Featured Gen 6 Prius engine will be a “game changer,” achieve a 53% thermal efficiency

BYD had a 43% efficient engine back in 2021. The next gen being better isn't a surprise.
Development of 43% Brake Thermal Efficiency Gasoline Engine for BYD DM-i Plug-in Hybrid

The not using traditional rounding is seems to be because of one up manship with Geely. They announced a 43.32% efficient engine soon after BYD released the 43% one. There is a government org certifying these values in China. BYD has a 46.5% thermally efficient engine; the 46.06% is the production model.
Geely and BYD argue over engines, plus other fallout from PHEV tech announcement

You were the one claiming it will be 50% here.

Currently, it is in a new Seal. BYD Seal 06 - Wikipedia
I'm sure BYD will use it elsewhere.

The Metro barely beats the Prius in the combined rating. If the EPA didn't have a thing for whole numbers, the difference could be as low as 1/10 or 1/100 of a mpg.

Cd is only part of the aero equation; the other is frontal area. The Metro is two size classes smaller than the Prius; it is a half foot shorter(height) and thinner. Wind may push harder on it, put there is good amount for it to push against. Then the Metro isn't saddled with ten more years of emission regulations. Fueleconomy.gov doesn't post emission ratings for cars that old, but it probably wouldn't pass the minimum of 2004, let alone the clean rating the Prius achieved.

It also had a 0 to 60 mph time of 15 seconds. With the patience required to drive that Metro regularly, a driver in the Prius could likely reach the original mpg rating

In that light, the two being close isn't that surprising. The Metro engine is working near its thermal efficiency peak most of the time. Which leaves it with little reserve power, and low power performance. The basic hybrid concept is to use such smaller engine, and have the electric side make up for the performance limitations doing so entails.

 

Yes, yes and yes.

But that wasn't the point.

A Geo Metro has around CdA of 6.24 (the 2-door Xfi may be different) whereas the 2nd gen Prius is about 7% better at 5.83 yet the Metro Xfi also has a new highway rating of 52 mpg whereas the 2nd gen Prius is only 45 mpg, 15% lower. How does a car with 7% better total aerodynamics get 15% worse fuel mileage?

Yes, emissions regulations and such has a lot to do with it, but efficiency is efficiency. Excuses, reasons or not, one is more efficient than the other. And if we compare that efficiency with, say, a diesel freight truck, it may be even more efficient. It may be apples to oranges, but if we get a +50% efficient engine in a 3-row SUV with a CdA of 28 and that costs $98,000, that efficiency is no longer all that impressive.

 

At first glance, looks like the Prius has about a 24% larger cross section, and is about 80% heavier. And in 1994, the National 55 MPH speed had not yet been fully repealed.

 

• The overall drag on the Prius is still less, despite being bigger.
  
• Weight isn't as much of a factor as one would think, especially for highway fuel mileage ratings, but maybe you have a point here to a degree.
  
• The 52 highway MPG the the current revised fuel mileage in terms of 2024 EPA fuel mileage testing standards, which has nothing to do with 55 mph highways. The original highway fuel mileage rating in 1994 in the Geo Metro Xfi when there was a National 55 mph highway law was 58 mpg.

 

Weight and smaller tires do affect highway fuel economy, aero isn’t the only consideration.

 

I'm not sure discussing a 'Press Release" performance spec makes a lot of sense. I'll wait until we can get test articles.

Bob Wilson

 

Back during the time frame of Geo's - the EPA test was much much easier to get high ratings on. That's why the Gen 2 Prius could easily get in the 60s MPG - even though it was a much much safer car to survive an accident in as well as many more modern features as well as roominess and comfort.
Fun backstory of the old EPA rules was, it's designed to make giant land barge's poor mileage not look so egregious. Then along came the the Prius getting 60's MPG & so the big 3 insisted that the EPA's get reworked to be more realistic & pull the Prius down into the 50's mpg. Cutting off their nose to despite their face.
.

 

The EPA city test cycle is nothing like what today's drivers consider highway driving. The max speed is 60mph, but the average is 48.3mph. It dips down to as slow as 30mph. At no point is there steady speeds. Acceleration rates are slower than what the Metro can do. While it is a lot slower than what goes on a highway today, the test is just fast enough that the Prius has to spin the engine for almost the entire time.

The test was almost custom made for the Xfi. Its lighter weight will play a part in the city rating with the slow downs and speed ups. Before we get to the tests, the weight difference will affect the cars' coast down results. The Metro likely ended up with lower resistance on the dynamometer. The Civic VX was available with the Metro. The Xfi could have been trading pollution for efficiency too.

For the Prius aero advantage to be seen, we would need to see the cars' fuel consumption at steady state speeds at 60mph and above. The Prius does beat the Geo in the city ratings. Enough that their original combined ratings were the same.

