Would a single rotor fuel cut at cruise help economy?
 

As I was puttering down the highway at a fairly slow speed for a FC (one moving violation, I lose my license) and unhappily watching my fuel gauge drop, I got to thinking.

My FC is most fuel efficient around 90-95mph - around 3500 RPM, right before the ECU goes from closed loop to "dump fuel" mode. The SAFC-II helps, but... not that much.

I've also noticed that, at 3500 RPM, I'm running significantly higher manifold pressure than at 2900-3000 RPM - ~50% on the SAFC-II readout vs ~25% at the lower RPM.

Engines get more efficient at higher manifold pressures, all other things equal. Also, with the throttle butterflies open more, pumping losses are lower.

Then, a Honda minivan passed me. For those unfamiliar with them, the V6 will cut out 3 cylinders at cruise (and pump white noise through the sound system to help mask the rather awkward sound of a V6 running on 3 cylinders).

Why can't we do that with a rotary? Some quick noodling around with SAFC readouts, and I figured out the following:

It should be possible for a FC to cruise, even at a high speed, on one rotor. People who have taken a '7 a good distance with a blown rotor can attest to this. With only one rotor, the throttle plates would have to be nearly wide open, but this is good.

With one rotor firing, and the other one dead, you'd be running almost full manifold pressure. This means less pumping loss going past the throttle plates, as well as a higher effective compression in the combustion chamber (which means more efficient burn).


Now, there would be a few problems with this. The biggest problem I can forsee is that those of us running premix would be without lubrication on one rotor for cruise. A short period with no lube is fine (decel fuel cut), but longer periods wouldn't go so well. However, it would be possible to cycle the rotor that was firing on a 5-10 second cycle.

Also, the O2 sensor, in the stock location, would be utterly useless in this mode. With all the air from the non-firing rotor, it'd be pegged lean. You'd need headers with 2 heated oxygen sensors to be able to use an O2 sensor effectively (heated to keep them warm while just getting air passed over them). It really doesn't matter, though, because the stock ECU won't go into closed loop mode at WOT (which is what it will be seeing). So, for optimum efficiency, you'd need a standalone, or at least a SAFC type device. The standalone would have two maps, a "normal" map, and a "cruise" map - when you went into cruise mode, the cruise map would deliver a lean or stoich burn at WOT in the cruise band, as opposed to the normal slightly rich tune.

Turbo guys probably don't want to even think about using this - cutting injectors out is bad for their engines - detonation kills.

So: How would this work in a real car?

A piggyback module to the ECU would be able to cut the signals going to either primary injector (and secondary, I suppose, but if you're cruising at 3600+ RPM, you'll probably need both primaries firing). There would be a button on the dash to enable "cruise mode". When cruise mode was enabled, the piggyback unit would alternate cutting injectors off, for 5-10 seconds at a time. By tweaking the overlap on the changeover, you should be able to change between injectors without any noticable power loss. The ECU, meanwhile, would be providing what it thought is wide open throttle fuel, being tweaked by a SAFC or standalone to provide a stoich or slightly lean burn on the injector.

I think the combination of reduced pumping losses & higher effective compression would lead to noticeable fuel economy gains.

Thoughts?

-=Russ=-

FYI, fuel milage is generally best at around 50 MHP, give or take 5-10 depending on drag and accesories. Theres a neat link explaining it if I can find it.

well at first glance it seems like a great idea. the problem with fuel economy in a rotary is that you have 2 (or 3) rotors spinning at x rpms and y afr (stock is 14.7:1, leanest you can go without bucking is middle 15's).

if you cut 1 rotor, thats 50% less engine to feed, granted the other rotor has to work harder, so you prolly wont see a 50% change in mileage, i only see a 20% change between a 3 rotor and a 2 rotor (19mpg with the 20b, about 23 with a 13b)

as they say your mileage may vary....

Why whould anyone consider this??? If you guys are willing to go through all this trouble to gain a few mpg's then you need to find another car... Sell your 7 or buy a daily driver... this is rediculous.

If you've ever driven on one rotor (fuel cutted) for extended periods of time it is very hard to maintain speed or even get moving. That one rotor will have to in a sense help spin the other one and wouldnt be efficient. And for the record you cant detonate if there is no fuel in there.

Get a beater if you wanna save gas like said above.

you willbe sevarly under powered ,adn you will pop teh rear rotor b/c gas is also a lubracte and you still need your oil injection too

Your FC is NOT most efficient at 90 mph, the amount of wind drag there is going to kill you, regardless of what you may think.

It may be possible to do what you're describing, but the work necessary to pull it off in a manner to take any advantage of it would exceed the cost savings for a LONG time.

Just buy a metro to drive daily. That'll keep you from driving too fast and save gas.

We've covered this several times before.

Long story short is that in a two rotor, you will be missing way too much power and it will become unbalanced. Also, the cut rotor will gradually build up oil from the MOP (or premix) and from that bypassed by the oil o-rings. Big cloud of smoke on restart.

On a three rotor, I could see idling the middle rotor or switching between the rotors in a staggered pattern. That would keep all rotors hot and prevent oil buildup...But this isn't really relevant in this case...more like something Mazda would do in a production 3 rotor Renesis to help with fuel consumption.

The step nobody covered was the air, Cutting fuel to a rotor will drastically increase drag (like engine braking) because the air is still being compressed. You would need to essentially have a spark plug hole with a valve to open to not have the compression drag of the engine. To agree with aaron Mazda would likely do this before anybody at home due to the complex valving, lubrication, and electronics.

whatever you are smoking...
let me have some!

