25cc Supercharged Two Stroke Engine - Fun Project

you could try a tuned exhaust, though i am led to beleive they work best within a small engine speed

http://microcarproject.tripod.com/html/tuned_exhaust_system.htm

the description you give is not of a supercharger bot a turbo charger.

a turbo charger would indeed give exhaust back pressure.

But a super charger which is driven by the engine and not by the exhaust, the supercharger could for a small engine even be elctricly driven if you have the power, or drive it from the output shaft

super charger

http://en.wikipedia.org/wiki/Supercharger

turbo charger

http://en.wikipedia.org/wiki/Turbocharger

sorry misred your post, what you describe is indeed a super charger.

the best way to find out is to try it, i think it will work

Object of a super- or turbocharger is to get as much vapourized fuel mixed with air^[1] as possible into the combustion chamber as quickly as possible.

Having difficulty seeing how you're going to do that with a fuel pump.

[1] in the correct proportions.

John, I'm going to put the fuel pump "supercharger" between the intake and the carburator, so it will suck the right amount of fuel to air ratio since the vacuum is going to increase.

The "electric supercharger" should work better blowing into the carburetor, as the restrictions of the carb would be better off being under pressure.You will need to increase the fuel pressure by a like amount if you do.

If I have no vacuum in the carb, There is going to be no fuel out of it. There is no way to put the pump before the carb.

The carb itself uses venturi principle to actually siphon fuel into the air stream.

If the fuel pump is not lubricated, it will wear itself out shortly.You might be able to mist some light lube through it while running.

Have you compared the volume of the fuel pump and the air consumption of the engine? At the engine speed I would expect from that 25cc engine, you might require more air than the fuel pump can provide. At that rpm, the engine would just run out of power.

Well, there isn't an easy way. I think it was Paxton or McCullough that developed a system where a plenum large enough the encase the carburetter was installed on the intake and the plenum was pressurized by the supercharger. Increased fuel pressure was required to get the fuel into the bowl.

Have you ever heard of the Detroit Diesel two stroke engines? All of the most powerful versions of them used turbo chargers. This was in addition to the required crank driven supercharger. Any time you can pump more air through an engine, you will have the ability to make more power.

How will you be able to determine if there is a gain in power? You might be able to connect an alternator to the engine and vary the field voltage to use it as a dyno.

You might be able to test the theory by using some shop air to help pressurize the carb. Good luck.

Bob, the designs of engines got to be different, they are made for that. That is a good idea to use the alternator as a dyno, thank man. My biggest concern is to know if the pump is going to get too hot since itis not submersed in liquid fuel to cool it. And the pressure that I am going to create behind the piston since it is going to have pressurized air from the intake(two stroke design). I believe that is going to reduce the power of the piston going down. just trying to do some research before having some fun.

25 CC engine even at idle speed is going to draw far more volume than what a electric fuel pump for a car delivers. Electric fuel pumps only need to move several gallons of fuel an hour while a 25 cc engine takes in several gallons a minute worth of air.

Yea bad volume equivalents but it give the ratios for comparison anyway.

You would be best off with one of those old smog pumps they used years ago for emissions systems. They are positive displacement air pumps capable of a many CFM at a few PSI and where capable of running at higher RPM's with out damage.

GA. I was afraid of that. I do like the idea of the smog pump. I knew a guy that was trying to put one on a moped. he was having problems with them seizing up. But he was trying to overdrive it at 2:1 on a Puch engine that already had a larger carb.

Most of the ones I removed over the years ran at roughly a 1:1 speed with the vehicle engine so I think that they only work up to around a 5000 - 6000 RPM.

For the most volume find one that was on a big V8 from a pickup or large car from the late 70's to mid 80's.

The smog pump is going to take a lot of power to run, and since this engine barely makes any power, I dont see how it can help. I was looking and most people said that it takes a little more than 1 hp to run the smog pump at a good rpm, and if you see that this engine makes 2 hp, I think I will probably loose power here.

