It's not a flat-6 that's for sure. I noticed it's a consistent icon in all cars. Sometimes it's got a lightning bolt through it, but other than that it's always the same.
So, which real life engine does it most resemble by it's outline? I figure it's something from the 70s because it's got a big circular air filter on top. But hey what do I know. Guesses?
For reference here's the icon in several different cars. Always the same.