Turbocharging carbureted petrol 2 stroke engines

[Luth]

Hi everyone, online I ve seen some images about 2 stroke carbureted turbo
(motorcycle derivation engine).
Now.. In the past in this forum some members spoke about turbocharging 2 stroke but not in sufficient detail.

The intake and the exhaust are open at the same time and there are no valves like a 4 stroke.
But if you search online you can find carbureted 2stroke turbo sled or the Am6 turbo.

The question is:

Is really possible turbocharge a 2 stroke carburated(NOT EFI)petrol engine and make more horsepower with these engine's config. ??
If yes how it can work??

I attached 2 images. Look at the Am6 (small bike engine) its a reed motor draw through.
And the other, the sled seems a piston port motor blow through.

Hope to see interesting answers ad learn something about.

Luth

 

[ChemAir]

As someone with turbocharging experience with V-8 4 strokes, I'd wonder first how you will get oil to the bearings in the turbo. 4 strokes have captive oil systems at fairly high pressure and good flow is required to keep the bearings cool. The crankcase will also have to be able to handle much higher pressures than normal. I'd also wonder if spark advance may need to change, and without EFI, I'm not sure how you'd manage this, unless it was mechanical.

The compressor side of the turbo will provide some back pressure that could increase power on the engine. The downside is a ton of fuel will be blowing out as well. This will have to be managed by pipe sizing (volume and diameter) and will have a limit, and may only be operable in a very narrow flow range. I don't know that the power gain will offset the efficiency loss and the weight increase for making the modification.

A draw through carbureted setup may work, but I suspect with all the exhaust/turbo tuning/sizing that will also influence this, it may take you a long time and lots of trial and error to find a workable combination.

Here's an old forum thread on this. I think the last post shows a two stroke outboard that has some kind of servo control that can change back pressure on the motor by some type of wastegate.

 

[Baluncore]

Maybe you could start with a 4 stroke engine, then convert it to 2 stroke by replacing the camshaft. That will have an oil pump, without pressure in the sump.

 

[Luth]

As someone with turbocharging experience with V-8 4 strokes, I'd wonder first how you will get oil to the bearings in the turbo. 4 strokes have captive oil systems at fairly high pressure and good flow is required to keep the bearings cool. The crankcase will also have to be able to handle much higher pressures than normal. I'd also wonder if spark advance may need to change, and without EFI, I'm not sure how you'd manage this, unless it was mechanical.

The compressor side of the turbo will provide some back pressure that could increase power on the engine. The downside is a ton of fuel will be blowing out as well. This will have to be managed by pipe sizing (volume and diameter) and will have a limit, and may only be operable in a very narrow flow range. I don't know that the power gain will offset the efficiency loss and the weight increase for making the modification.

A draw through carbureted setup may work, but I suspect with all the exhaust/turbo tuning/sizing that will also influence this, it may take you a long time and lots of trial and error to find a workable combination.

Here's an old forum thread on this. I think the last post shows a two stroke outboard that has some kind of servo control that can change back pressure on the motor by some type of wastegate.

Hi chemAir,thanks for your reply.
Yes, the thread you have linked is that's I mentioned in my post. But it didn't go into specific detail like I said. If you look around you can see 2 stroke turboing sled with big hp gain. And they have a blow through set also. Look at the pictures I've attached.
The turbo restriction is after the expansion chamber there exhaust gasses flow is not very speed like in a four stroke.
And what about the fuel blow out the exhaust port? Looking the old thread the simple backpressure can contain the boosted mixture??

 

[Luth]

Maybe you could start with a 4 stroke engine, then convert it to 2 stroke by replacing the camshaft. That will have an oil pump, without pressure in the sump.

Suppose the lubricate is done with an electric pump. The problem is what contain the fuel mixture?

 

[Baluncore]

Suppose the lubricate is done with an electric pump. The problem is what contain the fuel mixture?

