EVAP System

[MStiller]

I'm trying to find more information about the EVAP system, and I'm having trouble. In addition to the exhaust system which I'm designing (which a number of you have helped me out on, thanks for that) I'm also doing the fuel system, and the engine we're using (Rotax 4 cylinder 4-stroke Aircraft Engine, carbureted, not FI) has obviously never needed an EVAP system. If anyone could steer me in the right direction for some more info, as well as some sources for parts, I would greatly appreciate it.

 

[franzh]

One requirement of EPA testing is certifiable, repeatable, and usually independent testing, usually by a third party. You will not be able to do this testing yourself.

Southwest Research Institute in San Antonio, Texas, is a testing lab held in high esteem worldwide. They can do all emission testing for you, but at a cost, as will any lab.

Note for the record: I am not affiliated with SWRI but I personally know many of the automotive engineers there and have worked with them in the past.

There are other testing labs in the US that are recognized by the EPA.

Franz

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dgallup]

You can contact the EPA for emission exemptions. We have frequently gotten them for development vehicles. These always came with OE fuel injection & emission equipment which we were just modifying and/or calibrating so it was very straight forward. Don't know what you have to do for a ground up new vehicle but I'm sure the EPA can tell you. Probably only have to wade through a thousand or so pages of confusing & contradictory documentation to figure it out.

 

[MStiller]

I've talked to a few people at the EPA, and they said that they don't do emissions exemptions...given that our application would NEVER actually pass emissions, is there a chance of getting an exemption, or will it just have to be non-street legal for the moment?

Also, do you have any contacts at the EPA that could point me in the right direction with regards to an exemption?

 

[MStiller]

Also, I seem to have gotten a little sidetracked. I think everyone here acknowledges the fact that the vehicle I'm working on will NOT pass an emissions test in it's current iteration. But we'd like to at least show a good faith effort on our part. So, can someone give me a list of all of the parts that would be in both the EVAP system and the onboard fuel vapor recovery system (from the fuel tank)? I'm assuming that we could just use the same system and just have a line come into the Evap system from the fuel tank, but let me know

Thanks

 

[franzh]

Stiller:
I feel you are putting WAY too much emphasis on the evap system when that is only a small part of the project. Its sort of asking for an apple pie but only taking the crust and leaving out the apples.

Like the old song, "Love and Marriage", you cant have one without the other.

If you want to explore a vehicle for production, you need to explore the entire envelope, not just the evap.

Time for a project reassessment and identify the priorities. The last time I worked on a total vehicle package, the evap system was almost the last thing we did for the ECM programming.

Franz

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MStiller]

Franz-

I agree with you. Thus far, the pieces of the puzzle that apply to this particular vehicle (given that its going to have this carbureted engine for our proof-of-concept no matter what), are a cat (which wont work as well as it can, i know), the evap system, the fuel vapor recovery system....and thats all I've come up with. We're a startup, and the engine we have is going to be the one we use for the first prototype because we're too poor to go out and buy another engine. I've made it very clear to my boss that we will never pass emissions stuff with a carbureted engine, and he acknowledges it and agrees with me, and we know we'll switch to a FI engine for production, but right now I'm just trying to get a handle on all of the systems we'll need for production, even if we switch to a different engine. So, other than those three, is there anything else?

 

[BrianPetersen]

Okay, time to dumb things down. I hope this helps.

The evap system on a modern OBD-II vehicle includes complicated actuators and sensors to check whether the fuel system is sealed, and solenoid valves for purging out the evap (charcoal) canister, etc. In your carbureted proof-of-concept application, none of that is going to happen.

Here is the simplified "gist" of how the system works on a California motorcycle, which is not subject to OBD-II. There is an evap canister (charcoal canister) and a couple of check valves and a bunch of hoses, THAT'S IT. The fuel tank cap is sealed. There is a hose going from the top of the fuel tank to the "dirty" end of the evap canister (no check valve), and there is another hose going from the "dirty" end of the evap canister to a vacuum port on the throttle bodies through an out-only check valve that's usually part of another gizmo that I'll get to later. There are check valves letting air "in" if the system drops even slightly below atmospheric, and another letting air out of the "clean" side of the evap canister if the pressure goes much above atmospheric. These check valves may be built into the evap canister, or in the fuel tank cap, or as part of another gizmo to be explained later, or separate items, depending on system layout.

