67 hp (MG) + 76 hp (ICE) != 143

I am not an expert by any means, although as an engineer I think I know enough to answer your question. The horsepowers that are reported for each motor are the maximum horsepower at their respective optimum RPMs. That maximum happens at a different RPM for the electric motor than it does for the ICE. You basically need to plot a curve of hp vs RPM for each motor, and sum the two to get an approximation of the the total power vs. RPM. Since each curve has its peak at a different RPM, the peak of the sum curve is less than 67 + 76.

ICEs are notorious for delivering good power and torque over a very narrow range of RPMs, which is why they have such complicated 4, 5, or 6 speed transmissions. Electric motors have much better low-RPM torque and power than ICEs. I don't know this for a fact, but I suspect that this is why the Prius and other hybrids can get away with a simple CVT - because the electric motor provides a lot of torque at the ICE's weakest point in its power curve.

 

The Power Split Device is not a simple CVT.

 

Hi Michael...,

Some of the engine power goes to MG1 by way of the power sharing device in the transmission. From MG1, this power goes electrically to MG2. Think of MG1 as kinda an electric torque converter. The battery only has about 34 HP, the engine provides another 30 hp , so that makes up the 64 HP for MG2. So, the 76 hp from the engine is reduced by 30 HP. For a total of 110. This is what Toyota specs the drive train at.

If you doubled the size of the battery, and reworked the controls, then 140 HP could be realized.

But, your not thinking the way the Prius engineers were. The goal is accelleration, not power. Yea, you need power to get accelleration, but if the drive train limits power at low speeds, you do not get as much accelleration. Which is the case with standard cars with 4 gears. Car guys talk about torque, but what is really needed is full power convertered to the slow turning tires. And the Prius engine can rev quite high, get to full power, when the car is going quite slow. And its all that torque capacity of MG2 that allows this to happen. The Prius does not have any shifting delay either.

So, the synergy is that a small engine can provide accelleration that is well beyond what a standard car with that size engine could provide. Toyota says the Prius accellerates like a standard car of a 2.0 liter DOHC engine displacement. Which has about 140 HP. So, even though the car has just a 1.5 liter engine of only 76 HP and a total system power of only 110 HP, it performs like a traditional car with 140 HP already.

This kinda goes along with comments from various conservation people. People do not buy electricity, they buy lights at night. So, improving the light efficiency, allows for existing power plants to handle the load, rather than to build a new one. This is why power companies were giving away compact fluoresent lights at one time. They called this "negawatts".

In cars, people do not buy power, they are only marketed power ratings. What people are really buying is accelleration. In a Prius your buying "negaseconds", rather than ineffienct power.

 

This was answered already in the posts above, but I'm going to simplify it a bit for the non-engineers reading this thread:

The main reason total horsepower is less than the sum of MG2 and the ICE is that some of the ICE's power is routed through MG2. There is no way for the ICE to independently get all of its power delivered to the wheels.

Tom

 

Actually, the electric power is limited by what the battery can safely deliver. If the ICE is running, the system can convert ICE power to electricity using MG1 and feed it directly to MG2 (hence the high HP spec. for MG2). But if you're trying to run MG2 and MG1 as drive motors you quickly heat up the battery (and draw too much current from it for long life), so that is avoided.

You don't actually -need- any more power than the Prius delivers. If you want more, you bought the wrong car. You want one of those gas guzzling really powerful cars to compensate for something.

 

David, this is close, but not quite correct. It is true that battery capacity does limit what the electric motors can do, but that should make you ask another question: why wouldn't Toyota match the electric motor to the battery? The answer is that the electric motors are used as part of the transmission for getting power from the ICE to the drive wheels. MG2 needs to be more powerful than required for just battery operation, because it gets power from the battery and the ICE.

Let's look at the limiting case by replacing the existing battery with an infinitely powerful battery. For the sake of simplicity, I am also going to ignore the power contribution of MG1. It works the same way with MG1 added to the equation, but is conceptually more complex.

