Engine Balance Shaft?

Only the hybrids have an Atkinsonized engine. The others could get closer to an Atkinson cycle than previous engine models, but they are still an Otto cycle base. Dynamic Force engines have direct and port injection. Direct injections allows a higher actual compression ration without needing higher octane. The high compression ratios in an Atkinson engine are really just the expansion ratio of the power stroke; compression/charge stroke has a lower effective compression ratio.

 

Dynamic Force Engine means it can switch between Otto and Atkinson cycle for both standard M20-FKS and hybrid M20-FXE engine (Atkinson cycle for low power, Otto cycle for high power). Older Prius Engine had a different fixed intake valves timing but now it is variable as exhaust valves timing were. In fact, these two engines are mechanically more similar then ever! Now, drivers choose if they drive a Prius or a Celica! Software is limiting decompression ration to 1:13 for Standard engine while it is 1:14 for hybrid engine because it is assisted with an electric engine.

Balance shaft:
Hybrid engine run mostly at their peak efficiency, in other words at higher revolution (propulsion and charging). Balancing shaft is more useful when engine is idling as in standard engine. Prius Prime may have a different strategy. Maybe this is why Prius Prime has balancing shaft but not Prius. You can confirm presence of a balancing shaft with oil capacity in user manual: Prius Prime with balancing shaft 3.9 L, Prius without balancing shaft 4.2 L.

 

The purpose of variable valve timing is get power and efficiency performance out of the same engine. The technology has come a long way, but you can't get the full Atkinson efficiency benefit in an engine for an ICE car. The reason why is in that compression ratio difference. The higher ratio for the hybrid engines allows a little more energy to be captured from the fuel on the power stroke; the hybrid engine's thermal efficiency is 1% higher than the ICE model. The trade off is a lower power rating than what the ICE model gets. No real big deal with the electric motor to assist.
Toyota Dynamic Force engine - Wikipedia

Balance shafts are all about improving the NVH of the engine. You try to avoid using them as they do put some drag on efficiency, and they aren't a common feature of 4 cylinders. Testing likely showed that the engine NVH that wasn't an issue in the hybrid was something that was more pronounced when the PHV switched from EV to HV mode. So the balance shaft was added to get the engine NVH closer to what the driver experiences in EV driving.

 

Older Prius were admitting air and fuel mix and then exhausting this mix to another cylinder by intake manifold until it closed intake valves at a point where it would not reach more than 10:1 compression because fuel mix burn by itself over that. At low revolution, Prius 2001-2003 engine sounded like turbo one (These were 1:13 decompression)! With indirect and/or direct injection, it can get to 14:1 compression without this problem. In both cases, it gets 1:14 decompression to get more energy. Of course, there is less pumping loss with 10:1 compression!

 

The new hybrid engines are still doing this. It is a way of getting an Atkinson like cycle out of an Otto engine.

The Atkinson cycle has unequal strokes; the intake and compression are shorter than the expansion and exhaust. A true Atkinson does this with a set of cams between the piston rod and crankshaft. An Atkinsonized Otto mimics this by the aforementioned valve timing. That longer expansion ratio improves efficiency, but compared to a similar Otto engine, it makes less power.

This why the hybrid dynamic force engines have that 1:14 compression ratio, too get that longer expansion stroke for efficiency. The 1:13 ratio in the ICE models is the limit for a direct injection engine running on US regular octane gas. Mazda reduced the compression of SkyActiv engines to that for the North American market.

 

Correction: Software is adjusting compression ratio from 10:1 to 13:1 for standard engine while it is from 10:1 to 14:1 for hybrid engine. Toyota choosed a decompression ration of 1:14 because it is assisted with an electric engine (weaker but more efficient engine).

 

I have to assume you're looking for a more complicated answer, but the simple answer is they do it with variable intake valve timing. For the M20A-FXS, if the intake valve closes just as the piston begins the compression stroke, then the compression ratio will be 14:1(max power). But if the valve timing is adjusted so the intake valve stays open during the early portion of the compression stroke, then the compression ratio might end up as low as 10:1(max efficiency).

I'm pretty sure this wider range of compression is the reason the Dynamic Force family has switched to electric motor driven VVT adjustment on the intake side.



Thinking about this for a bit, it makes me wonder if the reason for the balance shaft in the PHEV is because that engine will spend more of its time closer to the 10:1 end of the range, while the HEV version will spend comparatively more time in the 14:1 end. And the PHEV balance shaft is there to smooth out any imbalances because the overall engine is more built for the 14:1 side of things. But that's just late night spit-balling.

 

It's just a basic understanding of engines, the different cycles, and (very)basic physics.

If Toyota is varying the compression on these engines, then something must be changing. It's not a turbo or supercharger because they don't have them(M20A & A25A). So it must be variable valve timing. And doing any research on the Miller cycle quickly shows what's happening.

I suspect you're looking at this from slightly the wrong perspective. Toyota isn't building a 10:1 compression engine and then boosting it to 14:1, they're starting with a 14:1 engine and then sometimes reducing it with valve timing down to as low as 10:1 for purposes of efficiency.

I don't know much about this stuff except for the very, very basics, but I don't think it's going to work the way you think it is(adding mpg by adding a turbo). I'm looking at the Crown Limited and Platinum. Both are hybrids and use Dynamic Force engines, but the Limited comes with the naturally aspirated A25A-FXS 2.5L I4, and the Platinum uses the turbo T24A-FTS 2.4L I4. The Limited gets 41mpg while the Platinum only gets 30mpg. Same thing with the Lexus RX 350h and 500h. Both are hybrids, but the 350h uses the A25A and gets 36mpg while the 500h uses the T24A and gets 27mpg.

I also doubt any economy engine designed for 14:1 compression is going to be able to handle any additional boost while at max compression. At least not for any real length of time. So I'm thinking you would need to do some sort of inverse connection between the valve timing and the turbo controller. As the valve timing adjusts for max compression, the turbo blowoff valve would have to release pressure to not over pressurize the cylinder. It seems to me that would totally defeat the purpose of having a turbo. This is also borne out by the base compression(before boost) of the T24A only being 11:1.

 

The non hybrid engines have a compression of 13:1. Even the hybrid ones won't exceed that on the compression stroke. Doing so risks knocking with US regular gas.

As valid as any other reason expressed here.

No big secret on adding a charger. A Miller cycle engine is basically a charger on an Atkinson engine. Usually a supercharger as the extended expansion stroke means less energy in the exhaust to spin a turbine.

Charged engines have lower compression ratios than naturally aspirated ones, even ones with direct injection. This is because the act of forcing more air into the cylinder can quickly raise the effective compression ratio to dangerous levels. Without adjusting the compression of the engine a turbo is being added too, will limit the amount of added boost, and thus the possible gains.

 

I'm going to quibble just a tiny bit here. It's a meaningless quibble in the real world, but I'm pedantic that way. lol

I hate that Toyota uses the Atkinson name for their hybrid engine cycle. An Atkinson shortens the compression stroke by physically altering the stroke length. A Miller shortens the compression stroke by leaving the intake valve open during the early part of the stroke. Obviously a Toyota hybrid engine is based off a Miller-style engine and not an Atkinson.

Yes, Miller also added a supercharger in order to overcome the disadvantages of the reduced compression stroke, but that means we should just call the Mazda and Toyota versions 'charged Miller' and 'uncharged Miller'. (or some variation of those)

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