[jrc347]

Hi, I've been kicking around an idea for a while and would like to get some feedback to see if it is actually possible or not. My idea is to make a scale 2-stroke v-8 that would look (and hopefully perform) like a top fuel dragster engine. The engine would use comercially available pistons cylinders and rods from let's say a .21 os buggy engine. It would have a supercharger to pressurize the intake tract, avoiding any scavinging from the crankcase -- ie the bottom end would act like a 4 stroke engine with either dry sump or wet sump lubrication. The major things to build would be cylinder block, crankshaft, and supercharger. I know people have built all of these seperatley, but I have not. I figure the hardest thing to make would be the supercharger. It would have to be a positive displacement either a "roots" style or a "twin screw" style. I would imagine that the twin screw style would be better suited due to it's efficiency advantage over the roots style. Has anyone evermade a scale supercharger that was efficient? How much boost could be expected? Am I crazy for even thinking of such a beast?

[NC Cams]

The next time you thinkabout it, look at what's in the fuel for a model airplane engine. If memory serves correct, there is nitro in it already which is why the model engines are so powerful.

The nice part about using the crankcase to pressurize the induction in a vavleless 2 stroke is that you have to move the piston and the air underneath it ANYWAY (think about it for a minute). Besides twice the power strokes, this is another reason why the 2 stroke makes more specific power than the 4 stroke - there is less pumping work loss.

You can easily make a boosted 2 sroke for inspiration - look into the workings of a 4:71 thru 8:71 Detroit diesel engine as they do exactly what you're proposing - compression ignition only (glow plug to start) with diesel fuel instead of nitro/methane.

It would be harder to make a sump drier than that of a model plane/car engine as it is pretty "dry" as the fuel not only cools and powers the engine but also lubes it.

A roots type blower takes LOTS of power to run. A turbo recovers wasted power. I don't think roots would be the way to go in a model. As your question is posed, it is impossible to answer. Suffice it to say, however, the boost would ultmately depend on blower sizing and pulley/drive ratio.

Then again, I"m an engineer and engineer's merely solve problems and do not necessarily come up with inordinately creative ways to do stuff. I believe that it was an engineer who proved that neither a bumble bee nor a humming bird could fly.... Another said that it would not be possible for a dragster to exceed 124mph or something ridiculous like that.

Yet, N/A pro stocks are running faster than the fuelers did in the 60's and fuelers are runnig 300 and change with gear and 90% nitro rules. All it takes is money and time and a group of engineers to design the hardware needed to go that fast - hardly DIY stuff done on cut-n-try basis....

[Geof]

....Another said that it would not be possible for a dragster to exceed 124mph or something ridiculous like that...

That was a physicist who 'knew' that the maximum possible coefficient of friction was 1, ergo; maximum possible acceleration was 1g.

[NC Cams]

I shoud talk.

When first told about "lofting" (intentioanly floating valves to gain more lift with very short duration cams), I said it was B/S, impossible.

We've since learned how to loft at 0.800" plus and can do so for 500+ miles with relatively good contro, very good actually - and good enough to win multimple times in NASCAR "cup" racing.

NOt for the faint of heart nor small budgets, but doable none the less....

[jrc347]

So, has anyone made a supercharger.... let's say 1/4 scale or less...... that actually works? This engine design would not really be "boosted" in the traditional manner, So I don't think the parrasitic losses from driving the charger would really matter a whole lot. I am really interested in hearing from someone who has made a working scale supercharger.......

[mrdj1234]

not quite 1/4 scale, but it does show that it can be done:

http://www.nvbackflow.com/engines/blowers.htm

on a scale V8 too no less! (can't wait to build something like that myself! )

[CrazyRonny]

This one is nice to

http://www.freewebs.com/motornisse/topfuel225cc.htm

CrazyRonny :cheers:

[jrc347]

This one is nice to

http://www.freewebs.com/motornisse/topfuel225cc.htm

CrazyRonny :cheers:

Wow..... Does that thing run????..... That's exactly what I want to build..... But a 2 stroke instead of a 4 stroke. I might be just dreamin', Here's my math.

21 O.S. buggy engine makes about 2 hp. (donor of sleeves and pistons)
x Multiply by 8 cyls
- subtract supercharger load 25% HP (on the high side)
= 12 HP.

