Author Topic: Sub Piston Induction (SPI) (Read 3205 times)

Larry Renger · « **on:** July 07, 2016, 09:49:17 AM »

Anyone know why this works? It seems to me that a larger venturi diameter would work better.

Tim Wescott · « **Reply #1 on:** July 07, 2016, 11:50:47 AM »

Well, you need a large air path throughout. I believe the idea is that instead of making the crankshaft bigger, or too thin, or going to a rear induction setup (or trying to make a reed valve pass all the air the engine needs), that you just make some sub-piston induction and call it good.

Fredvon4 · « **Reply #2 on:** July 07, 2016, 12:42:31 PM »

My experience with with the small Cox TeeDee's and SPI (and a few reed versions but no experiment with them)

Bottom line, on these engines, is the fuel load is heavy with castor and moves slowly..I suspect that adding high speed but very brief pure air into the rapidly spinning crank case volume--- offsets, or enhances if you will, the inefficient fuel/air charge from the venturi NVA induction

Important variables:

Atmospheric air density and pressure
Sub and super sonic air moving
Controlled duration of the clean air charge
Points of diminishing returns
Starting and unloaded NVA mixture and leaning
Muffler, vs open exhaust...SPI engines should not run with a muffler...too much dirty air in the SPI charge
Very brief timing of un-shrouding of bottom of piston before closing of fuel and boost ports... there IS a point where there is too much SPI

I have fussed about with SPI on Cox TeeDee .049 and .051 and there is a distinct minimum and max that yield increase RPM and torque

I have fussed with opening TeeDee Venturies, all on bladders, and the effects of SPI are more pronounced than the venturi drilling

One of the trials included increasing the SPI and Venturi...engine fell flat...went with smaller spi larger Venturi...xx rpm
then with larger spi and smaller venturi...xx more rpm

It would take a much more controlled environment than I was willing to deal with to determine bench vs in the air and prop load differences but my modest experiment proved to me there is a cross over relationship and a LOT of variables

In the end, I morphed 4 of my 7 TeeDee's to be able to swing a 5.5x 6 prop and haul a 9.7oz Lite Hawk variant out on 42" lines with authority---- similar to same birds with Norvel Aero .049s on the nose

Air Ministry . · « **Reply #3 on:** July 07, 2016, 06:34:20 PM »

Its like the two holes to the Transfer in a Torpedo 40 .

The gulp of fresh cold air cools the little end / piston crown .

Also the Cooler Charge induced raises the density of the mixture , presumably .

One or two people have played with ' dual induction ' ( Front & rear ) set ups . Dunno what results theyve had .

Youll get a extra 1.000 plus from a G-15 with SPI , properly done .
Biggest laugh was a 20/23 with a chunk out of the skirt , far healthyer runner than the stock one .

Phil Krankowski · « **Reply #4 on:** July 07, 2016, 06:43:01 PM »

On reed engines YES emphatically. Been used on motorcycles and more.

On rotary valve engines not so much, but there is often improvement. With the air going through the crankshaft there is only so much passage available.

Phil

FLOYD CARTER · « **Reply #5 on:** July 08, 2016, 02:18:08 PM »

On rotary valve engines not so much, but there is often improvement. With the air going through the crankshaft there is only so much passage available.

Phil
[/quote]

That's what I have found. On high-rev engines, the crankshaft port isn't open for enough time to ingest enough air.

The Super Cyclone , Orwick 64, and some Atwood engines had SPI holes in the crankcase/cylinder. I've tried covering them, with no change in RPM. That's because these engines are slow enough to permit adequate air through the venturi.

Floyd

Jim Kraft · « **Reply #6 on:** July 08, 2016, 04:54:10 PM »

I have found on my 60 size ignition engines that they run way more consistant with the holes blocked. Fuel draw is better and they run without ever running away. On free flight planes they add a touch more power as they are run wide open and fuel draw is not a problem. All of mine have plenty of power with the holes blocked in control line.

When I was flying R/C we found that engines with SPI did not like mufflers as it really knocked the power down.`

Tim Wescott · « **Reply #7 on:** July 08, 2016, 05:14:47 PM »

Quote from: Jim Kraft on July 08, 2016, 04:54:10 PM

When I was flying R/C we found that engines with SPI did not like mufflers as it really knocked the power down.`

Out of idle curiosity, was that just with the ones that pulled fresh air in through the exhaust, or was it the case with the ones with crankcase holes?

For some reason SPI reminds me of the Gnome-Rhone rotary engines, which drew in a good amount of their fresh air from the exhaust valve, and the rest through the crankcase. By today's standards it was a strange system, but it was certainly a world-beater for a while.

Jim Kraft · « **Reply #8 on:** July 09, 2016, 11:13:56 AM »

Tim, it was the R/C engines that the piston skirt went above the exhaust port. They did not like the back pressure from mufflers. I do not think the ones with the holes like the ignition engines would be effected by mufflers as the mufflers are in no way involved.

RknRusty · « **Reply #9 on:** July 09, 2016, 12:28:24 PM »

I am on the fly, so will re-read the thread for details later, but for now here's an explanation I wrote a few years ago on SPI for those who just want an overview. I hope it's not redundant:

EDIT: This was written with 1/2A in mind, so disregard minor differences like the prop size reference.