When a 4 stroke engine is converted to 2 stroke, the cylinders are used at twice the rate, so it has an improved power to weight ratio, but with higher emissions. A 4 stroke engine has separate inlet and exhaust manifolds so it can be easily turbocharged.

For inducted air-fuel mix engines the maximum compression ratio is limited by the octane rating of the fuel. The effective compression ratio is the product of the cylinder compression ratio, and the air pressure boost ratio. Most engines without superchargers are designed for optimum compression ratio, so there is little room for improvement. You might need to change from gasoline to ethanol for a higher compression ratio.

Fuel injection overcomes the compression limit. There have been many fuel injected two stroke diesels with superchargers.

 

[ChemAir]

But it didn't go into specific detail like I said.

I wouldn't expect it to, unfortunately. If I'd spent lots of time and money learning how to make something complicated work, I'd be unwilling to give out all my information for free.

I still say you will have to do a great deal of testing to get something working well, unless you can contact any of the people that have done it (and documented it) to see if they might give you some hints to get started. You may have to pay some one for consultation time.

A couple of thoughts just looking at the pictures (if the engines pictured actually run):
-As I'd expect, there seems to be a need for some more complicated cross section of piping between the cylinder and the turbo, but in some, it appears to look a lot like a typical motorcycle muffler. The appearance of what looks like a motorcycle muffler makes me think parts were re-used for economy. I don't know that this is the result of a calculated design as much as something that was put together trial-and-error.
-The outboard motor is more interesting. The diaphragm valve on the turbo outlet looks to be an exhaust bleed. Without knowing the internal plumbing, it is difficult to tell exactly what they are doing but I suspect the turbo has artificially high back pressure that can be changed by bleeding exhaust or reducing the turbo outlet pressure resistance in some way, or it may have unusually low back pressure and the valve can increase it.

If I were doing this, I'd choose EFI. Spark and fuel are considerably easier to manage with electronics. With four strokes, I've seen blow through carb users spend more than EFI costs to get their carburetor running right, sometimes breaking very expensive parts in the process. EFI will typically give you additional abilities, including wastegate/boost control, datalogging, etc..

My big concern with boosted carburetion is that it is a technical handful and not terribly reliable compared to EFI on a 4 stroke, at least in drag racing applications. Quick throttle position or load changes will require expert carburetor tuning for enrichment, bleeds, restrictors, cams, etc...

I'd look carefully at the options for variable exhaust pressure rather than designing the piping to do what you want. The piping design case will likely only perform right in a very narrow flow regime (RPM band).

In race cars, the transmission, gearing, and transmission plumbing can all have profound effects on how the engine behaves. Getting a turbo engine to crank and run is one thing. Getting it to run under the load you have is another, and it will be more difficult.

 

[Lnewqban]

The turbo-charged single cylinder two-stroke aluminium cylinder block, liquid cooled 49.7 cc (3.03 cu in) Motori-Minarelli AM6 engine shown in one of the OP's pictures in action an record:

https://www.southbayriders.com/forums/threads/117415/

 

[Luth]

Yes it's what I mean. The mistery is that thing seems work! But if we look the 2 stroke working process seems impossible. The return wave in the exhaust can put back the fresh charge expelled from the cylinder but if this fresh charge is boosted also how can it work? The gif picture is exactly the same engine principle of what I said. You can see carb Reeds and the ports open at the same time. Imagine to put a turbo at the outlet of the pipe.

Anyone can explain how a turbo can work on this?

 

[Tom.G]

The turbo has enough inertia so it keeps spinning/compressing between exhaust pulses. Also, whatever volume between the turbo and the carb will remain pressurized.

 

[bluechipx]

Luth, visualize putting the entire engine in a pressurized chamber that has twice atmospheric pressure. It would produce twice the HP. That is basicly what the turbo is doing at 14.7 lbs of boost.

 

[Baluncore]

Also, whatever volume between the turbo and the carb will remain pressurized.

Therein lies a problem. Raising the exhaust pressure will raise the inlet pressure.

