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Old 05-29-2009, 05:17 PM   #1
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Puzzling acceleration comparo of Prius and Insight

Apparently the 2010 Toyota Prius has a drag coefficient of .25 and the Honda Insight has one of .28. This, to my understanding, would mean the Insight would take longer accelerating to higher speeds than the Prius if they had the same power. Yet the Prius beats the Insight accelerating to 40mph, while the Insight then surpasses the Prius onto 80mph. Wouldn't the Insight only fall further behind the faster the two cars go since it has worse drag? How does it do better at high speeds??

Or perhaps it speaks more to the gearing? (They're both CVT tho).

Coefficient of Drag: .....Insight................Prius
....................................0.28.......... .........0.25
lower number = less aerodynamic drag

TEST DATA
Acceleration to mph

....................Insight................Prius

0-30............3.7 sec...............3.4 sec...... fastest (+0.3 secs)
0-40............5.4.....................5.3 ........... faster (+0.1 sec)
0-50............7.6.....................7.6 ........... same
0-60............10.4...................10.6 ......... slower (-0.2 secs)
0-70............13.8...................14.3 ......... slower (-0.5 secs)
0-80............18.1...................19.1 ......... slowest (-1.0 sec)

You'll notice how the Prius is faster accelerating when the aerodynamic drag is at a minimum and then is overcome by the Insight when aerodynamic drag is very significant. at 50mph the Insight catches up with the Prius and then progressively leaves it farther and farther behind the faster it goes.

http://motortrend.automotive.com/104...est/specs.html

more reading on the Insight
http://www.motivemag.com/pub/feature..._Insight.shtml
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Old 05-29-2009, 06:25 PM   #2
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who cares there both really slow and are meant for fuel efficiency
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Old 05-29-2009, 07:30 PM   #3
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Aerodynamic drag is frontal area of the vehicle multiplied by the coefficient of drag. If the Insight presents a smaller profile to the wind, it may actually have less drag than the Prius, in spite of the Cd numbers.

[edit]
Looking over the two vehicle's specs pages, it looks like the Prius has a frontal area somewhere around 2.3 square meters, while the Insight has a 2 to 2.1 square meter frontal area. So:
Prius - 2.3 * 0.25 = .575
Insight - 2 to 2.1 * 0.28 = .56 to .588

Anyone know where to get more exact numbers? :/
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Old 05-30-2009, 12:55 AM   #4
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Quote:
Originally Posted by 1993CivicVX View Post
This, to my understanding, would mean the Insight would take longer accelerating to higher speeds than the Prius if they had the same power.
Do they have the same power? I imagine this could be a bit trickier to compute for a hybrid, depending on what is running when.
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Old 05-30-2009, 04:56 AM   #5
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Different power curves, different drag, and different weight make this a very difficult comparison at best.
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Old 06-01-2009, 06:03 AM   #6
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Thanks guys for the replies. Motor Trend noted the difference, but didn't offer an explanation, just thought it was kinda odd that the Insight should do better at higher speed.

@domgg: you're missing the point of why I'm making the comparison

The Prius has a better weight to power ratio. It is heavier than the Insight, but I think it has over 10% advantage weight/power ratio to the Insight, which makes this comparo even more interesting. Insight's combined elec/comb. power is 111hp, while the heavier Prius is 136hp. The Prius has a 1.8liter and the Insight a 1.3l engine.
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Old 06-01-2009, 07:04 AM   #7
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Quote:
Originally Posted by 1993CivicVX View Post
Insight's combined elec/comb. power is 111hp, while the heavier Prius is 136hp. The Prius has a 1.8liter and the Insight a 1.3l engine.
I find it interesting how those numbers are distributed. Conventional automotive wisdom says that larger engines produce more torque, but it seems to be the other way around in this case.
Insight engine: 98 HP @ 5800 RPM, 123 lb-ft @1000-1700 RPM
Prius engine: 98 HP @ 5200 RPM, 108 lb-ft @ 4000 RPM

The Prius has a huge electric motor, nearly doubling the power output, and increasing the torque by 1.5x.
Insight motor: 13 HP @ 1500 RPM, 58 lb-ft @ 1000 RPM
Prius motor: 80 HP @ ?, 153 lb-ft @ ?

Electric motors tend to make their peak torque and HP at relatively low RPMs (many make peak torque at stall - zero RPM) where as engines, when unrestricted, tend to make more and more power the faster you spin them. This makes me wonder about the construction of the Prius drive train - do the engine and motor each have their own CVTs so they can spin at different speeds, maximizing power output? Or are they forced to spin at the same speed (as is the case with the Insight) before the combined output is fed to a shared CVT?
If they're forced to spin together, one or the other isn't going to be producing peak output (or possibly even any significant output) at any given RPM. I could see the interaction between the two - the swapping places in which is most effective as the RPMs change - making for some pretty interesting power output/acceleration graphs.
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Old 06-01-2009, 07:54 AM   #8
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The prius drivetrain is... interesting.

I can tell you right now that the better low-end acceleration comes from the Prius' big electric motor having so much torque. It starts to fall behind as the electric motor speeds up and becomes less efficient.

The electric motor is always connected to the wheels in the Prius. It has a fixed ratio. The gas engine is never directly coupled to the drivetrain. It has an electronic torque converter that is essentially an electric motor that gets loaded up more if they want less slip and loaded less for more slip. The part that connects to the rest of the drivetrain is at a fixed ratio as well. There is no 'cvt' in the Prius, it's just a variable, electronic torque converter going into a fixed ratio planetary gearset.
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Old 06-01-2009, 11:28 AM   #9
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That sounds like the kind of electric transmission they use in trains. So you're saying the Prius has a gas generator (engine connected solely to a generator head), and the only connection to the wheels is through the electric motor? If that's the case, comparing the power/weight ratio of the two cars should be a matter of comparing the Insight's engine+motor against just the Prius' electric motor. The Prius engine gets excluded since no matter how much power it spits out, power to the wheels can't exceed the electric motor's rating.

So that's:
Insight: 111 HP, 181 lb-ft peak
Prius: 80 HP, 153 lb-ft peak

So the question becomes: Why does the Insight lag behind off the line? CVT spooling up?
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Old 06-01-2009, 11:57 AM   #10
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Not exactly. The gas engine is connected to the same planetary 'power split device' that the electric motor is. Only, instead of a torque converter or a clutch, it has an electric coupler. The engine spins the rotor and the stator is connected to the 'transmission'.

It's an AC motor with a variable frequency driver. This variable frequency is how Toyota can get away with having only one gear speed to the engine. The AC coupler is actually responsible for the 'gears'. They are infinitely variable and never wear out! It helps a lot to read the wiki about AC motors.

Prius driveline:
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