It would be very easy to build a larger gas engine that could deliver all the required power and torque that’s needed. It’s been done before, but much higher fuel costs makes it impractical, so in my opinion it comes down to total costs.
Given same displacement size, modern turbo gas engines produce more power than diesel. The 2.7L Ford EcoBoost pulls a trailer uphill faster than the 3.0 turbo diesel. The advantage to 3.0 PowerStroke is lower fuel consumption, and probably longer life (which also lowers costs).
Just saying that if power and torque was only thing driving purchases, Ford could easily build an 8-liter EcoBoost V8, but who would buy it if it got 6 MPG?
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