hamster

Hey,

I have read in places on this forum that rx7exhaust is very hot. Hotter than a piston engine exhaust.

My question is this...why would the gasoline combusted in a rotary result in hotter exhaust than the gasoline combusted in a piston engine?

hornbm

My only guess is probably why our engines are so lound and burn so hot.

The only guess is that our exaust ports are so much bigger than a boinders exaust valve, the heat flows better, and the smaller opening of the pistons valve probably muffles the compustion to an extent.

Actually, I just thought of another idea. A piston engine has a power and an exaust stroke. The strokes probably give the gass some time to cool off and to quiet the compustion noise.

There is a lot less "space" between our power and exaust "strokes (if thats what you can call them)" The exaust is probably propelled out of the engine after the power occours much faster than a piston engine, resulting in higher temps and louder noise.

Just a therory here.

MarkPerez

because the rotory engine has more 'pulse's' then piston engines. piston is a 4 stroke, rotory is almost a 2 stroke. at least power band, and rpm wise. a 3 sided rotor ,going through 4 cycles. a rotory does 8 cycles
where pistons do 4, to make 1 rpm. i think thats right.

Suparslinc

I think it has to do with the unburnt (is that a word?) hydrocarbons.
The unique exhaust signature composes a much hotter composition that a conventional engine. Insert [bs flag] here. . .
Also, the unique combustion surface of a rotary engine is much larger than piston engine (think of the surface area as compared to a boinger) so the result is your inefficient wankel that disapates tons of heat through the heat sink.

Keaponlaffen

Less heat-absorbing material....

In a piston engine, there's a big steel block, aluminum or steel heads, pistons, valves that the exhaust has to pass through which are steel, etc..


In a rotary, there's only the rotor, seals, and the housings. the gas is just pushed directly out the exhaust ports IMMEDIATELY after being burned, whereas a boinger, the gas has to travel through the valves, and then out the exhaust...

Also, after being combusted, the exhaust gas in a piston engine has a chance to expand again in the power stroke, cooling it off quite a bit before being expelled without pressure in the exhaust stroke. In a rotary, the exhaust is still almost completely compressed as it's forced out the exhaust port. This also accounts for some of the extra noise from a rotary.

PaulFitzwarryne

The result is as Keaponlaffen expains is the high exhaust temperature, some 1600 degrees and you can tune the engine by reference to that characteristic
.
Above 1600 too lean, below too rich. However, if your A/F mix is right off, being far too lean then there is limited combustion and the temperature goes way down.

Terrh

iTrader: (7)

it has to do with the fact that the fuel/gas mix (ESPECIALLY at higher RPMS) doesn't have a chance to burn completely, so gets ejected while still making lots of explosion noises.
Port overlap has something to do with it to, as all rotaries (pre-renesis) have a good deal of port overlap. Renesises are QUIET. Ever seen under an rx-8? Theyres not much exhaust there, its almost straight through. There's also no port overlap, and the motors are quiet. I'd think that that has alot to do with it.

Rotary exhaust temps are high both because it's still compressed, as was mensioned above, and also because its still on fire...

purple82

Quoted from an earlier post with some modification:

1) combustion gas is cooled by convection of the hot gas to the metal parts around it. This is effected by the amount of metal the gas is swept past and how torturous the path is, the more time the gas spends next to metal, the more heat it will transfer. The spent gasses in a piston engine certainly have a more complex path to get out the the cylinder than a rotary.

2) Power cycles, the rotary has two complete power cycles to the one of a boinger. There is simply more energy spent per amount of time in a rotary.

3) The whole reason for combustion is to expand gasses. The more a combusted mixture expands, the cooler it gets and the more energy from the combustion process goes into making the output shaft turn. The compression ratio of a rotary just isn't all that good. There is alot of energy left in the combusted gas that is wasted out the tailpipe that could have otherwise been used to push on the rotor.

Mazda worked on a Miller cycle rotary where some of the inlet charge allowed into the engine was released back into the intake port before combustion. In a Miller cycle, the charge let in is reduced to more efficiently use the energy of combustion to turn the output shaft. The exhaust gasses are much cooler in a Miller cycle engine.

Spieder

Its a myth that rotory combustion gases are hotter than piston.

Heres how it works:

1) The rotary chamber has more surface area for cooling than a piston engine. One of the great advantages to a rotary is that the nitrogen output is lower (on a properly tuned endine) because the combustion ocurs at a lower temperature. The bad part is produced hydrocarbons. That's why on the Renesis they added side prots so that the gases remain in longer and burn more complete, thus reducing hydro's.

That is a fact, any one that tells you diff is wrong.

Here's why the exhaust system can overheat:

At high RPMS, because the rotaries don't burn all the hydro carbons, a lot of unburned gas can/will get dumped into the exhaust. At that point, you have a lot of combustion happening in the pre-monolith and later. That's why the racer's get those 3 foot flames.

The best source for info is magazines not the internet people dont site sources. I will look for some of my articles that discuss this in detail.

purple82

Sources? Read the Racing Beat catalog/manual. Listen to anyone who has tried a cheap muffler on their Rx-7. Talk to any rotary specialist. They'll all confirm that this is no myth.

PaulFitzwarryne

Simple test, spray high temperature paint on the header. It keeps on on a piston engine, burns off on a rotary.

One problem with the notburning fully theory is low temperature indicates too rich.

MRGSL-SE

the russian dude from CANADA that said about pist-on engines absorbing heat before exiting to the pipes was right... the rotary exhaust is still sort of compress and the housings really don't have that much cooling characteristics... since its plated with NITRIDE...