Archive for category g. Air tank and bypass valve

Air tank and bypass valve

The air tank on the bike serves multiple purposes. Beyond just acting as a storage tank for air, it buffets the air pulses coming out of the motor. Because the compressor pulses on an off, the air stream exiting is more sinusoidal.  Also, the intake  on the motor only opens for part of every other stroke, so the inlet creates pressure waves as well. To prevent this, typically a motor will use an intake plenum to buffet these intake pulses. Packaging constraints on the motor prevent the use of an intake plenum here, however, with proper sizing and placement, the air tank can satisfy these needs. Further, the use of an intercooler offers an opportunity to remove heat from the intake stream, offering higher thermodynamic efficiencies and as a result, more power and improved fuel efficiency.

Figure IC1: Intercooler placement

Figure IC1 shows the intercooler, a standard part from a Mazda Millenia sedan, in place on the frame under the seat. The positioning is a bit of a trade off. It places the air tank in close proximity to the motor intake, so it is usable as an intake plenum, however, air flow across the heat exchanger is reduced. Eventually, ducting will be added to deal with this air flow concern. This particular intercooler was used due to its small size. To act as an intake plenum, an ideal internal volume of 3 times the cylinder displacement is needed. The Millenia intercooler is a bit larger than this value, but not significantly. It is also the smallest non custom intercooler available at a reasonable cost.

The compressor system on this motorcycle creates pressure any time the motor is running. Because of this, there will be times when the throttle plate is closed and the motor is not consuming air, where this constant air build up would eventually cause problems. To avoid this situation, a blow off valve was installed on the charge pipe between the intercooler and the intake. This valve operates based off a vacuum signal from the motor intake. When the throttle plate closes, vacuum is applied to a diaphragm on the top of the blow off valve. This causes the valve to open and vent out from the pipe.

Figure BV1: Blow off valve in place

Figure BV1 shows the final placement of the blow off valve. This placement is the only place inside the frame perimeter the valve could fit. Also, putting the valve near the intake allows for a constant velocity flow during gear changes. At this time, the throttle plate is shut with the motor still spinning at high rpm. Without the blow off valve, air flow velocity would come to a stop. With the blow off valve, the only change in air velocity flow occurs in the short section of pipe between the blow off valve and the intake.


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