(edmunds.com) Fuel Sipper Smackdown 2

That's a good one! The Prius whupped 'em all by a fair margin in FE.

Prius also has the most cargo capacity of all the cars tested here.

I saw 3 more '10s today during an hour drive - 2 Blizz's and a Gray (Lots of Blizz's out there!). The more I see, the better.

 

There were 3 of us at the same intersection today! 2 Silver and a Red. Sadly, I had just passed another (Black) just a minute earlier. 4 at once would have been truly amazing.

Back on topic, gotta love those results... Prius - Jetta TDI:

City: 48.7 - 31.6

Highway: 47.4 - 40.6

Back Roads: 47.2 - 41.2
.

 

What I especially liked was:

My experience with after market tires strongly suggests anyone seriously measuring mileage needs to calibrate their trip meters. This can be done either using mile markers on a highway or a GPS. Regardless, it needs to be done.

The second is the accuracy of the Prius MPG gauge appears to be a function of the driving style. My wife, Edmunds and even Consumer Reports have from their low mileage, a tendency to be in the 'red bar' part of the HSI. Speculation on my part, I suspect the cooled exhaust gas operation at high power settings may be associated with this error.

There are two ways to measure fuel consumption: (1) mass flow, and (2) injector timing. I suspect mass flow measured fuel consumption is most impacted by cooled exhaust gas operation. My understanding is the fuel injector timing based scan gauge may be more accurate.

NOTE:


Bob Wilson

 

The lung-cancer factory has failed, AGAIN.

 

"The Insight showed the limitations of its mild-hybrid design, as its air-conditioner does not work when its engine automatically shuts off at stoplights."

OMG did they not think this was a slight design flaw? I have lights that do not change for almost 90 seconds. Admittedly in Southern California this is not as big of a deal (high temperature today: 85 F) but in places like Arizona or Las Vegas I imagine this would become uncomfortable quite quickly. Heck, even here we'll still get the occasionally 100 degree day.

 

Wow . . . I wonder how the poor TDI folks'll take this 'smack-down' ... sheesh ... even the guzzlin' Ford hybrid ate the TDI's lunch. Sure puts a different spin on some of those goofy TDI commercials showing that they get a bazillion mpg's above the EPA results.

'

 

The resident TDI folks are awfully quiet about this. Kinda sucks for them when the story they've been spinning gets blown apart so thoroughly. Emunds did what they could to throw the TDI a bone, but there's not any meat on it.

 

Maybe it can be a sticky for awhile and when needed to answer blah blah blah just point them to it.

 

What specifically about HCH-II's electric AC system is better than the GenII Prius' electric AC system? I'm curious, I haven't really studied either of the AC's.

 

Interesting, that would make it more efficient at times. But I wonder if there is any sort of persistent hit for whatever clutch/belt mechanism is running the compressor from the engine? It might be a small effect, but if it is always there it might offset or exceed the net of the conversion loss.

I remember having an underpowered K-car with a problematic AC clutch decades ago (the shaft had been messed up by the previous owner, probably during an accident repair, so sometimes the clutch would stick and I would have to fuss with it.) I removed the AC belt except in summer.

 

You are very close because the relative energy demands of an electric versus a mechanical system:

• electrical - the energy load is independent of engine rpm
• mechanical - a variable energy load following engine rpm

An electrical system will have an energy draw driven by the load and independent of how fast the engine spins. But a belt driven system, even with the clutch open, will have an energy draw proportion to the engine rpm. The bearings and belt flex will draw power in proportion to the ICE rpm all the time but under load, these losses become worse.

A belt drive alone does not give one A/C system an advantage over another without considering the whole A/C system. The Prius New Car Features discusses the 2010 air conditioner:

4,920 W - capacity
240 W - power consumption (~1/3 hp)
​

This is very efficient for an A/C system, which includes an advanced condenser with two distinct compressed gas flows.

BTW, a belt drive has losses too:

A Department of Energy study, ORNL/TM-2006/423, found typical Prius motor efficiency in the 90-93% range and it would reasonable to expect the generator efficiencies to be similar. A 2-5% difference in power transfer efficiency is pretty small especially when looking at a worst case 240 watts of the 2010 Prius.

So as the vehicle speed increases, the A/C load as a fraction of total power remains flat for the electrical A/C but a mechanical drag makes a belt driven compressor have a linear increase in energy losses. This data suggests a Prius with the windows up and A/C running may have measurably better mileage than with the windows down. The return, highway data suggests this may have played a part:

The Honda Insight showed an uncharacteristic loss of highway performance. Since they mentioned the heat in the City test, I suspect the mechanically coupled, compressor of the Insight might have been the reason for the dramatic loss of mileage. Now if the Jetta TDI had a better coupling mechanism or internal valving to reduce compressor load at high speeds, the data begins to make a lot more sense.

Bob Wilson

 

Bob, could the Insight's decreased highway efficiency also be due to a 1.3L engine coupled with a CVT having to maintain higher RPM's to maintain speed?

 

Don't think Insight owners will be happy to find their insides are comparable to a 1st generation Prius...

OTOH, the other day I parked next to an Insight, dark gray. Very attractive looking car. And 40 mpg is probably better than 95% of cars out there. There's nothing wrong with purchasing an Insight!

 

Hi Bob, I'm not sure if this is common on all vehicles nowadays, but the Jetta has a variable displacement A/C compressor that may help to reduce the load.

 

There are many possible sources of the lower Insight, highway mileage and the A/C was one. The Jetta TDI mileage improvement is disproportionate to what I would have expected and having the vehicle tuned for high-speed, highway driving would make sense including better accessories. But I am bemused at the prospects of Prius owners and advocates 'doing the engineering' for non-Prius vehicles ... in the absence of a test article. <GRINS>

Speculations, hypothesis, wild *ss guesses ... these questions really need to be addressed by the specific vehicle advocates in their own 'backyard.' I mean really, a Prius guy trying to figure out why an Insight didn't get better mileage compared to a Jetta TDI? ... What is wrong with this picture?


Bob Wilson

 

Bob, the ScanGage use's mass air flow to determine MPG. So far on my 2010 the ScanGage has read around 2 MPG lower than calculated while my HSI display has been 6-7 MPG higher.

Wayne