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Discussion Starter #1
Hello

Can someone explain to me in detail why the Prius is relatively inefficient on the highway than in the city?

Can someone also tell me what's the ratio between the motor output shaft and the driven wheel?

Thanks
 

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Whats your definition of "inefficient"? Have been very impressed by my highway driving experience. Both in terms of comfort and handling as well as mpg.
 

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In the city, it is normally lower speeds. In these conditions the electric motor can run more. At the higher speeds, the electric motor can't spin fast enough and mainly the gas engine runs. But it does help at various times.

Brian
 

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Discussion Starter #5
"In the city, it is normally lower speeds. In these conditions the electric motor can run more. At the higher speeds, the electric motor can't spin fast enough and mainly the gas engine runs. But it does help at various times.

Brian"

So that means the gas engine cannot take advantage of the CVT on the highway? (high speed)?


Does anyone know how efficient is the energy conversion from gasoline to the electricity?
 

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Step back and look at the big picture.

Going faster requires more energy, just ask anyone on a bicycle.

So hybrid efficiency should be common sense. Traditional vehicles are actually the counter-intuitive logic. But since that's all you've ever known, you just accept it without question.

The catch is that when you drive in a city, speeds slow down all the way to zero. It's the routine accelerations from a dead stop that kill MPG. If you could cruise at a constant speed for a very long time, like you do on a highway, efficiency would be fairly good in a traditional vehicle. But you don't. So it's not.
 

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Final gear ratio (product of all reductions from MG2 output to wheels) is 3.905 in the 2001-2003 Prius. I think it's the same for 2004, but am not sure.

The gasoline engine is sized to normal highway cruising with short bursts of high power assisted by the electrics, so highway cruising is more efficient than non-hybrids of the same size and weight. There are other cars that get close to the same efficiency as the Prius on the highway, but most of the current models opt for far bigger than necessary engines because that's what a lot of buyers are willing to pay a premium for.

The combination of Atkinson cycle engine and electric load smoothing allows for much higher efficiency at low power output (i.e. low speeds) than traditional cars. Since it's not burdened with the normal low efficiencies of slow speeds, you see the effects of wind resistance draining energy proportional to the cube of the vehicle's speed. This is why the EPA estimated low speed mileage is actually higher than highway mileage for the Prius. Still, even the Prius can't overcome the inefficiencies of really low speed such as stop and go traffic jams, traffic lights, and stop signs. So most people still find real life city driving to yield lower MPG than real life highway driving.

By the way, that electric load smoothing results in a huge reduction in Prius emissions. Most hydrocarbon, CO, and NOx emissions result from sudden changes in load on an engine. NOx is also cause by leaning out the mixture in an effort to improve mileage. The Atkinson cycle engine avoids the need for lean burn. The electric power system allows the use of the Atkinson cycle without lowering peak power so much that customers would reject it.
 

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Discussion Starter #8
Thanks for all the responses.

Now, as I understant it, all the power that the Prius get ultimately come from the gasoline engine ( it recharges the battery, powers the car directly, ect).

But to recharge the battery, it requires energy conversion from gasoline to electrical. Does anyone know how efficient this step is?

The thing that I'm not quite sure is:
the IC gasoline engine is about 20% efficient, so only 20% of the energy in the gas can be changed into useful work.

Now, this 20% has to be converted into electrical energy, which decreases the overall efficiency. Then finally, you have time take into account of the efficiency of the motor too, which is about 70- 80%.

So... the final percentage of energy from the gasoline converted into the motion of the car is substantially less than 20%. Right?

Am I wrong on anything?
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Now, compare this with the approach Honda took. Honda's hybrid powertrain is in series instead of parallel. The IC engine in the Insight are doing the most of the work, with the motor merely assisting in acceleration and functioning in the regenerative braking.

This means a lot less energy conversion (from gasoline to electrical) is needed in the Honda. This should saves the waste in the conversion process. But the disadvantage is that Honda cannot have a electric drive and have to use a mechanical transmission. This leds to loss.

But in the end, it's all comparable. Both approach have a motor to take advantage of regenerative braking. Toyota has a electric drive which have high efficiency, but Honda doesn't require as much gasoline to electric conversion which also saves energy.

Now, I can be wrong on a lot of things so I can appreciate it someone can correct me if necessary. Thanks.
 