To get more power from a two stroke you need more air to carry more fuel into the cylinder. One way to do that is to provide a higher air pressure through the carburettor into the crankcase. The maximum usable boost will be limited by pre-ignition, which will be more of a problem with higher pressure in the exhaust.

Should you run a turbocharger from the exhaust, or a supercharger from the output shaft? A supercharger driven from the output shaft would provide a speed independent boost. A turbocharger would tend to block the exhaust flow and increase back pressure. A turbo would under-perform at low RPM, and over-perform at high RPM.

So why run a turbo when you could run a blower?

 

[Baluncore]

Luth, visualize putting the entire engine in a pressurized chamber that has twice atmospheric pressure. It would produce twice the HP.

That would double the cylinder pressure, causing pre-ignition. The spark would not be needed since the fuel would ignite during compression, well BTDC. That would probably destroy the connecting rod, piston and bearings. It would also provide 14.7 psi of back pressure on the exhaust.

 

[Luth]

Luth, visualize putting the entire engine in a pressurized chamber that has twice atmospheric pressure. It would produce twice the HP. That is basicly what the turbo is doing at 14.7 lbs of boost.

Hi, so you think that this set up

can do more hp than the same naturally aspirated?

 

[Luth]

That would double the cylinder pressure, causing pre-ignition. The spark would not be needed since the fuel would ignite during compression, well BTDC. That would probably destroy the connecting rod, piston and bearings. It would also provide 14.7 psi of back pressure on the exhaust.

I this case a (if really works) lower compression ratio its mandatory...I think

 

[bluechipx]

Baluncore , your question about 'why run a turbo when you could run a blower' is a strong indication that you haven't quite had the 'eureka' moment yet about turbocharging two strokes. A blower would do exactly what everyone thinks pressure charging would do, simply blow through the open exhaust port with no pressure gain. I suppose a blower would work if it were coupled with a valve after the exhaust port to hold back the charge and actually create a pressurized cylinder. But why go through the trouble when a turbo does it naturally? I would think on a four stroke that any exhaust pressure created by a turbo would be detrimental, but on a two stoke it is absolutely needed. As far as preignition, the outboard I turbo'ed could safely run 15 lbs of boost on 100 av gas, at 17 lbs or more it would experience detonation.

Luth, what motor are you planning to turbo charge? There are turbos available that are self oiling for two strokes. Pretty simple to do. Matching the right size turbo to your engine might require a little research. The snowmobile I have that has seen 35 lbs of boost had turnbuckles instead of springs on the expansion chamber at both ends to deal with the high chamber pressure. If you get a reasonably close match as far as turbo size and use a draw through carb system, I would think your first trial will produce way more hp than n/a, mine was doubled the first time I started it on the Stuska dyno.

 

[Luth]

Baluncore , your question about 'why run a turbo when you could run a blower' is a strong indication that you haven't quite had the 'eureka' moment yet about turbocharging two strokes. A blower would do exactly what everyone thinks pressure charging would do, simply blow through the open exhaust port with no pressure gain. I suppose a blower would work if it were coupled with a valve after the exhaust port to hold back the charge and actually create a pressurized cylinder. But why go through the trouble when a turbo does it naturally? I would think on a four stroke that any exhaust pressure created by a turbo would be detrimental, but on a two stoke it is absolutely needed. As far as preignition, the outboard I turbo'ed could safely run 15 lbs of boost on 100 av gas, at 17 lbs or more it would experience detonation.

Luth, what motor are you planning to turbo charge? There are turbos available that are self oiling for two strokes. Pretty simple to do. Matching the right size turbo to your engine might require a little research. The snowmobile I have that has seen 35 lbs of boost had turnbuckles instead of springs on the expansion chamber at both ends to deal with the high chamber pressure. If you get a reasonably close match as far as turbo size and use a draw through carb system, I would think your first trial will produce way more hp than n/a, mine was doubled the first time I started it on the Stuska dyno.

