AT compression braking

I did not realize an exhaust gas charge at atmospheric pressure was a goal of the simulated Atkinson cycle. I do recall there being a ton of overlap during an unexpected stroke. Wonder how it would go if we programmed the exhaust valve to stay closed as long as possible within the bounds of the system during the compression stroke with the press of a button, like a half assed Jake Brake.

 

The goal of the atkinson cycle is to get more energy out of the power stroke by making it longer than the intake stroke. Venting compressed exhaust to the atmosphere is wasted energy.

Turbochargers take that otherwise wasted energy and turn it into a larger fuel air charge.

To simulate a compression braking device, you'd want the exhaust valve to open at top dead center of the compression stroke. I'm guessing that's beyond the ability of the variable valve timing (VVT-i) system. Interesting idea, but I'm not sure it's necessary or desirable for a mid sized truck with the GVWR of a cross-over SUV.

 

Given their GVWR's, it might make more sense to put it on the Sienna first.

 

It's not. The Atkinson cycle "shortens" the compression stroke by hanging the intake valve open at the start of it. This forces some of the air back into the intake manifold. He has it backwards on how it works.

 

You are referring to a particular implementation/variation of the atkinson cycle used in modern engines with variable valve timing. I was referring the idealized/generalized atkinson cycle. Either way, in both cases the power stroke becomes longer compared to the intake stroke so that less energy is lost to compressed exhaust gases. You can accomplish this by shortening the intake stroke or lengthening the power stroke. More to the point, the result is exhaust gases being at lower pressure and this theoretically will reduce engine braking.

https://en.wikipedia.org/wiki/Atkinson_cycle

 

What compressed exhaust gases? The only compression is of fresh clean air on the compression stroke.

 

At the end of the exhaust stroke, the exhaust gases are still partially compressed. They haven't been completely uncompressed. Venting them to the atmosphere is wasted energy that could have been used to push the piston further.

Read the wikipedia article for a more precise explanation. Here is one important quote:

"The goal of the modern Atkinson cycle is to make the pressure in the combustion chamber at the end of the power stroke equal to atmospheric pressure"

 

Wait a sec... I think I missed something. If no fuel is being injected and burned during engine braking (a la deceleration fuel cutoff), then the exhaust gas will be the same temperature as the intake gas and thus will be at the same pressure. Therefore, the atkinson cycle (if enabled) will have no effect on engine braking.

 

The CDL manual states engine braking effect is greatest at highest RPMs before hitting redline. Let the engine rev higher.

 

What is not clear to me is whether the engine goes out of the simulated Atkinson cycle during engine braking. I doubt that it does, but if it did, you'd probably get a little better engine braking. When accelerating, it is quite obvious with this engine when the valve timing changes.

Anyway, I just use S-mode, mostly when descending. But I've also found that the tranny can get into an annoying abrupt downshift/upshift pattern when climbing moderate trails in D, so I use it then as well.