Well, I was talking about cycling between rotors, specifically to deal with the lubrication/rotor temperature issue.

Addressing several concerns, in no particular order:

Done, and done. I have a Subaru that gets 30mpg highway if I'm gentle, and I normally ride a 1979 Honda CX500 that gets ~45mpg intown, ~42mpg highway, ~35mpg at 85mph highway.

This is more an engineering challenge type problem. And, if I'm right, I don't think it would take more than $50 in parts to make it actually work. That would pay for itsself fairly quickly with the amount of highway miles I put on my car.

I don't think you understand engines very well. The LAST thing you want to do is vent the compressed air - if you compress it, it pushes during the power stroke, and more or less gives you back the same energy. Venting it when compressed is how "Jake brakes" on a diesel work - they vent the compressed air instead of letting it push the piston back down. A diesel, with no throttle plates, has almost no engine braking without either an exhaust restriction or a jake brake.

The bulk of engine braking on a gasoline engine is pumping loss from the intake & throttle butterflies. Running with the throttle plates wide open will help reduce this. Also, you get more air into the rotors, so a higher compression pressure.

I'll agree with you there. But, given the gearing, 90mph is the most reasonable speed on the highways, as opposed to 3500 RPM in 4th, which gives right around 60mph. Any higher than 3500 RPM, the ECU (at least the S4 ECU) goes into open loop mode & dumps fuel, much lower and it seems the engine isn't as efficient. I know I get better economy at 85 than at 70, in 5th.

Remember, this is a cruise setting, not an "all time" setting. It's engaged once I get to speed on the highway. I think there will be enough power to sustain speed, though I'm not totally sure. I'll have higher compression pressures in the running rotor though, so I will be able to get a bit more power than at the lower manifold pressures.

-=Russ=-

4th at 3500?? i get into 5th way before 60.. more like 35 or 45 when trying to conserve...
like said above
what ever your smoking, pass it around :D

What I'm saying is that, if you're above 3500 RPM, the S4 ECU is in open loop mode.

Therefore, if you're steady state crusing above 3500 RPM, your fuel economy will suck.

3500 RPM in 4th is around 60mph - if I needed to cruise a distance at 55, I'd do it in 4th. But on the highway, 5th is the gear to use, and the closer to 3500 RPM you get, in my experience, the better the fuel economy.

-=Russ=-

#2 Make it so...

anyone that thinks you don't get the best milage at higher speeds in an FC is ignorant.

I used to drive mine about 400 miles on the highway every week so I had a lot of time to experiment - and 3400-3500 in 5th you get both best milage and least time!

Any time i go on the highway in my car I cruise at 85-90mph now. I averaged 1-2MPG better at 85 than I did at 70.

I also know that my car will go 105MPH on 1 rotor, at least. But I'm turbo powered so your results may vary :)

Glad to know I'm not nuts. Though, to be fair, 3400-3500 RPM in 4th (60mph or so) would probably net slightly better economy than in 5th. It's just not very useful.

-=Russ=-

I personally see it the other way around. It is the simple nature of wind resistnace. Above normal around town speeds, the wind resistance increases greatly. I, too, have done "experiments" with hiway driving, and they come out to about 60 or as the best seeds for me. I have done trips cruising at 90 or so there and 65 or so on the way back, the way back was definately better milage, despite being into the wind on the way back.

The FC is an oddity. Normally, I'd agree with you - wind resistance DOES increase with speed. But, the FC is a very slippery shape, and the rotary engine gets significantly more efficient at higher RPM for a variety of reasons.

-=Russ=-

Hmmm, I'd be interested to see a technical article on the subject.

Regardless of the cause, it's fairly well documented on the forum that a FC will get better fuel economy at a higher speed in 5th gear. WAYNE88N/A commutes some insane amount of distance each day, entirely highway, and has seen this. I've seen it. Many other people have seen that the closer you cruise to 3500 RPM without going over (the closed loop cutoff is somewhere around there), the better the fuel economy. Repeatedly and consistently.

-=Russ=-

You cannot maintain 95 MPH on one rotor, without going WOT. On An N/A, even at WOT, going 95 mph on one rotor is impossible.
Try it.

Seriously, try it.

While tuning for fuel economy, and netting over 30 mpg, I found that optimal fuel economy happens 65 or so MPG. Driving faster and maintaining similar EGT's and AFR's and the fuel economy would drop.


You guys are really fooling yourself if you think the FC is some wind tunnel god send. Most newer commuter cars can boast a lower coeffient of drag. Even though it has a rotary engine, the laws of physics still apply to the FC.
It will not be most fuel efficient at speeds 50% higher than every other car.

Youre full of it man. EVERYONE knows that the fc is above God and no laws apply to it!

How about holding 70 or 80?

Interesting. I know you have a full standalone. Why would the stock ECU, in closed loop mode, regularly offer higher economy at higher speeds? (curious, not saying you're wrong)

Yet, people are reliably seeing this effect. Why is that?

-=Russ=-

^^^ it doesnt. it offers most economy at 65mph in 5th or lower.
to think you get the best mph at 90.... you got alot to learn... grasshopper...

pass that shit you are smoking...

I think you have a good idea and that it may work well as long as the one rotor has the power to maintain the 65-70Mph you are after, the other problem would be having to cycle the rotor that is droped and making the transtion smooth.