[kennyb]

Why doesn't Toyota put the Atkinson cycle engine in other Toyotas?

From what I understand, all the Atkinson cycle is is the late closing of valves in the compression stroke. Is that right? So what is to prevent Toyota from their other engines close valves late on compression cycles also? Especially at highway speeds when you're looking for efficiency and not power.

Also how much of the Prius gas mileage advantage at highway speeds over say, the Camry, is due to the Atkinson and how much to lower drag coefficients?

 

[SLowen]

Because the Atkinson cycle provides high efficiency in exchange for torque. A Hybrid has the traction motor to provide torque and only needs the horsepower at speeds greater than 16 mph.

If this statement sets off any of this forums Toyota Pro's BS detector please correct me, but this is my understanding of the technology.

 

[hdrygas]

Also just to keep everything honest the Prius uses the Miller cycle not the Atkinson cycle. The difference is in the details but the Atkinson cycle uses a modified cam and is mechanical, while the Miller cycle uses modified valve timing to achieve the same ends. This is handled in software. Hardware vrs Software. The differences are geeky but this the Prius is a fairly geeky sort or car. The devil is in the details.

 

[kennyb]

Yes, Miller cycle. Very well then.

But why doesn't or can't the computer in say, a Corolla, tell the valves to start acting like a Miller cycle at highway speeds where you really don't need power per se, just efficiency. After all, the Prius at highway speeds is humming along just on ICE power not battery. The battery is even sometimes drawing power from it.

 

[flare]

atkinson/miller cycles have more torque on highway, less in city
therefore without an electric motor that provides most of the torque, it just wont happen with a corolla

 

[Jerry P]

I believe that the 1NZ-FE engine in the Echo and Scion xA and xB is Atkinson cycle, but 1NZ-FXE (same basic engine) in the Prius is Atkinson/Miller cycle. The Echo engine is very efficient- my son routinely gets 38+ mpg with the one I sold him when I got my Prius- but still has good low end torque. If you look at Toyota world-wide, the 1NZ-FE is one of the most commonly used engines because of it's great efficiency.
I occasionally check sites on the Echo/Scion cars and the 1NZ-FE is one of the most reliable engines in the world - no reports of trouble at all. I think that with the 'warm start' feature on the Prius plus the fact that it opeates in a very narrow RPM range, the 1NZ-FXE will last forever.

 

[kennyb]

Thanks all for replies.

I guess my question is: Given that all the Atkinson (or Miller) cycle is, is that valves close late, and given that Toyota uses something called VVTi (variable valve timing) on its regular engines, it sounds like they could easily make the valves close late at certain speeds on regular engines.

I looked up the Atkinson engine, but I didn't see it mentioned for the Echo. The Echo gets great mileage, but at highway speeds I think the Prius still comes out ahead. Which makes me thing that either:

1. The Prius has much less drag co-efficient (it does, I think)
2. The Prius uses the more efficient Atkinson (or Miller) cycle and that is what does it.
3. A combination of the two.

My 1995 Toyota Corolla doesn't have the Atkinson and I get 38mpg driving mostly highway. So yes, Toyota engines are great. I just wonder if they're missing out on an easy modification due to not wanting to trade torque for efficiency.

 

[Daniel]

Looks like the Corolla probably has either a 130 or 170 hp engine. Higher-horsepower engines are less efficient when that horsepower is not needed, as when cruising on the highway. One of the many ways the Prius gains highway efficiency is by having that relatively small 76 hp engine. The car can perform with such a small (and therefore efficient) engine because for those brief moments of acceleration when you need more torque, you've got MG2.

 

[mrv]

Echo

afaik the Echo uses the traditional Otto cycle engine.

 

[mrv]

more reading

http://home.earthlink.net/~graham1/MyTo ... Frames.htm
Go to "Understanding the Prius"
Go to "The Internal Combustion Engine"

 

[BIF]

I know this strays a bit from the topic at hand, but it kind of (sort of) fits, and isn't so big I want to start another thread...

I would love to see Mazda make a hybrid version of the Miata.

It's a really small engine bay, but maybe they could put a 3-cylinder Atkinson/Miller engine in with a Toyota-based MG1/MG2 arrangement.

Edit: A lighter engine might allow for a small battery pack someplace in the rear deck? Hmmm, maybe the car would have to be made bigger to accomodate an NiMH battery....

They would have to figure out how to drive the rear wheels with that arrangement, or maybe turn it into a front wheel drive car.

My old Miata used to get around 30 to 33 MPG, even with the top down. Heh, it would be neat to see what it could get if it where a gas/electric hybrid, PARTICULARLY if they could put a 3-cylinder engine in there!

Okay. I now return you to your regularly scheduled thread...

 

[clett]

I think I can guess at the answer to this one. If it's right, then there is a good reason why Toyota can't just add on the software to other similar engines without adjusting anything else. (I remember asking myself exactly the same question a while back).

When it comes to outright thermal efficiency of an internal combustion engine, you just can't beat raw expansion ratio. For the vast majority of engines, expansion ratio is the same as compression ratio. The compression ratio for a petrol engine is limited to about 10-11:1, any higher than that and the petrol detonates (pinks) before the spark ignites it and wrecks your engine in the process.

