Surasonac

So one of the biggest issues at least for me is the ungodly fuel consumption on the RX8. However I recently read about how modern V8's utilize displacement on demand which disables cylinders when power isn't needed. This got me thinking about how this can be applied to the humble wankel.

I thought about posting here because there are a lot of people far more knowledgeable than me to tell me i'm wrong.

My idea is that by disabling say the rear rotor, you would improve your mileage significantly for city driving and poodling around. The oil injectors would still be lubricating the apex seals so no problem there. The only problem I can foresee is you are essentially turning the unused rotor into an air compressor which would have a parasitic effect on power. But I feel like it wouldn't be too much.

I'm going to unplug the fuel injectors for my rear rotor tomorrow and see how the car runs. If its all okay then i'll probably just hook a relay up so I can flick the rotor off whenever I want. I really only need the full 230bhp when hooning around.

Has anyone attempted this before? 30+mpg sounds better than 18 for sure. This is only really an issue for me since I go to college and daily my 8' so it cuts into the finances. Don't get me wrong... its totally worth it. But an improvement is always welcome.

NotAPreppie

So, what happens when you get a bunch of fresh air going across the front O2 sensor that hasn't been used to combust fuel (i.e. from the rotor with disabled fuel injectors)?
What do you think the ECU is going to do when it thinks that the engine is running hella crazy lean?

200.mph

iTrader: (9)

Surasonac

Right yeah forgot about the O2 sensor. Definitely more complicated than it seems then. I would need an aftermarket ecu for sure then. I don't see myself doing that any time soon.

Aftermarket ECU aside... A mechanical solution is independent e-throttle bodies isolating the rotor not being used. Then you could cut power to the throttle body and injectors at the same time. The ITB's would connect to the regular intake tubing so the MAF sensor would still work. Excuse the poor paint work

IamFodi

iTrader: (1)

ITBs that work, without sacrificing all the optimizations of the stock intake manifold, would be a great achievement. But I don't think it'd work for what you have in mind here. You'd still have to completely reprogram the engine management.

AFAIK as a non-expert, big V8s and rotary engines are inefficient for different reasons. A big V8 has a lot of cylinders to fire, and it can be hard to operate at very low horsepower levels without running insanely lean or at RPMs too low for good efficiency. That's why cylinder deactivation works for them. Shutting down some cylinders lets the active ones run at the right AFRs and RPMs for high efficiency without producing too much power. I don't think rotary engines have the same problem. Their poor mpg comes from their low thermal efficiency, i.e. too much of the energy of combustion gets wasted as heat. I don't think reducing the rate of combustion events would fix that.

EDIT: Saw this thread with more (and likely better informed) commentary. https://www.rx8club.com/series-i-tec...t-demand-3691/

UnknownJinX

Not feasible on a rotary as far as I see it.

I actually owned a car with cylinder deactivation, a 2008 Honda Accord Coupe(North America, obviously) EX-L V6 with a 5-speed slushbox. It has the Variable Cylinder Management technology that can make the V6 run in 3, 4 or 6 cylinder mode.



You can read about it here: Honda VCM Technology

If you want such a system on a rotary engine, here are the issues:

- As far as I know, cylinder deactivation only works with cars with automatic transmission. If you choose a standard version of the vehicle(Corvette, Accord Coupe V6, etc.), you no longer have the cylinder deactivation. I suspect this is because the ECU needs to control the transmission as well in order for the cylinder deactivation to come on at the right time. With a manual transmission, this is just not possible. ECU doesn't really get much info from a manual gearbox.

- Like IamFodi mentioned, rotary's crappy gas mileage is from its inherit design, like long combustion chamber, greater surface area, etc. Cylinder deactivation helps with gas mileage because it reduces pumping loss. With only 3/4 cylinders working instead of 6/8, you need to open the throttle wider to maintain the speed, which means less pumping loss. This can help with gas mileage, but you know what else eliminates pumping loss? A blower. And it gives you way more power.

- Also, you need decent low-end torque for cylinder deactivation to work. A V8/V6 has that kind of torque. A rotary? Pffff.

- When you deactivate a cylinder/rotor, you need to make sure it's sealed so it's not robbing any power. On a piston engine, you can do it with some tricks on the cam lobe. Honda does it with a variation of their VTEC technology. Now, how do you plug up all of the intake and exhaust ports of a rotor? Another challenge.

