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Trend to Heavier, More Powerful Hybrids Eroding the Technology’s Fuel Consumption Benefit

28 March 2007

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Trends in average hybrid performance: curb weight, net hybrid system peak power, and fuel consumption. Variability across models is indicated by the error bars. Click to enlarge.

The market trend to heavier, more powerful hybrids is eroding the fuel consumption advantage of hybrid technology, according to a study done by researchers at the University of British Columbia.

The study, which compares hybrids for sale in the United States in 2007 to equivalent conventional vehicles, appears online in Environmental Research Letters.

Researchers Milind Kandlikar and Conor Reynolds used linear regression models to analyze how vehicle weight and power affect the fuel consumption of hybrid electric vehicles (HEVs) and internal combustion engine vehicles (ICEVs) independently, and then compared the set of currently available HEVs (2007 model year) against a functionally equivalent set of conventional ICEVs. The equivalent set are ICEVs of the same make and model, and with similar power.

From 2000 to 2006, the sales-weighted average hybrid-electric vehicle in the US fleet has changed significantly, driven largely by the introduction of new sports-utility and high-performance HEV models. The average curb weight has increased by 30%. Propelling this larger weight is a hybrid-electric system that delivers 60% more power. The gasoline engine component of this system is 43% larger in terms of engine displacement. Some of the observed net power increase is explained by the need to provide a larger vehicle with acceptable performance.

Over the same period, however, the manufacturer-reported acceleration times also increased: the average HEV in 2004 reaches 96.6 km h–1 (60 miles per hour) from a standing start in 20% less time than the average in 2003. Because vehicle weight and power both strongly influence fuel consumption, it is not surprising that average fuel consumption has gone up by 15% with the shift towards higher-performance HEVs.

Kandlikar and Reynolds chose two parameters to characterize the changing nature of HEV technology: the ratio of net system power to vehicle curb weight and the ratio of power to engine displacement.

Until 2003, with only the Prius and the Insight on the market, the parameters stayed approximately constant. Since then, the increased power per unit vehicle weight indicates a trend towards new vehicles having higher performance—similar to what has occurred across the entire light-duty fleet, conventional and hybrid. An increase in the power to engine displacement ratio indicates that the newer HEV technology has significantly improved.

Even while bulking up, HEVs still offer a benefit compared to their conventional counterparts. The fuel consumption penalty imposed by increased vehicle weight is significantly lower in HEVs than in equivalent ICEVs, according to the study, with a 100 kg change in vehicle weight increasing fuel consumption by only 0.4 l/100 km in HEVs, compared with 0.7 l/100 km in ICEVs.

Three different equivalence models in the study (based on different comparisons of HEVs against ICEVs) yielded average fuel consumption benefits ranging from 2.7 to 3.25 l/100 km for the hybrids, with varying effects of changes in weight and power.

When the hybrids are compared with their ICEV equivalents (grouped into cars and SUVs), the average fuel consumption benefit of an HEV was 2.65 l/100 km. However, the analysis found that an HEV that is 100 kg heavier than an identical ICEV, holding everything else constant, has a fuel consumption penalty of 0.15 l/100 km. Likewise, an HEV that is 10 kW more powerful than its ICEV counterpart results in a fuel consumption penalty of 0.27 l/100 km.

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March 28, 2007 in Fuel Efficiency, Hybrids | Permalink | Comments (21) | TrackBack (0)

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This is a somewhat obtuse way of looking at hybrid developments. The first hybrids were essentially technology demonstrators, and maximized fuel efficiency to prove hybrid capabilities. Now that more market vehicles are being hybridized, of course the weight and power is increasing. It shouldn't seem strange to anyone that manufacturers tend to concentrate on their high margin products for hybridization, most of which have a (justified) bad rap for their horrible fuel economy.

Yes, it would be great if the manufacturers were also decreasing the weight and power of these vehicles at the same time. Unfortunately they've been selling the "bigger is safer" line for years now, and they are afraid that lightening their vehicles will slow sales. Power is a similar conundrum for them. The fact that these vehicles are receiving hybrid treatment should be seen as an all-around good thing, as it decreases the cost of hybrid systems. This helps us toward a goal cheap light hybrid vehicles. The same process will probably occur with PHEVs.

Just my 2 cents.

Interesting demonstration that increased weight + more power + better acceleration generally = increased fuel consumption, for both ICEVs and HEVs.

However, if we compare apples with apples (same vehicle) Ex: Toyota Prius I versus Toyota Prius II, this mathematical model does not give the same result and the conclusion does not hold, because:

The Prius II is 5.9% heavier, 9.3% more powerful + 19.7% better acceleration than the Prius I but uses 14.6% less fuel instead of more.

