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

Discussion in 'Prius, Hybrid, EV and Alt-Fuel News' started by Gokhan, Jun 7, 2024.

  1. Gokhan

    Gokhan Senior Member

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    Thermal efficiency is thermal efficiency. There is only one definition. The parentheses is meant for laymen who might not know that it is related to fuel economy.

    Toyota already submitted a paper on October 24, 2022, reporting that they have achieved a 50% thermal efficiency in a super-lean-burn engine (stoichiometry ratio λ = 2.5) without generating excess NOₓ emissions. They seem to require some ethanol in the fuel to achieve that target though. They have developed new ignition, flame-propagation, and unburned-HC-reduction methods. The engine does not require rpm limitations as Nissan's does. There has probably been even more development in the year and a half that has passed.

    Development of 50% thermal efficiency S.I. engine to contribute realization of carbon neutrality—Toyota Motor Corporation

    Here is the first figure from the paper.

    [​IMG]

    They explicitly put the axis-break symbol ≈ in the vertical axis to show that it was not to scale, but they did not do that for the horizontal axis. So, if the horizontal axis is not to scale, it is deliberately misleading.

    [​IMG]

    Again, we won't know the actual number until the engine goes commercial, which is at least three years from now. In fact, even Toyota doesn't know the actual number, as the engine is still under development.

    Regarding hybrids, probably all Toyota cars using these engines will be hybrids, as Toyota will likely only make hybrids in the future.

    Gen 6 will likely be the last Prius generation, and it will also showcase the pinnacle of the internal-combustion-engine technology.

    Last but not least, the brake thermal efficiency seems to be directly proportional to the brake mean effective pressure (BMEP), which is the torque per engine displacement times a constant; so, a 30% increase in the engine output could indeed result in a 30% increase in the thermal efficiency. These are exciting times to come for the next-generation hybrids, and we will look forward to Gen 6 Prius, which will mark the sunset of the ICE in 2035.

    The following plot of the brake thermal efficiency (BTE) vs. the torque per engine displacement (brake effective mean pressure (BMEP)) is for a diesel engine.

    [​IMG]

    Here is a Youtube video of the conference.

     
    #41 Gokhan, Jun 7, 2024
    Last edited: Jun 8, 2024
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  2. Gokhan

    Gokhan Senior Member

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    It is no more or less “silly” than the current engine with a 41% efficiency or the previous engines with a 30% efficiency or a 20% efficiency. Thermal efficiency directly correlates with the EPA mpg numbers, especially in hybrid configurations.
     
  3. Mr.Vanvandenburg

    Mr.Vanvandenburg Senior Member

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    Ceramic engines could increase efficiency.
     
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  4. Rmay635703

    Rmay635703 Senior Member

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    There is a great deal of evidence that efficiency trumps pollution controls.

    ceramic engines pump out tons of nox a simple water injection system would solve this but that’s too complicated
     
  5. Gokhan

    Gokhan Senior Member

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    Here are more details on Toyota's all-new 50%-thermal-efficiency engine.

    Development of 50% thermal efficiency S.I. engine to contribute realization of carbon neutrality—Toyota Motor Corporation

    The idea is basically a super-lean-burn engine (stoichiometry ratio λ = 2.5) but without the excess NOₓ. That is achieved by ignition and flame-propagation control and unburned-HC reduction. They also suggest the use of ethanol etc. fuels to reach the target of a 50% thermal efficiency. The engine does not require rpm limitations as Nissan's does.

    Here is the the first figure from the paper.

    [​IMG]
     
    #45 Gokhan, Jun 8, 2024
    Last edited: Jun 8, 2024
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  6. Zythryn

    Zythryn Senior Member

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    I have to chuckle at their “Future Direction”.
    While that is their future direction, the manage to just crest 50% of currently available motors.
    Heck, they use some in a few of their vehicles:eek:;)
     
  7. ETC(SS)

    ETC(SS) The OTHER One Percenter.....

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  8. hill

    hill High Fiber Member

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    Not sure if this graphic is more bogus or Toyota's solid state battery that was supposed to come out is more bogus.
    Here's a great read on reciprocating engine thermal efficiency & how load / work / temps / smog devices play into theoretical Max's:

    Aircraft Reciprocating Engine Efficiencies

    Then you throw ethanol production on top of the formula? Petro chemicals for fertilizer production? For Insecticides? For running farm machinery? For Fermentation?

    Great, Toyota is trying to match 50% that's already achieved. Drawing a pretty picture of a yellow oval much much much higher on a pretty graph? It's another toyota solid state battery going into production.
    .
     
    #48 hill, Jun 8, 2024
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  9. Trollbait

    Trollbait It's a D&D thing

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    Toyota is using the strategies of the Honda Civic VX for lean burn in a turbo charged engine; tumbled air flow in the cylinder with a stratified fuel charge at the spark plug to get combustion going. The test engine was a modified one from the new Tacoma, Toyota Dynamic Force engine - Wikipedia. They are working on the tumbling through cylinder and piston shapes. The stratified charge comes from two direct injectors. One on the side provides the lean mix during the intake stroke, and the second squirts in a little fuel by the spark plug just before ignition.