About the fuel pump being slow. man, the fuel pump that I have can move about 120 gallons per hour, and I tested that (2 gallons per minute) and that is because it needs to move liquid which is a lot heavier, it will probably move twice as much if it is moving air. Besides it will be running oil from the two stroke gas, which is going to help lubrificate the system. And how can this engine draw several gallons of air per minute while idling from a hole on the butterfly that is half a millimeter? It is impossible, it can't have enough vacuum to do this.

They are electric so I am not going to loose a lot of power, and I can always put two or three if necessary.

Let's do the math...

Basic carburetor air flow calculation:
Cubic Feet per Minute (CFM) = RPM x Cubic Inches Displacement / 3456 x Volumetric Efficiency

I'll assume a maximum limit of 7000 RPM for your 25cc engine. And, for the purposes of this exercise, let's assume 100% volumetric efficiency.

1 Cubic Centimeter = 0.061 Cubic Inches, so 25cc = 1.53 Cubic Inches

7000 x 1.53 / 3456 x 1 = 3.099 CFM

1 Cubic Foot = 7.480 US Gallons, therefore 3.099 x 7.48 = 23.180 Gallons/Min

Even at a near-idle speed of 1000 rpm, it's still using over three gallons of air per minute.
(1000 x 1.53 / 3456 = 0.443 x 7.48 = 3.31)

If the engine makes 2 HP, and the air pump requires 1 HP to run, the engine will still run. But if the air pump can add another 2 HP to the engine, you will have a 3 HP engine. But all of this is useless if you are not going to open that throttle up and run it.

Hi All,

Once again very few [eople are reading the post and it seems the poster himself is "learning by playing". He definitely has too much time on his hands.Maybe working for himself?

The long and the short of it is quite simple. The fuel pump may well deliver loads of fuel, but it is a liquid that seals the vanes/gears in the pump, thus producing the required suction and consequently pressure. However to use the same pump for air, accept that you may have to turn it at maybe twice the speed, simply because the air cannot seal off any airgaps as is the case in when a liquid ring is formed. Thereafter you should start at you cylinder dimensions, ( pointed out altready) If you cannot achieve an airflow at full cylinder volume of at least 1,5 times, forget it all. Remember, best practice is to have excess air and waste it. However, excess air may also be a problem since you may not be able to supply the correct amount of air to combust properly ( typivally 13:1(lean)) After all this, laugh it off, you wont achieve your expectations, whatever that was in the first place

By the way, I am planning on putting this engine in my creeper seat at work. It will probably run at low rpm for a while, since it needs to move my weight and build up speed, and if it gets to high rpm, which it is not going to happen very often, I can open a valve that would bypass the pumps. That way I would have a high torque at lower rpm and have the same hp at high speeds, since I wouldn't restrict air. Of course, I will have to use some gears to lower the rpm from this thing and create even more torque. Does that sound good?

Sounds like you have to much time on your hands and a serious underestimate of Hp when geared correctly.

1 - 2 hp will accelerate and move a single person around a lot faster than you realize. Build it first then worry about the power later. Unless you weigh around 500 pounds and your creeper weighs 250 pounds that little 2 hp engines is going to shoot it right out from under you if its geared right.

If you do need more power just use a commercial chainsaw or weed whacker engine. The bigger 45 - 60+ cc engines in those usually can top 5 - 7 hp.

how carbs work.

http://www.youtube.com/watch?v=noOMaIHNknY&feature=related

coupled with the tuned exhaust link

http://microcarproject.tripod.com/html/tuned_exhaust_system.htm

i dont think a car fuel pump will work, but do try it.

Hi

Is this a crankcase compression two stroke? If so supercharging it won't work as the intake(tranfer port) and exhaust port will both be open at the same time and all of the charge will just go straight out the exhaust. A Further difficulty will be encountered due to the very high pressure needed to over come the throttling effect of the brief port action.

