 Many of these sources also suggest that modifications to the engine's air-fuel ratio, ignition timing, emissions control systems, electronic control systems and possibly other design elements, might be required in order to obtain any significant results.A modified vehicle in this way may not pass mandatory anti-smog controls.Due to the inherent complexity of these subsystems, a necessity of modern engine design and emissions standards, such claims made by proponents of hydrogen fuel enhancement are difficult to substantiate and always disputed.