Volumetric Efficiency
There’s a little chart included that typifies this engine speed vs. gas flow velocity. And a few extra minutes spent in understanding this relationship would be time well spent. You’re gonna hear about it again and again. Because it works.
And finally, we’d like to refer to a term first introduced in an earlier Shop Series: volumetric efficiency. Hopefully (if you passed the test that month), you’ll recall that this is a measure of cylinder filling efficiency. And since exhaust system efficiency (and operating pressures) can have an effect on volumetric efficiency, we thought a little discussion of how this happens might be helpful.
First, we know that exhaust gas residue left after blowdown is hotter than the incoming air/fuel charge. This results in a slight loss of volumetric efficiency on the order of 5-8% at wide-open-throttle operation. The assumption here is that the intake and exhaust gas pressures are about equal or that the relationship (ratio) of the two is about 1.0 (unity). But as the throttle is closed slightly, this ratio is changed in favor of greater intake pressure than exhaust, resulting in some gains in volumetric
efficiency, as intake pressure remains higher than exhaust backpressure. Okay. Now let’s slide back to the two sections of the exhaust system. The plan is now to include these terms and concepts as we work our way from the rear of the system to the front; sorta like we tend to write these stories—backward.
First comes the muffler. About one-third of the heat produced (or released in the engine) passes out into the exhaust system. A substantial amount of this heat energy is released during cylinder blowdown, so there is plenty of this energy available to set up high-level sound disturbances. Like noise. Exhaust gases have been measured to leave a combustion chamber at sonic flow rates (more than 1000 ft./sec), resulting in pulses of sufficient amplitude to require silencing by some sort of muffling device. Three basic types of mufflers are shown in illustration. But regardless of the method of silencing, the approach seems to be breakup or amplitude reduction of the high-pressure disturbances moving through the exhaust system.
Of course all this needs to be accomplished with the least amount of backpressure buildup back at the engine. And speaking of “back at the engine,” suppose we return there and examine the types of exhaust manifolds (or headers) in common use today.
At this point (even though you didn’t know it), we return to our “classroom engine” and find that it is a single-cylinder, 4-stroke-cycle design. There is one exhaust pipe leading from the exhaust port to atmospheric pressure at the open end of the pipe. The pipe is constant inside diameter (i.d.) and fixed length, so we know that gas flow velocity will be a function of engine rpm and piston displacement.
Since we have already established that at “critical velocity” the exhaust gas passing from the engine will experience a resonant condition (peak torque output), a test run of our engine shows a single torque peak at 3200 rpm.
E. The addition of a second “volume” (collector) at the end of a primary pipe (or pipes) tends to increase the amount of torque produced below the so-called high-rpm torque peak. F. A change in primary pipe i.d. tends to shift the rpm point at which peak torque is produced. Note that the basic shape of the curve remains unchanged, while the shift upward in rpm (based on an increase in i.d. size) takes place.