In case you wondered... What is the Atkinson Cycle Engine?

[Nick72]

Overview here from Toyota I just came across... Stroke asymmetry.

 

 

[Cyker]

I still find it bizarre that the Atkinson/Miller cycle even works - In my mind what's happening is no different than opening the throttle less so less air-fuel mix goes in in the first place

The Valvematic engines seem to operate on the same principles but in reverse - They run at full throttle but vary valve lift and duration to control how much fuel-air mix goes in instead of the throttle...

 

[Nick72]

1 hour ago, Cyker said:

I still find it bizarre that the Atkinson/Miller cycle even works - In my mind what's happening is no different than opening the throttle less so less air-fuel mix goes in in the first place

The Valvematic engines seem to operate on the same principles but in reverse - They run at full throttle but vary valve lift and duration to control how much fuel-air mix goes in instead of the throttle...

 

I thought it was just a valve timing thing like the VVTs but there's a different stroke length involved too. I hadn't quite appreciated that. 

[ernieb]

It's so clever and even seeing the animation find it difficult to follow what's going on. It just works, like a Toyota!

[RAV4-D4D]

This is really interesting and not something I've heard of. Did a quick google on it and seeing some of the animations of engines Atkinson designed shows what an amazing mechanical/thermodynamic engineer he was; humbling in many ways.

 

However, the Atkinson cycle is a 4 stroke cycle and that animation is showing a 6 stroke cycle. I was expecting to see the classic 4-stroke cycle of:

1. Suck (down stroke)

2. Squeeze (up stroke)

3. Bang (down stroke)

4. Blow (up stroke).

 

Instead there's an extra 2 cycles in before the exhaust gases are blown out. Animation shows:

1. Down - Suck in the fuel/air

2. Up - Squeeze the fuel/air

3. Down - Bang due to spark igniting fuel (assumed as they don't show the spark)

4. Up - This is when you'd normally blow the exhaust gases out, but note exhaust valves are closed so we're compressing burnt exhaust gases.

5. Down - Exhaust valves open so we'd be sucking 'stuff' back from the exhaust system but there's no reason to do that

6. Up - Blow the exhaust gases out

 

 

 

[Strangely Brown]

8 hours ago, Nick72 said:

I thought it was just a valve timing thing like the VVTs but there's a different stroke length involved too. I hadn't quite appreciated that. 

The stroke length is fixed, physically, by the throw of the crankshaft and the length of the connecting rods. There is certainly a lot of clever stuff happening with the VVT and the strokes are asymmetrical in that the valve timing and injection is different to Otto cycle, but the stroke length - i.e. the distance between TDC and BDC - is fixed.

Toyota may be playing a little fast and loose with terminology here.

ETA: If you define the "stroke length" as the distance travelled on the compression stroke with the inlet valve closed rather than the actual, physical length, then yes, I suppose it could be described as a different length.

[Strangely Brown]

This explains it a little better... IMO.

https://mag.lexus.co.uk/atkinson-cycle-engine-work/

 

[Cyker]

1 hour ago, RAV4-D4D said:

This is really interesting and not something I've heard of. Did a quick google on it and seeing some of the animations of engines Atkinson designed shows what an amazing mechanical/thermodynamic engineer he was; humbling in many ways.

 

However, the Atkinson cycle is a 4 stroke cycle and that animation is showing a 6 stroke cycle. I was expecting to see the classic 4-stroke cycle of:

1. Suck (down stroke)

2. Squeeze (up stroke)

3. Bang (down stroke)

4. Blow (up stroke).

 

Instead there's an extra 2 cycles in before the exhaust gases are blown out. Animation shows:

1. Down - Suck in the fuel/air

2. Up - Squeeze the fuel/air

3. Down - Bang due to spark igniting fuel (assumed as they don't show the spark)

4. Up - This is when you'd normally blow the exhaust gases out, but note exhaust valves are closed so we're compressing burnt exhaust gases.

5. Down - Exhaust valves open so we'd be sucking 'stuff' back from the exhaust system but there's no reason to do that

6. Up - Blow the exhaust gases out

 

I hadn't noticed that, but on re-watching the video I see what you mean; It's a bit deceptive as they don't show the complete cycle and jump forward half-way through to go through the steps again.

The cycle used in the engine is the same suck-squeeze-bang-blow, and is actually exactly the same as normal Otto-cycle except for one small difference, which is the intake valve is left open for part of the compression stroke - That's literally the only difference!

As an aside, a real Atkinson-cycle engine is actually very different; It has a complex linkage that actually changes the stroke length of the intake/compression and expansion/exhaust strokes - It's quite clever, but very fragile because of all the extra linkages.

The reason I keep adding Miller-cycle, is the way Toyota have done it is more similar to the way the Miller-cycle works than with the Atkinson-cycle (i.e. just using valve timing to fake the compression/expansion ratio differences). The only difference is Miller super-charged the intake to make up for the power loss.

 

[Flatcoat]

Mazda use or did use, the Miller concept on their engines at one time and Ford have a not dissimilar approach with their Kuga PHEV. I personally favour supercharging over turbo’s (hence in part why my other car is a Merc Kompressor engined CLK) and the Toyota/Mazda approach of large capacity but relatively unstressed engines to maximise fuel use efficiency, give good real world economy and still provide sufficient power. 

[NASY]

Always understood Atkinson as 4.5 stroke ie the compression stroke is effectively partly interrupted by open valves to reduce the load on the upcoming cylinder initially and hence lower opposition to travel, though reduced compression before ignition being a price paid in low end torque.

[NASY]

Meant upcoming piston.

[Nick72]

13 hours ago, RAV4-D4D said:

This is really interesting and not something I've heard of. Did a quick google on it and seeing some of the animations of engines Atkinson designed shows what an amazing mechanical/thermodynamic engineer he was; humbling in many ways.

 

However, the Atkinson cycle is a 4 stroke cycle and that animation is showing a 6 stroke cycle. I was expecting to see the classic 4-stroke cycle of:

1. Suck (down stroke)

2. Squeeze (up stroke)

3. Bang (down stroke)

4. Blow (up stroke).

 

Instead there's an extra 2 cycles in before the exhaust gases are blown out. Animation shows:

1. Down - Suck in the fuel/air

2. Up - Squeeze the fuel/air

3. Down - Bang due to spark igniting fuel (assumed as they don't show the spark)

4. Up - This is when you'd normally blow the exhaust gases out, but note exhaust valves are closed so we're compressing burnt exhaust gases.

5. Down - Exhaust valves open so we'd be sucking 'stuff' back from the exhaust system but there's no reason to do that

6. Up - Blow the exhaust gases out

 

 

 

There's still unburnt fuel in the exhaust gases so get some of that back into the cylinder 😉

[Nick72]

12 hours ago, Strangely Brown said:

The stroke length is fixed, physically, by the throw of the crankshaft and the length of the connecting rods. There is certainly a lot of clever stuff happening with the VVT and the strokes are asymmetrical in that the valve timing and injection is different to Otto cycle, but the stroke length - i.e. the distance between TDC and BDC - is fixed.

Toyota may be playing a little fast and loose with terminology here.

ETA: If you define the "stroke length" as the distance travelled on the compression stroke with the inlet valve closed rather than the actual, physical length, then yes, I suppose it could be described as a different length.

Mechanically fixed stroke length yes. But it's the ratio of time with respect to valve position defining the what constitutes stroke in this case. AFAIK at least.