It greatly depends on where you are located. In my area of the US furnace oil is just red dyed #2 low sulfur diesel fuel blended with kerosene. You would need to ask the fuel distributor what the exact composition of their furnace oil is. It may be suitable and it may not. If your not sure then don't use it. If it does not meet diesel fuel standards it could end up causing more damage and repairs then you would in fuel cost savings.
Furnace oil is basically the same as diesel. But, for furnace use, the cetane rating and the wax content is not an issue. It is for your diesel. On a slow running diesel, where the FO is at room temperature, It will run fine. Not in a high performance road diesel. If you are looking to reduce your cost, use coloured diesel (detaxed for off road use - see your local regulations).
Sulfur is a lubricant. Not a problem for the engine, screws up the environment... ;( (acid rain).
Can FO be used to fuel IC engines? Is a very interesting query!! Let us first understand what is FO? In the India context Furnace Oil or FO is a residual blended fuel available in a number of viscosities. Furnace Oil is a blend of residual fuels generally offered in different viscosities and its nomenclature indicates its viscosities, LV= 80 cSt (80 cSt @50 oC), MV1= 125 cSt, MV2 = 180 cSt and HV 370 cSt. Its Flash is 66 oC. As it is a residual fuel,( has high viscosities) and its costs is very low, BUT, has very high Sulphur (4 to 5%), and thus preferred by many bulk users to fuel their (Low RPM large engines) at the Captive Power Plants (CPP). Typically large engines sub 450 RPM engines made by Wartsila, MAN, Pielstick, etc.. typically as 18V32 or 36 that can generate upto 8 to 10 MW or more.
On the other hand, medium RPM engines upto 750 RPM can be fuelled by LDO as in some CPP’s, HEMs, Off The Road vehicles, Agriculture vehicles etc.. These have medium viscosities 2 to 15 cSt@40 oC. flash of 66 oC and a Sulphur level below 2%.
Most of the commonly available commercial applications use engines with above 800 and generally between 1100 to 1500 RPM ( and increasing RPM’s available nowadays) and use the popular High Speed Diesel (HSD). These have a Flash Point - 35 to 36 oC and most important viscosities of 2 to 5 cSt. And under Euro IV (BS 4) regimes 50 ppm Sulphur. Thee have centane value between 48 – 51.
TYPICAL SPECIFICATION OF FUEL OILS Properties Furnace Oil LS.H.S. L.D.O. HSD Density (Approx. g/cc at 15°C) 0.89–0.95 0.88–0.98 0.85–0.8 0.82 -0.85 Flash Point (°C) 66 93 66 34 - 35 Pour Point (°C) 20 72 18 3 - 15 G.C.V. (kCal/kg) 10,500 10,600 10,700 10,800 Sulphur Total, % Wt. Max. Upto 4.0 Upto 0.5 Upto 1.8 0.5
In summary, FO may be used in CPP where selected slow speed sub 500 rpm engines are installed.