Atkinson vs Otto

trish_auto · December 20, 2018

Dear All Experts,

This has been discussed under several topics and this is a continuation to the feedback provided by Rumesh, back in 2016 June. ☺️. Hope you can remember it. I thought of shifting this to a new topic since it was under a topic not relevant to the content.

This was your feedback

On 6/14/2016 at 4:23 AM, Rumesh88 said:

On 6/14/2016 at 8:53 AM, Rumesh88 said:

If I may answer your questions, yes, original Atkinson cycle engines were a failure. However, modern Atkinson engines you find in Toyota hybrids are in fact normal Otto cycle engines with all VVT-i features but with further modified valve timing to provide Atkinson cycle functionality on top.
As for the energy saving on the compression stroke, to compress a gas you need a certain amount of work done. Lesser the compression ratio lesser the amount of work done. For example if you compress 1000cc of gas to 100cc the compression ratio is 10:1 but if you compress 800cc to 100cc the ratio is 8:1. So the amount of energy to compress at 8:1 ratio is less than that required for a 10:1 ratio. In a modern Atkinson engine this functionality is achieved by delaying the closure of intake valve and thus expelling a part of gas (let's say 200cc) back into the intake manifold and compressing remaining 800cc. (please note that the figures used are for ease of explanation only). Expelled portion of gas mixture is sucked into the next piston on suction stroke while ECU makes necessary adjustments in EFI to take the expelled gas mixture into account. Hope this clarifies your query.

Edit: I did't realize that the topic was on taxation. My bad

This is some addition to what you have replied above. Other than the lower compression ratio (delayed intake valve closure thus less compression) following video explains the modern Atkinson engines use higher expansion ratio after the combustion, creates less pressure in the cylinder during exhaust. Technically both compression and exhaust strokes are extracting lower power from the engine during its compression and expelling cycles. This offers a better fuel efficiency. But it is under powered compared to Otto cycle engines.

Further to that, you mentioned that " Expelled portion of gas mixture is sucked into the next piston on suction stroke while ECU makes necessary adjustments in EFI to take the expelled gas mixture into account.", is there a big advantage of sucking the expelled gas back into the next cylinder ? Aren't these expelled gases contain very high amounts of CO2 , which reduces the efficiency of the firing ?

However, I agree with you the point that engine saves power by utilizing the pressure from the expelled gas to filling the cylinder in inlet stroke.

Edited December 20, 2018 by trish_auto

Rumesh88 · December 22, 2018

On 12/20/2018 at 1:33 AM, trish_auto said:

Further to that, you mentioned that " Expelled portion of gas mixture is sucked into the next piston on suction stroke while ECU makes necessary adjustments in EFI to take the expelled gas mixture into account.", is there a big advantage of sucking the expelled gas back into the next cylinder ? Aren't these expelled gases contain very high amounts of CO2 , which reduces the efficiency of the firing ?

However, I agree with you the point that engine saves power by utilizing the pressure from the expelled gas to filling the cylinder in inlet stroke.

Expand

The higher expansion ratio in an Atkinson engine compared to that in a Otto engine is a direct consequence of the lower compression ratio. Apart from that, there is nothing particularly done to achieve a higher expansion ratio in an Atkinson engine. The mechanical (ie the effective piston travel ) difference between two engines is in the compression stroke.

As for the your comment on expelled gases, it does not contain a particularly high CO2 content. It is just the same air/fuel mixture at the intake manifold.

Edited December 23, 2018 by Rumesh88

trish_auto · December 22, 2018

On 12/22/2018 at 3:30 PM, Rumesh88 said:

The higher expansion ratio in an Atkinson engine compared to that in a Otto engine is a direct consequence of the lower expansion ratio. Apart from that, there is nothing particularly done to achieve a higher compression ratio in an Atkinson engine. The mechanical (ie the effective piston travel ) difference between two engines is in the compression stroke.