Quote from: RknRusty

How SPI(Sub Piston Induction) works

The upstroke: On the piston upstroke the crankcase pressure is relieved by the action of the piston moving up to compress and burn the previously introduced charge. In a reed valve engine, the drop in crankcase pressure releases the reed which finally opens fully drawn by the vacuum created by the displacement of the rising piston. Fuel/air is drawn into the crankcase through the venturi behind the reed. In a rotary valve engine such as a Tee Dee, the intake valve opens while the piston is rising and the vacuum it creates draws the air/fuel mix through the carburetor venturi into the crankcase.

TDC and the downstroke: As the piston goes through top dead center, the piston skirt clears the bottom of the exhaust port, opening a gap exposing the crankcase to cold air(Not recommended for engines with mufflers). The crankcase vacuum created by the rising piston causes fresh air to rush into the crankcase adding additional air as the now descending piston compresses and forces the freshly inducted air to combine with the existing fuel/air mix.** This gives the crankcase a belly full of extra pressurized f/a mix ready for the next charge. The increasing pressure of the descending piston forces the mixture to scoot up through the bypass ports until the crankcase pressure is diminished. It starts over again with the upstroke and Kaboom, it's a wild cycle. With SPI more fuel/air mix is available for combustion power. It only has its full effect at high RPM. Usually a small prop, less than 6"(for 1/2A), is needed to achieve useful RPMs necessary for SPI to have a beneficial effect.

**Obviously, the bonus air pressure added to the existing air from the venturi leans out the mixture, so that the needle valve must be opened more(richer) to let in extra fuel to balance the mixture.

In summary, the crankcase is charged with fuel/air mix from the venturi plus the SPI inducted air and then compressed by the descending piston. All that, plus the extra fuel from the richer needle setting to balance the A/F ratio, now you have a higher charge in the crankcase than if there had been no sub piston induction.

It is hard to believe any useful amount of air enters. But considering the piston just pulled a vacuum inside the engine during the upstroke on a reed valve engine, I can believe it does gulp some. On a rotary valve engine, not so much, since it has that gaping venturi venting most of the vacuum to the atmosphere.

Rusty Knowlton

Chris Wilson · « **Reply #10 on:** July 11, 2016, 05:47:39 PM »

Quote from: Larry Renger on July 07, 2016, 09:49:17 AM

Anyone know why this works? It seems to me that a larger venturi diameter would work better.

Sub piston induction only works IF the normal intake system is compromised.

And it can be compromised beyond venturi size with other gas passages being too small , notably the crankshaft journal.
So IF SPI has an effect on an engine than the normal intake could be bettered.

SPI only induces air and not fuel and thus widely effects the normal fuel/air ratio and leads to claims of needling problems - in as much as it is not linear and predictable.

Some Jett engines have the cut outs that look as though they would act as SPI but do nothing other than lighten the piston because the normal intake is very efficient and there is no pressure drop available for it to work on.

Cheers.

rustler · « **Reply #11 on:** July 12, 2016, 04:08:29 PM »

Quote from: Chris Wilson on July 11, 2016, 05:47:39 PM

SPI only induces air and not fuel...

Best proof I know of this is the old 1940's Howler engines. Fuel in by the conventional route, but no air. ALL the air is admitted by SPI.
A later model had a small conventional air intake added, was about 1k rpm faster.

RandySmith · « **Reply #12 on:** July 12, 2016, 04:16:27 PM »

SPI is NOT anything you want in a Stuntmotor or anything you run a Muffler on, You can get an advantage with SPI running a pipe or open exhaust

Randy

RknRusty · « **Reply #13 on:** July 12, 2016, 05:51:17 PM »

Quote from: RandySmith on July 12, 2016, 04:16:27 PM

SPI is NOT anything you want in a Stuntmotor or anything you run a Muffler on, You can get an advantage with SPI running a pipe or open exhaust

Randy

Yeah, I always got the best use of it on bladder fed 1/2A engines with small props. Run them screaming fast and it's great for maximum thrills. If you ain't smiling when you finish those flights, something's wrong with you. Boy did I have a bunch of bad habits to break and new things to learn when I found PA, Lol.
Rusty

Author Topic: Sub Piston Induction (SPI) (Read 3205 times)

Larry Renger

Sub Piston Induction (SPI)

Tim Wescott

Re: Sub Piston Induction (SPI)

Fredvon4

Re: Sub Piston Induction (SPI)

Air Ministry .

Re: Sub Piston Induction (SPI)

Phil Krankowski

Re: Sub Piston Induction (SPI)

FLOYD CARTER

Re: Sub Piston Induction (SPI)

Jim Kraft

Re: Sub Piston Induction (SPI)

Tim Wescott

Re: Sub Piston Induction (SPI)

Jim Kraft

Re: Sub Piston Induction (SPI)

RknRusty

Re: Sub Piston Induction (SPI)

Chris Wilson

Re: Sub Piston Induction (SPI)

rustler

Re: Sub Piston Induction (SPI)

RandySmith

Re: Sub Piston Induction (SPI)

RknRusty

Re: Sub Piston Induction (SPI)