In the Miller cycle, however, the inlet valves close late, and a little of the air initially sucked in is pushed back out again. This effectively lowers the compression ratio, while the expansion ratio remains the same as has been built into engine naturally, in the form of the length of travel of the piston. Thus, the Prius engine is HARDWARE set to a compression ratio of ~13:1. It would self destruct itself with pinking were it not for the late closing of the inlet valves reducing the effective compression to only 10:1; while the expansion can remain at the hardware set 13:1. In this way, the best of both worlds is achieved. A high expansion ratio for decent thermal efficiency (36% vs maybe 30% for an equivalent otto ICE), but a low compression ratio so that petrol can be used without pinking.

When it comes to thinking about other engines in the Toyota range, however, they all have a HARDWARE setting of 10:1 for the compression stroke. Thus, the expansion stroke can only ever be 10:1 also (ie the expansion ratio can't be increased with clever valve timing, only with a longer piston travel), so there is no gain in trying anything Miller-cycle like.

What do the techies on here think, is this about right? :?

 

[clett]

By the way, I read somewhere that the Toyota techs were considering putting a real Atkinson cycle engine (like this one) in the next Prius!

 

[kennyb]

Well, Clett, you and I are thinking alike.

OK, then. But why not put a Prius like Atkinson engine with the "hardware" (I assume you mean the physical engine) into a Corolla? It wouldn't be that expensive, I mean all it is is extra long expansion room and some software telling the valves to close late.

Does it have such abysmal torque at low rpms that it just wouldn't work? I guess that's my question.

But I hadn't really though of it the way you said. I was thinking they could take a normal engine, say, make the expansion longer and then have the valves close late. If you wanted acceleration (at low rpms and low speeds) then have the valves NOT close late.

But what you said makes sense. If the valves don't close late, then PING, knocking. Very bad. So the valves MUST close late otherwise disaster. Maybe it is the case that there is very little torque at low rpms. Maybe so little that it wouldn't make sense.

Maybe they could have say a 100 wh capacitor to accelerate quickly and STILL have the valves close late. It would probably be simpler than the current hybrid. 100wh would take an engine only a few seconds to charge. That way you'd get around the low torque.

But thanks for replies.

 

[Daniel]

Thank you clett. That all makes perfect sense to me. Very well and clearly put.

 

[frenchie]

I concur. Clett's explanations are perfect.

I'm not sure whether the high expansion ratio is responsible for low torque or low maximum power (because of low max rpm : 4000 for early Japanese Prius, 4500 for classic Prius, 5000 for 2004), but I know some people refer to "Miller cycle" only for supercharged engines, like the original Miller design (and the one Mazda used for the late Eunos 800 M / Xedos 9). Supercharging was a way to compensate for relatively low power, the electric motors play the same role in the Prius.

 

[RSnyder]

Clett is correct, it would be difficult to turn a 10-1 compression ratio engine into a 13-1 compression ratio engine. Better to build it that way from the start. But if you're not going for super high peak power, why not just build a diesel engine from the get-go. They have compression ratios in the 18-1 neighborhood.

The other minor detail is that it would not just be a computer software tweak. The input valve timing is programmable, but the dwell is not. Even at it's earliest intake valve closing, the Prius engine is still way into Miller territory. Closing earlier would imply opening further back into the exhaust cycle. While a little bit of EGR is good, a lot will cause Bad Things (TM) to occur.

So the engine designers would have to build a reliable cam that allows computer control of intake valve dwell. They can't just reuse the existing VVT-i design.

 

[ammiel]

AFAIK. the reason regular cars don't use the miller engine is that altho it is more efficient it has this quality only over a limited range of RPM. This is no problem where there is a CVT and a computer to keep it in this sweet spot. Peace, Ammiel

 

[rdrast]

Wasn't just supercharged because of low power (the Mazda Millenia-S Miller was a 2.3 liter engine, about 230hp), but the superchargers (two on Mazda engines, one for each cylinder bank) were required in order to get enough fuel/air into the cylinders during the relatively short compression stroke.

Without pumping up the incoming fuel rail pressure to the cylinder's, the engine probably would just barely turn over, then starve out and die.

Very nice running engine though.

I concur. Clett's explanations are perfect.

I'm not sure whether the high expansion ratio is responsible for low torque or low maximum power (because of low max rpm : 4000 for early Japanese Prius, 4500 for classic Prius, 5000 for 2004), but I know some people refer to "Miller cycle" only for supercharged engines, like the original Miller design (and the one Mazda used for the late Eunos 800 M / Xedos 9). Supercharging was a way to compensate for relatively low power, the electric motors play the same role in the Prius.

 

[KiwiPrius]

Drive train link

Mrv,

Thanks for the link to Graham's website, found it absolutely fascinating. Do you know if all that applies to the 04 model?
Also, after reading through the different packages available in the US, I am VERY jealous! In New Zealand we only get one package, it has some 'extras' but I would really have liked the cd changer and voice control systems. Not so interested in the navigation and bluetooth as I don't have a compatible cellphone and NZ is a small place anyway - hard to get lost!!