[bwilson4web]

Hi,

I'm beginning to form a new model for car mileage performance that sums all energy sources and includes what to me is a new element, running engine drag. It then becomes fairly easy to figure out how much energy is needed at any speed by using:

• rolling drag (at ordinary speeds)
• non-linear transmission drag
• 2nd order aerodynamic drag
• vehicle systems drag (aka., 12 VDC power load)
• engine running drag consumed producing energy

Now the only sources of energy are:

• engine
• stored 'hybrid' energy system
• kinetic energy
• potential energy

What I'm starting to do is what in electronics is using a similar to 'Thevenizing' a circuit only applied to a mechanical system. All sources of drag including internal engine drag, have to be accounted for since they rob energy. All sources of energy have to be accounted for including kinetic and potential energy.

The beauty of this approach is a few, fundamental measurements should allow fairly accurate predictions of vehicle performance. It also explains whether or not any 'proposed' hybrid has a chance of delivering acceptable mileage. But let me give a more practical example.

The diesel hybrid could use one of two architectures:

• IMA - single motor/generator turning all the time
• dual MG1/MG2 - with the ability to turn off the diesel and run on stored energy

If the diesel hybrid goes IMA style, it will be a disappointment because it does not have the ability to stop the diesel mechanical drag and sustain speed. But if it goes with the dual MG1/MG2 style, it would pose a significant challenge to the Prius because at low speeds, it could generate excess power saved in the battery and then play back that power with the engine off and the 'round trip' generator-traction-motor losses would be less than, significantly less than, the diesel mechanical energy loss. This is the part that was missing from my earlier understanding.

Bob Wilson

[jdenenberg]

Bob,

Don't you have to also account for the losses that occur when moving energy into and out of the traction battery?

JeffD

[bwilson4web]

Yes, all energy conversion losses have to be accounted:

• mechanical-to-electrical - both MG1 and MG2
• electrical-to-mechanical - same
• battery-charge and discharge losses - the absolute value divided in half
• transaxle gear losses

Most of these losses have been discussed in the commonly available postings. What is new, unique, is treating the engine internal losses, the mechanical losses as another energy drain.

Because we think of the engine as an energy source, much like we consider batteries to be an energy source, we know but often ignore that there is an internal, mechanical drag associated with the engine turning at any given speed and throttle position. We tend to ignore this energy loss much as many folks are willing to ignore the internal (I**2)*R losses for ordinary circuits. But if we are trying to predict how long a circuit will run based upon the stored energy of a battery, we have to account for the (I**2)*R losses.

What gave me a clue was thinking about diesel performance in City driving or low speeds. This is a low-power region where diesels do terrible. Then it hit me that diesels should have a fairly flat, internal overhead, power drain that should be proportional to their starter motor power. If I turn over a diesel with a 5 hp starter motor and it spins at say 800 rpm, that 5 hp energy loss does not go away simply because I now inject diesel fuel and produce just enough power than it is consumed turning the mechanical engine. Yes, the idle fuel consumption will be low but that is a Carnot effect having nothing to do with the actual loss of mechanical energy.

In past Prius analysis, we've been happy to include the vehicle 'electrical' overhead in the models, which has worked well. But I'd forgotten to account for the engine overhead, which the Atkinson cycle reduces. So to properly model either a Prius, Honada IMA, diesel or any other vehicle, we need to account for all energy losses including the internal, mechanical losses.

Yes, I know this seems obvious but it wasn't clear to me until a couple of weeks ago when I was discussing vehicle efficiency with a diesel advocate. Sad to say, I didn't explain it well enough to enlighten the 'oil burner' but then I don't have to. <grins> Once I understand it, I can then work on a model and gain insights.

Empirically, we know our 1.6L Prius have a maximum range at 18-20 mph. With a better model, we can be more precise as well as undertand the vehicle efficiency fall-off as a function of speed.

Bob Wilson

[FastMover]

It really ticks me off that GM is tacitly supporting the claims for several hundred miles per gallon on on Volt in the media. Such claims do not consider the wall power used for electrical recharge. It amounts to the same thing as omiting losses in a system, but sort of in reverse. It is not the math either - it is a relatively simple process to figure out how much electricity a gallon of gas can generate at power plant efficiencies. Then, just plug in the other standard EPA numbers and do the reverse math for the watts back to "virtual gallons" to get a more realistic picture of the Volt's efficiency.

I have no problem with the claims that the Volt is more efficient, as I simply view it as a next generation hybrid dispite what Chevy says. I just have a probem with the outlandishness of the claims being hyped in the media -- without a more realistic response from GM.

RFB

[bwilson4web]

I agree. GM has history doing this sort of bogus math with their E85, 'flex fuel' vehicles. They build and sell a vehicle in the USA market that really is designed for Brazil. Then they get the law written that allows them to claim some sort of fleet advantage because the vehicles have to potential to use E85. ... Hokum.

BTW, my studies of hybrid energy flows suggest the Volt should be a Prius contender but that is by no means assured. It has to do with the mechanical energy losses of the engine. We often neglect them, assuming they are part of the Carnot cycle but in reality, they are different.

Regardless, the first Volts won't be out until November, 2011, if then. So I'm not likely to 'hold my breath.'

Bob Wilson

[FastMover]

I notice that the all of the new GM adds featuring Ed Whitacre (Jr) inviting comparison and inviting competition always have a Volt in the background during the course of his narration. This when they know that a sellable production model is over two years away. Whitacre is not even including it in his plans for financial performance. Business Week (David Welsh) stated that Whitacre has said internally that he wants to see positive sales results in three months or there will be consequences for executives. Instant gratification or none at all?

RFB