Why hybrids get better fuel economy on highway?

A Miller cycle without a supercharger *is* an Atkinson cycle. Perhaps what they are driving about is that the true Atkinson cycle uses a dual connecting rod linkage with a lot of extra monkey motion to separately control the compression and power strokes. Atkinson did this principally to work around Otto's patent, but the net effect was good efficiency. The modern Atkinson cycle engine uses variable valve timing to accomplish the same thing without out all of the mechanical mess.

Tom

 

Bob,

MG1 only needs minimal power (enough to overcome losses maybe a few hundred watts) to provide the counter-torque if it is rotating at low speed.

The road speed for MG1 not rotating can be anywhere from 25 to 79MPH according to the simulator - for this range of speeds (assuming the road load matches the engine output) all of the engine POWER will go through the mechanical path with a small amount being converted by MG2 to cover the losses required to maintain the low speed rotation of MG1. It is not the case that 28% of the POWER goes through MG1.

The current through MG1 (equivalent to the torque it is providing) will probably not vary by very much under most conditions as the ICE is mainly operating under high-torque conditions to provide optimum efficiency.

I don't know about MG1 speed being unimportant; the ICE rotation speed is completely controlled by MG1 rotation speed (for any given road speed) to provide the ICE output power demanded by the driver (or cruise control). The throttle plate is not used as the prime power control mechanism.

kevin

 

This is cool. An argument that I have no idea who is right. Can we call Hobbit to join the fray ?

 

I enjoy reading these types of threads and hope one day to understand more of it!

 

Well, curses! The speeds where I do much of my driving seem to be exactly the speeds (c. 63-70) where the old NWH 20 does better than the new ZVW-30!!!

 

Sounds like we need some data:

No problem, I've got a Fluke Hall effect, current probe. I'll see if I can rig up some way to record MG1 current along with the Graham scanner data:

	Column 1	Column 2	Column 3
0	data	source	use
1	MG1 Amps	Fluke current sensor	calculate MG1 power
2	MG1 Volts	sensor coil	calculate MG1 power
3	traction battery volts	Graham scanner	calculate MG1 power
4	ICE rpm	Graham scanner	calculate ICE power flowing to MG1
5	MG1 Nm	Graham scanner	calculate ICE power flowing to MG1
6	MG1 rpm	Graham scanner	calculate MG1 mechanical power

.
Can you think of anything else needed?

I'll feed the Fluke meter data into a microphone input and record the analog values. Then using a 100 W. incandescent light, figure out the calibration constant and convert the Fluke data into a current flow. Now I'll only have one leg of the three phase motor but it should be easy enough to work up the total power.

Since the laptop has a stereo microphone, I can use my earlier coils to measure the voltage on that leg. Again, I'll use the 100 W light to figure out the calibration.

I'll have to write some software to calculate the power, the RMS power, but this should be fairly easy.

Bob Wilson

 

Bob, you my man are a bloody legend!!!

 

Hi folks,

This is how engineers figure out the world ... when theory fails ... grab your instruments and take measurements. I too puzzled over the power split device and that led to what I think is a better model. Let me explain the problem:

MG1 power = F( MG1 rpm, MG1 torque)
​

power = (force * distance) / time :: definition

In the case of rotary motion, it is

power = (torque * rpm) / K :: the "K" value converts units

So if the rpm goes to zero, the power required should also go to zero regardless of the torque. If a mechanical brake reached down and froze MG1 shaft, I would agree but there is no mechanical brake holding MG1 nearly motionless.

MG1's torque is generated by current flowing through the stators. This takes power and the question is "How much power?"

To the best of my knowledge, no one has successfully measured MG1 torque and mapped it to current and rpm. We have studies that show the back electromotive force (EMF) as a function of rpm. But I don't think anyone ever measured the power needed across the operating range of MG1 vs. ICE power. This will be a first, unless someone else has a study I've missed.

Before I got my Graham mini scanner, I had started developing my own instrumentation. I needed metrics on MG1 and MG2 and the coil allowed me to track the voltage applied. What I didn't have was the current and it is not just the average but also the phase angle. The power is the sum of the dot-product of voltage and current. Then I got my Graham scanner and a lot of things began to make sense except low rpm, MG1 power flows.

What changed it for me was realization that the torque is applied at the contact point of the gear teeth. At that moment, the forces are equal but opposite and the velocities are equal. Their respective power values have to be identical. This is the "conservation of energy" that this experiment will address.

Change you inertial reference to the ICE shaft and have 28% of the torque flow one way and 72% flow the other. The ICE rpm remains the same and the power flows will be directly proportional to the torque. The only way this can occur is if the electrical circuit is either generating or applying the same power in opposition to the ICE power. This experiment will map that electrical power.

It will take me a bit to configure and run the test. But it is new information and I wanted to make sure folks had a chance to comment on the procedure and methodology. BTW, normally these are the types of questions we puzzle over in "Prius Technical Stuff" but in this case, it 'leaked out.' The experiment will resolve the question.

Understand this is advanced stuff. I picked up a GM SAE paper, $15, that attempted to compare and contrast the "two mode" and "Toyota" transmissions. After reading the paper, I realized why GM "doesn't get it" and this paper was published April 2009.

One tricky part is the power is pulse-width modulated feeding an inductive load with a moving magnetic rotor. Inductors tend to react to sharp voltage changes with high voltage spikes. Then the rotating magnets add their part. This is not a trivial problem.

Bob Wilson

 

Check out my signatures. The today's Kelly Blue Book Value, $7,000, paid off, 2003 Prius is getting withing 1 MPG of the 2010 Prius with $19k remaining to pay off. But the 2010 Prius replaced a 2001 Echo that was getting 32-35 MPG ... except when my wife drove it.

Bob Wilson

 

What a great and fascinating thread!. Your comment re MG1 requiring ELECTRICAL power to maintain counter-torque to the ICE is the key, even though MG1 may be in a zero rpm state and thus not transmitting MECHANICAL power..
Surely Toyota themselves have mapped these variables somewhere? Is this something they keep under wraps for competitive reasons?

I'd be interested in reading the GM SAE paper you reference Bob .... is it currently available anyplace?

 

Toyota has had enough problems with patents and patent law, I'm just happy they've published as many papers as they have. But they are under no obligation to reveal unique, proprietary aspects of their systems. Some of the patent suits are still under limited access. I know, I was looking for every bit of information about how they worked in the past.

The SAE paper is at:

An Analytic Foundation for the Two-Mode Hybrid-Electric Powertrain with a Comparison to the Single-Mode Toyota Prius THS-II Powertrain

Correction, it was a Georgia Tech authored paper and they really got it wrong. I would give you mine but they have electronic keys so it is difficult to share. Hey, $15 isn't so bad but I regret buying it.

Bob Wilson