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Some corrections first :
At the higher speeds, the electric motor can't spin fast enough
The "big" electric motor (MG2) always spins proportionally to the vehicle's speed. The faster you drive, the faster it spins.
Final gear ratio (product of all reductions from MG2 output to wheels) is 3.905 in the 2001-2003 Prius. I think it's the same for 2004, but am not sure.
It's 4.113 for the new Prius.
wind resistance draining energy proportional to the cube of the vehicle's speed
Aerodynamic force is proportional to the square of the speed, and so is the energy required for a given distance. Only the power is proportional to the cube.
So... the final percentage of energy from the gasoline converted into the motion of the car is substantially less than 20%. Right?
It's much better than that. See for instance :
http://www.toyota.co.jp/IRweb/special_r ... hs2_1.html
(beginning of the expos�
and more specifically :
http://www.toyota.co.jp/IRweb/special_r ... s2_10.html
(bottom of the page)
where Toyota claims the tank-to-wheel efficiency is as high as 37%. By the way, if you consider well-to-wheel efficiency (including fuel production efficiency) the Prius is more efficient than Toyota most recent fuel cell cars...
This means a lot less energy conversion (from gasoline to electrical) is needed in the Honda. This should saves the waste in the conversion process. But the disadvantage is that Honda cannot have a electric drive and have to use a mechanical transmission. This leds to loss.
Just the opposite. The pure mechanical transmission of the Honda is more efficient than the electro-mechanical one of the Prius. But it does not allow the engine to operate always at its best RPM where the engine efficiency is maximum. Except if you consider a mechanical CVT, but this one is not very efficient.

So, to come back to your first question, highway efficiency is not lower than city efficiency, but you need more gasoline at highway speeds because you need more energy at high speeds as long as you travel through the air...
And for the second question, note that the unique 4.113 gear ratio makes the Prius a one-gear automobile. :wink:
 

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Discussion Starter #12
Just the opposite. The pure mechanical transmission of the Honda is more efficient than the electro-mechanical one of the Prius. But it does not allow the engine to operate always at its best RPM where the engine efficiency is maximum. Except if you consider a mechanical CVT, but this one is not very efficient.
Why is the pure mechanical transmission of Honda more efficient than the electro-mechanical one? I thought the electric mechanical one has the motor directly connected to the wheel so there is not much mechanical loss. So for a given output from the shaft, the transmission to the wheel should be about equal.

Right, the real life mechanical CVT is very dissipative so it cannot benefit a electric motor. But how much do you think a theoretically perfect (0% energy to operate) mechanical CVT can benefit an electric powered vehicle? Maybe 5% - 8%?

Does anyone know how the efficiency curve for the Prius motor look like?

Thanks.
 

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hou_ge2000 said:
Why is the pure mechanical transmission of Honda more efficient than the electro-mechanical one? I thought the electric mechanical one has the motor directly connected to the wheel so there is not much mechanical loss. So for a given output from the shaft, the transmission to the wheel should be about equal.
It appears you're talking past each other. I think you're calling the Prius transmission just the mechanical parts between the larger electric motor (MG2) and the wheels, whereas frenchie (and me for that matter) consider the Prius transmission to be the two motor/generators and the power split device. I call the mechanical parts between MG2 and the wheels the final drive assembly and differential. Actually, MG2 could be considered separate from the transmission, but it makes more sense to include it when discussing electrical losses.
 

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hou_ge2000 said:
Can someone explain to me in detail why the Prius is relatively inefficient on the highway than in the city?
Wind resistance (drag) is greater at higher speed, causing more energy loss. Also, highway driving is done without much stop-and-go, so there is less opportunity for the battery to recover energy of motion.

*All* cars are inefficient at high speed. Old-style cars get worse fuel economy in city driving than on the highway because they have no way to recover energy during stop-and-go. Also gasoline engine is inefficient when it has to run at many different speeds. Prius transmission allows gasoline engine to run at most efficient speed.



Can someone also tell me what's the ratio between the motor output shaft and the driven wheel?
There is no fixed ratio between the gasoline engine and the driven wheel. For detailed explanation see links at http://home.earthlink.net/~graham1/MyToyotaPrius/Understanding/
 

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Yes, see Graham's site at the URL that Richard published above. It's fascinating reading...yum, yum. And I say this honestly, with no sarcasm or anything. It's excellent reading, very informative, and....it DID NOT bore me (an amazing feat, to be sure).

Be sure to look at his moving GIF diagrams depicting the various functions of the PSD (Power Split Device; AKA, the planetary gear "CVT").
 

