airraptor

My Feedback: (66)

Why do all most all gas engines have these carbs from the chain saw and weed eater days. The front mounted carbs on some engines like normal glow ones doesnt allow to fit in cowls very well.

I would guess they are cheap and work well and no one wanted to redesign them. I would think you could make a smaller carb and a remote mounted pump.

Anyway just wondering.

acerc

You make it a habit toanswer your own question? they are cheap and work well and no one wanted to redesign them

NM2K

My Feedback: (14)

The hobby market isn't large enough to pay for reengineering engines for such few sales.

On the other hand, there are a few gas engines being made that offer carbs on the front/back of their engine, such as MVVS/Evolution, RCS, Moki (I think), etc. The latter engines are great for tightly cowled models. Mine run well.


Ed Cregger

aussiesteve

Before I answer. I will ask you a coupel of questions.

What is your purchasing criteria when buying an engine?
Is Price even remotely a factor in the decision as to what you purchase?
Are you aware that any "redesign" of somethign liek a carb will be copied very quickly by others so the cost of doing so needs to be amortised over a very few items?
What is stopping you from finding a better alternative?

Ok - to answer the question

What we now get is inexpensive and it mostly does the job required as long as the engine "manufacturer" chooses the right model for their goods. To use an alternative that wil do the job better will significantly add to the selling price of the enigne and will limit the market to very few users.

airraptor

My Feedback: (66)

Price doesnt matter to much. I do plan on a MVVS/EVO 58 for that new warbird racing and a couple of DLE30's this year.

I was just wondering like you said I answered my own question.

mrbigg

My Feedback: (21)

Didn't Saito design their own gas carb?

Lnewqban

That is actually a very intelligent question.

378

My Feedback: (4)

OS GT55 has a front mounted carb. Not sure about the GT33 though, think it's a normal reedie.


I prefer the front-mounted carb because it removes the reed valves and replaces them with something that just doesn't fail.

Turk1

NGH engines are also having front mounted carb/rotary valve system.

w8ye

My Feedback: (16)

A retired Walbro engineer designed it for them

speedracerntrixie

My Feedback: (29)

This brings up a question I have been asking for years. What we really need is fuel injection. We already have RPM information being read by the ignitions. It really would not be too difficult to upgrade the ignitions to monitor EGT as well. Between RPM and EGT a fairly good fuel injection system can function. Once the initial engrichment VS RPM curve is figured out the system would only have to make small changes based on EGT. I think there are small injectors available and the Jet guys have usable fuel pumps. Seems we need an EE to step up to this one.

HunkaJunk

I'm not at all convinced that the benefits of fuel injection would outweigh the added cost and complexity for our application. We won't get any noticible increase in power from it.
The engine temperature may not be stable enough to reliably use fuel injection also.

speedracerntrixie

My Feedback: (29)

Not for power increase, think linear throttle response. Something lacking in a Walbro or Tillitson. We make due with throttle curves but it would be so nice to have a flat, smooth transition. Not to mention fuel economy. We could carry 1/3 less fuel, that will make up some for the added weight.

airraptor

My Feedback: (66)

Well Fi is a whole other ball game. Yes you will gain some power. Fi will allow you run a larger throttle body which could allow more power.

I should have stated this question is for the smaller GAS engines coming ou like the Magnum 52, and some of the other 10-20cc engines where the carb is the size of the cylinder. I would think a glow carb would work just as well on these smaller engines with out a pump.

speedracerntrixie

My Feedback: (29)

Sure just as long as the O rings on the high and low needles are of the proper material. The only issue I wouls have with it is that as long as the carb is open fuel can leak out. The Walbro type carbs don't do that. I have actually thought about putting a gasser carb on a glow motor so as not to have to run a seperate pump to get a fuel tank on CG

MTK

GT33 is a rear intake engine like so many others in that size range. The 55GT has a walbro carb on it and the ratory shaft valve usually does not fail. MOKI's (glow 180's) had a run of bad luck with their shafts breaking several years ago right at the valve, so this isn't fool proof either.

I use both types and don't think about either a reed or a rotary valve failing, ever. If one has to worry about something failing on their engines, why even bother?

378

My Feedback: (4)

Poppycock! Fuel injection conferred significant boosts to car engines because of it's ability to maintain an ideal fuel mixture, whereas a carb is never quite there. It will do the same for us as well. It will also make engines as trouble-free as brushless, the only fuel mixture control needed would be a break-in switch that fattens it up a bit during the first few tanks for glow applications. After that, the computer will adjust the fuel mixture on the fly. You'll have an engine that's peaked out 24/7 with no worries of the engine leaning out at high altitude or due to the tank draining.

Engine temperature won't matter. For starters the ideal fuel mix will keep temps in check, but on top of that the computer would be able to richen the mixture when temps got high to act as a somewhat active cooling system. The main stats required are exhaust oxygen levels, where the piston is, the RPM, the throttle position, and how much air is going in. Measure those things and the computer will be able to give you a good fuel mix.
If the rotary shaft on a GT55 breaks you've got bigger issues, namely, a snapped crankshaft. That's why the front carb setup just doesn't have issues and is why it's used on glow engines. A hollow crankshaft with a timed port under the carb will not cause induction issues like worn reeds will. It only fails due to operator error or a defect in materials, if it's well made and not constantly subject to prop strikes and detonation it will outlast the rest of the engine.

Why intentionally complicate something that doesn't need to be complex? Rotary induction works, it's fool proof, the fixed intake timing renders them more resistant to running backwards(My 46AX likes to start backwards but if I drop the idle enough it will bounce forwards and stabilize, something reedies do not do), and they last the life of the engine. The only way they fail is if you get a lot of prop strikes, let the engine suck in tons of dirt, or the crank is poorly made and not strong enough to survive the engine running. If it truly isn't superior, then why did Cox opt to use a hollow crank front carb setup on their tiny little 0.010 when everything else they made was a reedie?

aussiesteve

There is the real reason that the majority of RC Engine "manufacturers" (based on numbers of units sold) don't use the Rotary intake.
Most of them have anough problems with conventional crank failures - It is a scary thought what would happen if they then hollowed them out and removed part of the front shaft. - VERY scary indeed.

I do like my rotary intake engines but they are ZDZ and MVVS so I don't worry too much about that.

Back to the OP - it all comes down to what the users expect to pay. Nowadays, the majority of RC buyers don't expect to pay much so there is no reason for the manufacturers to add cost (Even if they knew how to do so). Where the majority of the upper end manufacturers sell their engines to, Fuel injection is in fact used - and quite a lot more extensively than many realise. But th price of those engines is very much higher than what most would be prepared to pay (plus those engines are not made available to RC'ers for a multitude of reasons).

378

My Feedback: (4)

That's not a fault of crankshaft induction though. That's just lazy design and skimped-on materials.

aussiesteve

Yep - that is pretty much exactly what I was getting at.
It scares me to think of the amount of people who would be ducking for cover if some of the manufacturers tried it. One in particular had enough problems with a conventional crank design letting the prop hubs go a flying all on their own (I am specifically banned from posting their name).

Like you, I am a fan of rotary valving as long as it is done right. Pretty much all high performance 2 stroke engines use it nowadays.

MTK

Same reason his larger Tee Dee engines were all front rotary induction. No valve float at the very rpm these engines operated at. And in the case of the 010, that was something around 35,000 rpm if I recall.

I just got a few of these little jewels from a guy's estate after he died. A couple 020's and 049's too....Marvelously simple little pumps