Cam regrinds for Atkinson cycle? - Page 2 - Fuelly Forums

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Old 07-10-2007, 12:50 PM   #11
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Originally Posted by Bill in Houston View Post
Really? Wow, I did not know that. Seems pretty wasteful.

Is VVT on the exhaust side usually made to keep the exhaust valve closed a little longer at low loads?
I think wasteful for FE use, especially. They build the engines to endure the extremes that 95% of the time never happen. They also have to allow for drivers that know absolutely nothing about cars. Some high performance cams have the exhaust valve opening half way into the power stroke, extreme cams open even before that. This is not so much that the exhaust requires opening that early as the desire to get the valve lift underway. When you have high lifts, you have longer durations to accomodate the design limitations inherent in valve train design. At high rpm you gain more than you lose, generally.

The same scenario exists with intake opening well before TDC while the exhaust valve is still open. Not so much required but done just to start the valve opening early to have it open a significant amount at and shortly after TDC on the intake stroke, the most powerful area in the intake stroke. Textbooks carry on about the necessity of overlap but don't seem to know the real reason it's there.

On my car, the VVT does open the exhaust valve later at low rpm, which allows the power stroke to continue longer. Concurrently it closes later. The camshaft duration and lift remain constant, the whole camshaft advances and retards.

Don't get me started on engine design. You asked for this.
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Old 07-10-2007, 02:42 PM   #12
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Originally Posted by CO ZX2 View Post
They build the engines to endure the extremes that 95% of the time never happen.

One think I like about hybrids is the ability to avoid that 5% of extreme operation and still have reasonable performance.

They also have to allow for drivers that know absolutely nothing about cars.

Ha, that is almost all of them, right?

design limitations inherent in valve train design.

Ya, a step change in that technology would be helpful, huh?

Textbooks carry on about the necessity of overlap but don't seem to know the real reason it's there.

I heard that it was for exhaust valve cooling. But I suspect it's just to give time for the valve to open.

On my car, the VVT does open the exhaust valve later at low rpm, which allows the power stroke to continue longer. Concurrently it closes later. The camshaft duration and lift remain constant, the whole camshaft advances and retards.

Interesting. A little more power extracted, and a little more exhaust gas recirculation. Sounds good.

Don't get me started on engine design. You asked for this.

Ha, I enjoy it.
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Old 07-11-2007, 12:32 AM   #13
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@CO Yup, totally.

What I've found interesting is the difference changes make in terms of efficiency at load/speed. For instance, offset cranks allow for much more efficient high rpm operation (it pushes out the BSFC ovals to the right), but don't really benefit much in terms of low load. And the four stroke Atkinson cycle allows for a very efficient BSFC map (below) from ~5-70hp in the case of the 1NZ-FXE, almost comparable to the equivalent load map in the TDI from ~5-90hp. Course, 70-90hp isn't enough for most people, so we need electric assist but for some of us it's acceptable. I can't really justify grinding a cam for economy, since once I pass ~50mpg, it's just not worth the time/effort/cost imo.... yet. However, when gas is $6+/gal I'll probably be whistling a different tune.

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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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Old 07-11-2007, 06:03 AM   #14
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Overlap is for exhaust scavenging and cylinder filling. Most beneficial on the high side of the RPM band. It helps fill the cylinder with the fresh charge, and the fresh charge makes it easier for the exhaust to leave.

Ya know, I think you guys could really benefit from cruising around the high horsepower sites, because a lot of the tricks used to get max power out of an engine could be used to improve fuel economy. For example, I'm looking at a header install on my engine, supposed to add a full second and a 20 percent fuel economy increase to my engine. I believe it too, because my manifolds are horribly restrictive.

If you were to get a cam that has less overlap and were timed to open the intake as close to TDC as possible on the intake stroke, and be completely closed just after the piston started moving back up on the compression stroke, it would be the equivilent of adding about 1 atmosphere of compression to the engine. On average each atmosphere in compression will add 30HP to the average Chevy 350, so would add a smaller amount to a smaller engine. Each atmosphere theoretically would also add another MPG or 2 to that 350, should add more to a smaller engine. I have no proof of this, someone would have to test it out.

Here's a site with a lot of math on how cams work and the different effects. And, this article describes the effects of using a longer connecting rod, shorter stroke and larger piston on an engine. By building a 350 using a 4.155 inch piston, 6.2 inch connecting rods and a 3.25 inch stroke instead of the factory's 4 inch bore, 5.7 connecting rods 3.48 stroke, they were able to run a much higher compression with no detonation on cat piss fuel. Run the same compression on a standard 350, and you have to run octane booster. In theory, since the new long rod config makes more power, it will use less fuel to maintain a specific speed than the short rod version would. Both have the same cubes, but one is more efficient. I have no idea how the smaller engines most people here have would be able to use all these tricks without custom parts, but no telling what someone might be able to come up with.

Food for thought.
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Old 07-11-2007, 07:03 AM   #15
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Food for thought.
Telco, do you have any knowledge pertaining to the Atkinson Cycle Engine? That is what is being discussed here. We are exploring exceptional mpg capability.

Most of us have spent time in the high performance world and can't remember any mods that produced exceptional mpg. The complete opposite is what would be expected and IS what happens. Claims are very easy to make and most times are not easy or are impossible to back up.

We didn't just ride in here on a load of turnips. Most here have a very good understanding of the inner workings of engines.

Take the time to read the posts contained in this thread.
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