Slow acceleration, manual only, possible lack of A/C lead to low sales for the Xfi, and GM canceling it before the rest of the line up. The gen2 Prius had waiting lists because it gave the efficiency without the compromises.

 

Looks like our posts were mere seconds apart. Great minds rebut alike

.

 

The Metro isn't leaving anything in the dust.

The only reason in approaches the earlier Prii in fuel economy is because it is a tiny, low power, two door hatchback. The Prius c is a spacious hot hatch in comparison, and it does better on fuel use. The c has a Camry's trunk worth of added space. A hybrid Metro's fuel economy won't be much better than the Prius c, cause the electric side is going to be used to do better than 15 to 16 seconds to reach 60mph. Don't do that, and its fuel economy won't matter as no one will buy it. The original Insight would close to what a hybrid Metro would be.

 

Again, the point never was that the Metro was the greatest or worst car in history. All I wanted to say is that posting an efficiency number without context is worthless. Engines don't run at the same efficiency all the time, unless you have constant load and RPM.

 

Not quite. There have been several methodology updates over the years, and the older vehicles have been updated only part way, not all the way to the latest methods. E.g. the 1984-2007 model figures have been updated only to the 2008 method, not to subsequent updates.

IIRC, up through 2007, there were only 2 tests. 3 additional tests were added for 2008, and old ratings were 'updated' to 2008-equivalents by ad-hoc adjustment factors, not by re-testing any old models.

Also IIRC, 1983 and older models used just the original CAFE results, without any EPA de-rating.


EPA: 2017 Ratings Changes

"EPA updated its method for calculating the fuel economy shown on new-car window stickers starting with the 2017 model year.

EPA periodically updates its methodology to account for changes in vehicle technologies, driver behavior, and/or driving conditions. The 2008 changes (see below) were broad revisions to the entire methodology that affected every vehicle.

The 2017 change updates some of the calculations used to estimate fuel economy. The new calculations are based on test data from model year 2011–2016 vehicles. ...

...

Comparing New Vehicles to Older Ones
You may see 2016 vehicles with the old fuel economy estimates on the window sticker along side 2017 vehicles with estimates based on the new calculations. To help you compare vehicles with new and older ratings more easily, the estimates in Find and Compare Cars have been adjusted:

• The original estimates for model year 2011–2016 vehicles have been adjusted to match the revised 2017 calculations.
  • The ratings for most vehicles are unchanged.
  • Ratings changes are small for affected vehicles. Some go down by 1 to 2 mpg.
• Ratings for 2008–2010 model year vehicles are still based on the 2008 methodology.
• Estimates for model years 1984–2007 have been adjusted to be consistent with the 2008 methodology.

2008 Changes
EPA updated its methodology in 2008 to account for (1) faster speeds and acceleration, (2) air conditioner use, and (3) colder outside temperatures. Since all of these factors lower fuel economy, the 2008 changes lowered fuel economy estimates for all vehicles.

See How Vehicles Are Tested for more information."

 

There are plenty of other factors to go into the fuel economy rating. That doesn't make the thermal efficiency of the engine worthless. Much of the improvement the Prius saw between generations came from using a more efficient engine. Then full hybrids can maximize the amount of operation time the engine is running at that efficient point.

I thought there was always an adjustment to the number on the window sticker. They were aware the CAFE value was too optimistic from the beginning.

 

A quick search is not yet finding an answer. Different pages do indicate that ratings began in 1974 for model year 1975, with first updates in 1978 and 1986 (but not 1984 as I wrote earlier), but manage to avoid answering this particular question.

 

Without context it does.

The fuel efficiency in my 1985 VW Golf diesel is 100% efficient. I'm serious, it is. Some 25% might get to the wheels, the rest turns into noise energy and heat energy or chemical energy. But 100% of the fuel that goes in ends up leaving the engine in one form or another at some point, even if some of it leaves in the oil during an oil change.

 

Unless you have evidence that it is 100% reversible, no change in entropy, then it isn't 100% efficient by anyone else's definition.

Don't confuse Conservation of Energy (First Law of thermo) with 100% efficiency (Second Law ...)

 

Now that's what I call context!

Still:

• Efficiency under what circumstances (average, maximum, minimum, etc.)?
• What is the energy being used to overcome (how much air drag, how much rolling resistance, how much is needed for acceleration and lost during deceleration, etc.)?

I'm not trying to start an argument, but throwing out a number on efficiency all by itself is mainly just click-bait, a bragging right, something that might indicate a great advancement to lower overall fuel consumption, or is just put in there to make the company look good even though the reality is that the overall fuel consumption is still poor for what can actually be done.

I'd like to see what kind of energy is spent to move stuff in a vehicle on a graph, divided into people and weight, starting from 1) just one person to 2) a full vehicle.