jono20

I have some questions about how the rotary (and any other engine really) works. Ive heard different things from different people.

Regarding fuel efficiency, what determines how much fuel is injected: RMPs, or how far down you push the pedal. Ie... Is it more fuel efficient to be at 3000rpm in 3rd as opposed to 5000rpm in second (with the pedal pushed down the same amount..)

does letting the engine slow you down consume fuel? like when it is slowing dropping down to your stop is it consuming fuel even though your foot isnt on the gas pedal?

if rpms are climbing at the same rate, does it matter how far the pedal is pushed down?

and finally, what is the most fuel efficient speed? Ive heard 100km/h (about 60mph) but im wondering if you folks think differently

AREITU

EPA testing is at 55mph. You lose something like 10% for every 5 or 10 mph above that speed. No matter what, if you place load on the engine, it's gonna be sucking gas. If you read the owners manual of most cars, it'll tell you to floor it in each gear to a certain speed and change gears.
Basically internal combustion motors like gasoline ones, operate at full efficiency at wide open throttle, not part throttle. So flooring it until you get up to a cruising speed is the best compromise between gas mileage and acceleration.

My POS mazda mpv minivan has gotten 15 mpg when I drive like a jackass and try to break it. It'll get 18mpg if I accelerate briskly until 40 (in city driving) and just cruise at that speed, accelerating slowly if I need to. It'll get 17 mpg if I feather the throttle really lightly. My parents' V6 Accord does the same thing.

I'm not sure how rotaries work. My friend has an A/F meter in his FC and he can watch it go from open loop to close loop. When he accelerates, even just a bit, it tends to run extra rich, then when he's cruising, it goes back to the mode where it doesn't read the 02 sensor.

jono20

so on piston engines its best to floor it until crusing speed then let off?

can anyone confirm if this is also true for rotaries?

Wankel7

When you let off the gas pedal completly.....the injectors are cut ... no fuel on decel.

Easy enough to test your theroies.

Just top off the tank and drive 50 miles fill up.....

Do the math to get the MPG....wash rinse repeat for the different secarnios....

jono20

a very plausible solution wankel, but asking here will result in a much faster resolution

ive only had the car for a little while but it seems the heavier I put my foot down when I accelerate the more gas it takes. If I go 100% grandma and accelerate very slowly it seems to save gas. I donno though.

this is bascially all I want to know: what controls fuel flow, how far the pedal is depressed, OR how high the rpms are with the gas on. (does 7000 rpm take more gas than 3000rpm with the pedal depressed the same amount?)

gingenhagen

Um, rpm.

jono20

for sure?

so with the pedal depressed 50% at ALL times, running 6000rpm in second gear will consume more fuel than say 3000rpm in 3rd?

NZConvertible

Both. The amount of fuel required is based almost entirely on how much air[ is entering the engine. Airflow is dependant on both throttle opening and revs.

maxcooper

Yes, absolutely.

But don't take that as an indication that it is only RPM that determines fuel flow. Fuel injection systems essentially measure(*) how much air is flowing into the engine and then deliver fuel in a quantity that corresponds to the air flow. Air flow increases when you press the gas pedal down. It also increases as the RPMs rise.

(*) Some EFI systems (like the one in the FD) don't measure the flow, but instead measure intake manifold pressure and then lookup how much fuel to deliver in a table. There is an assumption about how much air is flowing into the engine at various RPMs and pressures baked into the table. But the air flow is not measured directly. These are called "speed-density" EFI systems.

-Max

jono20

ok well what about the acceleration part then. if your accelerating from 1000 to 5000 rpm, will it consume more fuel if you have your foot down than if you depress it only 50% (taking longer to reach that top speed)

thanks a lot guys

Shinobi-X

The more revolutions your engine is turning, the more air flows, the more fuel has to be injected. Cruising at lower rpms means your engine does less work, which means less fuel is being consumed. Engine RPM is also related pretty closesly to how heavy your foot is, so "how far the pedal is depressed" has its bearing on fuel consumption as well.

In general, if you're trying to increase the amount of miles you're getting, drive at a constant speed when possible at low rpm, and don't go heavy on the throttle.

maxcooper

Tough call. By accelerating slower, you also end up driving a lot further. Your MPG will definitely be better with the slow acceleration, but it might actually take you more fuel to reach 50 MPH.

If you drive 20 laps on a race track, you will use X amount of fuel. If you then put a block under the gas pedal that only allows 50% throttle and go out and do another 20 laps, you will use less than X amount of fuel in the second session.

-Max

Syonyk

The other catch to this, ESPECIALLY with stock rotary ECUs, is that wide open throttle tends to run, eh, a bit on the rich side. I've heard of NA ECUs getting into the 9:1 range (stupidly, insanely, crazily rich... heavy boosting turbos don't even run that rich).

