DarkWingDuck

My Feedback: (1)

Any advantages or disadvantages of piston port induction compared to engines that use reed or rotary valves?

Dave G.

tande

My Feedback: (48)

"Piston port" Usually= less efficiency (pumping losses)---ALTHOUGH some do run very well, with a good exhaust system---On HIGH performance two/cycle engines, the exhaust system is a HUGE part of the overall performance picture---Having said that; HIGH performance two/cycle engines also don't use "Piston Port" design---Also on "Piston Port" designs, the induction port takes up cyl. wall area that could otherwise be used for "Transfer or Boost Port" locations---

Antique

My Feedback: (4)

I guess piston port Zenoah GT80s turning 11,500 rpm and Quick Draw water cooled piston port 25cc boat engines turning 18,000+ rpm in model boats at 111 mph don't count
Not efficient enough ?

davewallace

My Feedback: (29)

Hi everyone,
We still havn't answered the why question. A modern two cycle engine is a careful balance of several competing factors which require compromises in order for the engine to produce good power throughout its rpm range. Each intake method has advantages and disadvantages. The piston port is simple and provides a large port, but it doesn't allow independent timing of opening and closing of the port. You are restricted on how early it can open, because it can cause it to stay open too long losing pumping efficency. This limits maximum duration possible. The reed valve operates on crankcase pressure, opens early and closes early as a result of the piston coming down and compressing the case volume. It also requires a more complicated assembly to get the port area large enough for better performance. In addition it doesn't take advantage of the momentum of the incoming charge past TDC. The rotary valve is the most effcient, because the timing can be optimized for opening and closing. This gives the maximum duration possible with pumping efficency. As pointed out in a earlier post the piston port gives up cylinder wall area which can be used for transfer porting. That being said, any of these intake methods can be used to creat a fantastic engine when the designer reaches the correct balance of all factors.

Dave

tande

My Feedback: (48)

Maybe we should tell the "Japanese motorcycle racing industry" about this!---NO "Piston Port" engines in that business!---

Geistware

My Feedback: (16)

What is ment by transfer porting?

tande

My Feedback: (48)

The "Route or Pathway" that the fresh air & fuel mixture "Transfers" from the crankcase to the combustion chamber---

davewallace

My Feedback: (29)

Hi,
We used to call them bypass ports. From all of the post that I have read recently, everyone now calls them tranfer ports. We are talking about the passages from the crankcase area to the combustion chamber.

Dave

Hobbsy

My Feedback: (102)

They are also noisy, I have a Maloney 125 and a Seuvia 1.50 both piston port and the intake noise is at least as loud as the exhaust on both.

TLH101

My Feedback: (90)

In a given engine of any certain size, a rotary or reed valve will have more power. In almost all 2 stroke racing, go kart, outboard, Personal watercraft etc, Rotary valve motors are in different classes from piston port, or are restricted to smaller sizes. Reed motors, under the right conditions are comparable to rotary valve.

Diablo-RCU

My Feedback: (1)

Advantages to a piston port. Less parts = cheaper to make and can be a little lighter, and lower long term maintenance costs because there are a few less parts than can wear out.

B.L.E.

When a piston port engine opens the inlet port, there is already a good vacuum in the crankcase causing an implosion type of intake, hence the noisy intake. This however ensures a high air velocity through the carburator even at very low rpm's and as a result, the carburator can be quite big, yet the engine does not "fall on it's face" when the throttle is opened at low rpm's because the airflow is too low to draw fuel.
It is for this reason that many two stroke Grand Prix racing motorcycles employed rotary valve timing similar to what piston port engines had.

dlwood

I have an old (1990) 650 CC snowmobile that produces a dyno proven 130 horsepower.
This is a mild trail ridden sled perfectly happy on 87 octane gas. Peak horsepower is at 8000 rpm.
This engine has a combination reed valve/piston port induction with 7 transfer ports. Exaust is
muffled expansion chambers. To equal this a 50 CC model aircraft engine would have to produce an honest 10 horsepower. It seems there is a lot of room for improvement in the model aircraft engines.

David_Moen

Yes but your snowmobile engine benefits from infinite gear ratios made available to it by a torque converter. With an airplane engine, the maximum speed that it can turn is whatever speed it takes to get the tip of the propeller up to the speed of sound. That speed limits the amount of power an engine can produce in our application. Of course you could install a gear drive and run the engine at much higher speeds, but the extra weight and complexity of the system would not likley be offset by additional thrust produced (at model airplane sizes anyway).

dlwood

Agreed David, the final drive is more or less a constant RPM at speeds of 40 or so MPH, but clutch engagement is 3000 RPM and the engine has to grunt it up to higher RPM. I'll grant you there is some (minimal with the heavy weights I prefer) belt slippage to aid it from standing starts. On an aircraft I believe the power requirement to drive a propeller is squared for a doubling of RPM. A lot of gas engines peak in the 7 to 8K range, some even higher.

B.L.E.

