David Parkers Vega Twin Fourstroke – Air intake /carburettor question

[Steve WithnellParticipant @stevewithnell34426]

I've been pushing the plans around for a long time now, whilst others have actually gone on built some nice versions:

**LINK**

However, they are quite a few comments about lack of power and carburettor issues. When I look at the air intake, the carbs (Using two carbs instead of one was David Parkers first fix of the carburation problem) are facing backwards in what I guess is a low pressure area. The second thing is that the air flowing into the engine has two 90 degree corners to navigate to get into the cylinder.

If I modify the cylinder heads so the inlet angle is reduced from 90 degrees to say 60 degrees and then I make an inlet manfold that sweeps round smoothly to face into the airflow ( so the carbs then face into the airflow too), will that make any material improvement to power output and help the carburation problem (which I think is that the carbs are tricky to optimise for best running).

Unless there is a clear cut case to make the changes, it makes sense to stick to plan.

Interested to hear from anyone who has experimented with this area of model engines and what their experience was.

Regards

Steve

[Steve WithnellParticipant @stevewithnell34426]

[JasonBModerator @jasonb]

Think I would be tempted to put the carb straight into the head and not use an elbow. You could have a straight shank on the carb that fits a plain hole in the head and just a small grub screw in the top of the head to retain the carb. Make them as a mirrored pair.

Something like this

[Steve WithnellParticipant @stevewithnell34426]

Wow, that's pretty impressive, not so much you can produce the drawing, but that you can do it at lightning speed!

That's a very easy way of getting rid of one of the 90deg bends, thanks.

Steve

[JasonBModerator @jasonb]

I wish, someone gave me the Vega in Alibre CAD files some time ago. I just added a couple of Nemett carbs for you.

[Steve WithnellParticipant @stevewithnell34426]

Should have kept that to yourself!

[Johan van ZantenParticipant @johanvanzanten]

Hi Steve,

I have no special knolidge about the Vega but I am afraid the carbs getting too hot, causing vapour lock. You have to use an insulating sleve between the head and the carb. This is the usual practize on model aircraft engines.

Best regards, Johan.

[Allan BParticipant @allanb]

Not having ever built a model aro engine, or even any model 4 stroke engine, the only reference I can take is off motorcycle engines (I teach motorcycle repair, maintenance and diagnostic) the air intake really needs to be in a steady state, any disruption to the air will cause low pressure point then disrupting the venture effect of the carbs, but ram air is a good way of increasing power, so if you could make some sort of airbox with a basic filter, but then the air intake to the airbox facing the propeller side, you can then raise the air pressure in the airbox, but still keep the air with minimal disruption, hence raising the power of the engine.

To give you an example of the sort of power increase, when Honda did it to the CBR600 it increased the power of the same engine by 15% which is no small increase

[Wayne WardParticipant @wayneward51978]

I think that Honda got 15% out of that engine by doing much more than just putting ram-air (forward facing intakes) on it. At 300kph (186mph) the velocity effect is worth just over 4%, and that is if you can convert the velocity head to pressure. To get 15% relative to still air, you need to be going about 350 mph. I suspect that their whole intake system previously had problems. Less kinks in the inlet will be better than 90 degree bends. If you have to have the bends, make them as large a radius as possible.

Edited By Wayne Ward on 25/03/2017 18:54:00

[CorRadParticipant @corrad]

Having looked at several twin, 4-stroke, aero engines, I came to the conclusion that the VEGA Twin is the most suitable project for my level of experience and equipment. Could someone please direct me as to where to get the drawings please. Thank you.

[JasonBModerator @jasonb]

Probably best to get hold of the old copies of Model Engineer that the Vega was published in as that will also give you some pointers towards construction. You should be able to get the whole of volume 156 for under £5.

Part 1, 3 Jan. 1986, Vol 156, No 3768.

Part 2, 7 Feb. 1986 Vol 156 No 3770.

Part 3. 7 March. 1986 Vol 156 No 3772.

Part 4. 4 April. 1986 Vol 156 No 3774.

Part 5. 2 May. 1986 Vol 156 No 3776.

Part 6. 6 June. 1086 Vol 156 No 3778.

[Howard LewisParticipant @howardlewis46836]

+1 for avoiding heat into the carbs, (risk of vapour vlocking, and loss of volumetric efficiency, due to heating of ingoing air)

A thought, with the carbs so close to the head, the carbs will be subjected to quite violent pulses during the induction cycle. If the carbs could each be mounted on a long induction tract, assuming no icing takes place, there may be benefits.

Firstly, the "sharpness" of induction pulses will be slightly softened, secondly, there may be a slight ram effect from the inertia of the ingoing column of mixture. AS long as icing does not take place, it is possible that the atomised fuel, may be better vapourised by any heat from the induction tract (a form of surface carburation).

This may improve both power and steady running, maybe even improving fuel consumption?

Just a few thoughts. Also, if bell mouth pipes can be fitted outboard of the carbs, air flow and possibly atomisation may be improved. It is important that the carbs are not allowed to flap about, that will not improve matters and will very likely result in fractured pipework.

Just a few thoughts!

Howard

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