So, wide open throttle stuff sucks a lot more fuel than it should with a properly tuned fuel computer.

Also, rotary engines run more efficiently at higher RPMs. Again, with standalones (and proper tuning), it's quite possible that a higher RPM/lower manifold pressure will lead to better fuel economy.

-=Russ=-

maxcooper

I agree with your assertion that rotaries run more efficiently at higher RPM, but my experience watching injector duty cycle readings (a very good relative indicator of mileage) is that higher gears pretty much always give better mileage. Lugging it in 5th will give better mileage than revving it in 4th.

-Max

jono20

cool thanks for all the info guys. I knew when I bought this thing that it was going to guzzle gas so any driving tips I can get will help me out

anyone ahve any suggestions for aftermarket ECUs that will stop it from running so rich (thus saving me gas...) but wont hurt performance immensly?


and aswell, I dont think the woman I purchased the 7 from did much to change the stock engine components, so I was jsut wondering: would a new higher quality O2 sensor possibly better my gas milage?

BoostFrenzy

I just put in a new Bosch 11027 universal O2 sensor and checked everything over, new plugs etc... was getting 10-12mpg city with 3k rpm shifts and low tps (no boost) with 5-8 mile trips, going to recalculate and hope things get better now

usma_2006

iTrader: (3)

the O2 sensor won't get you better gas mileage. The ECU only uses it at idle. Once you begin cruising it uses fuel and ignition maps and ignores the O2 sensor.

maxcooper

The ECU does use the O2 sensor for light cruise as well, and it can help mileage if you stay closed loop while cruising. A cheap A/F ratio gauge can tell you if you are closed loop or not.

-Max

jono20

cool...

jsut to ressurect a little bit...

lets say your going in 5th at like 25mph. obviously your in too high a gear and you have almsot no power, but thats my point. if you were to pump the gas pedal from full on to full off, it would not effect the rpms at all really, but woul the gas comsumption go up and down with those pumps?... what im trying to ask is if the rpms arent changing, but you still have the pedal all the way down, is it using more gas than if you only had the pedal HALF way down (and still no rpms change)?

neit_jnf

iTrader: (17)

at 25mph in 5th you'll probably stall... so there would be zero gas consumption

but yeah I was wondering the same... if I want to accelerate say from 40 to 80mph, will going wot (and max boost) in 5th gear and take forever to reach 80 but keeping the rpms down spend less or more gas than downshifting to 3rd and doing the same but with much higher rpms and way faster reaching 80??

jono20

no it doesnt stall if you have the momentum. does putting your fopot down consume a lot of gas tho even though the rpms dont change?

maxcooper

Yes, the amount of fuel injected will increase when you put the pedal down, even if you don't accelerate much (note that you WILL accelerate, just not much).

-Max

maxcooper

You will get better MPG accelerating from 40 to 80 in 5th gear with the pedal down than you would get in third gear doing the same. However, you will travel further and will most likely use more fuel with the 5th gear run.

Imagine a car named the Econosippa that gets 40 MPG. If you drive it 400 miles, you will use 10 gallons of gas. Now imagine the Speedyguzzla. It gets 10 MPG and will burn 5 gallons of gas if you drive it 50 miles. The 5th gear run in your question is like the Econosippa -- it used more gas (10 gallons) than the Speedyguzzla (5 gallons), but it also went a lot further.

-Max

Syonyk

Ok, this thread is full of various bits of misinformation and other crap, so I'll correct a few of them.

My statements are based on observation of the S4 ECU via various test connectors (the O2 sensor being one of them), as well as watching readouts on my SAFC-II wired to the pressure sensor. I believe the S5 ECU is much the same in terms of operation, but I can't speak for the S6 ECU.

First: The ECU does NOT use the O2 sensor at idle. Idle is quite rich of stoich. The O2 sensor is used at steady low power cruising. The ECU drops into open loop mode at 3500 RPM, or when the manifold pressure gets beyond a certain point (I'm not sure exactly what it is). Cruising around in town and on the highway will generally use the O2 sensor, though going up a hill on the highway can easily drop to open loop mode (it's rather touchy about that).

Second: The throttle position doesn't matter. MANIFOLD PRESSURE matters. The throttle is just a reasonably effective way to control manifold pressure. However, at low RPMs, the throttle opening needed to achieve full manifold pressure is fairly low. Specifically, in the above mentioned case of running at 25mph in 5th gear (somewhere around 1200-1300 RPM), half throttle is going to be at full manifold pressure already. Going to full throttle will have a barely noticeable change, if anything. So it won't affect fuel delivery.

-=Russ=-

jono20

cool. thanks russ.