Reply #9: If you used diesel engines coupled with hybrid technology, you could achieve greater fuel efficiency

[Selatius]

Diesel engines are more fuel efficient by design, even though a 5.7 liter diesel engine may sound louder. One motorhead managed to decrease the amount of fuel injected per cycle down to like 50% by introducing hydrogen gas into the combustion chamber to squeeze more energy out per unit of diesel fuel injected, effectively doubling fuel efficiency. It burns hotter and thus is more efficient in consuming the hydrocarbons, leaving less CO2 emissions, but at the same time it can increase horsepower as well. I'll have to dig up the article, but the only problem is that hydrogen gas isn't so easy to come by.