Atkinson vs. Otto Cycle

[David Beale]

In the "Hybrid Synergy Drive" (HSD) one of the "Motor generators" is used to start the engine (MG1). It uses about 550 V, and that is obtained by electronically stepping up the 201VDC from the "traction battery". It's a three phase "A/C" system - pulses actually, not sine waves, generated electronically. It takes a little less power to spin up the engine - you must overcome friction in both cases, but the higher compression of an Otto cycle engine requires a little more power to overcome. The "amps" (current) is much lower, however, due to the high voltage.

The Prius 12V system is there to power the electronics and make the ancillary systems less expensive and therefore more practical (lights, radio, etc). It is not involved in cranking the engine. In the Hybrid Civic you -can- start the engine with the 12V system, though it usually uses the "traction battery" to do so.

I mention the whole drive system because you couldn't operate the car without the whole thing. It would be undriveable.

It doesn't matter what kind of engine it is, by the way. In the HSD you could have an Otto cycle engine, as they do in the Lexus vehicles. It would be less efficient, but much more powerful.

One final point I would like to make - it's important that the HSD spins the engine up -before- it injects fuel. This allows the engine to build up oil pressure before the parts have to withstand the torque that will be produced once the controlled fuel explosions start. If it didn't I suspect the engine wouldn't last long in the frequent start/stop mode it is operated.

[usbseawolf2000]

MG1 also properly stop and park the ICE. Everything is computer controlled. I believe Bill Gates calls HSD the software car because the complexity is in the software with simple hardware setup.

[FL_Prius_Driver]

There is one possible downside to the Prius Engine. The attenuated intake of the Atkinson stroke forces some oil mist back into the intake manifold and may contaminate the throttle, requiring cleaning.

[MSantos]

Originally Posted by David Beale

...
The Prius 12V system is there to power the electronics and make the ancillary systems less expensive and therefore more practical (lights, radio, etc). It is not involved in cranking the engine. In the Hybrid Civic you -can- start the engine with the 12V system, though it usually uses the "traction battery" to do so.

Just to clarify... the Civic hybrid will only use the redundant legacy starter when the hybrid system is in "managed mode" due to low temperatures (or extreme high temperatures) but you do not choose when it actually starts the engine.
As usual, the 12V subsystem is only there to power-up the legacy devices and hybrid subsystems and therefore the engine cranking ability is rather limited in intensity and frequency of use.

Since we're also talking of Atkinson cycle engines, has anyone noticed any Otto power plant that can seamlessly switch to an atkinsonized mode? no?

Cheers;

MSantos

[Ichiro]

Suck squeeze bang blow!

[David Beale]

Originally Posted by FL_Prius_Driver

There is one possible downside to the Prius Engine. The attenuated intake of the Atkinson stroke forces some oil mist back into the intake manifold and may contaminate the throttle, requiring cleaning.

It does this. You will find, however, a "highly tuned" Otto cycle engine also does this, and worse than the Atkinson one. This is due to the higher amplitude pulses in a "high compression" engine and to the larger "valve overlap" of a high performance Otto cycle engine. The oil mist is minimal, as it's only the oil that appears -above- the piston rings and remains unburned and the oil that leaks past the valve seals.

There must be some oil on the cylinder walls to lubricate the rings, and yes, the "oil control ring(s)" do scrub most of it off, but not all of it.

I wonder if there might be a small side benefit to the intake charge flowing backwards - might it dislodge dirt from the air filter and let it drop to the bottom of the airbox?

[Patrick Wong]

Hi FL Prius Driver and David B.,

It's not totally obvious that the fluid accumulating in the intake manifold is engine oil, since the fluid color is lighter than used engine oil would be.

My guess is that the fluid is unburned gasoline coming up from the intake valves.

Another disadvantage of Atkinson cycle is that the engine produces very low torque at low RPM; hence this is not suitable for non-hybrid automotive use.

[Stefx]

I thought the Prius ICE was actually using an "Atkinson-Miller" cycle, because it's a hybrid version mix of both. It doesn't have the complex linkage of the real atkinson concept (it uses a delay on the closure of the intake valve) and it doesn't have a supercharger like a Miller cycle does.

[donee]

Hi Stefx,

The Prius engine uses the Atkinson Cycle, but the Miller Valve Train. Miller invented the way to control the valves to decrease the compression ratio, while maintaining the hight expansion ratio. Miller's engine used a turbocharger, however, so the Miller Cycle is different than the Atkinson Cycle. There have been some production cars which do use the Miller Cycle, although I do not believe any have been sold in the US.

Ooops. It looks like you already understand this. So, you answered your own question. You could call the Prius engine a Atkinson-Miller engine, but there is no such thing as an Atkinson-Miller cycle.

[FL_Prius_Driver]

Originally Posted by Patrick Wong

It's not totally obvious that the fluid accumulating in the intake manifold is engine oil, since the fluid color is lighter than used engine oil would be.

My guess is that the fluid is unburned gasoline coming up from the intake valves.

All we need to do is find someone who has done an analysis of the residue and we would know. Any clues where to look?