Q: Why not just allow fluid form of oils to be directly usable as fuel?
A: While no vanilla machines accept fluid fuel directly, it is possible in modded machines. Please check my fluid boilers in another mod: Schall Machine Modification.

Q: Do you plan to provide a vehicle/locomotive taking fluid fuel directly as fuel?
A: No, I am not interested in that. If devs will provide this kind of machines in base game 0.18 or 1.0, I may reconsider that. But for now, NO, please do not ask me.

Q: The fuel value are too low/high. The oil barrels are underpowered/overpowered!
A: The fuel values are NOT arbitrarily chosen, but based on vanilla recipe and fuel values. Please see the below explanation (in mathematics) on how the values are chosen.

Technical Details

Setting Fuel Values

Oil as fuel are designed to work similar to existing fuels, particular in line with solid fuel, which are assumed to be an oil fuel product (from the recipe ingredients).
At the creation time of this mod, the game version 0.17.66. Therefore, all the balances are based on it, as listed in the tables below.

The solid fuel is assumed to be highly similar to light oil, the "reference" fuel. The acceleration and top speed multipliers of light oil are thus identical to that of solid fuel. A solid fuel item (12 MJ) is made from 10 light oil, so we define 10 fluid unit of light oil to be 12 MJ. A barrel contains 50 fluid unit of light oil, so it is 60 MJ per barrel, or 1.2 MJ per fluid unit.

Heavy oil is, historically (and currently), the fuel for large machineries like furnaces and large ships. It is the form that provides the most heat value (fuel value). So it is set to have a slightly higher value of 1.4 MJ per fluid unit. No speed bonus for simplicity and balance.

Crude oil is the unrefined form of oil. The advanced oil processing gives more of less the oil volume. So for simplicity, crude oil is set to act identical to heavy oil when used as fuel. (Though using crude oil as fuel is a bit wasteful, since refining it into products like sulfur and plastic, would be much better uses. Crude oil as fuel should be just an emergency measure.)

Petroleum gas would better resemble the "clean" and "high-quality" fuel in household and vehicles, which are more on civilian side. As a result, the performance is tuned up a bit, with reduced total heat value to compensate (and balance). Therefore, it has set to only 1 MJ per fluid unit.

From the above table, there are some useful (and unexpected) results:

• Recipe for rocket fuel has a net loss of fuel value, which is desirable in the sense of balancing. When speed is not a concern (say, furnaces and boilers), burning the light oil directly will be more efficient. Rocket fuel still has a place for the much better performance, obviously the superior fuel when used in tanks and locomotives. (If considering productivity bonus, there is not as much loss, may even be a net gain.)
• When performance is not a concern, it is most fuel-efficient to burn the oil directly. If not considering productivity bonus, every conversion to solid fuel or rocket fuel has a net loss in fuel value.
• Coal liquefaction looks like too beneficial in terms of fuel value! (Steam are assumed to be provided by 165°C boiler, which is 200 J/°C × (165-15)°C = 30 kJ per fluid unit.) But since coal is involved in other recipes, it is not simply just a matter of fuel value. Thus this (or any other) recipe is left untouched by this mod.
• Oil crackings are no longer a necessity, and actually net loss in fuel value. For excess heavy oil, it should be used directly as fuel, rather than lossy cracking. But of course, they are still needed if you are in dire need of petroleum gas, for sulfur and plastic that are of high importance.

Rebalancing Fuel Pollution

Since 0.17.4 / 0.18.3 / 1.0.0, a new "fuel pollution rebalance" option is introduced.

In vanilla game, there are no differences among any chemical fuels regarding amount of pollution generated, when burned in the same vehicle or burner machines. It does not resemble their real-world properties: definitely burning coal generates a lot more pollution (due to impurities within), than the refined fuels.

Turning on the rebalance option (under "Options" → "Mod settings" → "Startup" tab) allows my "experimental" attempt to make the fuel pollution settings a bit more realistic. Specficially, this option will:
- Increases pollution from dirty fuels (e.g., Coal, Crude oil.)
- Decreases pollution from cleaner fuels (e.g., Rocket fuel, Petroleum gas.)

The exact figures can be found in the above table, "Emissions" column. (If not stated, it implies the default value of 100%.)

Full List of New (or Changed) Items

Fuel

• (4 changed) item:
  • Wood
  • Coal
  • Rocket fuel
  • Nuclear fuel
• (4 changed) fluid:
  • Crude oil
  • Heavy oil
  • Light oil
  • Petroleum gas
• (4 changed) oil barrel:
  • Crude oil barrel
  • Heavy oil barrel
  • Light oil barrel
  • Petroleum gas barrel