- Lastly, there are a lot of ways cylinder deactivation can go wrong. This is actually a part of the reason I learned how to drive a stick shift: I will take the slightly crappier gas mileage over a pricey repair bill any day. Look up "Honda VCM lawsuit." Honda V6s around 2010(like mine) all had issues with oil burning. How? The VCM comes on way too often on some earlier flashes of the ECU and it makes the seals lose their effectiveness over time. My Accord didn't burn any oil, but even then, you can see the problem on the spark plugs. The left most two are from the cylinders that fire all the time, while the right most one is from the cylinder that gets deactivated the most.



GM learned and made sure the deactivated cylinders actually fire once in a while to keep the pressure up so the seals still work. But who knows? Maybe something else can **** up.

In short: Buy an old beater to daily if you want to save money on gas.

mazdaverx7

iTrader: (6)

I would just stay away from the idea. A lot of great things mentioned above already. however, remember that the RX-8 does not have a lot of low end torque. The car would be pathetically slow running on one rotor. I had to drive an RX-7 home from work once in the late 90's after losing compression in the rear rotor. It was horribly slow and ran like crap. But I made it home. If you own an RX-8 and drive it regularly, you learn to live with the mileage. I get 18 mpg in my 04 that I drive almost every day and I'm happy with that. If I wanted good mileage I would drive my 01 MR2 Spyder to work.

UnknownJinX

Actually, I just noticed that the OP said he wants the rotor deactivation to work around city driving.

On cars with cylinder deactivation stock, they rarely turn on when you drive around city. My V6 Accord has an ECO indicator and it mostly turns on when I am on a highway.

Depends on how insurance works in your neck of wood, a beater could work pretty well, definitely a lot better than all the work you want to do. I had a 1999 Corolla 5-speed standard. Cost me CAN$1200 and I sold it for CAN$1100 when I was done with it. Purchase price is more like a deposit since they don't depreciate anymore unless you crash it. It has 120 BHP(quicker than an 8 running on a single rotor, I imagine) and could get 30+ US MPG without even trying. As boring as a Toyota may be, keep the oil changed and it will never leave you stranded. Then you can keep your 8 as a fun project car.

Alternatively, you could hypermile... But for people who don't like it, it's very boring and I am not sure if some of the hypermiling techniques are that great for a rotary engine.

NotAPreppie

OP, I feel like you're trying to tighten a bolt with a hammer.

sonicsdaman

iTrader: (3)

step 1 : update car insurance policy
step 2 : disconnect rear rotor injectors
step 3 : send it!
step 4 : ????
step 5 : buy a civic

RX0004

Take an engine which already has low torque and reduce that again by at least another 50%....

I've driven early RX rotas that were running on one rotor and it wasn't fun. They run impressively down steep hills with a tail wind, but in all other scenarios you'll get to your destination faster by walking...

Loki

iTrader: (1)

Keep in mind that cylinder deactivation systems have different flavours of keeping valves open so the deactivated cylinders don't act an an engine brake. Not super possible in a rotary.

However, what you could do is skip every other face. Just don't fire it, no fuel, no spark. Not sure it will be as smooth, but better than a single rotor?

Oats

Put some batteries in the trunk and some small electric motors in the front to make it a hybrid. How hard could it be?

Surasonac

Not a bad idea, if you made sure one rotor is firing when the other isn't then it shouldn't be too rough. And I'm only talking for city driving anyway. Cruising around the MPG is manageable.

Surasonac

There are actually systems that do this you can buy. They connect onto a modified driveshaft and assist the engine. But I don't think I'd go down that route. Quickly gets into snake oil territory.

Loki

iTrader: (1)

As someone said above, cylinder deactivation only makes sense when you're cruising. Engines are most efficient at low rpm, high load, so deactivating cylinders forces the remaining cylinders to work at higher load to maintain constant speed. The fact that the cylinders arent firing doesn't actually figure into fuel consumption equation all that much.

In the city, the load is constantly changing, so you're not winning anything by deactivating cylinders. You already have everything you need to be as efficient as the car can be in the city: the gas pedal.