The Prius III will also defeat this mathematical model by a wide margin because it will use significantly less fuel than the smaller, less powerful, slower Prius II.

Many (most) second generation Toyota HEVs will defeat this mathematical model in 2009.

Hi Zach,

Its not that they are not decreasing power. They are increasing it, using the hybrid as a kinda of eco-turbocharger.

Some of this has to do with the chasis being designed for a V6, instead of a large I4. The I4 hybrid system can have the same accelleration performance as the V6, and allot better fuel economy.

That will probably take another design cycle to be able to accomadate the smaller engine. Hopefully.

If Toyota used the same 3'rd generation tech to make a same sized/weight vehicle as the current one it would get even better mpg then the 3'rd gen will get.

We still don't know exactly what the 3'rd gen will be, though we are told may be a more powerfull Lith Ion battery, a turbo charger or turbo generator and a larger 1.8l engine.

Well of course. Technology cannot solve economic and political problems. Switching people to 50 mpg vehicles will not affect total energy use or ghg production, if they just use their more efficient vehicles to drive farther.

No they won't.  People who are already spending several hours a day commuting are not going to have any more time to drive, and nobody with a brain is going to try to adopt a longer commute over already-packed roads.

What Toyota and all the other carmakers are truly afraid of is a widespread exposure of the auto buying public to true series-hybrid electric vehicles. By maintianing 6 cylinder engines in their larger hybrids Toyota prevents the conversion to PHEV's aka the Calcars modified Prius.

Producing power-tuned hybrids allows them to fake green credentionals along with their consumers while still producing oversized intimidation machines.

The Toyota Prius has already been proven to be an major improvement in maintenance costs over a same year Camry. The truth is that electric drive systems remove the neccesity to replace cars as often as the automakers would like.

Without government insistence automakers are not going to produce battery electric vehicles or PHEV's simply because they would destroy long term sales. Electric power and traction systems are simply superior to ICE engines for most applications.

This study fails to account for several factors...
For example, as these hybrids become more powerful, more consumers buy them and they displace other less fuel efficient cars in the same market segment... thus resaulting in a net DECREASE of fuel consumption.

Also it fails to take into account the % of change vs non-hybrid vehicles of the same class. Example: the Hybrid SUVs have MORE of a gain vs thier non-hybrid counterparts than say, a Honda Insight vs a CRX HF, or a Prius vs an Echo, or a Cvic hybrid vs a Civic lx. And thus these SUVs have a greater impact on net fuel consumption, despite that they get less MPG than an Insight... because of the vehicles they displace in the marketshare.

You aren't going to reduce net fuel conumption by just making hybrid gokarts and scooters... people just aren't going to buy them... Honda cancelled the most fuel efficient hybrid on the market because only about 500/year were bought... pure MPG doesn't sell if the car won't suit people's needs and interests, and if people don't buy the cars, then no difference is actually made.

"Without government insistence automakers are not going to produce battery electric vehicles or PHEV's simply because they would destroy long term sales."

Pangolin,

Consumers offered PHEV/BEV with a 60 MPG equiv, a 250 M range, and significantly reduced maintenance over an extended vehicle life - will make the pocketbook choice. Big automakers are not going to leave it to startups and the Chinese to grab this huge market share.

As for replacement sales -- improvements in the above specs, expanding world markets, design elements and consumer demand for "new" product will keep volume sales on a par with the designed obsolescence of ICE vehicles.

gr:

It seems that Toyota forgot to incorporate enough 'designed obsolescence' into the Prius I and II. Many Prius are still going strong after 300 000+ Km. That is almost 2X the average ICE units.

Will the Prius III (and other good ICEHV) reach 450 000 Km?

Will Series PHEVs last longer yet?

Will mass produced PHEVs and BEVs be designed to last 20+ years and 500 000+ Km or to fall apart after 10 years and/or 160 000 Km? Only time will tell.

I agree with most posters above. This is not really a good comparison.

Another way to do this study better would be to weight the data by number of cars sold. Currently, the powerful Lexus models and the Accord are totally skewing this analysis. Those sell only a few hundred per month (at most), while the Prius sells on the order of 10000 per month.
I bet the data would look a LOT better if the just weighted it for actual sales.

The general theme here seems to be, sure, the heavier, faster vehicles get worse gas mileage than say, the Prii of the world, but that's just the way the market is. The market is a complex beast but consider that most of the auto advertising on TV emphasizes monster trucks, monster SUVs, power, and speed. The advertising emphasizing efficiency is rare. Speed and size are a cancer on this nation and the world. We took cigarettes off the air; perhaps we should pull these irresponsible speed and size ads off the air. Or, are people arguing that advertising makes no difference. If so, that means that auto companies are awfully stupid wasting tens of millions of advertising dollars per year.