On to how it works. When the vehicle is sitting still, and gets parked out in the sun, fuel evaporation goes out from the fuel tank into the "dirty" side of the evap canister. No check valve in that hose, air can go back and forth between fuel tank and "dirty" side of evap canister freely. The air has to go somewhere, and the check valve on the "clean" side of the evap canister is designed to be the first one to open. So, fumes go from the tank, through the canister which absorbs them, then the air gets vented out (usually this connection is to the clean side of the engine's air filter housing). The purge connection stays closed because there's not enough pressure/vacuum for that system to do anything.

When you start the engine, vacuum gets applied to the connection to the throttle bodies and this opens the check valve and sucks fumes from the "dirty" side of the canister. This puts the canister under a very slight vacuum, which opens the "in" check valve on the "clean" side, and this draws air through and purges the canister. The air is drawn in from the engine's air filter housing so that dirt doesn't get in.

When the engine is stopped and the fuel tank cools off, the suction gets applied through the evap canister and a check valve mentioned above breaks the vacuum by letting air in through the "clean" side.

On to one more complicating factor; on a bike the design of the filler neck doesn't completely eliminate the possibility of liquid fuel spillage into what's normally the vent line. Liquid fuel into an evap canister is BAD news. Usually there is a liquid-vapor separator housing in the line from the fuel tank to the evap canister, and a little vacuum-operated diaphragm pump to pump the liquid back into the fuel tank. On motorcycle engines, they almost invariably use one throttle per cylinder, giving lots of vacuum pulsation in that connection to the throttle body, and this gizmo uses that same pulsation to operate the return pump and to activate the purge system.

With that ... here is an example from a 1994 Kawasaki ZX9R.

http://houseofmotorcycles.bikebandi...-1994-fuel-evaporative-system/o/m4233sch66094

Part number 16165 in that diagram is the vapor/liquid separator.

The place labelled "Ref. Cylinder Head" is the connection into one of the engine's intake ports downstream of the throttle - with pulsating vacuum to operate the pump inside that separator.

Part number 16164 is the evap canister itself.

 

[patprimmer]

The epoxy resins will out gas even after curing. You never ever get a 10% complete chemical reaction. That requires that the exact number of molecules of each component are directly and correctly aligned in the exact right ratio at the time of the reaction. The molecules have some mobility, but as the cure proceeds the mobility is reduced, and there are always some molecules from each part, left in a position with no molecule of the other part being close enough to react. These then very slowly migrate through the material until they find another molecule to react with or until they reach the surface, from which they are slowly released. This rate of diffusion and release is accelerated by heat.

Any other additives like stabilisers and release agents and possibly colours and flame retardants as well are also migrating and being released. This is also happening from paint, other plastics and fabrics and lubricants and fuel.

As well as volatiles migrating and releasing, the more solid components are degrading due to exposure to air, light and heat. Some of the products of decomposition are also volatiles and therefore emitted from the surface as gas or even as tiny solid particles.

This applies to all components from tail light lenses to windscreen wiper blades to insulating materials on any electric motors, to door seals and upholstery fabrics to name just a very few.



Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MStiller]

Brian-

That was very helpful, thank you.

Pat-

I agree that there will be some off-gassing even after we cure, but we're using high-temp pre-preg, so I'd think that most of it is going to get cooked off, and its not like we have a super resin-rich part like we would with a wet layup. As a result, I'm curious as to how huge a problem the carbon is actually going to be for a SHED test, as you're not the first person to have expressed some concern with that regard...there are other fully composite vehicles on the road (granted, not very many, but there are a handful) so I'm assuming they have passed the SHED testing, so what did they have to do special, or did they just get lucky and managed?

 

[patprimmer]

I don't know the spec and I don't knowthe level of outgassing of epoxies after curing. I just know you will get some and I expect it will be highly variable depending on how well the resin is mixed and how well it is cured.

Treating at moderately elevated temperatures under vacuum would certainly help, but the cost would be through the roof I expect as it would require a very large vessel and vacuum pump.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MStiller]

We already have the oven and vacuum pump, so that end is covered...we were planning on doing the cook under a vacuum anyway...our composites expert spent the last 30 years building racing boats for the America's Cup, and this is how they did it, and he seems fairly confident...Plus, the fact that we're using pre-preg should go a long way to reducing any post-cure offgassing

 

[patprimmer]

Prepreg in itself does not help, but certainly a very high reinforcement to resin ratio will help. Prepregs certainly help to obtain that.

You will still get out gassing, but you certainly have taken some very serious steps to minimise it. Time after treating under vacuum will also have an effect. I expect out gassing will be at a minimum immediately after vacuum treatment, then will slowly increase of a moderate time then will slowly decrease over a very long time

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.