Can VVT-e vary compression ratio?

[Zynerji]

Can VVT on the exhaust cam be used to hold the valve open during the compression stroke to effectively vary the compression ratio? If it closes at BDC (like normal) and has 14:1 compression, can the exhaust be retarded 20 degrees to have it close after BDC, effectively shortening the stroke and generate an 11:1(ish) ratio?

My brain says yes, but for N/A only.

Thoughts?

[Dr. Acula]

Simple answer, no.

The problem is, that you would have to basically bridge somehow one stroke. Because after the exhaust stroke, the intake stroke happens before the compression stroke starts. Although it is not unusual for the exhaust valve to be still partially open when the intake stroke starts it would be quite unusual to be remain open until the compression stroke starts. I mean that would mean it stays open for more than a full crank revolution. Also the cam profile you would need for that would be...unusual to say the least.
What you can do for the effect i think you discribe is simply keep the intake valve open longer. Toyota for instance does this in many of their modern engines and this is called an Atkinson-cycle.

[Zynerji]

Simple answer, no.

The problem is, that you would have to basically bridge somehow one stroke. Because after the exhaust stroke, the intake stroke happens before the compression stroke starts. Although it is not unusual for the exhaust valve to be still partially open when the intake stroke starts it would be quite unusual to be remain open until the compression stroke starts. I mean that would mean it stays open for more than a full crank revolution. Also the cam profile you would need for that would be...unusual to say the least.
What you can do for the effect i think you discribe is simply keep the intake valve open longer. Toyota for instance does this in many of their modern engines and this is called an Atkinson-cycle.

The engine I'm playing with currently is vvt-e only... And I appreciate your response.

I also see that it isn't impossible, just need a double-lobed exhaust cam, and only use the cam retarding on the off-side lobe to just "bump" the exhaust valve a bit while using the VVT to control the duration, and changing the effective CR.

So, would this be Atkinson-E?

[Dr. Acula]

Simple answer, no.

The problem is, that you would have to basically bridge somehow one stroke. Because after the exhaust stroke, the intake stroke happens before the compression stroke starts. Although it is not unusual for the exhaust valve to be still partially open when the intake stroke starts it would be quite unusual to be remain open until the compression stroke starts. I mean that would mean it stays open for more than a full crank revolution. Also the cam profile you would need for that would be...unusual to say the least.
What you can do for the effect i think you discribe is simply keep the intake valve open longer. Toyota for instance does this in many of their modern engines and this is called an Atkinson-cycle.

The engine I'm playing with currently is vvt-e only... And I appreciate your response.

I also see that it isn't impossible, just need a double-lobed exhaust cam, and only use the cam retarding on the off-side lobe to just "bump" the exhaust valve a bit while using the VVT to control the duration, and changing the effective CR.

So, would this be Atkinson-E?

Technically yes, but i will be honest, this is not something i would call "A smart idea!" Without knowing what kind of engine it is, i already can see a whole bunch of problems with such a system. I mean i'm not an expert when it comes to engines, i believe i have good understanding how things in them work though.

• Lets say it's port injected or it has a carburator, you will push some unburnt air/fuel mixture directly into the exhaust.

• Direct injection isn't much better because now, you give the fuel a lot less time to properly vaporize because you can only start to inject when the exhaust valve is closed.

• If you use an O2 sensor, well it will probably generate some funny readings because again you push unburned air into the exhaust.

• A conventional catalytic converter would have problems to run like this also because of the air.

• Bumping the exhaust valve open twice as many times, even if it's with much less lift, will potentially require stiffer valve springs.

[Zynerji]

Simple answer, no.

The problem is, that you would have to basically bridge somehow one stroke. Because after the exhaust stroke, the intake stroke happens before the compression stroke starts. Although it is not unusual for the exhaust valve to be still partially open when the intake stroke starts it would be quite unusual to be remain open until the compression stroke starts. I mean that would mean it stays open for more than a full crank revolution. Also the cam profile you would need for that would be...unusual to say the least.
What you can do for the effect i think you discribe is simply keep the intake valve open longer. Toyota for instance does this in many of their modern engines and this is called an Atkinson-cycle.

The engine I'm playing with currently is vvt-e only... And I appreciate your response.

I also see that it isn't impossible, just need a double-lobed exhaust cam, and only use the cam retarding on the off-side lobe to just "bump" the exhaust valve a bit while using the VVT to control the duration, and changing the effective CR.

So, would this be Atkinson-E?

Technically yes, but i will be honest, this is not something i would call "A smart idea!" Without knowing what kind of engine it is, i already can see a whole bunch of problems with such a system. I mean i'm not an expert when it comes to engines, i believe i have good understanding how things in them work though.

• Lets say it's port injected or it has a carburator, you will push some unburnt air/fuel mixture directly into the exhaust.

• Direct injection isn't much better because now, you give the fuel a lot less time to properly vaporize because you can only start to inject when the exhaust valve is closed.

• If you use an O2 sensor, well it will probably generate some funny readings because again you push unburned air into the exhaust.

• A conventional catalytic converter would have problems to run like this also because of the air.

• Bumping the exhaust valve open twice as many times, even if it's with much less lift, will potentially require stiffer valve springs.

Thank you for this info!