But how do you define it then? A 1.0L 3-cylinder engine that emits 90g of CO2/1km that goes into 1 million cars is still more harmful statistically than a 7.0L V12 which emits 600g of CO2/km engine but only goes into 300 limited-edition ferraris. So maybe all engine engineers are "morally responsible" for air pollution, no matter what engines they build, or whether they lie or not?

It doesn't have to be that difficult, you can just think about these separately.

Is the V8 designer responsible? Yes if people die as result of pollution from his engines.

Is the 1L designer responsible? Same thing.

What about the scenario where the designer did nothing and people still bought engines by other makers? But this is a moral fallacy, "I'm relieved of the responsibility of doing <bad thing x> because another morally irresponsible person would seize the opportunity if I didn't".

What if the 1L engine displaces another engine product in the market that would result in more people dying. It's similar to tobacco companies introducing "light" cigarettes to market. "Lesser evil".