Why Carbon Build-up
 

The conventional wisdom is that we should reasonably frequently high-rev the Renesis rotary to "blow out the carbon". I like to high-rev this terrific engine for fun, but why the need to blow out carbon? What is the difference between burning fuel in a rotary and burning it in a reciprocating piston engine that makes it produce carbon which has to be regularly blown out in the one, but not the other?

You can read around and find many references to this, but in a nutshell, the rotary requires a richer air to fuel mixture (10-14 to 1) than a piston engine (that mostly stays at stoich, 14.7 to 1).

This rich fuel mixture causes most of it - in addition the rotary has some oil injection for seal lubrication and this will add to it a little.

The reason the mixture is rich is because of the hot exhaust. This causes premature failure of the catalytic converter. The richer mixture cools the exhaust which lengthens the life of the converter.

We get alot of carbon from the oil metering system. There are 2 oil injection nozzles on each rotor housing. This supplies a very small amount of metered oil into the system to lubricate the apex seals. The nozzles on the Renesis are actually aimes towards the outsides of the rotors. They are also trying to lubricate the corner seals which now see hot exhaust gasses and have different wear characteristics from older non side exhaust port engine corner seals. Since alot of this oil goes towards the sides of the rotors, it doesn't get burned. The hot gasses of combustion do not get to the sides of the rotors. This carbon buildup is more of a problem on the Renesis than the older rotaries as they did not direct most of the oil injection towards the sides of the rotors. Carbon does still build up on the rotor faces as well.

We want to get rid of this carbon. It can do a number of things. Carbon buildup can lead to unstable combustion as it physically takes up space in the engine and it's rough texture can lead to flame propagation issues and internal airflow during combustion. Carbon also holds alot of heat. It is due to high carbon buildup that old car engines will "diesel" and keep running after being shut off. This hot carbon can spontaneously ignite whatever is in the engine. We don't want this to happen. We only want the spark plug to do that. It can also lead to detonation while running.

The carbon that builds up on the sides has a tendency to collect on the ports. This affects airflow into and out of the engine. It can also cause seal breakage in severe cases. Seals can get stuck down in their grooves and in extreme cases an engine could carbon lock. Carbon holding seals could lead to low compression.

Overall it's a bad thing and we don't want it in our engines.

Also remember the rotors turn 1/3rd the speed of the shaft. Taking it to 9000rpm to clear it out is like clearing out a piston engine by reving it to 3000rpm:lol:

There's far more to it than the 1/3rd argument. Keep in mind in a piston engine you are trying to get carbon to go up to get out of the engine and are relying on exhaust velocity to pull it out. In a rotary we can at least physically scrape it across the exhaust ports. Pistons change direction. Rotors don't. Piston engines (modern 4 stroke car engines) don't inject oil into the combustion chamber. There are definitely pros and cons for each engine when it comes to it.

Keeping on the echoing and hopefully clarifying some of rotarygod's message theme; carbon build up is not just an issue on rotaries. Conventional piston engines can also have carbon build up. One piston engines, carbon tends to be either on top of the piston and on the backside of the valves. This is the basis behind the "Italian tune-up", i.e. running an engine hard will sometimes smooth it out as the carbon that was built up gets blown out. Older piston engined cars really did require the occasional redlining, but with modern computers the air-fuel mixture is much better controlled and so this is not as big of an issue. However, there are still situations where the computer will run fairly rich AF ratios (Subaru boxers for example, love doing this, even on the normally aspirated units) and so contribute to carbon build up. It's good to occasionally redline a piston engined car as well!

Slight segway to the above post on redlining the engine. In order to blow-out the carbon one must be in WOT. Not just let it lazily rise to redline.

OK, I am going to ask a question that may get me flamed, but idk...how good are the fuel additives, such as Chevron Techron, at removing carbon build up? I know some ppl love the BG44k, but are other fuel additives effective?

Still somewhat confused about it all myself. 2 exhaust ports, 2 primary intake ports, 2 secondary intake ports, and 2 auxiliary intake ports. How each fits into the magical rpm 'ranges' I still don't fully understand.
Another thread indicated to me the following:
SSV opens @ 3750
VFAD opens @ 5500
APV opens @ 6000
VDI opens @ 7250

Now just how this four items 'fit' into the whole 6 port thing (on a 06 or later) rotary is still...I'm afraid a mystery to me. Plus toss in the WOT that seems to be needed....and long timers on the site wonder how us newbies can get confused.