200.mph

iTrader: (9)

buy a civic or w17 swap that bitch for the mad torquez and mpg. ya digz mane? this is the dumbest thread ive read in a while

Surasonac

I actually considered K20 swapping it because its only £130 for the K20 to RX8 transmission adapter.
But then I factored in everything else i'd need to do the swap and it came out as FAR too much money. Also i'd never get a civic, I dislike FWD.
If my engine ever blows I'd consider an engine swap, but I do love the rotary so it would be a hard decision. The RX8 chassis is so good, its still a pleasure with any engine.

UnknownJinX

When I had my Accord, it was possible to keep the ECO light on when accelerating, but then you may as well buy a car with a smaller engine to begin with.

It's not hard to make a hybrid, but it's hard to make a good, reliable hybrid.

Well then stop whining about the gas mileage. Or buy an old Miata as daily(which would be more expensive to purchase). There aren't many cheap RWD cars that's good on gas.

Surasonac

I already had an MX5 before my RX8, it was great fun but pretty slow. And i'm not whining about anything.... i'm actively trying to find a solution which would benefit myself using the community as a guide which seems to just be full of needless hostility. Its just ideas. I can afford to run the RX8 with its current MPG no problem but who doesn't want better mileage. Its not so much of an issue for people in the US/CA who pay 1/3 the price we do for gas and then say people in other countries are whining about gas mileage.

UnknownJinX

Well, a car that's good on gas is not supposed to be fast. My old Corolla has okay acceleration in the first two gears and that's it, but again, 30 US MPG is easy.

In order to tackle this, you have to understand how and why cylinder deactivation works. Read the Honda article for starters. Like I mentioned before, it mostly only works when you are cruising around or accelerating very, very lightly. If you accelerate like this off a stop, expect road rage behind you. When you pull away, you will still want all cylinders/rotors firing.

People are bashing you because it's not realistic. 200 can talk pretty rough but his idea that this isn't feasible isn't wrong.

As for the firing every other face idea... I am not sure if that would have any consequences on the engine. Not a terrible idea, though, but still quite a bit of work and challenges as I see it.

Gas is not exactly cheap at where I live, either, but the only feasible option I have is to sell my 8 for something else and that's not happening.

200.mph

iTrader: (9)

ms miata will crush an rx8. i babysat riwwps for 9 months and it was a blast to drive


da faq? wash the sand out of your front hole lol

UnknownJinX

They are also rare AF. I think there were only thousands of them ever produced.

NA(1st Gen) Miata is also starting to get more expensive as it gains the classic status.

You gotta admit that you were pretty harsh to OP...

Surasonac

MX5 NA is still cheapish here. No idea about MS Miata though. For a decent rust-less NA bone stock its 2-3 grand. The one I had I picked up from an older couple, it was their sons car. Cost me £550 Been sitting in their driveway for several years. I fixed it up in my garage over the course of a few months. Paintwork was in terrible shape, I bought a rotary buffer and various compounds and spent a couple days completely doing that. Fixed up the roof which the rear window was broken on. New rocket cover gasket and painted it. New coolant hoses. It had lowering springs and a racing beat exhaust on it too when I bought it. I was planning on turboing it but underneath it was rusted out pretty bad, it was really heading for the scrap yard since it was beyond repair. Its a shame because the interior and exterior were almost mint condition after I finished restoring it. Any way, I don't really have an issue with constructive critics. But people who just get dismissive over ideas because in their opinion its stupid grind my gears. Everyone has a different situation and still wants to enjoy a nice car. For me, I chose the RX8 because its one of the very few RWD cars I can afford to insure because its technically a 4 door family car. My only other choice was a MR2 Turbo but those have plenty of issues on their own, not to mention a greater insurance and buying price.

From what I gather here, you can't do rotor deactivation without serious ECU and Mechanical modification.
The every other side misfire idea sounds like it might work, could be a cheap way to reduce mileage when cruising.
Hybrid is the best bet with motor assistance and possibly a bump in torque from that motor. But thats requires sinking a lot of money into the project.
I know Mazda are toying with running rotaries on hydrogen. I don't have a hydrogen station near me.
LPG is much cheaper and I have one locally but it doesn't last nearly as long. I don't want 50 miles per tank.
And of course there is always the old engine swap. Also a lot of money.

I think the RX8's MPG fate is sealed really.

NotAPreppie

Welcome to 10+ years ago.