Those supercharged or turbo charged two strokes don't work on the crankcase compression system and a lot of the air is just used for scavenging to get the exhaust out. tuned exhaust or expansion chamber would give a bit more power but not at low revs

Cheers for now

Peugeot Man

Good and accurate post Mr Peugeot Man!

Why not sign on as such? You appear to be an expert that CR4 could use.....

I actually wanted to post the same important point. You beat me to it!!

It would appear that this 2 stroke is "Piston Ported" as you pointed out, and does not appear to have some sort of rotary valve that might allow a pressure build up in the combustion chamber....

The tuned exhaust is the method most often used to increase the power of such engines (as you and several other good posters mentioned previously), but if I remember correctly, it makes the power curve a bit strange, loss of power either side of the optimum power RPM, on 2 stroke motor bikes its makes them difficult to drive as the narrow power band is so small......

If a narrow power band is required, which I seriously doubt, then that is probably the right way to go....

There are some 2 strokes with a single exhaust valve (I just remembered!!), that would allow a pressure buildup from a super charger or Turbo etc....

Also not to be forgotten and mentioned by someone else as well, is that a fuel pump will be most inefficient working dry and will probably fail within a few hours of use.....

In this example, what is needed is a pump (Turbo as a prime example) that will a) run dry and b) provide large quantities of air at low pressure, proportional to the requirements of the engine.....a tiny turbo would do it, but it would need to run at extremely high RPMs to work properly, probably totally impractical.

Here are some interesting things to look at:-

http://www.youtube.com/watch?v=S3Ymdn5XeV0

http://www.youtube.com/watch?v=NvkV-wrmkDE&feature=related

So there are some tiny turbos around once the motor is converted to allow proper usage (assuming that it is piston ported.....)

Hi

I am registered as Peugeot Man but I don't seem to know how to log on so I just sign off as Peugeot man. The reason for the name is that I own three of them 2 504 diesels and one 505 SRD Turbo diesel one of the 504's is a genuine one owner car

Cheers for now from Adelaide South Australia

Peugeot Man

Good to see you are a regular. Have you tried telling the site to remember you. Once I did that, CR4 always knows when I log onto the site.

Same comment as Andy Germany, why don't you use the name Peugeot Man

you sound knowledgeable, Just the soret of person CR4 Needs

I agree, on a piston-timed two stroke engine most of the pressurized air-fuel mix will be blown into the exhaust. It will improve performance only slightly because burnt gases will be driven out more completely. As I see it adding pressurized mix (or fuel only?) in such engines would only make sense if injected directly into the cilinder during the upgoing stroke immediately after all ports have closed. This requires more pressure as the piston rises. So I'd rather imagine a well-timed diesel-like injectionpump than a blower of any kind.

Peugeot Man,

I had a friend that ported a Suzuki X6 Hustler (piston-port) to a point where there was a "fog" of fuel/mixture at the carb inlets at low speeds.

Quarter-mile performance for this 6 speed was 96 mph at 14 seconds on pump gas.

Factory horsepower was 27 for a 250cc, this bike was never dyno tested so horsepower increase was an unknown.

He rode this bike to and from the drag strip, so fitting expansion chambers was not an option (stock exhaust).

My point is, depending on how you utilized the torque-band (gearing), you can get a piston-port engine to perform without super charging, provided you can keep it from over-speeding, and ignition of a pressurized fuel/air mixture in the crankcase, such as would be possible with a super charger, could possibly turn the crankcase into a grenade.

Again I will agree with those that have said any gain in intake pressure would just be blown out of the exhaust port(s) if nothing else was changed. But the answer here is to block the exhaust port earlier than the intake port. With the unproved exhaust removal from the blower, the loss of some port time on the exhaust should not decrease power.

I don't know about anyone else, but I am getting anxious to see this little buzz bomb run.

much can be gained by exh timing, the pressure pulse timing is the
key to preformance, timing the pulse to hit the end of the exh chamber and return to hit the now just closed port is critical, had long lessons with two stroke ultralite engines and preformance exh timing, the key was the lenght of the exh header pipe, much trials and struggles, found the prime lenght but the noise was incredible, have a speed boat with a weedwacker motor and head carb exh are all special, can be found in model mags or online. good luck, never give up just because it is hard. Mitch, retired Peugeot mechanic/

Some things to consider.

