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"grams of CO2 per km" is a very strange way to rate an engine.

It makes sense for a complete car. But just an engine by itself? Without specifying the speed of the car, weight of the car, aerodynamic shape, type of fuel?

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They have a model to estimate various features of the car or they take it live from real cars, and deduce the resulting opposing force at speeds to counter-apply on the engine when bolted on a test bench.

I guess if you only have the engine to work on you don't have full data so you're comparing what you can.

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