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Discussion Starter #16
Thanks, nice website.

I still have one more question that I didn't find the answer to on the website.

It's my knowledge that all the power of the Prius ultimately come from the IC engine. The engines powers or wheels or charges the battery.

But since ultimately all the power come from the engine, shouldn't the power of the car simply be limited to the power of the engine? How can a batteries' power count since it needs to be recharged from the engine, which takes power.
 

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The answer is that you don't need all the power all the time. Yes, the "real" power of the Prius is the power of its engine alone, and that's why its speed limit is relatively low : it corresponds to the maximum speed available with the engine power only, on a level road. But during normal driving the actual power you need fluctuates enormously, from high (positive) values to negative values, when you need to slow down the car or are driving downhill, so that you rarely use the maximum power of any car.

Think about a fluctuating curve that represents this power, as a function of time. Now smooth it, to erase the sharpest peaks and fill in the gulfs. If you remove from the peaks as much as you put in the gulfs, you haven't changed the total mechanical energy needed for the trip. But considering that the engine has better efficiency when it's operating smoothly, with few RPM variations and low or high rotations, you will actually use less gas. That's what grandmas are doing usually, compared to youngsters who drive more agressively.:wink:

Now can you make the engine think you're driving like a grandma while you're still quite young ? :idea: Simple, if you've got a Prius. During decelerations, you will store energy in the battery, that otherwise would be wasted (transformed into heat). That's for the lowest part of the power curve. Or even during cruising sometimes, the engine will deliver more power than necessary and one part of it will charge the battery, because doing so will make the engine operate in a better efficiency zone. Now when you need maximum power, the engine will not deliver all the power, instead some power will come from the battery. Of course this can not last forever, since the battery capacity is limited. But in everyday driving bursts of maximum power are usually quite short and you will never see an "empty" battery if you don't race with your car.

Furthermore, smoothing the engine power demand has other benefits : you can use an Atkinson cycle engine, which is thermodynamically more efficient but not good at producing high power, and therefore not used in conventional cars (or combined with a supercharger to compensate for this weakness). And if you combine mechanics and electricity you can make a CVT which is totally different from conventional CVTs, without any clutch or torque converter and achieves great efficiency.

What the Prius proves, is of course not that you can create energy from nothing, but that most cars are really wasting too much energy. 8)
 

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hou_ge2000 said:
But since ultimately all the power come from the engine, shouldn't the power of the car simply be limited to the power of the engine? How can a batteries' power count since it needs to be recharged from the engine, which takes power.
Simple: the engine does not charge the battery all the time. The engine charges the battery when the engine isn't doing anything else. The battery also gets some charge from the brakes when slowing down the car.

The total power available to the drive wheel is therefore the power from the engine plus the power from the battery through the electric motor. It should be clear that this maximum power can be sustained only until the battery is discharged.

My earlier reply over-simplified the operation of the engine. For more details see the Toyota technical website http://www.toyota.co.jp/en/tech/environment/ths2/index.html
 

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If I may add to Richard's comment....that there are two motor-generators.

Sometimes, my MFD shows the ICE driving the wheels, and sending electricity to the electric motor, which is also driving the wheels (with no juice coming from the battery). I assume that it's because MG1 is playing the part of G1 (generator) to provide electricity for MG2, who is playing the part of M2 (motor), and that there is neither any "leftover power" from the ICE to recharge the battery nor a requirement for "battery reserves" to suppliment M2. I'm guessing that the computer is treating it as a state of "equilibrium"...of sorts.

In these cases, encountering a steeper incline or stepping on the accelerator sometimes causes electricity to be drawn from the battery for extra assist. Conversely, letting up on the pedal slightly, or cresting the hill (and going "level" or "slightly down") may cause ICE-to-battery recharging to take place.

Ooooh, now my head is spinning. I should just let you engineers argue these fine points.
 

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Don't worry BIF, your head may be spinning, but it landed on the correct spot :)

The engine is a source of power. The power enters the power split device (the two motor/generators) and normally heads out toward the wheels. The battery is a reservoir, if the engine is producing more power than the accelerator pedal has directed to the wheels, the excess is sent to the battery. If the engine is producing less power than requested by the accelerator, the battery makes up the difference.

The engine changes power output slowly and the battery acts as a buffer to allow the quick changes in power we expect when we change the accelerator position.

When braking, the wheels become a source of power into the power split device. Since the engine cannot absorb power, all of this reclaimed energy goes to the battery.
 
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