It might be possible to adjust the supercharger outlet pressure to match a custom tuned pipe, to tune for exact fuel/exhaust mixture.

A high mix of nitro would only help things in the power department.


Basically in theory you could raise the intake port timing above the exhaust port and build all the boost you wanted.

[VTLguy]

I really hate to pop your bubble but a 2 stroke 6-71 detriot diesel isnt actually boosted... meaning that without the blower, the engine wont actually run because it wouldnt move enough air by itself. so in the case of a detriot, its more of an air pump rather than compressing it. the reason that 2 stokes make more power is that there are twice the power strokes. oh and there was no 8-71 either... it was an 8v-71(as in v8. and 8-71 would be inline, but they didnt make one). the 71 means 71 cubic inches per cylinder. I would say you see more 92 series detriots now days anway. Rather than using a roots or screw style, i would say it might be easier just to use a centrugal style supercharger... otherwise you'd have to come up with a manifold. i think your biggest problems would be getting your port area and timing correct. on a detriot the exhaust is run through normal valves and valve train, and only the intake ports are in the cylinder... that might make design a little simpler since you could adjust your camshaft to suit your intake port "timing"

[Geof]

QUOTE ".......on a detriot the exhaust is run through normal valves and valve train, and only the intake ports are in the cylinder... that might make design a little simpler since you could adjust your camshaft to suit your intake port "timing"......"

This is really only applicable on a compression ignition engine because your incoming flow is only air. Don't these engines actually have a large valve/port overlap to scavenge the combustion gases thoroughly? With a spark ignition engine either you close the exhaust valve early and have a very large combustion gas residue or you maintain the overlap and flush a lot of your incoming fuel-air mixture out the exhaust. So you are either going to have a very inefficient engine or blow your exhaust system apart.

I suppose you could tune everything for a particular rpm and throttle opening so your valve closed just in front of the incoming charge but a constant speed constant power engine has limited applications.

[vulcom1]

I am a retired ex/heavy mech. and have worked on my share of Detroits. As I remember the valves are intake and exhaust is sleeve ported direct to manifold. The blower charges the cyl. on the upstroke valve's open and before tdc the valves close and the injector puts in the fuel charge. Ignition and power stroke and at the bottom the exh. is released out the port. Getting the pressure from the blower and how much would be the hard part. You can put a carb on it to get away from the injector's. As mentioned also you need separate oil supply and pressure or some other type of lubrication.
Model glow fuel would also work and I have run 65% nitro while racing .40 size. I have also used nos but that is another story. Just be advised my luck was out of sight rpm for a few seconds before it blew.
To me if you can get a blower or make one that will put some psi out it would be easier to get it to work as a Detroit style 2 stroke especially in the 6/71 category.
John

[Geof]

QUOTE: ".....As I remember the valves are intake and exhaust is sleeve ported direct to manifold....."

I googled Detroit Diesel Two Stroke and found this:

Definition: A type of engine where air enters the cylinder through cylinder liner ports and is compressed. Fuel is then injected and burned.

Which seems to agree with the exhaust being through the valve.

[jrc347]

The diesel 2 stroke has been around for long time... They come in many shapes and sizes. Some have only an intake valve with exhaust ported through the sleeve, some are the exact opossite. Some also have both an intake and exhaust valve, just like a four stroke. The hypothetical engine I dream of building will be Shneurle ported... Both intake and exhaust will be ported throught the cylinder sleeve. By changing the intake/exhaust port timing I believe it will be possible to scavange the cyl almost completely... and also build positive pressure in the combustion chamber. Theoreticaly the best way to make the most power and efficiency would be to directly inject the fuel into the cyl just after the piston has closed the exhaust port. This would eliminate the need for any tuned pipe, and would be the most powerfull / fuel efficient way to charge the air/fuel into the cyl. The only problem is the size/ complexity of the injection system. Is it possible? Yes............ I guess I just need to get off my butt and start making Parts.

Btw.. The diesel 2 strokes are very similar to what I am talking about here. I guess this "hypothetical" 2 stroke takes a few of the good traits of these designs and puts them into one engine. I just wish I had the time to model this engine in 3d to kind of have a proof of concept... But I can see each part in my head... Putting it down on paper is alot harder to do.

Thanks for all the input and keep it coming so I can refine my ideas even more.