As for the your comment on expelled gases, it does not contain a particularly high CO2 content. It is just the same air/fuel mixture at the intake manifold.

Expand

Confused...!

1. "The higher expansion ratio in an Atkinson engine compared to that in a Otto engine is a direct consequence of the lower expansion ratio. "

I suppose you wanted to mention, " direct consequence of the lower compression ratio" . is that ?

2. "Apart from that, there is nothing particularly done to achieve a higher compression ratio in an Atkinson engine"

this is other way round. Atkinson engines are lower compression that of Otto.

3. "The mechanical (ie the effective piston travel ) difference between two engines is in the compression stroke."

You mean piston travel distance is different from compression to exhaust ? How come it

4. "expelled gases, it does not contain a particularly high CO2 content. It is just the same air/fuel mixture at the intake manifold".

You mean the air-fuel mixture is same even after combustion ?

trish_auto · December 22, 2018

On 12/22/2018 at 4:12 PM, trish_auto said:

Confused...!

1. "The higher expansion ratio in an Atkinson engine compared to that in a Otto engine is a direct consequence of the lower expansion ratio. "

I suppose you wanted to mention, " direct consequence of the lower compression ratio" . is that ?

2. "Apart from that, there is nothing particularly done to achieve a higher compression ratio in an Atkinson engine"

this is other way round. Atkinson engines are lower compression that of Otto.

3. "The mechanical (ie the effective piston travel ) difference between two engines is in the compression stroke."

You mean piston travel distance is different from compression to exhaust ? How come it

4. "expelled gases, it does not contain a particularly high CO2 content. It is just the same air/fuel mixture at the intake manifold".

You mean the air-fuel mixture is same even after combustion ?

Expand

Found answer for above 3.

"Due to the unique crankshaft design of the Atkinson, the expansion ratio may differ from the compression ratio. By adjusting the linkage to allow a power stroke that is longer than the compression stroke, the engine can achieve greater efficiency than with the Otto cycle engine. The exhaust expansion stroke was 1.78 times longer than the intake stroke."

How about 4?

Edited December 22, 2018 by trish_auto

Rumesh88 · December 23, 2018

On 12/22/2018 at 4:12 PM, trish_auto said:

Confused...!

1. "The higher expansion ratio in an Atkinson engine compared to that in a Otto engine is a direct consequence of the lower expansion ratio. "

I suppose you wanted to mention, " direct consequence of the lower compression ratio" . is that ?

2. "Apart from that, there is nothing particularly done to achieve a higher compression ratio in an Atkinson engine"

this is other way round. Atkinson engines are lower compression that of Otto.

3. "The mechanical (ie the effective piston travel ) difference between two engines is in the compression stroke."

You mean piston travel distance is different from compression to exhaust ? How come it

4. "expelled gases, it does not contain a particularly high CO2 content. It is just the same air/fuel mixture at the intake manifold".

You mean the air-fuel mixture is same even after combustion ?

Expand

I can't believe I typed 1 and 2. I must have smoked something. Corrections done. Thank you.

3. As for the piston effective piston travel yes the travel is shorter because the intake val.s are open.

4. You don't get exhaust fumes in the intake manifold unless you have an EGR.

Edited December 23, 2018 by Rumesh88

trish_auto · December 27, 2018

On 12/23/2018 at 8:25 AM, Rumesh88 said:

I can't believe I typed 1 and 2. I must have smoked something. Corrections done. Thank you.

3. As for the piston effective piston travel yes the travel is shorter because the intake val.s are open.

4. You don't get exhaust fumes in the intake manifold unless you have an EGR.

Expand

Hi Rumesh,

Thank you for the reply. Please explain the purpose of below process,

" In a modern Atkinson engine this functionality is achieved by delaying the closure of intake valve and thus expelling a part of gas (let's say 200cc) back into the intake manifold and compressing remaining 800cc. "

Google says:

" it reduces the load on the piston as it rises to begin the compression stroke.."

is that so ?