High speed fuel economy

[ksa8907]

My 1st gen volts would go from 45mpg @ 60mph to 32mpg @ 85mph.

[Hersbird]

Well I made another run, which will probably be an every other weekend thing now, and it was colder and windy and my mpg dropped to 36.5 mpg. The one direction I was fighing a straight 25 mph headwind and was only getting 34 then the return trip had clamer wind and I was about 40. Still not as good as the first trip.

It will be interesting to see how much it changes with the weather.

[fusion210]

I recently found out that Car and Driver now includes a 75mph test for their fuel economy numbers. Some might find it useful.

Highway Fuel-Economy Test
We run all our tests at a GPS-verified 75 mph on the same 200-mile out-and-back loop on Michigan's I-94 highway. Our consistent procedure includes a methodical fill-up process, following a specific route, using cruise control, and setting the climate control to the same temperature (72 degrees auto). We also correct for odometer error, and we don't test in heavy wind or rain or with extra passengers. In the event we encounter too much traffic or unusual conditions, we abort the run and try again later.

We follow the same procedure for electric vehicles and plug-in hybrids, except for these, we have additional steps that include making sure the battery is fully charged before starting and recording the kilowatt-hours (kWh) needed to fill the battery after the drive loop. Plug-in hybrids also get a highway EV range and MPGe economy for those miles. MPGe is calculated just like miles per gallon of gas only using the EPA's equivalence factor of 1 gallon = 33.7 kWh of electricity to arrive at the result. For plug-ins that can't hit 75 mph in electric mode, we instead first drain the battery and then start the test in charge-sustaining (hybrid) mode. Since those plug-ins don't use any electricity, their results are in miles per gallon rather than MPGe. Likewise, we have to shorten our route for EVs that don't have the range to complete the entire loop. We still give them an MPGe number, though.

[Piotrsko]

Umm stupid question: if you force the fuel motor to start does that not include energy to recharge the battery which is reflected in the economy?

[aerohead]

Originally Posted by Piotrsko

Umm stupid question: if you force the fuel motor to start does that not include energy to recharge the battery which is reflected in the economy?

Very good question!
1) Since the engine runs @ constant rpm, and is optimized for this specific rpm, it's likely to have one of the highest thermal efficiencies around. Perhaps 42%.
2) And since it's directly-coupled to the 3-phase alternator/ rectified generator, it has no 'powertrain' mechanical losses 'in between.'
3) The 'generator' might have a mechanical efficiency equal to the motor, @ 97%.
4) And GM's motor efficiency is right up there with Tesla and Lucid Air, around 97%.
5) If you're burning REGULAR UNLEADED, E10 gasoline, it's specific heat content is 111,836- Btus/ gallon.
6) From each gallon of gas, you'd have 46,971.12-Btus worth of energy from the engine.
7) 45,561.986-Btus from the 'generator,' into the motor. About 13.349-kWh available/gallon.

[Piotrsko]

I take that answer is "YES" then.

[aerohead]

Originally Posted by Piotrsko

I take that answer is "YES" then.

Exactomundo!

[CVTCivic]

Originally Posted by Hersbird

What's the best 80 mph average others have gotten? Even the TDIs seem to drop off pretty hard after 75 mph.

43 mpg is pretty impressive for that speed. I can see what my 1.5l Turbo (non hybrid) can make on a 60 miles trip, but I have a lot more evaluation and even though I'm driving on the Autobahn, it's hard to maintain 80 mph as average speed. Because you're getting constantly slowed down to 40 mph (construction zones, semis and slower passing vehicles) and then I have to hit very high speeds to reach an average speed of 80 mph.

I'd say my fuel consumption could be arround 35 mpg (6,8 l/100km)

TDIs (1.6, 1.9 and 2.0) would probably do 46 mpg

Did you use cruise control most of the time?

[Piotrsko]

My 2.0 golf tdi does 40 mpg at 80 with the neutering software, was about 10% better originally. The way I drive it, anything above 75 means an extra fuel stop which isn't lesser time overall

[Hersbird]

I just set the cruise at 85 mph, but there are places where you have to slow down to 70 mph. The passes, the couple cities. Now that summer hits, road construction starts so averaging 80 is going to be tough.

I swapped vehicles with my girlfriends 2005 3.0 liter awd Subaru outback wagon and even slowing down 5 mph, it could only manage 22.9 mpg on the same run.