Hypothetical 2 stroke v-8

[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

[jrc347]

This one is nice to

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

CrazyRonny

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.