Intake Flow Visualization

Ever wonder why dirt bikes’ adoption of EFI was delayed by nearly half a century compared with passenger vehicles? Why do so many rely on open-loop fueling strategies that often require remapping as the setup and conditions change? Part of the explanation can be seen in the intake-flow visualization above.

The simulation methods and computing power required to produce this level of detail were emerging technologies at the turn of the century. Now we can display the result casually in a browser. Each line represents about 22 mL of air at standard conditions flowing through the intake ducting of a simulated 450cc four-stroke dirt bike engine; its color represents air temperature, and its height represents pressure.

Try the different RPM and throttle positions in the dropdowns. Notice the following:

• In the slowed animation, flow reverses more than once per second—more than 300 times per second at real speed.
• Flow speed can exceed 200 mph forward and sometimes exceed 200 mph in reverse.
• Air from the cylinder often pushes back into the intake ports.
• The mass of air trapped in the cylinder and the power output can change from one cycle to the next, despite constant simulated RPM, constant simulated throttle position, and constant simulated combustion parameters.
• A 10% throttle opening produces more than 10% of the available power, and this throttle-to-power relationship changes with RPM.

This chaotic, slinky-like effect is strongest in single-cylinder and high-performance engines. The intake and exhaust ducting trade in the energy of these pressure waves to shape engine character and response. The same behavior that materially affects performance also complicates conventional EFI strategies, especially when hardware or operating conditions change.