Rotec Engineering TBI ?

[SvingenB]

With a separate (from the cowl) ram air to the engine, and adjustable cowl flaps, I doubt the Rotec TBI will function properly. I think in those cases you would need a separate sensing tube installed directly upstream the TBI like the Ellison. The ellison is not perfect either, one single tube is not good enough. According to common engineering practice, 4 tubes shall be installed separated 90 degree and leading to a plenum, to nullify as much as possible skewed inlet profiles. Common in most fuel injections. Maybe it doesn't matter, maybe the meetering bar needs a good velocity profile in any case?

[ronschreck]

Originally Posted by ratc

Whilst Looking at alternatives to carburettors and FI on Lycoming 320 engines I came across the Rotec TBI.
Has anyone used the TBI 40-4?
It appears to be the only one which is approved for mogas (which has great appeal to me). Not to mention that it looks beautifully engineered.

For those who have gone before the pros and cons of TBI operations and installation would be useful. Thanks.

Airflow Performance FI is also approved for mogas.

[rocketbob]

Originally Posted by SvingenB

With a separate (from the cowl) ram air to the engine, and adjustable cowl flaps, I doubt the Rotec TBI will function properly. I think in those cases you would need a separate sensing tube installed directly upstream the TBI like the Ellison. The ellison is not perfect either, one single tube is not good enough. According to common engineering practice, 4 tubes shall be installed separated 90 degree and leading to a plenum, to nullify as much as possible skewed inlet profiles. Common in most fuel injections. Maybe it doesn't matter, maybe the meetering bar needs a good velocity profile in any case?

I asked why the metering tube is not there when I bought the two I have and they told me they tried it and found it made no difference in operation.

[Don at Airflow]

Technically, the Rotec and the Ellison are not injectors. This is because the fuel is discharged into the air stream at a pressure below the ambient static pressure. The fuel in the discharge tube is slightly below the static pressure entering the throttle body. The fuel pressure regulator is what we call a negative head flow demand regulator. That is it controls the down stream pressure and sets the fuel pressure to the discharge tube slightly below ambient pressure. As air flows over the metering tube the air is sped up lowering the pressure at the discharge holes, ?sucking? the fuel out. As the discharge tube holes are rotated toward the incoming air stream the air pressure at the discharge holes on the tube approaches static pressure so the fuel will not flow out. Since the metering head across the discharge tubes is so low the fuel metering is sensitive to inlet conditions like ram scoops and turbulence. In theory this ?carburetor? operates like a chain saw carb. No float, operates in any position, except it uses a flat slide with a discharge tube with holes at are exposed as the slide opens instead of a tapered metering that changes the area of the discharge nozzle as the slide opens. And of course the fuel distribution is at the mercy of the intake manifold design. There is no way to adjust for rich or lean cylinders because of uneven fuel distribution. Also because there is fuel being discharge in the vicinity of the slide and evaporating, there is a possibility of carburetor icing.

If this device were truly an injector, the fuel would be discharged with a fuel pressure higher than ambient pressure and discharged behind the throttle, more like a PS-5 pressure carburetor.


Don

[SvingenB]

Originally Posted by rocketbob

I asked why the metering tube is not there when I bought the two I have and they told me they tried it and found it made no difference in operation.

Maybe the pressure in the venturi is so low compared to any variations in total pressure that is doesn't really matter.

[Snowflake]

Originally Posted by SvingenB

The Rotec TBI sort of assumes that the total pressure is whatever pressure is inside the cowl (like fig 14 in Ellison patent). This is probably good enough in most cases, but I wonder what happens when flying without a cowl ?

That's a good question, but it may not matter... There are many Rotec radials running without cowls.

[Alan Carroll]

Originally Posted by Don at Airflow

...Since the metering head across the discharge tubes is so low the fuel metering is sensitive to inlet conditions like ram scoops and turbulence...
Don

This is the clearest explanation I've heard yet for why the Ellison-type carbs are sensitive to inlet conditions, thanks. The appeal of these units, at least to me, is simplicity of installation - no high pressure fuel system, no injector lines, no return lines, etc. On the other hand I wouldn't expect it to perform the same as a true injection system.

[ratc]

Thank you all for your views and info.

Rocketbob, have you installed both of yours yet? Pictures would be useful when you have time (certainly no rush).
Thinking about Mixture and Throttle cable routings are these both on the same side of the TBI as delivered in the standard configuration, if not, could they be re-positioned?

[smokyray]

Andy,

Three years ago I helped my late friend Arvil complete my Dad's RV-6H (hybrid) with an Ellison TBI equipped 0-320. My only performance comparison was with my RV-4, flown for nearly 13 years with a Carb-equipped 0-320. The Carb-RV4 had very few worries including 10 years and 1000 hours running Mo-Gas prior to installing a newer 0-320.
I purchased the Ellison TBI off the classifieds on this site five years ago, removed running from an RV4 and included the Van's FAB. Initial testing showed the little RV-6H to be a lightweight prize fighter with very smooth running and much more leaning capability than the MS Carb. The only difficulty was setting the low idle. I finally called ETBI INC. and talked with them and they patiently went over the procedure
with me. Once set, the ETBI starts very easily hot or cold, even without a primer or accelerator pump. Throttle response is quick, so I installed a vernier throttle control which helped. So far, I have 25 hours on the -6 and like it, light, responsive and smooth in aerobatics with the MT FP prop and ETBI. In hammerheads my RV4 would sputter and quit, not so with the ETBI, excellent in aerobatics.

If the Rotec works anywhere near as well, it would be a home run setup on any RV, especially for the price.

Smokey
HR2

[ratc]

Smokey,
As always thanks for the input. I do think the overall package of 0-320, dual Pmag's and Rotec TBI would be a good all round package. If only MT would support their Fixed Pitch propellers with engines equipped with dual EI we would all be home free!

I believe Jim Ayres is looking into the problem with Bart at AeroSport power so a solution may be on the horizon.