Rotor deactivation for fuel savings
 

I'm doing a long distance trip over flat ground and thought I could save some fuel by deactivating one rotor by pulling the spark plug, plug lead, disconnecting the coil and disconnecting the fuel injector in that runner. I'm looking for people to actually explain why this is a terrible idea so I get it out of my head.

For a start I think the oil will continuously inject into the rotor chamber and it won't get burned off, filling the chamber with excess oil.

There is the issue of exposing the rotor to debris through the plug hole, maybe some kind of breather could be screwed in there

I want to do something I can undo on the side of the highway with basic tools should the need arise.

Pointing out why it's idiotic is appreciated, but making suggestions to make it less idiotic is preferred. :)

Plus it would run like crap. Haha.

The other problems:
- that the pressure on the rotors help to balance each other, so running on 1 rotor is incredibly rough. Proven by plenty of people with 1 blown rotor or 1 rotor dead on ignition
- assuming the rotor is still in good condition, the compression/expansion stroke will still be occuring, which means that the rotor will be not just not providing power, it will be actively soaking power, the same way as engine braking does, as it will still draw in air on the intake stroke, still compress air, etc... The amount of power needed to hold an RX-8 at steady speed will go up, as will the fuel consumption.
- oil injection would need to remain active, otherwise the seals are just sitting there wearing themselves out. Because it's not getting burned off, your plugs will foul heavily, and your cat (if you have one), will get destroyed quickly.


It's not a stellar idea :)

i talked to a guy recently that drove his fd home from dgrr last year on one rotor. he said he had to downshift to 1st gear to pull the slightest incline and couldnt go past 5mph. bad idea

Bad. Idea.

Hmmm, I thought the rough running would be somewhat alleviated by the plug holes being open, there would be essentially no power sapping compression in that rotor. The plugs won't foul because they're not installed. that just leaves the pressure balance you mentioned and fouling the cat, which at the age of the car is inevitable anyway:)

Maybe I'll try driving around the neighborhood like that and see how the power loss is and let you guys know how it worked out, and also how much blue smoke I belt out after reinstalling everything!

Once around the block can't hurt too bad right?

The fuel injector is still active and you will pollute the cat with unburned fuel. The cat will over heat and burn out.
Be careful where you park.

Please see original post about disconnecting injector in that runner

With the plugs open, that fuel will just be hosing down the side of the engine, the frame, and the left front tire.

Basically, in order to get this to work you have to:
- Solve the dramatically increased air compression/expansion pumping loss
- Stop the ignition from firing
- stop the fuel injectors from firing
- reduce the oil injection severely, but not stop it
- dynamically re-balance the rotor that is firing.

Doesn't matter. See RIWWP's post above. The OMP still needs to run to lubricate the rotor seals or they'll just burn themselves up.

Game over.

Nice thought, though... :rollingla

Ummm, thanks for your input, but if you see the original posti was looking more for quantifying why it's bad idea. I already know its a bad idea, I just want people with the proper know how to break it down

So if I disconnect the fuel injector the OMP is disabled also? If they can run independently of one another then it's still doable

do you know how difficult it is to get to the injectors? if you did you wouldn't be asking this question just in order to save a few bucks and create a virtually undriveable car. not to mention the destroyed cat in the process.

If you solve each of the points, then yes, you can "get it to work", however the only difference will be that you will have to run 1 rotor at a much higher throttle to move the car at the same speed.

It will still take ~40g/s of airflow into the engine to maintain ~70mph. It doesn't really matter if this is on one rotor or two.

Basically, in order to get this to work you have to:
- Solve the dramatically increased air compression/expansion pumping loss(plugs out)
- Stop the ignition from firing(coils and leads disconnected)
- stop the fuel injectors from firing(disconnected)
- reduce the oil injection severely, but not stop it(ermmmm....)
- dynamically re-balance the rotor that is firing.(yeaaaah...)

