Sci-Fi Terminator 4: Salvation (2009)

[Kevin]

Black smoke is partially-burned fuel. An engine that's running properly will burn all of the diesel fuel completely, making CO2 and water. So black smoke means something is causing the fuel to not burn completely. ... Not enough air means incomplete fuel combustion.

But why would it be using what would appears to be a liquid fuel at all? None of the other machines in the franchise seemed to be using a similar fuel source.

 

[Tom]

Why do writers and directors do half the silly stuff they do?
Just guessing, I figure it was added for effect so the audience felt amazed in its size or threat.
Something people could identify with.

Harvester

The Harvester is a large, bipedal Non-Humanoid Hunter Killer unit produced by Skynet sometime before 2016. Its primary function is to capture humans for the machines to do lab testing on their stem cells. This research on humans led to the creation of the Series 800 Terminator. When a Harvester has captured any human test subject, it calls a Transport, which arrives and lands somewhere near the Harvester. After a Harvester has collected the humans, it drops the humans into the large...

terminator.fandom.com

The Harvester is a large, bipedal Non-Humanoid Hunter Killer unit produced by Skynet sometime before 2016.

Pay special attn to the Notes

It appears that the Harvester runs on a diesel power plant or possibly a diesel generator as there is black smoke that billows from of the Harvester's back at times.