Variable displacement V8 - thoughts?
 

I got the bright idea today to see if I could retrofit a V8 engine (specifically, the 4.7L engine in my Dakota) with variable displacement. It would be beneficial to be able to recover between 8% and 20% of fuel economy in this manner.

Electronics is not a concern for me, since I have a BSEE and some experience with building and programming automotive engine electronics. It should be a simple matter to cut power from the necessary ignition coils and fuel injectors. Granted, the stock engine computer very likely would throw codes for multiple cylinder misfires and loss of continuity of said coils and fuel injectors, but this could be overcome in time with a standalone engine controller (like the Megasquirt).

It seems the big obstacle to this project would be shutting off airflow to the deactivated cylinders. The proposed solution came to me as I was reading ways on how this was done with engines that already had this capability.

I remember that there is a company out there who came up with an adapter kit to fit a Ford 4.6L intake manifold to the Chrysler 4.7L engine, and it was basically a pair of adapter plates that mated the intake manifold flanges of the Ford intake manifold to the intake flanges of the 4.7L heads.

It should be possible, then, to fabricate adapter plates to go between the stock 4.7L intake manifold flanges and the 4.7L head intake flanges. These plates would have butterfly valves to shut off airflow to the desired cylinders. This should work inasmuch as it would allow the O2 sensor to continue to correctly read the lambda of the exhaust gases, instead of trying to read exhaust gases excessively diluted with fresh air.

Thoughts?

Would it be possible to move your oxygen sensor upstream on the exhaust such that it only measures the exhaust on one bank of cylinders? This would allow you to run the air through the the non sparking bank of cylinders as normal.

I had thought of doing just that, in order to completely shut off one bank. Ideally, in order to have a more-or-less evenly running engine, you'd want to have -X-X-X-X, where the "-"s correspond to deactivated cylinders, and "X"s correspond to active cylinders. This is to maintain, as much as possible, the existing engine harmonic arrangement.

However, the problem with this approach, of shutting off a whole bank, is that my engine has a firing order of 1-8-4-3-6-5-7-2. That translates to a LRRLRLLR pattern. If I were to shut off one bank (e.g., the left bank), so as to facilitate using the O2 sensor on the other bank, I would have -RR-R--R, which looks very uneven, and could shortly lead to destructive bearing wear or even a snapped crank.

Seems like peeing into the wind to me.

Variable displacement systems shut down intake and exhaust valves on the disabled cylinders. If it was as easy as adding a butterfly valve to the intake, that's the way it would have been done OEM. If the valves are still working, there will still be pumping losses from the intake tracts and from sucking/pushing into the exhaust tracts.

So the trick will be to disable valves. I believe it can be done at home, but it would be quite involved.

The firing order for the 4 cylinder 2009 yamaha R1 motorcycle has these intervals; 270° –180° – 90° – 180° which is actually the same as you would end up with. They did this for better traction.

Don't let the perfect become the enemy of the good.

OEMs don't use butterfly valves to deactivate individual cylinders because they cannot allow the exhaust to be contaminated at all with inlet air before the pre-cat O2 sensor(s) can read it. There will be more leakage with butterfly valves than with the poppet valves in the heads. This is the reason why OEMs tend to use the poppet valves - the OEMs can comply with EPA regulations and the Clean Air Act. Granted, some vehicles inject air into their cats, but that's a different matter.

That aside, pumping losses will offset much, if not all, of the gain provided by causing the remaining cylinders to work more closely to their full potential. Therefore, minimize these pumping losses, and you minimize the drag induced on the engine itself.

I would have to design some of the hydraulic lash adjusters to be able to collapse more completely than normal, on command. After that, I'd have to figure out a way of keeping the rocker arms in place, since they are only secured onto the valvetrain by the lash adjuster at one end, the cam at the center, and the valve itself at the other end. Sounds like a lot of trouble.

On the other hand, it's a compromise to put butterfly valves on the intake side. I should still see some meaningful gain.

...contact the SAE for papers on (A) GM's variable (V8-V6-V4) engine (fiasco) and (B) Chryslers's Multiple Displacement System (MDS) Hemi engine(s).

...and, I'll bet, FoMoCo has dabbled in this arena too.

Sounds like a cool idea, but also quite involved.

My wife's '09 Impala SS has GM's 5.3L with cylinder deactivation and it does seem to work nicely on the highway. We averaged 29 mpg on a recent 1100-mile round trip. That was with two adults (and I weigh 240), 150 lbs of kids and the trunk loaded full-to-bursting with coolers and luggage going 75-80 mph the whole way (speed limit is 75).

That said, I'd be willing to bet that you'd be better off just changing out the rear gears to something taller. It would have a similar effect...reducing the number of cylinder fires per mile.

the best, most reliable, version of variable displacement is a smaller engine with a turbo. I wish it were different.

Variable displacement schemes are often talked about on EcoModder, but I don't think we've seen a DIY success yet. Probably because of the enormous engineering effort required to extract fairly small gains.

Using intake parts from an engine that leaves the factory with variable displacement is a great idea. A better one, that involves less fabrication and doesn't require a custom controller, is to transplant a variable displacement engine + ECU into your target chassis.

The biggest problem with MegaSquirt is it's not OBD-II compliant. In my state, I would be unable to install it in a road-going vehicle '96 or newer.