Engine Balance Shaft?

It reduces engine vibrations that come from the varying speed of the piston depending on where it is in the cylinder.

We need a little more data about when it's used and when it's not to see if there's a pattern. I did a quick check and it appears the Prius, Corolla, Camry, C-HR, and RAV4 have it, while the Corolla Cross does not.

 

https://en.wikipedia.org/wiki/Balance_shaft

https://en.wikipedia.org/wiki/Engine_balance#Secondary_vibration

 

Correct:

"The balance shaft was invented and patented by British engineer Frederick W. Lanchester in 1907.[1][2] It is most commonly used in inline-four and V6 engines used in automobiles and motorcycles."

 

Is its use linked to higher output engines, or dependent how an individual engine performs in quality testing? That is, it is only installed on engines that vibrate too much when tested.

 

It's just there to reduce vibrations.

Interestingly, the gen5 Prius Prime has it but the regular Prius doesn't.

Well, there's some conflicting info on that. The Prime definitely has it. The regular Prius hybrid probably doesn't. When you look up the engine disassembly procedures on Toyota TIS(what the dealership service departments use), the PHEV has the balancer while the HEV doesn't. But if you plug in the part number for the balancer on autoparts.toyota.com, it will come back as an exact fit for both the Prius and the Prius Prime.

Also, according to TIS, it looks like the two cars have different oil pans(to accommodate having the balancer or not). But the parts site says there's only one oil pan option and it's the same for both cars.


Maybe I'll confirm this next week when I have my first oil change. The difference between the two engines is externally visible from the oil pan.


With balancer:



Without balancer:

 

All Priuses sold in the US have the M20A. The old 2ZR will only be found in Japan, Mexico, and maybe other smaller markets. Europe, US, and Canada are only getting the M20A.


*tiny caveat: Europe might be getting the 2ZR, but I think it's only an option for fleet sales. The general public will only be getting the M20A.

 

@Hammersmith hit it on the head.

All your typical inline 4 engines (which includes all inline 4 Toyota engines that I know of, including on all Prii) have perfect primary ballance but terrible secondary ballance.

If you want a smooth running car, then you either need an engine that has great primary and secondary ballance (like a cross-plane V8, a V6 or an inline-6) or you need ballancing shafts. Sadly, ballancing shafts create more friction and therefore less fuel economy. But it is minor compared to changing the inline-4 out for a 6 or 8 cylinder engine.

In other applications you may want vibrations. I'm not sure if the GR Corolla and GR Yaris have ballancing shafts, but in a sports car you may want more vibrations since comfort isn't really the main goal. And inline-3 engines, like in the GR Corolla and GR Yaris, have primary ballancing issues, which can feel great as you drive around on a track and can help you know when to shift. The newest Corvettes also have ballancing issues (flatplane crank V8 which has the same secondary ballance problems as a typical inline-4) yet they didn't add any ballancing shafts on those either. So a Prius with ballancing shafts should, in theory, be smoother running than a 2022 or newer Corvette.

It's all these interesting quirks and pros and cons of each internal combustion engine type that I will miss when we go all electric some day. Instant torque cannot replace sounds, vibrations and manual shifting IMO. My personal prefference, considering everything, is an inline 4 without ballancing shafts and a manual transmission. It gets the best of all worlds, good fuel economy and moderate vibrations that are hard to feel while cruising, yet enough to help you know when to shift on track day. Although if I went to a track regularly, then the inline 3, like in the GR Yaris and GR Corolla, would be my preferrence.

 

The Prime to Prius difference makes sense. Some vibration with the hybrid is a fair trade off to get the best fuel economy possible. With a PHEV that spends much of its time in EV mode, those additional vibrations can be more noticeable, and a negative, to the driver.

 

Yes, I suspect the same reasoning why Toyota did that. There's also no manual transmission, so having sounds and vibrations are basically pointless IMO.

 

I thought the differences of why vibrations were deliberately avoided on the Les Mans Cadillac (a race car where you'd think it wouldn't matter) yet weren't on the Corvette (a consumer car that people might not want their car vibrating while driving) is pretty interesting in this beginning of this video:

 

Might as well also ask why the engine has a power rating of 143 to 176 hp?
Depends on the specific application.

 

Do all have the atkinson cycle?

 

According to the Wikipedia, the M20A-FXS is only in hybrid models.
https://en.wikipedia.org/wiki/Toyota_Dynamic_Force_engine#M20A-FXS

 

M20 Family:

M20A-FKS - standard application (168-176hp)
M20C-FKS - Chinese version* of the standard M20A
M20D-FKS - Chinese version* of the standard M20A
M20E-FKS - Chinese version* of the standard M20A
M20F-FKS - Chinese version* of the standard M20A
M20A-FKB - flex fuel(E85) version for Brazil
M20A-FXS - Atkinson cycle version for hybrids (143hp in the Lexus UX 250h up to 176hp in the European Corolla Cross Hybrid; 150hp in Prius)
M20B-FXS - Chinese version of the M20A-FXS

*each of the Chinese versions are manufactured at different plants by different subsidiaries/licensees