Atkinson vs. Otto Cycle

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.

 

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.

 

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.

 

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

 

Suck squeeze bang blow!

 

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?

 

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.

 

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.

 

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.

 

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?

 

I'll bet it's mostly fuel, with some oil dissolved in it. Easy to test. Take a "Q" tip and sample some. Smell it. If it smells like gasoline there is some in it. Let it evaporate. Is there oil left? If so, it's as I think.

This is called "fuel standoff". All engines do it, but some do it more than others. Two stroke engines have enormous fuel standoff - I've seen it projected 2 feet above an open carb., which is why the intake plenum/filter is mandatory for safety when they're running. Otherwise it's quite a fire hazard! Rotary engines also have high fuel standoff. Low compression Otto cycle engines have very little fuel standoff, but high compression engines have more, not so much as the first two I mentioned though.

In all cases there is some oil mixed into the fuel. The first two mix the oil in themselves, but the rest get it from the previously mentioned paths.

 

Look here, it is different. WOW!

 

ww.keveney.com/Atkinson.html

 

With Atkinson a NA engine of 2L ACTUAL displacement only produces the POWER of a ~1.5L engine. Intake cycle cylinder "filling" is limited to ~70%.

 

About 30% of the gasoline-air mixture flow back into the intake manifold so only 70% is left to ignite. When it combusts and the cylinder expand 100%. Therefore, you get 100% of work done with 70% of the fuel.

 

Shouldn't that be more like 80-90%? (these numbers are just guesses) of the work for 70% of the fuel?

The reduced air and fuel charge means less pressure in the cylinder, therefore less capacity to do work. But coming from overpowered fuel hogs, we find this a worthwhile trade.

And at lower power levels where the Prius keeps the intake valves open even longer, reducing the fuel charge even more (to 35-40%?) without becoming a giant Otto-style vacuum pump, the difference gets even larger.

 

We have owned a 2008 Prius for a year, and we really love it. Very economical and very reliable. I have read this thread with great interest since I am trying to learn more about the car and this unique engine. I have one question. The compression ratio for this engine is apparently very high but it burns regular gas. Why does it not knock? Is the timing retarded?

Thanks for any information.

 

Since 30% of the fuel air mixture flows back out the intake valve, it is as if it had 30% less compression. So the 15 to 1 mathematical compression is about 10 to 1 atmospheric compression.

[ame]http://en.wikipedia.org/wiki/Compression_ratio[/ame]

The math is from Bottom Dead Center to Top Dead Center, but if your intake valve is not closed at Bottom Dead Center, you are not really compressing any air until it does close.

 

The expansion ratio is high. The compression ratio isn't all that high. It's one of the main characteristics of an Atkinson cycle engine.

Tom

 

Since 30% of the fuel air mixture flows back out the intake valve, it is as if it had 30% less compression. So the 15 to 1 mathematical compression is about 10 to 1 atmospheric compression.

Thanks for the reply. This leads to the next question. What is the advantage in pushing 30% of the fuel air mixture back out the intake valve?