Ram-Air Induction
NEW FOR '93 RAM-AIR INDUCTION
KAWASAKI'S SPEED SECRET?
KEVIN CAMERON
WHAT, YOU MAY BE WONDERING, is Kawasaki's Suzuka 8-Hour racebike doing with a huge hole in the front of its fairing? Well, in effect, that hole is a simple compressor, without moving parts, whose function boosts airbox pressure and, with it, power. The gain is small but significant at racing speeds. Called ram-air induction, this concept was first used in 1990 by Kawasaki on the ZX-1 1, and similar systems are now employed on the company's new-for-'93 ZX-6, ZX-7. ZX-7R and ZX11.
Ram-air induction takes advantage of Bernoulli's Principle, which states that as air is accelerated, its pressure falls, and as it is decelerated, its pres sure rises. The classic example is flow through a carburetor venturi. The flow, accelerating through the venturi, loses pressure, and this low pressure is what causes fuel to be drawn up from the float bowl, through the nee dle jet and into the airflow. Now turn that venturi inside-out so that the flow streams around an object moving through it. To pass around the ob ject's bulk, the flow must accelerate, losing pressure.
The accompanying illustration of this flow displays the streamlines so beloved of textbook writers. But what happens to the streamline that hits the object front and center? This stream line (and those nearby it) stops. Ac cording to Bernoulli, the air therefore converts all its kinetic energy into pressure in that central region, called the stagnation zone.
Ihere are ways to determine the pressure generated here from the ob ject's speed. One is to use a formula to calculate the pressure; the other, easier way is simply to refer to a sci entific table, called "Pressure of Air on Coming to Rest from Various Speeds," published in numerous engi neering handbooks.
As we move away from the stagna tion point, the oncoming flow is slowed but not stopped, so it gener ates positive pressure, but not so much as develops at the very center. Moving farther around the nose con tour of our object, we come to a re gion in which there is neither gain nor loss of pressure. Beyond that is the re gion of negative pressure, where the flow is accelerating around the width of the object.
Now comes the question: Where shall we locate the air intake for our bike's carburetor airbox? In the old days, the intake snout was located under the seat, but this just sucked in hot, expanded, low-density air fresh off the engine's egg-frying cooling fins or from its radiator. To escape that, and gain the power available from cooler, ambient air, air intakes were relocated to the shoulders of the fairing-approximately where pressure is neither higher nor lower than atmospheric.
But designers of racebikes are greedy individuals, tempted to use any sly device to eke out even a tiny advantage over one another. They read scientific tables, too, from which they learn that if they could feed their engines air at full ram or stagnation pressure, then at, say, 175 mph, their airboxes would gain pressure to the tune of about six-tenths of a pound per square inch. That translates to al most 4 percent more horsepower.
The bad news is that ram effect is proportional to the square of vehicle velocity. This means the effect really drops at lower speeds. On a streetbike operated at legal speeds, the compres sion obtained in the fairing's stagna tion zone is tiny-too small (just six-hundredths of a psi at 55 mph) to make a difference. But up at a race bike's top speed, which often exceeds even 175 mph, the ram intake can make 10 times that gain.
How much speed advantage is there in 4 percent extra power? Well, the force needed to overcome air drag rises as the square of the speed, so the power needed (power is force times speed) rises as the cube of speed (setting aside rolling friction for the sake of simplicity). The speed advantage factor in our 4-percent ram-intake power gain therefore is the cube root of 1.04, or 1.013 per cent. That, multiplied times 175 mph, yields 177.3 mph-a gain of 2.3 mph. That might seem only a leisurely walking pace; but, as racers so often say of a faster machine, "He walked away from me on top end." An ad vantage is an advantage.
If the ram-air passage is made too small, the engine's demand for air will turn the passage into a venturi, pulling the pressure down and reduc ing the gain achieved. Such was the case with the smallish passage on one privately built Superbike racer, which achieved only about two thirds of the maximum possible ram pressure at 160 mph. No wonder the intake opening on Kawasaki's 8-Hour bike is so big.
A favorite misconception is that the best intake is a funnel, oriented with its large end toward the oncom ing flow. What we want to do is de celerate moving air smoothly, converting its energy into pressure. But the funnel does just the opposite: It accelerates air through its venturilike narrow end, causing pressure loss. Therefore, a workable ram-air intake is simply a good-sized, round edged opening in the stagnation zone, ducted at large diameter all the way to the engine. This keeps the air flow at low velocity so its pressure remains high.
As a ram-air-equipped bike acceler ates, its airbox pressure rises-just as though you were riding down a moun tain into ever-denser air. And, just as in the case of zooming down the mountain, your engine is getting more air without also getting more fuel.
This is a potential problem, be cause carburetors measure pressure difference, not absolute density. Ram-air therefore causes a slight, high-speed lean-out. And the effect is worse if the float-bowl breather lines are not routed to the airbox along with the intakes. Fuel delivery is based upon metering signal-the pres sure difference between float-bowl pressure and venturi pressure. If air box pressure rises, that raises venturi pressure, too. If float-bowl pressure doesn't rise in proportion, the fuel metering signal will decrease and so will fuel delivery.
There are a number of ways to compensate for this high-speed leanout, but not all are acceptable. Sim ply enriching carburetion will make the bike sluggish at lower speeds. When operating near top speed. however, carburetors are calibrated to deliver an enriched peak-power mixture anyway-maybe enough to cover the4 percent.
If that doesn't work, there are other options. The neatest is to use fuel in jection, which has an air-density sen sor that will take care of the problem automatically. A second method is to connect the air-correction jets (which are there to prevent carburetion from becoming ever-richer as airflow in creases) to some pressure lower than what's in the airbox. As airbox pres sure rises, progressively less correc tion air will be supplied, causing a compensating enrichment.
For street motorcycles, ram air has damned little significance, unless ei ther (a) you spend a lot of time in close competition up beyond the val ley of the ultra-speeding ticket, or (b) you just like the look, and the idea, of having what the racers have. The real, highway-speed value of Kawasaki `s intake scheme lies in positively pre venting any heated, low-density en gine-bay air from entering the carbs. Up to now, the so-called cold-air sys tems on sportbikes have either been functionless dummies or have simply dumped their puny flow of cold air near the airbox, and have not been its sole supply.
With its ram-air system, Kawasaki has changed all that.
AIRFLOW AROUND A MOVING OBJECT