The torque (and thrust) is proportional to the square of the rpm. The power is proportional to the cube of the rpm. In other words, if it takes 8 HP to turn a prop 8000 rpm, it will only take 1 HP to turn it 4000 rpm and only 1/8 HP to turn it 2000 rpm.
Not understanding this important relationship between rpm and power/torque is a major factor in the erroneous belief that four-stroke engines make more torque than two-stroke engines.

rmh

That concept completely escapes a vast number of hobbiests.
The most common error is thinking that torque is a fixed figure for a given engine or that
adding a pipe increases only rpm -not torque- Therefor - if the pipe only adds 300 rpm it has not increased torque.
So why change the exhaust system if what you are really after is torque?

Flip and Fly

So what about tuning an engine, as we say a tuned pipe? From my dirt bike days I know first hand that some after market "tuned pipes" could increase top end power and rev like crazy, and you could pull wheelie's all over the place, but only when run close to flat out. Not the greatest when you are exiting a corner. And they would also develop a big flat spot at mid range power, so you had to shift like crazy! A nice fat torque curve was much better, tractor like power!

So as stated "The torque (and thrust) is proportional to the square of the rpm", on paper maybe, but there is more to it I think, just getting lots of revs don't do it for me, and porting, breathing, and bore and stroke plays a big factor in what the final outcome will be.

Dave Wallace said it all.

rmh

Your piped bike is a perfect example of the common conception of pipes
On aircraft however , the prop load is not the same as being hooked to the ground.
The prop load is piddlin at low rpm -irrespective of the pitch-so pulling "off th line" -like a bike -simply is not a problem .
But as the rpm increases (the formula as you mentioned), the load increases sharply .
So the pipes we use are really not a peak tuned device .Unless
The best ones are a fairly shallow divergent cone-into a longish constant tube -then to an end plate of some kind - a dish, a cone a flat plate etc.. A second type is a long tube -which goes into a constant diameter can -to within aprox 1" of a reflector .
The first type is the better -in my opinion- as it has really good low speed to high speed transition-not at all peaky and in most cases actually has better off the line response -compared to any exhaust setup .
Having fiddled with a variety of these -on the bench and in the models - Our personal opinion is still that in cowl mufflers are the worst setup for power -in any part of the power band.
They are however - very easy to do .
They also reduce torque in almost all cases.
They do improve idle over open stack tho.
Any exhaust system which increases rpm -on a given prop -over open stack--is tuned to some extent . Don't lump pipes into one single catagory.
My 25/ 26/35/40/50/60/80/100/160-all showed improved low throttle response on correct pipe setups. Sometimes I got good tuned responses out of very cheap tubing and paint can thingys I made. You don't have to blow the bankbook to get a good workable setup.
If you are into really top notch pipes super quiet smooth curve - the Carbon fiber one shown here is excellent.

B.L.E.

Maybe someday, somebody will build a header pipe with a servo controlled trombone slide to keep the engine "on the pipe" at all rpms.

rmh

Well -for our use the current well designed pipes ar e just fine - peakiness is not a problem if you first do some basic test bench setups and establish best operating band and prop.
I have just recieved some new mild tuned mufflers (cans) which are purported to be extremely good at full rpm band running -under load.

Flip and Fly

Well thanks for that explanation Dick, and I see your point.

But what about when you try to pull out of a hover for instance. The motor has to go from 2000rpm to max power, or is it torque, at once. It still has to over come the load on the prop, from the weight of the plane, air resistance, and all that.

Now if the pipe makes a good transition from low to high rpms in this example I guess it would not bog down. We often hear of people being impressed when they hear the motor unload as it goes vertical. Still you are piping for a particular power curve, and not all pipes are made equal, and do need to be setup. Torque is good.

I am no expert on this, just a inquiring mind!

RTK

My Feedback: (1)

You will probably be a lot higher than 2000rpm in a hover, that's almost an idle. So the spread would not be that great.

Flypaper 2

Used to drive a little 250 RV Skidoo racing sled 1975 vintage. Rotary valve with twin tuned pipes. Clutch came in at 6000, Clutch set up to accelerate through the speed range at 9000 RPM . When it reached about 60 MPH the clutch was all the way in and RPM started to climb it felt like you turned on an afterburner as it came on the pipes. Would keep climbing to 11000 RPM. Cool when you can sit on it and feel what the motor was doing under you Carb throats were almost as big as the cyls.When you boil it down it's volumetric efficiency. How much air, fuel mix you can ram through in one revolution.

rmh

correct -at least 3000-when people say they hover "just off an idle - well -it may be low on the tx stick but the engine is not Just off an idle.
Also - the prop load on a hover is never as great as it is, simply sittling still on the ground.
So If the pipe responds perfectly on the ground from 2500 to full rpm - you are fine - actually 3500 is more like hover power requirementon a excellent ,very high power setup. Recovery fom a slide takes a lot more power (thrust) - but still th load on th engine is not as great as simply sitting still on the ground .
The larger the model and engine and prop -the lower the rpm range but the higher the power requirement
The 160cc's in a hover , holding 40 pounds sound like they are hardly turning but they are at at least 50% of available thrust .
The ear is really tricking you.
It all reminds me of the guys who once said " yas - I was going 90 MPH in my Caddy n and I still had 2" of gas feed left ".
I always wanted to say -floorboard it and see how much faster it goes -lucky if you pick up 10 mph.