[jrc347]

I really hate to pop your bubble but a 2 stroke 6-71 detriot diesel isnt actually boosted... meaning that without the blower, the engine wont actually run because it wouldnt move enough air by itself. so in the case of a detriot, its more of an air pump rather than compressing it.

Your statement is exactly correct...... For that style of diesel engine.
But by the very design of any 2 stroke engine, the piston cannot "pump" air like a 4 stroke because the exhaust and intake ports (or valves) are open at the same time. This is also the reason that you cannot build a whole lot of boost in a 2 stroke.

Think of any model, or weedeater 2 stroke (shneurle ported) , They use a sealed crankcase and the downstroke of the piston to make positive pressure in the crankcase which in turn pressurizes the intake charge to flow into the cyl. Lets say just for an example that this pressure is about 3 psi. This pressure is enough to help scavange the exhaust and fill the cyl with a fresh air charge.

Now take 3 psi from any type of air pump. And instead of routing the air charge through the crankcase, put it direclty form the blower outlet into the intake cyl ports....... besides the parrasytic losses from driving the air pump......Would'nt you theoretically have the same thing?

What if you could change the port timing so you COULD build boost in the 2 stroke.... and make it direct injection.......

[mdreitzusa]

have you seen this
http://www.coatesengine.com/index.html
it looks like it would be easier to build than a stand valve system.
they say this systen is used in 2 and 4 stroke engines.
looks like it might be fun.

[VTLguy]

wow. just glanced at that rotary valving and it looks astounding. havent read it yet, getting to it. also must say its a pleasure to converse with you guys! you 're all really knowledgable!

[vulcom1]

After reading the info you could build the v8 configuration but you still have to get around the induction. As mentioned without the sealed crankcase and wanting to use the .21 sleeves you are going to need pressure on the intake. The type of pump and how much psi would be the problem. If you use a carb on the intake like the old days then no injectors or valves which is what would be ideal. Injectors are a very precision device and they also require there own pump.
I was thinking of this a few years ago using 40 size sleeves. Then there was no net to search or forums. What I was thinking about at the time was a blower in the saddle driven by the crankshaft with an adjustable waste gate which would have to be plumbed back to the intake.
If you ever figure out the pump it would be appreciatted if you let us know.
John

[CrazyRonny]

And what about "Direct injection" two stroke... I know the problem will be with the fuel pump setup.... but...

http://www.envirofit.org/files/publi...20Strategy.pdf

http://www.synerject.com/di.html

CrazyRonny

[vulcom1]

I read the posted info and agree it would be nice to be able to go that way. I think as a home builder it is very complicated actually to make all the parts. They are still using crankcase scavenged for the air and oil all be it with no fuel for mix. Not sure about todays injector's but I have hand lapped a few in my days and the spray pattern at the nozzle would have to be figured out. If you had a hole plugged or worn in the tip you then you get the hole in the piston which does not take long.
I still like the original thought of the carb/blower combination with maybe a splash lubrication in the crankcase. Have you figured out the pump side at all.
Any idea's or thoughts would be interesting.
Just reading this is interesting and would like to help but I have a few projects on the go. I am into building an rc 87"ws model and also working on a electronic ignition for the 42cc engine. When I get finished would like to do basicly the same but use 4 30cc homelite or similar jugs horizontaly opposed.
John

[jrc347]

I agree about the injectors.... They are in the too hard pile.

I was talking about the design with my dad the other day, and he saw a problem and also came up with a good solution to it.

The problem is the raised intake port relative to the exhaust... the exhaust would try to push out through the intake tract (I think the scientific term is reversion).

The solution he came up with was simple, yet highly effective and really easy..
Put a check valve ( either a check ball or reed valve ) in the intake tract..

To put it another way.... lets say the power stroke has just occured and the exhaust gas is at some pressure ( 100 psi... just a guess at this point)
The piston opens the intake port before the exhaust, The exhaust would naturally flow against the intake flow from the supercharger. So the check valve would keep the exhaust from flowing into the intake, and when the exhaust pressure is equal to or less than the intake pressure, then the intake would flow into the cylinder. I think this is a very solid design of a supercharged 2 stroke nitro powered engine...........

I hope this makes sense to everyone.

Thanks again for all the input. And keep it coming. I hope I can get this idea off the ground in less than 10 years.....