Wow.. Amazing so you have a direct experience on turbo 2 stroke?

I want turbocharge a small 50cc 2 stroke just for understand and learn something about it. The project I want to do is similar to the image I've attached in the previous message.

You think it can work?
Draw through, carbureted , 50cc,two stroke, small turbo to the outlet, gas powered.

Luth

 

[Luth]

But if is possible why are so rarely or practically inexistent in the motorcycle's world?

 

[bluechipx]

My outboard was a relatively low hp per cc engine, 44 ci or 720 cc and it developed 40 hp, but it had a very wide power band with torque at low rpm's. A propeller is quite a load at idle and higher performance engines wouldn't even start unless a couple of pit crew lifted the prop out of the water while a third person started the engine.

On a hydroplane the 40 hp would push the hydro up to 80 mph on a good day. Compare that to a jet ski I have that has 300 hp and barely makes it to 80 on a good day. The turbo easily had double the 40 hp and still had acceleration in the 90's but over a hundred it would began to get very unstable.

I'm not sure what is available as far as turbo's for a 50 cc engine. A turbo with an oil pressure system can be used with a little modification, but it is not as easy as my Aerodyne turbo's with precision bearings and a small oil reservoir and a wick oiler.

I have done many projects for hp gains, nitrous, methanol, expansion chambers and such but have NEVER had a project that greatly exceeded all expectations I may have had. On my very first try as I was pulling back to shore, my friends stated "it must have worked, he has an ear to ear smile".

As far as two stroke tubos being slow to catch on, I think the concept that most of us had about them simply blowing through the exhaust port and not working, slowed us down from trying them. For several decades I assumed they wouldn't work and never took them seriously myself until I attended a grass drag and saw a turbo snowmobile and asked the guy what major changes he had to make to get it to work. He said the engine was unchanged, to which I replied "it won't work" and he asked if I just got there to the races. He told me to go to the starting line in ten minutes and watch.

He was running the unlimited cc open fuel class against extremely fast machines and made it all the way to the finals where he was running against a 1300 cc sled with his 700 cc turbo. Running 35 lbs of boost he would only get about 3-4 runs before destroying a crankshaft. A broken crank put him in second place that day. He stated that every race he had with the turbo he would be catching up to his competitor near the end.

Here's a good one, a friend with a trail looking snowmobile that was turbo'ed went to a national drag race on ice for unlimited sleds and he would get along side of them as they were returning on the return road back to the pits and nod for them to hit it. They assumed it was a nothing trail sled that just wanted to see a super drag sled go. When they both hit it, he would pull away from them and they couldn't believe a trail sled could possibly beat their unlimited drag machines. They would find him and insist on looking under the hood to see what he had and get their first look at a turbo two stroke.

 

[Luth]

My outboard was a relatively low hp per cc engine, 44 ci or 720 cc and it developed 40 hp, but it had a very wide power band with torque at low rpm's. A propeller is quite a load at idle and higher performance engines wouldn't even start unless a couple of pit crew lifted the prop out of the water while a third person started the engine. On a hydroplane the 40 hp would push the hydro up to 80 mph on a good day. Compare that to a jet ski I have that has 300 hp and barely makes it to 80 on a good day. The turbo easily had double the 40 hp and still had acceleration in the 90's but over a hundred it would began to get very unstable. I'm not sure what is available as far as turbo's for a 50 cc engine. A turbo with an oil pressure system can be used with a little modification, but it is not as easy as my Aerodyne turbo's with precision bearings and a small oil reservoir and a wick oiler. I have done many projects for hp gains, nitrous, methanol, expansion chambers and such but have NEVER had a project that greatly exceeded all expectations I may have had. On my very first try as I was pulling back to shore, my friends stated "it must have worked, he has an ear to ear smile". As far as two stroke tubos being slow to catch on, I think the concept that most of us had about them simply blowing through the exhaust port and not working, slowed us down from trying them. For several decades I assumed they wouldn't work and never took them seriously myself until I attended a grass drag and saw a turbo snowmobile and asked the guy what major changes he had to make to get it to work. He said the engine was unchanged, to which I replied "it won't work" and he asked if I just got there to the races. He told me to go to the starting line in ten minutes and watch. He was running the unlimited cc open fuel class against extremely fast machines and made it all the way to the finals where he was running against a 1300 cc sled with his 700 cc turbo. Running 35 lbs of boost he would only get about 3-4 runs before destroying a crankshaft. A broken crank put him in second place that day. He stated that every race he had with the turbo he would be catching up to his competitor near the end. Here's a good one, a friend with a trail looking snowmobile that was turbo'ed went to a national drag race on ice for unlimited sleds and he would get along side of them as they were returning on the return road back to the pits and nod for them to hit it. They assumed it was a nothing trail sled that just wanted to see a super drag sled go. When they both hit it, he would pull away from them and they couldn't believe a trail sled could possibly beat their unlimited drag machines. They would find him and insist on looking under the hood to see what he had and get their first look at a turbo two stroke.

