motor home mpg

Floridastorm wrote:
Hello Folks,

I am a mechanical idiot (Contracts Manager my entire career)and know next to nothing about engines with the exception to what I read. Am still considering a motor home for getting around Florida and Georgia in retirement.

Can you help me with the reasoning behind the inability of engine manufacturers to produce an RV engine that can get decent gasoline mileage. I worked with engineers my entire career and don't really understand why they would not want to build this type of engine considering the hundreds of thousands and possibly millions of motor homes built every year. Is it a physical problem or scientific? I would imagine this would address the trucking industry also.

Also, is it better to use premium gasoline compared to regular in a motor home? How about synthetic oil as compared to standard oil?

The problem isn't that the engines are particularly inefficient (though they likely aren't up to the standards of the best auto engines), but rather that moving a heavy non-aerodynamic vehicle around at highway speeds requires a lot of energy. If the engine puts out more energy, it needs more energy (fuel) put in to do that.

There are some theoretical limits on combustion engine efficiency related to thermodynamics (the specific math for which I don't care to take the time to understand, but it's based on the intake and exhaust gas temperatures and pressures and similar operating constraints). There are practical limitations to how efficient an engine can be made based on materials science (how hot or stressed parts may be without failing), operating conditions (the exhaust temperature isn't lower than the ambient temperature), compression ratios (limited in gasoline engines by the tendency of fuels to start to burn prematurely), etc.

Higher octane gasoline is only advantageous if the engine is designed to need or use it. The higher octane rating means that it can have a higher compression ratio before it detonates (i.e. engine knocking). Modern engines with computer controls vary the spark advance and other things to generally prevent knocking, but at the expense of power and efficiency, so some engines will run somewhat better or more efficiently with higher octane gas, and that should be specified in the owner's manual as recommended or required. The gas engines used in RVs generally don't specify anything above regular octane gasoline.

In some cases, premium gas may have more or better additives, or may not have ethanol blended in, which can have their own advantages.

Synthetic oil has somewhat better properties typically than conventional oil (greater resistance to burning/decomposing and such), but would have little if any effect on gas mileage for the same viscosity rating. Using the oil viscosity and service grade as recommended by the maker is all that's required, synthetic or not.