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

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.



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.



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.



Here is a Youtube video of the conference.

 

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.

 

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.

 

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.

 

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.

 

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.

 

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.”