Diesel generators for Data Centers as backups... are surprisingly efficient.

So much so, that I know of a couple that use it as a part of their "green push", rather than the Turbine Jet Fuel powered ones.

Of course, you only can get *so far* with Diesel as a Generator, once you get to certain point... (around 7000-10000 cu inches) the sheer weight of the engine becomes prohibitive.