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Toyota targeting thermal efficiency of more than 45% for next-generation gasoline engines for hybrids

Among the R&D projects Toyota Motor is exploring to further lower fuel consumption and emissions are two concepts on a pathway to deliver gasoline engines featuring more than 45% thermal efficiency for application in its future hybrid vehicles, according to Koichi Nakata of Toyota in his presentation today at SAE 2011 High Efficiency IC Engines Symposium in Detroit.

The engine used in the first- and second-generation Prius (the 1.5L 1NZ-FXE) had a thermal efficiency of about 37%; the thermal efficiency of the new 1.8L unit in the third-generation Prius (2ZR-FXE) has a thermal efficiency of about 38%. Toyota is targeting a thermal efficiency of more than 40% with what Nakata called its Future Concept 1, followed by thermal efficiency of more than 45% in Future Concept 2 (which is based on concept 1).

In the 2ZR engine (third-generation Prius), some of the main technologies Toyota applied are the Atkinson cycle with variable valve timing to control intake valve timing, cooled EGR, and lowered friction. (In the 1NZ engine, Toyota reduced friction 21.1% compared to an engine for a comparable conventional vehicle; the 2ZR engine in the newest Prius drops that another 26.8%, in large part by the removal of parasitic loads via the hybrid system (e.g., no alternator).

Concept 1 is a cooled EGR stoichiometric spark-ignited direct-injection concept, featuring a long stroke design (stroke/bore=1.5) and cooled EGR with an EGR ratio of more than 30%. The long-stroke design (lengthening the stroke while maintaining displacement), reduces heat loss and also increases piston speed, creating more turbulence. A high tumble ratio intake port (TTR=3.0) and a high-energy ignition system (100 mJ) also contribute to improved combustion. Toyota is continuing to reduce friction.

Concept 2 is a turbocharged lean burn concept, built on the base of concept 1. It also uses the long stroke design, with a high tumble ratio and a higher-energy ignition system (150 mJ).

The high tumble ratio intake port extends the lean limit from 19 to 23, Nakata said. In addition, the lean limit is also increased by using a spherical face on the piston. Furthermore, the high discharge current in the ignition system also gives a higher lean limit.

Nakata said that the engine team has currently delivered a 42.4% thermal efficiency in concept 1 and 43.7% thermal efficiency in concept 2.

Work is ongoing, focused on increasing the expansion ration and decreasing pumping loses. Toyota is also considering a variable super high expansion ratio cycle for further improvements. Nakata suggested that such an engine applied in a hybrid would result in total lifecycle greenhouse gas emissions comparable to that of an electric vehicle.



So, 45%/37%=1.21! Would that mean a Prius would go from 51 Mpg highway to 62 Mpg?


That sounds about right, the 60 mpg car may be possible soon. The PNGV cars all got 70 mpg as four seat sedans in 1999 with a diesel hybrid configuration, so this sounds reasonable.

Roger Pham

Good work, Toyota!!!

However, applying the above principles to a direct-injection Hydrogen fueled engine, thermal efficiencies at 45-50% can be realized without needing cooled EGR nor necessarily high energy ignition system nor turbocharging. H2 has much higher lean-burn limit than gasoline without requiring turbocharging, and requires much less energy to ignite. Furthermore, H2 combusts very rapidly, leading to isochoric combustion for maximum power and efficiency harnessed by the piston. When high power is momentarily needed on rare occasions, the H2 can be combusted stoichiometrically, resulting in high power density.

If the H2 is produced from renewable sources such as solar and wind generators, then the life-cycle GHG produced would be zero.



From well-to-wheels, don't you have to count the storage and transportation losses, not to mention the latent energy content of the as-yet-to-be-created H2 infrastructure? H2 could easily give back that 5% and more when those values are counted.

Roger Pham

One day, with solar PV's on every roof top and a wind turbine on every acre of farm land, H2 can be produced and stored locally in the same community where it will be consumed, thereby no transportation losses. For agricultural waste biomass, the raw material can be transported in dried form to be gasified to H2 near the point of consumption.

The building and maintenance of H2 infrastructure will amount to millions of jobs created locally. The H2 will be produced locally, resulting in more local that can't be exported or nor outsourced...! The elimination of oil importation will keep hundreds of billions of USD here at home to invigorate the US economy!!! A win win win combination, as far as I can see it!!!


You could use the new MIT method with built in solar cells and get hydrogen at the station from water and sun. The whole quarter acre gas station property would have to be covered by awning, but you could fuel a few cars.


    thermal efficiency of more than 45%

That's the way to go! :-)

Good work, keep raising the bar.


BTW, range-extenders as in a GM VOLT v2.0 should do the same.

It should be easier in a Volt-like than in a Prius-like setup, as with bigger batteries and electrical motors Volt relies less on ICE for the dynamics.

Couple it to Lund's PPC findings and deliver a flex fuel with a thermal efficiency north of 50% !

(RangeExtender: 2cyl-Turbo-DI-EGR-FlexFuel-Miller/PPC-LowFriction-60kW @ > 50%TE !)


Gee, if i had solar panels and wind generators on my roof i'd just be turning that electricity into..... electricity! forget the intermediate H2 step.

Battery tech continues to improve and energy density is going up year by year. I'll place my bets on EV...


I vote for neptronix



"One day, with solar PV's on every roof top and a wind turbine on every acre of farm land"

Long before that could happen there will be a battery tech that could store 100kWh in a shoebox size container weighing 20kg. Why bother with the H2?

Roger Pham


H2 technology is available here and now. The 100kWh battery the size of a shoe box is still science fiction that may or may not materialize. Oil is running out and becoming more and more expensive...We also need more jobs, good-paying, local jobs that can't be exported overseas. We have no time to wait nor to waste!


Roger Pham,

Electrics and Biofuels are already more developed and more economical than hydrogen, we should not waste time and money on hydrogen.



H2 technology is pie in the sky.

45% efficiency for a gasoline engine is quite an outstanding achievement indeed, and will probably help us to be patient in our wait for the perfect electric car...


Two comments:

1. 37% thermal efficiency sounds very high. At what kind of load and what kind of on/off duty cycle is this achieved?

2. The hydrogen economy: Please read

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