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Old 01-11-2008, 08:09 PM   #11
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Well here goes. I think your cylinder deactivated cruising mileage gain will be at best only a little more than the best that can be achieved by pulse and glide near the speed you want to drive. If you are driving 60 mph this will often be about 10-15%. 10% gain is about all the big factories claim for mileage gain with cylinder deactivation.

When I was in college I simulated extreme compression and late valve closing in a simple basic program. Using a 15:1 compression ratio and closing the intake valve at about 85-90 degrees before top dead center resulted in the same peak cylinder temperatures and gained about 3% in thermodynamic efficiency in the simulation. Assuming a 25 hp load this resulted in about a theoretical 10% mileage increase.

Both ideas are valid but the gains are incremental. If you look at a PV diagram, Pressure vs Volume plot of a working cylinder what you are trying to recover is often the tail end of the plot and there really is a small amount of work that can be recovered in that area.

I have driven vehicles with cylinders deactivated and I have driven the old Ford Turbo-coupe with the cam retarded three teeth. A person can do these things. The retarded cam experiment would have been more valid with a twin cam engine or a special grind cam. One can do these things but the gains are not as big as you expect when you work for a day or so to set them up.

Higher compression is good. Higher compression with a little extra duration before intake valve closing is better. Combine that with Somender Singh grooves might even be better.

If you want to deactivate by shutting off injectors I think you need to move the O2 sensor to a location where it is not affected by the excess air. That would involve a little fancy work in the exhaust system.

I think I am kind of pushing the practical limit on simple engine experiments. I am thinking of returning to where I was going before I got involved with my Saturn. If you really want to go a long way on a bit of gasoline you need excellent aerodynamics and a plug in hybrid. Maybe that is cheating but that is where I think it is going to end up. If you start with an efficient car such as a Honda, Saturn, or Corolla there is not a lot of low hanging fruit left to pick from those engines.

Just some rambling thoughts as I read thorugh this thread.
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Old 01-12-2008, 06:03 AM   #12
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Originally Posted by usedgeo View Post

I have driven vehicles with cylinders deactivated and I have driven the old Ford Turbo-coupe with the cam retarded three teeth. A person can do these things. The retarded cam experiment would have been more valid with a twin cam engine or a special grind cam. One can do these things but the gains are not as big as you expect when you work for a day or so to set them up.
Retarding cam timing is an interesting concept, I've heard of advancing the cam timing to get more low rpm torque for better mpg with cars that have a high geared trans, but I've never heard or retarding the cam timing.

So how did the engine run after retarding the cam timing that much? Did it sacrifice any drivability?
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Old 01-12-2008, 08:36 AM   #13
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Retarding cam timing is an interesting concept, I've heard of advancing the cam timing to get more low rpm torque for better mpg with cars that have a high geared trans, but I've never heard or retarding the cam timing.

So how did the engine run after retarding the cam timing that much? Did it sacrifice any drivability?
Without changing compression ratio the advanced profile cams are an effort at increasing cylinder pressure. I ran one with dismal results. Opening an intake valve too soon can allow exhaust gasses into the intake. Hence the ratty idle. A problem I have experienced with this type of cam is increased heat at the intake valve leading to coked up intake ports. For the optimum benefit of the advanced profile the engine should be ran at higher load. These cams were sometimes called RV (Recreational Vehicle) grinds in the first oil crisis. They can help but at very light load they offer little gain. I ran this kind of cam in a v-8 I drove for 5,000 miles on 4 cylinders.

Drivability was very poor at three teeth retarded. I usually ran one tooth retarded. Retarding the cam opened the exhaust later to try to get a bit more work out of the gases before the exhaust opened. Even two teeth retarded exaggerated turbo lag to unacceptable levels. I could play with turbo boost level to make up for the lower volume trapped in the cylinder.

Naturally aspirated you really need the combination of higher compression ratio and late intake closing which is primarily what Miller cycle or Atkinson cycle is. Here are some links that explain it pretty well.

http://en.wikipedia.org/wiki/Miller_cycle

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

What you really need is an engine with some sort of VVT technology. That way you can have the best of both.

WARNING: Perhaps I should have mentioned that one must take care not to run a valve into a piston when doing things like this.
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