Yes, a hybrid SUV is probably an improvement over a conventional SUV but we are beyond the point at which these marginal improvements will get the job done with respect to energy security and environmental security. If mpg get marginally better and the planet warms up by 5 degrees or more, the whole exercise is a waste of time. Besides, these marginal, pathetic improvements in gas mileage will take years to have much of an impact on the overall mpg of the average vehicle.

And I am certainly not just picking on the domestic auto companies. The most obscene advertisement this year on TV is the one for the Toyota Tundra truck. The message is, the bigger the better. Screw the planet. They'll do anything for more market share.

Where are Toyota's hybrid trucks?? GM and Chrysler are close to production. Toyota is silent???? Starting in fall you can buy the same GMC and Chevy to pull your boat or use it on the Job site and get significantly better mileage especially in the city. Also what about Toyota's lack of cyclinder deactivation??

By the way the CRV, Hylander and Lexus 350's are NOT trucks but tall cars. They have no utility in work terms and only move people.

Maybe I am not reading the graph properly, but it looks to me to declare that current hybrid technology on average improves fuel efficiency by 15%. So you can drive a vehicle that weighs 15% more and still get the same mileage. That is pretty good for an average which may include the hybrids in name only vehicles. As others keenly pointed out, we need to see the Prius III in the mix with the first Lithium Ion battery. This should allow perhaps a 25% improvement in fuel economy, ie. instead of the Prius II 45 MPG, the Prius III may get 52 MPG. And if a Hymotion mod was done, to increase the battery capacity to 12 KWH, with a plug, the annual fuel cost (gas and electric costs) would drop by perhaps 50%, because 60% of the miles would come from the plug.

...almost the same story can be told about the Diesel.

In Europe they are expierenceing the following phenomenon: more Diesels on the street mean more bigger and heavier cars, more power under the bonnet, more oel consumption per car, more harmful emmissions in the air, increasing C02 emissions per Diesel but more profit for the car makers.


They tried this with the Accord V6 hybrid and the sales were low. The Highlander V6 hybrid has had better sales, but it is an SUV. It would be better if they asked the people what they want to buy before they go to all the expense and trouble to make something that does not sell well.

sjc,

You don't think they have a planning department that tries to assess the needs of the market? They just messed up on the Accord Hybrid...I wonder if they all still have their jobs.

Product Planning is my bread and butter. It certainly isn't as easy as talking to 5 or 6 average joes and asking them what they want...in fact, their samples were probably off and focus groups consisted of too many like minded people which lead them to believe the Accord Hybrid is what the people wanted. You don't just slap together a product without doing some research ...their research was obviously faulty or a sudden change in the mind's of consumers (sudden high gas prices??? and the idea that hybrids should save you a bundle of money on your gas expenditures) caused a bit of a shift.

I know a bit about marketing, but it seems like the people that do it have their heads up where the sun does not shine. It is about time they did a better job, or start looking for real work.

I live in NY and both the people driving here and the commecials I see on tv are ridiculous. Marketing of cars is focused on the fantasy whims of post-pubescent kids under the age of 25. They drive like they are playing a video game and will contiually want more speed. Hybrid technology should be ADVERTIZED... it is not. I own a ford escape hybrid to drive my 3 young children around and love getting over 30+ mpg (in a nice size vehicle) when I hit a stop sign and light every block.

Hybrid technology is not needed on military sized vehicles that get 10mpg (and would increase to 13mpg with hybrid). Also, the net dollar savings between a honda civic that gets 35mpg (my other car)and a prius that gets 45 is not that great to justify all this controversy.

What this country needs is hybrid technology on familiy-oriented, people-mover vehicles like small SUVs, and minivans that make short trips with frequent stops, and is the bulk of america's vehicles. The folks are the ones who have the most money to spend, because they must spend it on their family. This type of vehicle has the most to gain because my ford escape is getting 30+mpg rather than the 15mpg the gas version gets (50% savings!). Also, the batteries and other mechanics could easily be added to minivans with the long, flat floors.

HOWEVER, THIS WILL NOT HAPPEN BECAUSE THE CAR COMPANIES AND THEIR ALLIES HAVE TOO MUCH TO LOOSE! TIP: WATCH THE DOCUMENTARY... "WHO KILLED THE ELECTRIC CAR"...

i think cars are fabulous! yes tremendous! *smiley face*

Amen, Brother! and every other car in the fleet, too.

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