    Lean improves efficiency, and actually produces less NOx. The ratio with HC and CO is just out of whack for the 3 way cat to clean up the NOx though. That's the problem Toyota faces with just modifying their current engines for stricter regulations. Making them run lean for better efficiency and carbon emissions is easy. Cleaning up the exhaust means adding SCR and other equipment used on diesels. Burning less fuel per cycle will also mean less power.

    Higher compression ratios also increase thermal efficiency. Cylinder temperatures rise with that, leading to more NOx and knocking. Ethanol has a cooling property on the cylinder. Toyota probably is using it for that reason in the 50% thermal efficient engine.

    This paper didn't go into the combustion properties and emissions, but from figures 2 and 3, they needed to use hydrogen injection to hit the target cylinder temperatures and NOx production.

    The paper is from last year. These prototypes are using that knowledge, but unless they are using an eturbo, they'll need not readily available fuel additives to reach Toyota's claims. The prototype with high efficiency claim isn't a turbo though, which is why I believe it will be used in series hybrid with a narrow operating range like the Nissan e-Power.
     
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  10. fuzzy1

    fuzzy1 Senior Member

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    Many bad conclusions are based on such assumptions, which then turn out to be faulty.

    Until I see it in actual print from Toyota ...
     
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  11. austingreen

    austingreen Senior Member

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    There was a lot of talk of this when battery prices were higher. Mazda did a capacitor based regen braking. I think battery prices just dropped too much but there is hope for a battery/ultra cap hybrid system in the future to produce higher power levels.
    https://www.caranddriver.com/news/a18739871/mazda-introduces-i-eloop-capacitor-based-regenerative-braking-system/

    Today BMW uses a lithium ion 0.5 kwh 48v battery which only weighs 10 kg and costs a lot less than capacitors and the additional electronics they would need.

    The 50% e-power engine from nissan was turbocharged for heat recovery. You aren't going to get there without some heat recovery. Hydrogen injection is an option but water injection (water + methanol). Until I see toyota with the engine in a car, I do not believe they will get past 46% without heat recovery. It is likely some marketing person misunderstood and ignored that their was water injection or turbo charging involved.

    When you are leaning out the engine then it needs to be physically bigger or have higher compression. That higher compression will lead to more pollution if the air is not already compressed when it gets to the engine. I do believe that they are close to a 53% efficient engine but do not believe at all the graphs for normally aspirated engines. We will see in 3 years if Toyota actually provides one.
     
  12. Gokhan

    Gokhan Senior Member

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    Note that hydrogen was used in an engine with homogenous lean burn, as otherwise, they couldn't achieve combustion at a stoichiometry ratio λ higher than 1.8. They first wanted to see what happened when they increased λ before they developed the test engine. The test engine did not need hydrogen, as it had turbulent lean burn.

    Google AI counts many advantages for ethanol in lean burn:
    • High laminar velocity: Ethanol burns fully during combustion
    • Low stoichiometric fuel-air ratio: Creates a dense air-fuel mixture
    • High latent heat: Creates a dense air-fuel mixture
    • High self-ignition temperature and octane rating: Allows engines to run at higher compression ratios without knocking
    • Minimum ignition energy input: Can operate in leaner conditions without misfiring
    • High flame speed: Can operate in leaner conditions without misfiring
    • Wide flammability range: Can operate in leaner conditions without misfiring
    Also note that the test engine had a longer stroke-to-bore ratio than the original engine. The all-new engines Toyota showcased have short stroke-to-bore ratios. So, they are not quite the same design.

    Toyota is developing both the naturally aspirated and turbocharged version of these engines. It will be interesting to see if they can achieve super lean burn without a turbocharger and reach 50% or higher thermal efficiency. I am guessing more developments have happened since this paper.
     
  13. Gokhan

    Gokhan Senior Member

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    It is a heat engine. The second law of thermodynamics limits the maximum theoretical efficiency to the Carnot-cycle efficiency. Therefore, it is not easy to reach a 50% or higher thermal efficiency in a heat engine. You would have to have a very high ratio of the combustion temperature to the exhaust temperature, as the efficiency for the maximally efficient Carnot cycle is given by 1 − (Tcold/Thot), and for the realistic so-called endoreversible cycle known as the Chambadal–Novikov–Curzon–Ahlborn cycle running at maximum power output, it is given by 1 − √(Tcold/Thot), with temperatures expressed in kelvin. So, considering that the temperatures are expressed in kelvin, to archive 50% is very difficult with practical ratios of the combustion temperature to the exhaust temperature.

    Carnot cycle—Wikipedia

    In fact, even natural-gas power stations run at a thermal efficiency of slightly below 50%. See Table 5 in page 16.

    https://efiling.energy.ca.gov/getdocument.aspx?tn=233380

    So, considering this, what Toyota has been developing is truly amazing.
     
    #53 Gokhan, Jun 8, 2024
    Last edited: Jun 8, 2024
  14. Zythryn

    Zythryn Senior Member

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    No doubt, pushing an ICE engine to that level of efficiency is impressive.
    Making a bridge out of legos that will support 600 lbs is also impressive.