Hi bluechipx, thanks for your answer, definitely you confirm me that it can work?

Small turbo are available for 100cc 4 stroke I could use that with a sparate electric pump oiling system.

So, seems there is a good hp gain but it's a very rarely set up. Looked around I ve seen many banshee guys That have divergent opinion about. Some of them say "I've try it and without turbo going faster" other "spooled up quickly it's a beast" I'm so confused!

Theorically people say can't work but
Phisically should be work??

What's the truth??
Luth

 

[bluechipx]

Luth, a tuning must is pressure gauges in several places, if the guys that tried turboing that said it didn't work simply stated pressures in three places, it would be simple to see where they went wrong. Here are the places, one, in the intake after the turbo where 4 stroke guys check boost, two, in the exhaust system after the exhaust port and before the turbo, three, the crankcase (with a one way valve to check the high side of the intake pulses). You would be guessing to try a 100 cc four stroke turbo on a 50 cc two stroke, but if they are cheap enough and you don't mind the work of fabricating, it would be a good guess. Are there any other choices for small turbos?

 

[Luth]

Hi bluechipx, thanks for your explanation, mine is just a test to learn something about it.

Anyway the smallest turbo on the market is that I said.
But I think it's not a problem.
The quantity of exhaust flow produced by a 50cc 2 stroke is about the same or a little bit more than a 100cc 4 stroke...Maybe...
For the oiling, I can use an electric pump. But before to start and buy some parts
I need the securities that this set up can work:

50cc ported piston 2 stroke (intake direct into the cylinder) gasoline powered

Carburetor on the turbo (draw through)

Lower compression to prevent detonation

Turbo after the expansion chamber.

Stock engine produce 5 hp

With the properly settings (right jet, right expansion chamber dimension) can it produce more than 5hp??

If yes I'm really curious! Why I never seen one of them aeound??
Why is so uncommonly??
All very strange.

Your outboard produces 40hp but what was the stock power?

Thanks again
Luth

 

[bluechipx]

A common four cylinder Mercury outboard motor produced for decades starting in the 50's advertised as 50hp were 44 cu in and produced a surprisingly low 40-41 hp at the engine, before gear box. This is a freshly rebuilt, stock exhaust motor. The very first time I started it on the dyno, it showed 85hp with the turbo. Pretty much simply removing the stock exhaust cover and putting an aluminum plate with the turbo on it and plumbing the intake side with a draw through carb. The Merc was 720cc and I figured due to it's low hp, I used a turbo model for 500cc snowmobiles instead of the 700cc model. Later I tried the 700cc model only to find the boost limited to 7-8 psi whereas the smaller model would go over 20 psi and very quickly. Both werer Aerodyne variable vane models.