With the infinitely powerful battery, your Prius can now motor along in EV mode using the full 67 hp of MG2. Now let us bring the ICE on line. This should add another 76 hp, for a total of 143 hp, but it doesn't. Why not? The answer lies in how the ICE gets its power to the drive wheels. MG2 is directly coupled to the final drive train, so all of its power goes directly to the drive wheels, but the ICE sends its power through the PSD (Power Split Device). The PSD is a torque splitting device that splits a fixed ratio of ICE torque to MG1 and the final drive train. This is a key concept, so I'm going to say it again: the torque split from the ICE to MG1 and the final drive train is fixed; there is no way that the driver or control system can vary this ratio. The ratio of transfered power can be varied by changing the amount of power indirectly transfered through MG1 to MG2. Sending part of the power from the ICE through an electrical path is how the Prius varies its virtual gear ratio.

So now we have 67 hp from MG2 and potentially 76 hp from the ICE, but as we discovered in the previous paragraph, the ICE must send part of its power through the electrical path of MG1 and MG2. At 67 hp MG2 is already maxed out, so it is unable to safely accept more power from the ICE, but in the general case the ICE cannot transfer its full power to the drive train without sending part of it through MG2. In this case the control system has no option other than limiting the power from the ICE or the battery. The maximum battery driven contribution for MG2 is 67 hp minus the electrical power from the ICE. In the case of the real battery, Toyota's engineers have sized the battery to be consistent with this limitation.

Tom

 

Hi All,

I realized what David Beale is talking about after posting.

One could still realise 143 HP, but because of the size of the inverter, and motors, the accelleration would not be that much better. Because you would loose the torque multiplying capability. As the inverter is limited to 60 HP too. And with the torque split in the PSD, the engine RPM at low speed would be limited by that torque split.

So, you just be back where other cars are, accelleration for HP wise.

In other words, the 143 HP would only be useful at high speeds.

To get better accelleration with two batteries, one would need 1/3 bigger MG1 and MG2 and Inverter.

 

In normal driving, my Prius accelerates as well as my Boxster (2.7 L 6cyl). That sounds unbelievable, but the low end electric torque of the Prius is probably better than the Boxster. You don't really see major torque in the latter until 3K rpm or above. The Prius drives more like a 6 than a 4. So, pricewise you should compare it to a 6 cyl., not a 4 cyl. car.

 

It can't do that without being part of the chain - you can't have it both ways. MG1 is absolutely part of the primary means of getting power from the ICE to the wheels. The percentage of power passed through MG1 varies by speed and load, so yes, in this way it is a bit like a low gear, but it's not like MG1 is cut out at anything other than low speed. MG1's contribution is more of a smooth curve.

Tom

 

The ICE needs some of MG2's 67 hp for normal operation. That's the main reason you can't simply add the two horsepowers together.

Tom

 

In normal driving, my Prius accelerates as well as my Boxster (2.7 L 6cyl). That sounds unbelievable, but the low end electric torque of the Prius is probably better than the Boxster. You don't really see major torque in the latter until 3K rpm or above. The Prius drives more like a 6 than a 4. So, pricewise you should compare it to a 6 cyl., not a 4 cyl. car.

 

The motor develops full torque at zero rpm.

Tom

 

Hi Michael,

The Prius is not bad at standing starts, but no, its not great either. Primarily because engine takes seconds to come on and spool up. Its 10.5 to 11 seconds to 60. And from the standing start it does not feel like its accelerating fast. Remember it does not waste shifting time, so slower accelleration still gets the car up to speed quick.

The engine has no enrichment, to avoid the emissions that enrichement causes, so it starts at about 1000 RPM and then ramps to 3000 RPM slowly. Usually by 15 mph the engine is on and over 3000 RPM and then the Prius will accellerate crisply from there.

There are a few people who have done walk-on drag racing with the Prius, and they can tell you what works best. 17-18 seconds I seem to remember.

In side-by-side merging, the Prius is a match for allot of cars, if you keep the engine running. The electric pedal response is quick, and that is usually all it takes if you have a line-of-sight advantage over the in-line stubborn driver. It can confuse these drivers if you attempt a merge in electric only, and they close the gap, to then see what the Prius will do once you have the engine active.

 

Keep in mind, hiremichaelreid, that the Prius was designed for minimal emissions. Great gas milage is a byproduct of very low emissions. Low 0-60 or quarter-mile times are not fuel efficient, low emission modes.