Thanks for the comments - rich fuel/ air mixture + oil does sound like a carbon recipe. One wonders why the carbon caution and advice to high-rev is not in the owner's manual. Seems as though it should be.

Oh but it is. When you get your 8 you get a small three page book that states;"And remember Rotarys like to beep. So beep everyday!" :)
-Gil

I never got that!!! Must be a US thing (I'm in Australia).

BG44k is the best cleaner out there - very strong.

Next up is Techron, Gumout Regane, and Redline SI-1, which are all PolyEthylAmine (PEA) based fuel system cleaners. PEA was developed by Chevron and is sold to several companies for their use - it is also used in Chevron/Texaco gasoline. These are all good at removing built up carbon.

Most of the rest are very mild cleaners, mostly kerosene based. Can be useful, but not nearly as effective as the ones listed above.

Now for the catch - all the cleaners listed above when in use, will hurt your oil film from your OMP injection and/or premix - that is why they should be used sparingly, because higher wear will occur during use.

That is also why I always suggest continuous use of a lubricious cleaner that does dissolve or break down carbon, like FP Plus, FP60, MMO, or Lucas UCL. These will provide continuous cleaning and add some lubricity, WITHOUT impacting your oil film from your OMP oil/premix.

Note though that Lucas UCL should be used alone in your fuel (if you like this product) as it does not seem to mix well with other 2-cycles - the others will all mix well with other premixes if you so choose.

Man, I had a dream last night that when I shut my car off it still continued running for awhile and was dieseling. I was kind of freaked out by the thought of all that carbon build-up!

I just put a bottle of Techron (the one for 20 gallon one), cost me 11 bux :(, with 4oz Premix cuz I know that Techron will clean my OMP stuff out. and I dont want that.

Then I got 5 bottles of Prestone Complete Fuel system cleaner, 7 bux each. for my next 5 tanks of gas (or maybe 1 after another)

I want to get REdline but Autozone does not sell them, and Im lazy to order it online :(

Uhhh Awkward :uhh: :uhh: :uhh: :uhh: :uhh: :uhh: :uhh: :uhh:

Did you buy your 8 used or new? If you want PM me and I'll send it to you via digi pic so you can read it :)

i do have a ?

btw, thanks for answering my pms..

would it be better to skip premix when running 44k???? the whole glop issue is a concern of mine.. :) as in i am 500miles from 44k and beat the f out of my car!!!!!


beers :beer:

OK, I can clearly see how the carbon cooks up through the combo of oil and rich mixture. But can some tech-savvy please explain why normal cruising around lets the carbon build up? I mean, why aren't the flames and the pressures in the working chambers at low or mid-revs (which must be pretty powerful, lets face it) enough to blow the crap out? After all, at high revs you're using MORE fuel and MORE oil, thus creating even MORE carbon. I shouldn't think about it, coz its driving me nuts. :Eyecrazy:

Depends on you priority:

- 44k alone in the fuel (no premix) will likely give you a little better cleaning out, but with some sacrificial wear (albeit for only one tank). You would still have some oil from the OMP coming in, so the additional wear should be minimal (of course very slow wear is happening all the time, so you are slightly increasing the wear rate for the one tank that the cleaner is in use).

- If you would like to clean but want to minimize any wear impacts, then you can mix in your normal premix as well - you will just have slightly more carbon generation during this tank that will slightly reduce the cleaning gains.

The key is not to use a non-lubricious cleaner very often - no more than once every 5000 miles - and hopefully use a lubricious cleaner in every tank so that the need for this additional cleaning is reduced and is performed even less often (say every 15k or so).

End of a long answer - personally, I would go ahead and add my premix as well.

as i thought. i just wanted to put that out for everyone.. btw, i agree the 15k miles is the spot with 44k..

will order some fp plus this weekend.. jax thanks for all the great info..

when you make it to orlando the cocktails are one me!!!!!! ;)

beers :beer:

so do you guys take it real easy with the 44k in it? or do you drive more "beepy" with it in the tank?

You want to find a hill that will challenge your car near red line and push it till it beeps a few times.

Here is a question, I put a complete fuel system cleaner in almost every month, says every so many miles but I disregard that, been doing it for about 5 months owned the car for 8 months, is this doing me any good with the carbon?