 

[Luth]

A common four cylinder Mercury outboard motor produced for decades starting in the 50's advertised as 50hp were 44 cu in and produced a surprisingly low 40-41 hp at the engine, before gear box. This is a freshly rebuilt, stock exhaust motor. The very first time I started it on the dyno, it showed 85hp with the turbo. Pretty much simply removing the stock exhaust cover and putting an aluminum plate with the turbo on it and plumbing the intake side with a draw through carb. The Merc was 720cc and I figured due to it's low hp, I used a turbo model for 500cc snowmobiles instead of the 700cc model. Later I tried the 700cc model only to find the boost limited to 7-8 psi whereas the smaller model would go over 20 psi and very quickly. Both werer Aerodyne variable vane models.

Wow it showed 45hp over the stock!
The intake design was a ported piston or rotary valve?

 

[bluechipx]

It was a reed valve with steel reeds and the one downside I saw was short reed life. The reeds were cracked and the ends were chipped and the indication was the engine began to start hard. On the Merc the reeds are in the heart of the engine, very hard to get at, so my new engine has a reed setup that is easily accessable and phenolic reeds are used. This a pic of the crankcase I made from a block of 7075 alum, notice the reed setup in the back ground.

 

[bluechipx]

Here's a shot of the assembled Merc with the Aerodyne turbo.

 

[bluechipx]

Another view

 

[bluechipx]

Here's the turbo

 

[bluechipx]

Here is the unique way the Aerodyne controls boost, from the outside, the ring of vanes can be changed to direct exhaust gases towards the outside of the driven turbine to increase boost.

 

[Luth]

Hi bluechipx, awesome pictures! Extremely interesting set up!
In that engine there is not an expansion chamber turbo is directly placed on a cylinders common exhaust manifold? So in this case the resonance wave is not used like in a tuned pipe. It's correct?

Luth

 

[bluechipx]

No tuned exhaust on this one, very broad torque curve from idle to max rpm. Here is a pic of a tuned exhaust expansion chamber with a turbo.

 

[Luth]

This is not your sled correct?
You think there is a big difference between the two set up?

Manifolf>turbo
Manifold>Tuned pipe>turbo

??

Luth

 

[bluechipx]

I own the sled in the pic, not me driving though. This was a blow through carb set up that seemed to work but on my outboard it never worked out for some reason. Draw through is real easy, carbs are sensitive to the pressure of blow through. You either have to pressurize the entire carb with the boost pressure or pressurize the top of the float bowls through the top air vents. A tiny change in external carb pressure means a lot of fuel mix change. The area that I choose to get the boost pressure to feed the float bowls had a simple tube installed in part of the intake that had boost. Depending on the slight angle the end of the tube was cut at and by rotating the tube would cause a big variation in jetting, if I am clear in explaining it.

 

[Luth]

I own the sled in the pic, not me driving though. This was a blow through carb set up that seemed to work but on my outboard it never worked out for some reason. Draw through is real easy, carbs are sensitive to the pressure of blow through. You either have to pressurize the entire carb with the boost pressure or pressurize the top of the float bowls through the top air vents. A tiny change in external carb pressure means a lot of fuel mix change. The area that I choose to get the boost pressure to feed the float bowls had a simple tube installed in part of the intake that had boost. Depending on the slight angle the end of the tube was cut at and by rotating the tube would cause a big variation in jetting, if I am clear in explaining it.

Yes understand, thanks again for the explanation. I think we can find the same problem of the carb position blow or draw in the 4 stroke carbureted turbo too.

Why blow through never worked on the boat engine? Humidity in the air during the the working of the mercury play a problematic role in the carburetion? Maybe?

Luth

 

[bluechipx]

My possible guess as to why draw through seemed to make more hp was the charge was drawn through the turbo which is rotating at a very high rpm and possibly atomised the fuel air mix better? Also with blow through the carbs are in the original location, close to the engine whereas draw through the fuel/air mixing begans far from the engine, in my outboard case nearly 36 inches away. Poor mixing at idle but higher rpm, much better.