(Scientific) study on the factors that affect fuel economy

Very detailed report- you obviously put a lot of effort into it!

Not really too surprised by the conclusions that tire pressure and ambient temperature effect fuel economy that drastically.

For years we've had the mantra of "tire pressure, tire pressure, tire pressure" drummed into our heads.

Not too much we can do about seasonal temperature changes- other than grill blocking- which I see you're planning to investigate.

Didn't see anything about the engine air filter? Maybe I overlooked it.. but have always been told that a partially clogged engine air filter also kills fuel economy.

So, in reality- if you keep your tire pressure up and don't drive like an A-hole (gunning it at green lights and skidding up the the red lights) you should see very good results from the Prius without jumping through hoops to chase it.

Thanks for the report SCote!

 

I too would love to see some data on grill blocking. Great work on the stuff you do have though.

 

Great study. I was surprised by the minimal contribution of speed to the variance. I think you would see more contribution at speeds higher than you tested (ie >60 km/h).

BTW, what kind of commute is this? City, interstate hwy, or rural hwy?

You have to study tire pressure next, please!

 

For traditional engines, this is just common mythology. The air flow restriction from a clogged filter is really no different than a partially closed throttle plate, so for a driver who never floors it, there should be no loss of fuel economy. Loss of top end power is another matter.

But this particular view doesn't necessarily apply to most hybrid engines, which do want to run at low RPM with the throttle fairly wide open. So it may be worth more investigation for the Prius.

 

+1 It's a myth like the need for 3000 mile oil changes. At one time there was a reason for it but not now.

Back in the olden daze of carbureted engines it was true because a restricted air filter enriched the mixture and there was no way for the engine to compensate. With modern fuel injection systems that run closed loop mixture control most of the time, the mixture is adjusted to compensate for reasonable amounts of filter restriction.

 

Aerodynamic drag goes up by the square of speed. The power required to overcome aerodynamic drag goes up by the cube. Aerodynamics make little or no difference at low speeds, but are the dominate factor at high speed.

Tom

 

And in terms of energy required to push the air out of the way per distance, also the ratio of the speeds, squared.

Work = Force*distance

The distances are equal, the force (drag) as Tom wrote. I think.

 

Stephane, another fine report.
You talked a bit about AC fan speed, and its low contribution to FC variation. Ditto with the ECO button. You didn't have many datapoints for it, but for the next study )) perhaps you can look at the two together and see how effective the ECO switch is at controlling the AC and its effect on FC. That may be of interest to folks who have to use their Prii in warm climates.
I suspect there is some contribution of error in the weight calculation technique contributing to the anomalous uphill result -your next study will fix, that, I'm sure.
Anecdotally, I can tell you that hauling around 400+ lbs of extra passengers makes a big hit in FC, especially uphill.

 

beyond warm driving temperature, over inflated tire pressure, a top/average speed under 35mph, not having the ac/defroster on, not having to stop, driving longer than 30 minutes, and not carry extra payload/weight...

going downhill and having a tail wind helps too...

 

Interesting and nice report. Thank you.

 

Initial HV battery SoC would be a good independent variable. You could do 40%, 50%, 60%, 70%, 80%.