bart_heemskerk

Why are rotary exhaust gasses so hot compared to piston engines? It seems to me that the combustion temperature should me the same. Is it because on a piston engine the valves and cilinder head are cooling the gasses down? Don't think that would make a lot difference compared to a rotary.

I try to understand why the gasses are too hot for a normal muffler. It travelled through several foots of pipe and a pre-silencer. That should cool it down. Or?????

steve84GS TII

Its partly because of the large amount of fuel that is wasted.
The long,sliding combustion chamber tends to allow a lot of fuel to build up in the rear since the fuel is heavier than air.The left over fuel that is not ignited by the trailing sparkplug is shoved out the peripheral exhaust port and left to be ignited in the exhaust system,either by exhaust heat or the cats.The igniton of this extra fuel is evident by the very high temps and backfires/flame throwing that rotaries are known for.Simply leaning out the fuel mixture going into the engine wont help because it doesnt change tha fact that a set percentage of fuel is still going get trapped at the back of the chamber and spat out the exhaust port.

Thats partly what the Renesis is all about.Its side exhaust port sit above the housing's running surface a little.That gap allows the heavier wasted fuel to bypass the port while the lighter,hot gasses are pushed out by the rotor face.The fuel that bypasses the port is brought into the next intake cycle.Since there is now residual fuel in the intake cycle,less fuel needs to be injected for the next go round.Less fuel wasted means less exhaust heat and better fuel economy,both traits of the Renesis.

RotaryMelon

iTrader: (7)

I believe the other part of why they are so hot is because the exhaust gasses exit the engine into the header as soon as the combustion happens.. where as a piston engine has exhaust valves in the head which let out the gas when appropriate, there, cooling it slightly.

correct me if i'm wrong please

PercentSevenC

That's the reason, mostly. The exhaust gases have lots of energy exiting the exhaust ports, causing high heat and very strong pulses (also helped by the fast opening/closing action of ports as compared to valves). That's why a good-flowing exhaust is so important on rotaries.

steve84GS TII

And its why rotaries and turbos are such great friends!

Jeff20B

iTrader: (13)

And why direct fire works so well; to burn some of the fuel mixture at the trailing edge of the rotors where it tends to build up causing - you guessed it - backfires and stuff.

Directfreak

iTrader: (2)

Also because each rotor sees three different combustions per cycle. It never gets a chance to cool down. That's 6 different "booms" in one two rotor. Like forcing 6 cylinders to fire into two valves.

Vashner

You ever drive old school GM's with V8? It will melt your toes...

12a exhaust is not too hot or out of the ordinary. Any exhaust manifold
will burn your BEEP off when warmed up...

Jeff20B

iTrader: (13)

When a rotary is idling the exhaust temps can get to around 200°F. I checked it with a digital infrared heat checker dealy.

speedturn

iTrader: (1)

the reason the rotary has a hotter exhaust is because the rotary has to open up and dump the exhaust gasses before they are fully expanded. If held in longer like a piston motor does, the gasses begin to do negative work on the face of the rotor, and power is lost. So, the rotary exhaust port opens up sooner and dumps out the gasses while they are still much hotter. In a piston motor, the gasses are expanded further inside the motor, doing a little more work on the piston and crank. The old Pressure versus Temperature gas laws always hold true, so further expanded = lower pressure = lower temp relationship holds true, so the exhaust is cooler when it comes out of the piston motor. This is also the one basic reason why rotaries get worse gas mileage than a comparable power piston motor - the rotary has to open up and dump the exhuast before extracting the full amount of work out of it. Wasted energy is going out the exhaust port.

Vashner, if you have ever used a thermocouple to measure exhaust gas temperatures, you would know that rotary exhaust gas temps are hotter than piston motors, even hotter than a small or Big Block Chevy.

djessence

Old school v8's do not burn through their exhaust pipes very quickly which rotaries will if you use shitty systems or flexpipe. All exhaust is hot, obviously but rotarys are VERY hot.

Vashner

Stock 12a too hot? It's not hotter than any other engine that is the same weight and hp ratio.

(By hotter I mean outside of any reasonable operating range for a OEM automobile).

steve84GS TII

Not hotter in regards to the engine OPERATING temps.Coolant and oil heat exchangers in rotary cars are quite large to keep things in check,but our ENGINE operating temps are typical for most internal combustion engines.

But with regard to the exhaust gas/system temps....they are significantly hotter.Its been known for decades and its not really debatable!
Its why the older RX's had 80lb thermal reactors that did the job of a catylist without needing ANY catylizing elements inside and had an air circulating system dedicated to cooling the exhaust system.Its why our cars have several square yards of exhaust heatsheilding and triplewalled stock mufflers.Its why RB exhaust systems are so damed expensize and my tailpipes glow dull red at night, even when I drive normally!

bart_heemskerk

Thanks Steve, Learned a lot from you.

What's the difference between direct fire and conventional fire? Both are sparking the leading and trailing? What makes direct fire to do a better burn?

680RWHP12A

nuff said!

boyee

iTrader: (3)

did some researching... this is what i found. http://tprmag.com/issue/11/11_rotary_1.shtml
i read it a while back. a pretty good article. it has all the technical info you need.
i will be accepting $100 each as a thank you

purple82

There are a number of reasons, most of them have been mentioned, but speedturn has pointed out the major reason.

Yes, the hot gas passes heat onto the valves and has to contract and expand to get out of the combustion chamber in a piston engine, and that has an effect, but it is mostly due to the energy left in the exhaust that has not been used to move the rotor and is instead dumped directly into the exhaust system. As he says, this is why rotary engines aren't as efficient.