Cool learning article. I read it before, but it's been updated with more info. Some see the technical marvel that is the A25.

For reference, the engine in our hybrid models is the A25A-FXS. The "FKS" is the non-hybrid variant, such as ES250 AWD.

https://toyota-club.net/files/faq/18..._df_r4_eng.htm

 

So here's a question related to the hybrid - it seems the difference between the ICE and hybrid ES MPG is pretty material - not so much on the RX. Why is that?

 

I haven't done a deep dive into the 450h tech and a lot of this is likely beyond my comprehension since it's really deep-level engineering stuff, but I'd suspect two main reasons:

1) The 450h powertrain is an older-generation design

2) Weight/Performance. The 450h powertrain being a V6 is more geared towards the performance side of things. So naturally being a V6 and being more capable, it will just eat more fuel.

So I think it depends on the design goals for the particular application (frugal hybrid vs performance hybrid) and the generation of hybrid technology being used at the time. I'll bet the next RX will use a more advanced V6 hybrid system (LC500 system maybe?) assuming it does not use the NX system.

Now that I think about it how does the LC500 system compare to the 450h? Is it a new generation V6 hybrid tech? I haven't bothered to look.

 

Why they say it is a Miller cycle engine?

 

I think the fact that the RX 450h uses the same 6 cylinder (just detuned) suggests their priority was trying to generally match the performance of the 350. My last tank in good weather (city) was 36 MPG but last week my fill up only netted 29. So I agree with your conclusion with respect to the RX, the mileage difference on the ES hybrid platform is considerably greater. While I am personally satisfied with the performance of the hybrid ES performance for my needs, it does not match the performance of the ES 350. I would have been disappointed if there was as much of a power difference in the RX as an SUV just needs more power - especially if driven in the mountains. This was really my main complaint with the hybrid NX, which was just fine on city streets and most roads.

 

if i'm not mistaken the engine can dynamically switch between two cycles = miller or atkinson depending on load. or you mean why did they use one label and not the other?

 

This engine is akinson only. Engines with supercharger and atkinson are called Miller.

 

It's a little bit more complicated than that. In essence, the difference between what Toyota names it and the cited article names it boils down to one of those engineering students sitting around the beer keg arguments on who should get the named inventor credit - the person who came up with the thermal energy efficiency theory, Atkinson, that Toyota is implementing in their engines or the person who partially came up with the practical mechanical implementation, Miller, that Toyota is implementing in their engines. (Partially because Toyota's implementation uses variable valve timing to switch between Atkinson and Otto cycle operation which Miller's design and implementation didn't have.)

While Atkinson came up with the idea of different "lengths" for the compression and power portions of the engine cycle for increased energy efficiency, his late 1800s implementation were mechanically complicated.

On the other hand, Miller came up with the idea of changing the effective length while keeping the same, relatively simple, mechanical length of the compression portion of the cycle by merely permanently changing the intake valve timing albeit at the reduction of output power (due to the reduction in air mass available for fuel combustion) from what would be produced from a non-altered valve timed Otto cycle engine of the same size. Unsatisfied with the smaller power output, Miller then worked on means of increasing the compressed air mass available for combustion to increase output power by means of superchargers or turbochargers; however, from a purist standpoint, the Miller cycle doesn't require either of those add ons - just the altered intake valve timing to change the ratio between the effective lengths of the compression and power cycle portions.

What name to use for what Toyota has implemented is, I think, one of those things you can argue about until the heat death of the universe without any final resolution. If one wanted to stir up the argument pot I'd throw in the name of the person who came up with the whole design/idea of combining Atkinson, Miller, and variable valve timing into Toyota's present day implementation.

From a practical standpoint, Toyota calls their implementation Atkinson cycle so to avoid confusion in general discussions it is best to go with the name Toyota uses.

 

 

The real world MPG difference between RX and RXh is significant.

 

Even with mainly highway miles?

 

Yes. I find mine better at highway than in the city.

Easily getting 32 mpg highway, the Rx350 will not average that. Not even close.

 

If you go to Fuelly, you can see the RX350 typically averages about 19 mpg while the RX450h averages 27mpg. that's over a ton of user input over time. I guess that's a decent difference