How much does Hybrid really contribute to mileage?

I should disclose that my Prius is not beating EPA by as much as its non-hybrid predecessors.

My alltime best performer, as a percentage of EPA, was an '86 Accord, the car that this Prius replaced. It did 10-18% over (2008 restated) EPA Highway right out of the box in winter, and averaged 30% over in summer. I didn't drive it much after starting hypermiling last year, but did get consecutive tanks 40% over before disposing of it.

The '97 Subaru didn't do as well. For years it could barely beat its original EPA Highway sticker, and only in summer. 2008's high fuel prices prompted me to start mild hypermiling, pushing it to 30% over its restated-2008 EPA Highway. If fell back to 6% over EPA last winter, but it had never been over EPA any previous winter.

My Prius averaged just 14% over EPA Highway (54.8 actual, 48 sticker) for the main part of the summer while I was learning to drive it, though some partial tanks were far higher. The fall family road trip was lower due to heavier load, higher speeds, and sharing driving with a non-hypermiler. And now that winter is here, it is significantly below EPA. I can't give you a real number yet, as it hasn't been filled in more than 5 weeks, but this tank appears to be roughly 8% under.

Based on your past results, you must already have the general hypermiling principals down pat. So look for the Prius-specific stuff here and on cleanmpg.com.

 

So, what is the highway speed at which one would get the best fuel economy inder these conditions:

No wind, level road, tires at specified pressures, ICE (the only way to extract energy out of the fuel) running at peak efficiency, all of ICE output going to the wheels (batteries are fully charged).

That would be the ideal cruising speed for long distance trips at highway speeds ...

 

I believe the minimum, posted speed on interstate highways is 45 mph. So I would recommend 46 mph.

Bob Wilson

 

That may well be, but what is the most efficient speed based on the back-of-the envelope theory (or the fancy add-on instrumentation that some folks use for hypermiling)?

You are implying that the most efficient speed is less than 45 mph.

My Q is simple: What is the linear speed when the ICE is running at the most efficient RPM and the CVT is at it's lowest speed ratio (most linear travel per engine rotation)?

 

I measured it:


And you?

Bob Wilson

 

[snipped -- Bob posted before I could]

Note that the upper limit on Bob's graph is the Prius display limit. That actual figures at 20 and 25 mph are likely higher.

 

Hi All,

My experience in my slightly aero-modded Gen II is for highway driving (> 45 mph) the best speed is about 53 mph. I have a hard time terrain (mild hills around here) driving much below that speed with a steady-speed driving goal, getting better mileage than at 53 mph. Bob's chart withstanding, driving at 45 mph steady state will give worse mileage than 53 mph, on the order of 55 mpg for 45 mph, versus up to 70 mpg at 53 mph (summertime moderate temperature conditions).

One has to switch to pulse and glide to do better, and in the Gen II that means below 41 mph (indicated on the speedometer). Of course that is not highway driving then.

In a Gen III its probable that the engine has to make a certain RPM to be efficient (due to internal aerodynamics) as with all piston ICE's. So the steady-speed conditions where aero-drag drops mileage and engine efficiency increases mileage results in a peak mileage is going to be a little higher than 53 mph due to the higher power output of the engine / rpm. I wonder if Dan down in Texas has figured out that speed yet?

 

Yes you're probably right...on the highway. In the city portion the non-hybrid Prius that you envision would be somewhat better than than the Matrix or Corolla. BUT ... your theoretical 'Otto cycle' Prius with all the benefits you envision would be a relative dog to the current or Gen 2 HSD Prius.

You've gotta look at the whole picture. Hey if they could have made an Otto cycle Prius that got real world 50 mpg in the city it would have been built a long time ago. It's not possible now. No vehicle maker in the world has made such a vehicle.

 

You are correct. This is my 1.5L Prius chart for the NHW11, 2003 Prius:

The green diamond points are what I've measured over the years. Something 'not good' happens in the area around 42 mph, the control laws change. But if you squeek up a little faster, things get much better. There is a 'knee in the curve' at 65 mph, which is my preferred cruise speed in the 1.5L, NHW11.

I have a few NHW20, 2004-09 Prius, data points:

The 'knee in the curve' at high speed appears to have moved to higher speeds.

Bob Wilson

 

Bob,

Are your charts for the ICE alone? or is the electric motor also contributing, especially at lower speeds?

