A Plan to Save Oil

August 30th, 2006 by Potato

After scouring the web for information on hybrid cars and civics in general while trying to make up my mind, I noticed a trend: a lot of people complain about not being able to get the rated fuel consumption figures for their car (and it really doesn’t matter which car). There are a ton of articles explaining the EPA/Transport Canada testing procedures, and how very little basis in reality they have — which explains why so few people manage to make those figures. The problem seems to be more prevalent in hybrids (and there are numerous “are they worth it?” opinion pieces out there that use the fact that you won’t make your EPA mileage as a reason not to buy a hybrid), and it seems to be for one very simple reason: hardly any cars have milage/consumption instruments, but most hybrids do.

I think that mandatory fuel economy gauges could end up saving a lot of gas for the country at a whole, since it really seems to give people the real-time feedback they need to see how changing their driving habits (driving slower, or at least accelerating slower) impacts their fuel consumption.

Another thing that would help a good deal in getting everyone, no matter what vehicle they have, to save on gas would be a set of fuel efficiency vs. speed curves for cars. These would also be subject to a fair bit of variance, since temperatures, use of air conditioning, and a host of other factors can impact mileage, but since they’re for steady-state driving they should be a lot closer to the truth. These curves could also be immensely useful for comparing cars for the way you intend to drive them, instead of the two mileage figures found with short example test drives used now.

For example, I know that my ’97 Accord gets in the neighbourhood of 10-12 L/100 km in city driving, and about 8.2 L/100 km on the highway at a cruising speed of 125 km/h. As almost everyone knows, typical peak fuel efficiency comes at around 90-100 km/h… but it really isn’t all that much better than at the more typical highway speeds. Whether this is due to the constant load of the AC, or the engine/transmission design, I don’t really know. I do know that up to about 130 km/h, the fuel consumption curve is surprisingly flat. After that, it gets bad pretty quickly due to air resistance, and the engine having to rev above 4000 (I know there’s a point where the engine gives you diminishing returns for additional RPM, but not sure where that is). In comparison, the hybrid systems on the Civic Hybrid even out a lot of the start/stop losses seen in city driving, so its city and highway estimates are very close. However, it’s been suggested that since the car has fewer linear losses (friction, A/C, etc.), and since it has a smaller, less powerful engine, that its high-speed fuel economy will flag faster. This is in part because the smaller engine has to increase RPMs faster for increasing speed, and quickly gets out of its efficient operating range (and at constant highway cruising speed, the engine must work unassisted by the electric motor). The larger engine of the gas-only Civic may be better able to stay close to its highway rating above the speed limit. Using my own data and that of some of the members at greenhybrid.com, particularly “NASAgineer”, I’ve come up with the following graph:

Chart of estimated fuel consumption vs speed for a few cars

The big jump around 80 km/h for the gas-powered cars comes thanks to getting into 4th gear. Also note that my own data is far less reliable below that speed (it becomes much less about cruising and more about stopping and starting in that range), and I’m just pulling numbers out of my ass for the gas Civic — I don’t even have RPM vs. speed data to make educated guesses from. So, I really don’t know if the curve for the hybrid will cross that of the gas model, but you can see how I would be very interested if they did. (Since most of my driving is at around 120 km/h, I might save the air just as well with a gas-only Civic, and have less initial outlay to boot).

Also, I’ve updated my previous post on car considerations with my experience actually test driving a Honda Civic Hybrid. See the comments for details.

4 Responses to “A Plan to Save Oil”

  1. Wayfare Says:

    Sweet mother of god, you made a graph. You’ve now officially done more work on this than on your PHD.

  2. Potato Says:

    I’m telling you, instead of just doing a “sample city drive” and a “sample highway drive” and listing the consumption for both, Energuide/EPA/car manufacturers should publish these graphs!

    I’m going to look up who to start sending letters to and let you know.

    …and graphs aren’t all that much work to do…

  3. Netbug Says:

    What do you need to do to actually get a requirement like this passed as a law?

  4. Potato Says:

    Now that I have my ScanGauge, I’ll be able to really fill that graph in with real data… :)