1. The electric fuel pump will not put out anything remotely close to the required intake air volume.

2. As others have correctly stated, the mix will be blown through the transfer ports and out the exhaust port, even if the pump were of adequate size. This is also true in rotary valve engines and reed valve engines. There is therefore no gain from supercharging to compensate for the lost power in driving the supercharger.

3. The crankcase seals are not designed for a lot of pressure.

4. The expansion chamber (the tuned type lacking on a 25 cc engine) is the single largest determinant of power. In two stroke engines, there are no free lunches, and to gain high hp, you will lose low rpm torque.

5. Any change you make to port timing must be done in concert with the timing (acoustics) of the expansion chamber.

6. With a correctly designed expansion chamber, retimed ports, larger carb, and a slight milling of the head to restore or slightly raise the effective compression ratio (which drops as a result of the port modification), you could double the hp of your engine. Reliability will, of course, suffer. 25cc is enough to produce 12 hp, in road racing trim -- but nothing in your engine will handle those stresses.

7. Forget supercharging in two stroke spark ignition engines. There are far easier, cheaper, more effective ways to produce enough hp to cause engine failure.

OK, that's what I feared; the pressure would leak out the exhaust. Then I guess there is no point in doing it.

Just so you guys know: I am 21 years old, I don't have a lot of time; full-time employee(mechanic, 7 ASE certified, and going to do more 7 in a month), full-time college student, and that is not counting about 8 hour/week of training(brazilian jiujitsu)(btw, i'm brazilian an live in US for 4 years), and going out with friends and stuff like that. I just graduated and have a automotive technology associates degree. And I am starting having some classes for an bachelor degree in physics engineering, which I love to death. I don't know a lot, so I like to learn some stuff by trying it out, and doing some research(that's why I came here).

Btw, I am going to put a larger(in diameter) spark plug so I can raise the compression ratio, since the combustion chamber is so freaking small. If it is no good, please tell me.

Thank you, everybody.

Btw, I am going to put a larger(in diameter) spark plug so I can raise the compression ratio, since the combustion chamber is so freaking small. If it is no good, please tell me.

This would have the reverse of the desired effect. A larger spark plug has more volume between the insulator and the housing, which would reduce compression ratio. To increase compression ratio, you want to make the combustion chamber (with piston at TDC) smaller. The standard way to do this is to mill the head, or in some cases, to use a thinner head gasket (if there is one). You want to keep the "squish" area contoured so that the head in that area is parallel to the piston crown. That usually means, that in a small two stroke, you do not simply mill (or plane) the head flat, so this job is often easier to do well on a lathe, with the head bolted to a face plate, and shimmed to get the gasket surface parallel to the face plate.

The pink above is the metal to be removed. In my drawing (side view of head and piston), the squish area is asymmetric, but often, in very small engines the entire head is symmetrical.

Up to a point, the closer the piston is to the head in the squish area (the area outside the combustion chamber cavity in the head) the better, because it reduces the amount of mixture cooled in the space and also allows the piston to cool better from proximity to the head. On the other hand, you want enough clearance that the piston will not hit the head as the crankshaft flexes at high speeds. On an engine as small as yours, if all the bearings are needle and ball bearings, (and if there is no detectable slop in them) then .020" clearance in the squish area is probably OK.

Gordon Jennings wrote a good book on two stroke engine tuning many years ago. It's worth looking for if you are interested in the theory and practice of such things. It includes formulas for dimensioning expansion chambers, etc. I may still have mine, and I think that the "5" in the lower right corner of the Amazon picture was the original price in USD. Now the cheapest used ones are $76, with others being over $100.