Aloke

 

On the highway, regeneration is insignificant (obviously). For around town driving, Toyota claims about 40% of the mileage improvement comes from regeneration.

Tom

 

With my 2006 Gen II Prius, I see the best mileage at about 43 mph for non-P&G driving. At that speed you have to be careful to avoid going in and out of the speed range where the ICE is required to spin. The transition has a detrimental effect on mileage.

Tom

 

I wonder what that number is for the Gen III Prius (with bigger ICE).

 

We don't really have good ways to force either in any practical sense. What I've found experimentally is after ~10 miles, the MPG indicator begins to wander up and down around a given MPG. The vehicle has achieved a stable mileage at that given speed.

It is my practice if there is any doubt to do one pass as 'practice' to verify speed and identify traffic hazards (aka., road construction or others) that might make the test route impractical. During the test runs, the speed is totally controlled by cruise control. If I have to use the brake or accelerator, the data is thrown out and another run in that direction made. If everything is OK, I go to the next exit and record the data. I then do a second pass in the reverse direction and average the two, minimum 10 mile legs. This second pass helps eliminate altitude and most wind effects.

At lower speeds, the traction battery state of charge entering the course has a significant impact on the mileage. Since the critical speed is 46 mph, at lower speeds, I do one 'practice' run at the target speed and then go for the numbers. Although there is a risk of hitting a 'sweet spot', my regression testing using a circular loop suggests this is not the case:

This is a photo of the GPS display of my circular test track, ~1.1 miles (~1.76 km) that has a speed limit of 25 mph (40 km/h) and a maximum altitude change of ~5 meters:

34 38' 59.18" N 86 38' 02.17" W (Google Earth center)
​

I also have a higher speed circular test track that is a little shorter, a little faster, and includes a greater altitude change:

This is in Huntsville Alabama.

Bob Wilson

 

Would be nice to have percentages on that pie chart!

 

Without having researched it a bit, how different was the Echo from the Gen I prius? Would that serve as some basis for comparing a "non hybrid" prius to a hybrid one? (Although the gen 1 is a different beast from the car of today)

Bob?

 

Both the Echo and NHW11/NHW20 Prius have the same 1.5L engine block. But the Prius uses an Atkinson cycle and rev limiters on power. The vehicle differences:

	Column 1	Column 2	Column 3	Column 4	Column 5	Column 6	Column 7
0	hp	rpm	weight	MPG (users)	model
1	108	6	000	2	050	37.6 (7)	Echo
2	70	4	500	2	765	45.0 (22)	NHW11
3	76	5	000	2	900	46.7 (113)	NHW20

All automatics.

So the hybrid system appears to add 700 lbs to the vehicle weight in a sedan body but delivers a solid, 8 MPG of mileage. My wife's former Echo was a 'pocket rocket' around town but not at the fuel pump.

Bob Wilson

 

There are 2 cars that are have conventional drive and toyota hsd to find a closer answer.

The camry hybrid is 420lbs heavier than manual transmission conventional Camry. It gets 50% better mileage in the city, but only 1 mpg more on the highway (3%). The hybrid gets 2 mpg better than the automatic on highway.

The altima hybrid is 300lbs heavier than its cvt non hybrid. It gets 1mpg more on the highway and 50% more in the city.

So if the prius was built as a conventional cvt, but with the other advantages we would guess it would weigh 2750lbs and get 34mpg city/ 46 mpg highway. YMMV and some of the electrical tricks might not work with the smaller battery.

Thanks to bob for the correction

 

I don't have any percentage number.
But, please be aware that the pie chart shows efficiency improvement from THS(Gen1) to THS-II(Gen2), not from conventional vehicle to THS-II.
TOYOTA: Company > Technology > Output Enhancement

Ken@Japan

 

I concur.

There doesn't seem to have been any 1.7L engine on the generation that undrgnd's Civic belonged to. FWIW, when Conusmer Reports tested an 06 Civic 5AT (same generation), they got 18 city/43 highway, 28 mpg overall, 34 mpg for a 150 mile trip. I suspect (but haven't seen if he's posted) that his commute was mostly highway.

2010 Prius' numbers are at Most fuel-efficient cars: 32 city/55 highway, 44 mpg overall, 53 mpg for 150 mile trip.