| 123 / | HD | INTERNAL-COMBUSTION ENGINES |
| 434 | DF | CHARGE FORMING DEVICE (E.G., POLLUTION CONTROL) {29} |
| 536 | ![]() | .~ Combustible mixture ionization, ozonation, or electrolysis {3} |
| 537 | DF | .~.~> Before intake valve (e.g., in manifold) |
| 538 | DF | .~.~> Fuel only |
| 539 | DF | .~.~> Air only |