How Can We Accurately Convert MPG to Vehicle Efficiency?

[ISX]

This seems pretty hard to do but is there a way to figure out how efficient a vehicle is based on fuel used and MPG? I know there's about 1/3 gone to exhaust 1/3 through radiator and 1/3 is work but I want to get an exact percentage on the work part. So it would be 33% if it was like what I just stated. I know weights of the vehicles, everything. I want to factor in as much as possible so I can get the closest possible percent. Thanks for any help!

 

[brewnog]

Well you need to decide what you want to accomplish by this. The answer varies hugely depending on what the vehicle is doing at the time you choose to take your measurements.

When the vehicle isn't moving, your MPG is obviously zero, and your efficiency (in terms of moving the vehicle) will be zero, but some work is still being done for anciliaries which you may or may not choose to account for.

When the vehicle is moving at full pelt, the equation is pretty easy; just look at the power needed to maintain terminal velocity, the energy going in as fuel, and; well there's your answer.

At part load or part throttle you can do the same consideration under steady state, constant conditions. For transients you're pretty buggered.

What are you trying to do?

 

[ISX]

Yeah that will be good enough. Like at 70mph I get 20mpg so I could base it all off that, just don't know how to do it..

 

[brewnog]

Ok.

To work out the power required to propel the car at 70mph you need to calculate the power required to overcome rolling resistance and air resistance. Add a bit for drivetrain efficiencies if you want to take care of those, and ancilliaries. Do all this on a power (ie. watts per second) basis.

To work out the energy going in as fuel, first calculate the fuel flow rate (i.e. what's 20mpg in terms of litres per minute of fuel at 70mph). Then multiply this by the calorific value of the fuel.

Your efficiency is the ratio of the two.

 

[Ranger Mike]

also aero drag is major player in MPG..

 

[russ_watters]

To me this seems like a useless pursuit: any answer you get would only be a single data point, applying only to a specific set of conditions. The thermo eff of a car engine is nearly constant while the mpg varies wildly.