BTW,I just found these Jennings articles, which talk about some of the issues that appeared in Jennings' book. In these two articles, he describes the use of his formulas for expansion chambers. With very slight porting mods, a new pipe, and a new head that cooled better and had a higher compression ratio, the increase in hp was 46%, without needing to increase peak rpm by more than 500 rpm. This is a huge increase for very low cost. (This increase is in comparision to a bike that already had a tuned exhaust -- so the comparison to a weed wacker or chainsaw engine would be even more dramatic.)

I used Jennings' formulas many times for building pipes for our motocross team decades ago, and they worked well. Especially important was the ability to tailor a pipe to a rider's preferences and for use on particular tracks -- with different pipes for high hp tracks vs tight and twisty tracks.

I also made several pipes with inverted stingers (described in the article). With these, no silencer was required to meet the (admittedly approximate) sound level requirements -- the bike sounded as loud as a bike with a small fiberglass packed silencer, (but the character of the sound was different -- maybe a little raspier).

In road racing, you design the pipe to be peakier, and typically allow the engine to run much faster, figuring on rebuilds after just a few hours of operation. If you can improve breathing to enable 50% more torque at a 50% higher rpm, then you have (1.5 x 1.5 =) 2.25 times the original hp.

For a 25cc engine, maximum reasonable rpm might be around 12,000 rpm even with stock crank and bearings. (Maybe... it depends upon how conservative the original manufacturer was.) (Racing 50cc GP motorcycle engines turn twice this, and a smaller engine, other things being equal, can turn at even higher rpm.)

This could be a fun, loud, project. If you lie on your creeper (like on a luge), and if you put on large wheels (smallest, narrowest go kart wheels) you should be able to get better than 60 mph on 5 hp, given a CVT or similar transmission that will provide both good acceleration and good top end.

You should be aware, of course, that a collision with anything solid at that speed will turn most of your body into goo. You should also be aware that even a tiny engine self-disassembling from overrevving can throw lethal shrapnel.

I'm not sure a larger diameter plug will increase compression. The maximum that the threaded section should extend into the chamber is 0. Only the electrode should be exposed to get the spark in the path of the swirling combustible mixture. Additionally, a larger diameter plug might have a larger void area between the insulator and the inner wall of the threaded section, effectively decreasing compression.

I'm not really that familiar with 2 cycle performance modifications but a friend of mine always talked of the easiest way to increase power was to "stuff" the crankcase. He raced Saabs (ice racing) and snowmobiles. As I recall, someone correct me if I'm wrong, it involved filling the crankcase with epoxy in any area that didn't interfere with the rotating components. I guess that was like improving the sealing properties of a pump. Kinda like increasing the charge pressure coming from a supercharger. Best part was it didn't have moving parts, use horsepower, require extensive modifications to install or add significant weight.

He also spoke of "five porting" which I guess is the same as the porting mentioned earlier. That's probably a science in itself. I've seen various shapes of ports (triangular, oval etc.) that were tried in developing his racing engines. That was years ago and I don't remember any results. If you decide to try stuffing and have the engine apart, that would be a good time to port it, though a good rule of thumb is to test one modification at a time.

I guess that was like improving the sealing properties of a pump. Kinda like increasing the charge pressure coming from a supercharger.

Basically right. Although it is not so much "sealing" as compression ratio, which your second sentence addresses. The higher the crankcase compression ratio, the faster the flow through the transfer ports. Some of the old tricks do not work as well with higher performance engines, which are already pretty well optimized for a given operating condition. In high performance engines, the effect of the expansion chamber can be to draw substantially more mixture out of the crankcase due to the scavenging effect of the chamber. Then having a little more volume in the crankcase can be a good thing, because there is the potential for containing more total mixture. It is a little like the advantage of low compression in a turbocharged four-stroke engine: the larger combustion chamber volume holds more total mixture, with the pressure being governed by the boost.

He also spoke of "five porting" which I guess is the same as the porting mentioned earlier.

