RX8_Buckeye

This is the first rotary-powered vehicle I've owned, and I'm not too familiar with how the timing is set up. My question is this: how many times does the Renesis fire per single revolution of the crankshaft (eccentric shaft?)? For instance, an I-4 engine fires twice per revolution, a V-6 fires 3 times, etc. What about the rotary? I think I read somewhere that the rotors spin at 1/3 the speed of the crankshaft. Does each rotor housing fire once per rotor revolution? This doesn't seem right, because the engine would be firing twice for every 3 rotations of the crankshaft. If somebody could answer this question, or even better, provide a link to a detailed explanation of rotary engine ignition timing, that would be great. Thanks.

zoom44

for every rotation of a rotor it fires 3 times per spark plug. there are 2 plugs per rotor in the 13b-msp, leading and trailing. so by my math that makes it 12 sparks per e-shaft rotation.

IKnowNot'ing

I believe you omitted the 3:1 ratio between e-shaft and rotor rotational speed. It means it fires 4 times per e-shaft rev.

zoom44

yeah i knew i was goingt o get that wrong i should have said "12 times per rotation of both rotors"

also there is a new feature of the forum since the upgrade. at the bottom of a thread it lists similar threads. i noticed in the one at the bottom of this thread there is rotorygod's thread about th efiring sequence. you should give that a read.

Z00M RX8

There is one ignition event per rotor per E-shaft revolution. An ignition event is usually the leading plug firing followed by the trailing plug, however in the Renesis depending on the load, RPM, and other factors the trailing plug my fire first. For a two rotor motor there are 2 ignition events (one per rotor) per E-shaft revolution.

I always think of a rotory as being like a 2 stoke piston motor, every time the crank (E-shaft) comes around to TDC (right side of the rotor housing looking at the front) the plug fires.

On each revolution a different side of the rotor is used, so if you designate the sides of the rotor as A, B and C. On the first revolution of the E-Shaft side A is used, on the second revolution side B is used, on the third revolution side C is used and on the forth revolution side A is up again.

rotarygod

Each rotor fires once per eccentric revolution. Leading plug fires first (usually) followed by trailing. This happens for every rotor face.

It takes the engine 3 complete eccentric shaft rotations to fire all 6 rotor faces.

RX8_Buckeye

Okay, so in terms of firing frequency, the Renesis is similar to an I-4 in that the engine fires twice per revolution of the crankshaft/eccentric shaft. That's what I was wondering, thanks!

Mr M

Every 180 degrees of eccentric shaft rotation (remember the eccentric shaft places the rotors 180 degrees apart). Or putting it another way, every time one of the rotor faces (i.e. the combustion chamber) passes past the spark plugs.

No, the rotary is nothing like a recip engine for the above reason. The recip only has one power stroke every 720 degrees - lots of lost energy!

Watch out on the Renesis timing though, the firing order for the plugs changes from L/T to T/L at idle. Easy to confuse the beginner....
Cheers

RX8_Buckeye

Actually, from the information given in this thread, the Renesis IS similar to an I-4 reciprocating engine from a dynamic torque excitation standpoint, which is relevant to the NVH characteristics of the vehicle. The I-4 engine has 2 firing events per crankshaft revolution, and the 2-rotor engine also has 2 firing events per eccentric shaft revolution. The tuning of the powertrain mounting system is highly dependent on the frequency of the engine reactive torque at idle. While idling, the Renesis is around 900 rpm (when warm), so the dominant reactive torque excitation frequency will be (900/60 )*2 = 30 Hz. For an I-4 idling at 600 Hz, this excitation frequency will be (600/60)*2 = 20 Hz. The higher idle speed of the Renesis seems to be desirable from an NVH standpoint, because it is easier to tune the mounts to isolate higher excitation frequencies. The rough idle sometimes felt in the RX-8 is due to brief intermittent drops in engine speed, which excites the rigid body roll mode of the powertrain on its mounts.

shelleys_man_06

Sounds interesting RX8_Buckeye. How did you get 900 Hz and 600 Hz? Was that from experimentation? Your analysis is pretty far in depth. At least it explains the weird idling characteristics. I like your intelligence (I'm an ME student).

Has anyone figured out the RENESIS' weird low-rpm firing order? Also, remember that a Wankel rotary engine is NOT a two-stroke.

rotarygod

The rotary powerstroke is around 1080 degrees long as opposed to 720. This alone means it is nothing like a piston engine of any kind.

In terms of smoothness, the 2 rotor engine is closest to a V6.

zoom44

according to the service manual the rx8 idle should be between 750 and 850 rpm. this may have changed with the later flashes to smooth the idle feel of the car as you suggest.

RX8_Buckeye

I was just giving a ballpark figure for the idle RPM of the Renesis--I wasn't suggesting that any of the newer PCM flashes actually increased idle speed. As far as the similarities between the rotary and reciprocating engine, I'm simply making a statement about the frequency/order contents of the torque excitation relative to engine output speed. No matter how long the power stroke of the 2-rotor engine is, the dominant torque harmonic should be 2nd order since there are two firing events per eccentric shaft revolution. Correct me if I'm wrong, but if you were to look at a plot of engine torque output versus eccentric shaft angle, the plot would show increasing torque immediately after the first firing event (assume 0 degrees) eventually peaking and starting to drop. The cycle would repeat at 180 degrees when the other firing event occurs.

I would agree that the 2-rotor is probably closest to a V-6, which is dominated by the third order torque. For a V-6 idling at 600 rpm, the firing frequency would be (600/60)*3 = 30 Hz. This is equal to the firing frequency of the 2-rotor idling at 900 rpm. Assuming an 800 rpm idle, the firing frequency would be about 27 Hz--once again, very close to the V-6 firing frequency.

shelleys_man_06: I was just estimating idle speed by what I can read off the tachometer. no experimentation was performed . Generally, reciprocating engines will idle somewhere around 600 rpm. The lower boundary for idle speed is governed by the lugging limit of the engine, i.e. when it doesn't generate enough power to sustain a steady rotational speed.

shelleys_man_06

Oh man I'm an idiot :o. I had forgotten the simple formula for calculating frequency from engine speed,

1 rpm=(2*pi/60) rad/s

I think. Late-term epiphanies hurt this ME student's brain .

Anyways, what can we make of these numbers? I'm not too familiar with ignition timing.

RX8_Buckeye

It's not really ignition timing that I'm talking about, just basic firing frequency. Ignition timing deals with moving up or pushing back ignition events by a few degrees of eccentric shaft rotation to increase power or reduce preignition of the fuel.

The frequencies that I calculated are important for tuning the powertrain mounting system for acceptable noise and vibration characteristics of the vehicle at idle. For example, if the powertrain system resting on the mounts has a natural frequency that is aligned with the dominant torque excitation frequency (firing frequency) at idle, there will be an excessive amount of vibration transmitted to the vehicle body. This will be perceived as very rough idle. So, the powertrain mounts need to be tuned such that the powertrain vibration modes aren't aligned with any significant excitation frequencies. Basically, this means changing the rubber compound in the mount to achieve varying static and dynamic stiffnesses as well as damping rates. Sometimes rubber mounts don't provide enough damping, so fluid-filled mounts (hydromounts) are used, but at a higher cost.