    That is, until you consider other building materials…
     
  15. Trollbait

    Trollbait It's a D&D thing

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    Truly amazing in that others have done it, some with retail available fuel, in cars that can be purchased.

    What Toyota is doing isn't novel. Honda had very lean burn engines in production cars in the late 1980s and early 1990s. Gasoline engines the reach 50+% are already operating. Just getting a very efficient engine that is smaller, clean, and affordable would be great news for PHEVs and carbon neutral fuel adoption.

    I'm not objecting to Toyota being able to reaching such high efficiencies, just to claims that they have based on promotional material presented to the public and press. Toyota themselves haven't said anything about thermal efficiency. Their statements of improved fuel economy are based upon these engines allowing more aerodynamic body designs.
     
  16. Gokhan

    Gokhan Senior Member

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    Of course they have. They have provided figures showing the thermal efficiency in their presentation, but they haven't given exact numbers.

    It is not to mention that you cannot increase the BMEP by 30% as shown in their presentation without increasing the thermal efficiency substantially. I am pretty sure that through super-lean burn, ethanol, etc., they are aiming for 50% or higher thermal efficiency. Aerodynamic improvements to fuel economy will be small, especially in city driving, and even in highway driving for the Prius, which already has a small drag.
     
  17. bwilson4web

    bwilson4web BMW i3 and Model 3

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    Replacing poppet valves with rotary would make a step, efficiency increase. Rotary valves have non-trivial sealing problems that can be solved. This gets rid of the cam shaft and energy impulses needed to open the poppet valves.

    Bob Wilson
     
  18. Trollbait

    Trollbait It's a D&D thing

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    No, they haven't. You are reading it from a power point slide made for a public marketing presentation. It was not a graph for a technical publication. The lack of axis values means any reading beyond more or less pure speculation.

    Tertiary articles claiming 30% efficiency gains are likely telephone game errors with the 30% power difference Toyota talks about.

    They did not increase power.
    "This new engine complies with regulations that would require the existing model’s power to be cut by 30%."
    - The Engine Reborn--Three Companies Develop ICEs for Decarbonization

    Getting their current engines to comply with Euro7 and other upcoming emission regulations would result in having to depower them that much. The while circle of the presentation graph represents that hypothetical engine. It is a comparison like claiming mild hybrids increase efficiency by 20%. Which is true, when comparing a mild hybrid on a modern engine to an engine that is 20 years old.

    I'm sure they are aiming for that too, and it would great if they deliver that for a reasonable price. Toyota did not talk about that tough. The presentation was about them working on engines to meet stricter emission regs, and how these engines will be used in hybrids.

    "By contrast, the new engine retains output thanks to improved combustion technology. At the same time, its compact size allows for lower bonnet profiles, reducing aerodynamic drag. These changes are expected to yield 12% better fuel economy in sedan-class vehicles."
    - same source, which is Toyota owned, Launching Toyota Times!

    So sedan fuel economy improvements with these new engines will come from a mix of engine and aero improvements. How much of each contributes is unknown. Dropping it into an existing Prius will result in a less than 12% improvement.
     
  19. Trollbait

    Trollbait It's a D&D thing

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    I have been wondering how about the space requirements of the multi port fuel injection on the Dynamic Force engines. For the test engine in the SAE article, they got rid of it, and were using direct injection only.

    If the DF engines would need an exhaust particle filter under Euro7, why bother with the cost and complexity of the port injection?
     
  20. Gokhan

    Gokhan Senior Member

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    That's where you are mistaken. The English version of the Toyota Times news is wrong due to some mistake by the Japanese translator. If you Google Translate the Japanese version, you can see that the 12% improvement only comes from aerodynamics.

    ----------------------
    In order for existing engines to comply with the strict emission regulations that are scheduled to be introduced in Europe and the United States in the future, it will be necessary to reduce output and incur high costs for catalysts to purify exhaust gases.

    In response to this, the new engine maintains output through improved combustion technology, and is expected to achieve a 12% improvement in fuel economy in the sedan class by reducing the size and lowering the bonnet to reduce air resistance.
    ----------------------


    If you look at the thermal-efficiency plots in the presentation, the improvement varies depending on the engine, and in some cases, the thermal efficiency actually gets worse. The new 1.5-L NA is far more efficient than the old 1.5-L NA, but the new 1.5-L turbo is actually less efficient than the old 2.5-L NA.

    Toyota mentions the 30% a lot. They are saying that if they hadn't increased the efficiency, they would have to reduce the power by 30%. That sounds like they have increased the efficiency by 30% so that they are having 30% less CO₂ emissions with the same power output to comply with the greenhouse-gas-emissions regulations. That means that even if they have a 12% aerodynamic improvement, they would still have to increase the thermal efficiency by 16% from 41% to 48%.

    Again, we will have to wait and see for the exact numbers. I would bet that the 1.5-L NA engine will see a thermal efficiency over 50%, at least with some ethanol. Toyota is not joking about their new engines being nothing like before and a “game changer.”
     
    #60 Gokhan, Jun 8, 2024
    Last edited: Jun 9, 2024