Five porting was the addition of two more transfer ports to engines originally with 2 transfers ports and one exhaust port. The norm in motorcycle two-strokes since about the 70's has been 6 or 7 ports. But this weedwacker engine might have only three ports, in which case, adding two more could add some power. It would be a tricky job with such a tiny cylinder. One would have to be sure that the new ports are not located near the piston ring locating pin.

Two strokes are very strongly influenced by exhaust pipe dynamics. In fact, if you look into a two stroke cylinder with the head removed, you can see that the exhaust port is completely open any time the transfer ports are open -- so it is easy to imagine a condition in which virtually all the fresh charge goes right out the exhaust pipe. In a highly-tuned engine this happens (to a small or large extent) much of the time, and the engine must rely on a properly-tuned expansion chamber to return this mixture, under pressure, to the cylinder.

Earlier in the process, for scavenging of the cylinder, a negative pressure pulse is desirable, and this too can be delivered by the pipe. So a good first step is to design and build an expansion chamber -- because nothing else can work well without one.

In this article, some of the issues are addressed. The writer describes his Suzuki's 8500 rpm porting and 6750 rpm pipes -- a situation to be avoided if lots of hp is the goal. Ideally, every aspect of porting must be designed with the pipe in mind. As the writer of the article says, "Fiddle with two strokes at your own risk."

I guessed as much. It is good though to have a curious yet industrious mind. Wish we had more of those in South Africa.

I am also involved in Model aircraft and the standard/ best/most efficient way to boost these two strokes ( my own was a 31cc) was with a tune pipe. Tuned exhaust as some posters already mentioned. However it was painstakinf to get the exact length of the pipe. As mentioned already, different "duties" appeared at different lengths. The trick was then to decide what you wanted to do with your engine and then clamp, or cut the pipe to that permanent length. However, now you have an engine that will pull into the air like a bat out of hell, OR you have one that tkaes time and flies like a bat out of hell. I suppose ones efforts will be well used if you would ratherher develope a variable length pipe that you can use at the right aetting for the moment. Having said that, it may not be a bad idea to do it anyway, since you will have hands on the engine all the time, and find the best "spot" manually, or even vary as you go.

Just some comments which may have been placed already

Aren't there any motors around that are 4 stroke around (lawnmower?) that you could experiment with? Then a tiny Turbo might get interesting......

Just about all of the guys have hit it on the head with this project. but to throw my two cents in remember that supercharging is a process and that blower and turbos are a form of supercharging. Also, all two stroke engines are supercharged in some way.

I did a project like this as a little kid with a 80cc Yamaha 2 stroke engine. I used my mothers industrial hair dryer (with the heating element removed) and I spent about a month tuning the carb and exhaust. Once I finished, it screamed but had very little power in the low RPM's. Yes, allot of intake pressure was exhausted out of the exhaust ports but what remained was more than what was offered stock.

Sorry, but your comment:-

"Also, all two stroke engines are supercharged in some way."

Does not ring any bells with me. Would you be so kind as to explain to me what you meant as I am really interested to find out......thanks in advance.

Has anybody considered the possibility of using a vacuum pump (ie of a diesel alternator) the type used to operate brake boosters. i wonder if it would be plausible to generate vacuum on the exhaust port side in order to aid scavenging? may need to consider timing it to the exhaust pulses otherwise i would imagine that it would not run. Good luck getting your head around it I have had i little think about it and in theory it should aid air speed through the combustion process.

It wont work, since the exhaust port is the last one to close. It would end up removing my gas mix from the combustion chamber, and I would loose power. I did think about that too, I would do that if it was a 4 stroke engine.

Thanks for the thought man.

You've hit on one half of the function of a tuned expansion chamber. See Jennings articles linked in post above.

A scavenging vacuum alone would reduce power, because in a two stroke without an exhaust system, the cylinder is already too effectively scavenged -- much of the fuel/air mix goes right out the exhaust port. If you don't have something to stuff that lost mixture back into the cylinder at the right time, you loose power. The expansion chamber serves that second, stuffing, function.