That's for humoring me guys, I think there are too many obstacles in place.

It'd be nice to have a 'single rotor' push button but whatever, I've never given a crap about economy before, why start now? If I wanted to save fuel I'd drive slow and not autocross :)

Well, you found me out there, ive never had injector issues and haven messed around with them, I was kind of hoping they'd be at least as easy to reach as our ridiculously located oil filter :P

If you solve each of the points, then yes, you can "get it to work", however the only difference will be that you will have to run 1 rotor at a much higher throttle to move the car at the same speed.

This is true and where the fuel savings come from, since you are closer to peak BSFC the nearer to WOT you are, at least that's true for most piston engines.

Either way, Ive seen enough to write it off, thanks again guys

Cost/Benefit
 

In full recognition that I'm late to the conversation... I would also like to point out if you were driving a 3,000 MILE distance and DOUBLING your MPG from this proposed mod, you would only save $264 in gas.

What are the odds you'll break something more expensive than that? Pretty Good;)
Are you going to have ANY fun with your car performing worse than a Prius? :scared:
Consider that 8.8 cents per mile the cost of insurance and ability to have some fun.

X Miles . Gallons . Dollars = Savings
_______Miles___Gallons

I assumed:

21 and 42 MPG
My $3.64 Local gas price
3000 Miles.

I wouldn't do it.

i like how people always come up with something completely retarded , everyday.

Not sure i'm understanding your question. Even if you do let the OMP run for the "de-activated" rotor, where is all the oil going to go? Normally its burned off during the combustion cycle.

It's not the worst idea I've ever had, see my much older post about a power plant frame mounted electric motor inline with the driveshaft, that was a whopper :)

This isn't something I have any sort of real knowledge in, so if I'm incorrect, I apologize. But I remember hearing eco-modder type people talk about making hot air boxes (drawing air off the radiator) to increase the amount of throttle required to maintain speed, reducing pumping losses.

Would this achieve the same effect (if the compression stroke got vented to the atmostphere)? Granted, you would still be adding air into the exhaust, and messing with air fuel ratios...


Either way, there is too much dumb stuff to worry about. Suck it up, it's an RX-8, it doesn't get good gas mileage. We all know it. There are small things we can do to help, but either way, it's going to be bad.




*Edit* I can't even imagine the bearing wear from something like this.

Isn't this why we are still around the RX8 forum?

this made my brain hurt...

Yes, pumping losses run inversely proportional to how wide open the throttle is. A full open throttle has much lower pumping loss than a partially closed one.

That being said, it would be insanely difficult to reduce the pumping loss from one fully healthy rotor that is "shut off" far enough for this to actually produce a net gain.

A big portion of the eco-modder work is to reduce the amount of power needed to keep the car at speed, i.e. drag reduction so the air resistance isn't as great. Without doing any of that, the engine will still need ~a fixed amount of power to counter wind resistance to keep the car at highway speed (varies based on engine health, ignition health, and air density due to temp and altitude, plus more factors). It doesn't really matter how it gets that power, or how much power is sapped away due to other things (like pumping loss of one rotor spinning without power), after all the costs are subtracted, it still needs the same amount of power for the air resistance.

Someone who is REALLY serious about trying to get superior mileage out of an RX-8 would see a far greater gain from re-paneling the bottom of the car completely flat with a small front splitter and significant rear diffuser than just about anything else (Assuming their engine is already healthy). Significant gains can be had here, and it usually a grip improvement as well, since you can reduce lift without increasing drag (vs increasing downforce always increases drag)

Example of another car:
http://photos.motoiq.com/MotoIQ/Feat..._JEF6292-M.jpg

nah GM came up with the idea in the 40's and had it on a car in the 70's Variable displacement - Wikipedia, the free encyclopedia

http://en.wikipedia.org/wiki/Cadilla...368_and_V8-6-4

it works with modern EFI stuff, but i guess the 70's caddy system was pretty shite