>Vendors’ compliance is not disputed, and you have produced no credible evidence of cheating. If there were demand for the smaller, more efficient engines that you’re fantasizing about, someone would already be building them with no legislative restrictions necessary.

Maybe I'm missing your point, but the vendors non-compliance is documented fact in a court of law. Multiple individuals were indicted and the company itself pleaded guilty to multiple felonies (including fraud). Further, as with most collective action problems of this type, relying on individual choice to correct the problem all but ensures it won't be corrected. This is one area where the extreme libertarian mindset (which seems to be overrepresented on HN) tends to miss its mark.