (higher than atmospheric) can be produced on the engine’s cylinders. Since both impellers are situated on the same shaft, the rotational speed of one is the same as the other.
Exhaust gas, passing through the exhaust turbine side of the turbo-charger, spins the shaft, causing the inlet turbine (or compressor turbine) to increase the amount of air or air/ fuel mixture passed into the engine. In some cases, this positive pressure is applied to the entire induction system, carburetor and intake manifold. But in other applications, this pressure is introduced between carburetor (or injector) and the engine’s cylinders, creating a need for some sort of special back-pressure valving to prevent the carburetor from “flowing backward” under the influence of a pressure greater than atmospheric.
One particular advantage of turbo-supercharging is what this method of forced air induction does to the quality and condition of incoming air/fuel mixtures. For example, we earlier mentioned the benefits of high-mixture densities. Remember, the tighter air and fuel are packed, the more quickly and efficiently they tend to “burn.” Turbochargers, in fact, increase mixture density while increasing the amount of air and fuel delivered to the engine. And that spells both power and economy, particularly economy. For if an engine is receiving mixtures of high density and in greater volume than under the conditions of normal aspiration, less throttle opening will be required to sustain the same vehicle speed as without the turbocharger. And whether the engine is operating with or without a turbocharger, less throttle opening typically means less fuel consumption.
The other fuel-economy-related feature of turbocharging is what happens to liquid fuel as it passes through the inlet (or compressor) stage of the turbo. Operating at shaft speeds often exceeding five times engine rpm, the mechanical atomiza-tion (breakup) of liquid fuel particles is far superior to that of a conventional carburetor or fuel injection nozzle. This assumes location of the turbocharger downstream to the carburetor. Excluding any significant return of atomized fuel to liquid form by the time everything gets into the engine’s cylinders, the turbocharger breakdown of fuel stands to improve fuel economy. The reason is that small fuel droplets burn (oxidize) more quickly and efficiently than large ones. Add to all this the fact that many turbochargers can be installed within the confines of stock hood heights, and some of the attending engine alterations (such as the exhaust system) begin to diminish in importance.
There are two other critical areas with regard to turbocharging that should be mentioned, at least at the level of pure basics. One is the sensitivity of a turbocharger to exhaust system backpressure. This includes such factors as muffler flow efficiency at engine speeds where boost is produced. For as backpressure increases, the turbo becomes more of a heat pump (and less efficient), turbine speeds can decrease, less air is passed into the engine, basic carburetor mixture calibration can be upset, and power lost. The sensitivity of a turbocharger to even see backpressure created by improperly sized exhaust head and tailpipes is also widely recognized. So the mere installation of a turbo without consideration of exhaust system flow capability relative to engine size (displacement) and rpm is both foolish and expensive. Systems that are factory-designed (even aftermarket units) to include the exhaust plumbing (or come with dimensional recommendations) should be used to the letter.
The other area of some criticality has to do with detonation. Remember our discussions about this condition which amounts to spontaneous and uncontrolled combustion, often resulting in engine damage if allowed to continue. The use of water injection with a supercharger (especially turbo-superchargers) is required when boost pressure causes cylinder pressure to exceed the practical limits of combustion for the fuel being mathematically, the pressure ratio would be used.

C. Schematically, this is the way air moves into the turbine housing (exhaust side of the turbocharger) and out the exhaust system, having caused the turbine to rotate at a very high rpm. Then, because they are connected on a common shaft, the turbine end of the unit begins to spin, causing air to be drawn into the turbine housing and passed into the engine’s induction system. Turbine, compressor and the housing design for each are critical to how much air is moved, how fast the system comes to rpm, and other variables that cause design sizing of these components to be a science unto itself. Right, Hugh? D. General layout of turbocharger system showing typical location of waste gate. This is intended to show that many such installations sense intake manifold pressure (level of boost) which, in turn, is used as a signal to relieve some amount of turbine (exhaust) pressure so that a predetermined level or maximum boost point is held. In some race cars, this boost level can be adjusted from within the vehicle. Over-the-highway systems do not normally include such control. But boy, does it help make power!

Since most supercharged engines are built with mechanical compression ratios on the order of 7.0-8.5:1 (to allow for boost pressure to increase mechanical efficiency of the engine), there comes a time when either load or boost (or both) can place an engine in detonation, a condition relatively easily solved by the injection of water. This provides an amount of intercooling during combustion, thereby preventing excessive heat from combustion pressure causing spontaneous ignition of all remaining mixture after normal combustion has begun.
Should some limit to boost be necessary, a pressure-relief (blowoff or “waste gate”) valve can be installed on either the exhaust or intake side of the turbocharger. This allows excessive exhaust or intake (boost) pressure to be relieved when a predetermined level of pressure or boost is reached. In turbocharged race cars, this is often a manually operated valve which gives driver control of how much on-track power the engine is making.
From all the discussion this month (and it was almost too big a subject for one little Shop Series), there are some essentials you may want to keep jn mind. First of all, there are two frequently used ways to increase an engine’s volumetric efficiency by the methods of “artificial pressuriza-tion.” One is a positive-displacement blower that is engine-driven, absorbs some amount of net power, and is both rpm and boost-limited with respect to power increases at boost levels above 12.5-14,0 psi. The other does not require any mechanical link to the engine. But by the utilization of high-velocity exhaust gas passing across a finned impeller (whose design and housing size are a function of engine piston displacement and rpm range), fuel atomization, mixture density and increased volumetric efficiency are improved from the use of otherwise lost exhaust heat and pressure.
What it all boils down to can probably be summarized in this little analogy. Must of us have blown up a rubber balloon and released it into a room. The more air the balloon contained, the longer it stayed in the air. If you equate the balloon’s air time with the amount of air contained, you’ll see how an engine filled with more air than atmospheric pressure can provide will show increased power. As the Shop Series for this month suggests, you started with an empty balloon, but “forced air induction” caused it to have more “power.” If you don’t believe us, try holding an empty balloon open to atmospheric pressure. Supercharged balloons may not be worth much, but the principle sure applies to your favorite brand of engine. In fact, Sir Dugald Clerk would agree. But then again, who was . . .?

REVIEW QUESTIONS: True or False
1. Although not identified here, Sir Dugal Clerk first used a form of supercharging in the early 1900s (now you’ve got to make a trip to the local library, right?).
2. Large fuel droplets burn much more quickly than small droplets.
3. Roots-type superchargers tend to increase air/fuel mixture density more than turbosuperchargers.
4. Turbochargers, because they employ exhaust gas flow rates for boost pressure, cause increases in air/fuel mixtures as compared to Roots-type superchargers.
5. The higher the air/fuel mixture density, the slower the “burning” rate.
6. Engine backfires in a Roots-type supercharger system are much less a problem than with a turbocharger system.
7. In a positive-displacement supercharger, the greater the pumping efficiency the lower the cylinder pressure.
Positive-displacement superchargers enable an engine to operate at 100 percent volumetric efficiency throughout the entire rpm range.
Pressure ratio is a numerical comparison of how much mixture would pass into the engine (under conditions of supercharging) as boost is varied.
10. A positive-displacement pump is certain of what it is doing.
11. Cylinder pressure has little or nothing to do with a given engine’s ability to provide good fuel economy.
12. Supercharging an internal combustion engine means providing more air/fuel mixture than would be available under the influence of atmospheric pressure.
13. In the normal course of engine operation, as.much as 10% of combustion heat is lost to the exhaust system.
14. Supercharging is a method of applying self-imposed extensions of credit card application … for the purpose of improved engine construction.

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