Turbocharging carbureted petrol 2 stroke engines

[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.

 

[jack action]

Very interesting thread.

In my opinion, when you put a turbo, you change the pressure & temperature everywhere. Therefore a tuned exhaust is most likely untuned (detuned?) with the turbo. If it's really bad, then it won't work at all.

But it may give positive effects. For example, if the pressure wave doesn't come back at the "right" time, you can effectively lower the compression ratio.

The reason why the above engine without a tuned exhaust work is greatly due to the fact it uses a variable-geometry turbo. It allows to tune the restriction by modifying the back pressure in the exhaust outlet. Otherwise, you would have to match carefully the right turbo to the engine. Even then, it would probably not work very well outside a very narrow rpm range.

Supercharged diesel two-stroke engine were used on GM trucks and were apparently very good engines. To my knowledge all supercharged two-strokes used a valve to control the flow (uniflow scavenging).

 

[Luth]

Very interesting thread.

In my opinion, when you put a turbo, you change the pressure & temperature everywhere. Therefore a tuned exhaust is most likely untuned (detuned?) with the turbo. If it's really bad, then it won't work at all.

But it may give positive effects. For example, if the pressure wave doesn't come back at the "right" time, you can effectively lower the compression ratio.

The reason why the above engine without a tuned exhaust work is greatly due to the fact it uses a variable-geometry turbo. It allows to tune the restriction by modifying the back pressure in the exhaust outlet. Otherwise, you would have to match carefully the right turbo to the engine. Even then, it would probably not work very well outside a very narrow rpm range.

Supercharged diesel two-stroke engine were used on GM trucks and were apparently very good engines. To my knowledge all supercharged two-strokes used a valve to control the flow (uniflow scavenging).

View attachment 269395​

Hi, Jack action luckely another person interested! Me and bluechipx were ever alone!

Yes, it's a very interesting topic because online there is no documentation and the strange is that all the people say turbo on two stroke (gas engine) can't work because fuel and exhaust port are open at the same time but than very few people that try it like bluechipx that has a direct experience say can work and do horsepower. I hope in this thread we find "why can work". The mistery is deeper if we think that in the story a carbureted 2 stroke gas turbo engine in never factory produced.

Interesting the uniflow but there is used an exhaust valve that simplify the use of a turbo. But here speak about gas engine 2 stroke gasoline Reeds or piston port!

Luth

 

[jack action]

people say turbo on two stroke (gas engine) can't work because fuel and exhaust port are open at the same time

The best response to this is from @bluechipx in post #11. Yes, the pressure in the intake is higher and will tend to push the air through the exhaust port more easily, but the turbine creates a restriction that also make the exhaust pressure higher. When well balanced, the effects of both should cancel each other's out.

 

[bluechipx]

The best response to this is from @bluechipx in post #11. Yes, the pressure in the intake is higher and will tend to push the air through the exhaust port more easily, but the turbine creates a restriction that also make the exhaust pressure higher. When well balanced, the effects of both should cancel each other's out.

Jack, for possibly thirty years I "knew" turbocharging wouldn't work, it would simply vent all the possible pressure out the exhaust port. Then I saw it work with amazing results on the snowmobile I eventually bought, where the owner said there were no internal engine changes. I was with my wife and on the four hour trip home, I was unusually quiet, pondering how it could possibly work. Once you see it actually work, you have to start thinking differently and about halfway home it hit me, the simple concept that we all missed for decades and I almost had to pull the car to the side of the road to compose myself! I gave it a try and my first test was beyond my expectations. Oddly, it is so simple when you finally get it, but most of the time when I explain it to my racer-type friends the best I can, and ask 'now do you see' , most of the time they say they still they don't see why the charge doesn't just blow out the exhaust port. Another response when asked if they now understand, they sometimes reply"they sort of get it" which means they are embarressed to say they have no idea what you just explained. Well, anyway welcome to the 1% Jack!

 

[Luth]

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.

For what I have understood on four stroke or two stroke the Blow through set up is more complicated but can produces more hp if properly setted than the Draw through but this, the draw through is more easily to do.

Bluechipx had you problems to start the engine in the draw through config?

 

[Luth]

The best response to this is from @bluechipx in post #11. Yes, the pressure in the intake is higher and will tend to push the air through the exhaust port more easily, but the turbine creates a restriction that also make the exhaust pressure higher. When well balanced, the effects of both should cancel each other's out.

Yes but can the intake pressure generate from the turbo be the same of the exhaust?. It is no possible.
The exhaust pressure will be ever higher than the intake pressure because the petrol explosion cause other gasses.
So in the exhaust we have the explosion of the petrol gasses plus part of the boost blow off the exhaust port. Are we sure that the turbo hot side (plus the sonic wave if is used the expansion chamber pipe) can Re-put into the cylinder all (or just the necessary) of the fresh charge and increase hp??

Or I'm wrong?OT: this thread is awesome!

 

[jack action]

In the following figure I found on the Web, you have a compressor map and a turbine map:

Everybody is always focusing on the compressor map, but the turbine is important too. The pressure P1 (on the compressor map) is the inlet pressure of the compressor (usually the atmospheric pressure) and the pressure P4 (on the turbine map) is the outlet pressure of the turbine (also usually the atmospheric pressure).

The compressor can increase the pressure (P1 -> P2) by a factor of 3-4 (from 15 psi to 45-60 psi). But the turbine can also decrease the pressure (P3 -> P4) by a factor of 3-4. If the outlet turbine pressure is also 15 psi, then at the turbine inlet it will be at 45-60 psi as well. Note that the flat portion of the line is where the flow chokes in the turbine (more or less at an expansion ratio of 2.5 for this particular turbine).

Carefully selecting the correct turbine for the given compressor is critical. Hence why some set-ups may work and others don't.

 

[Baluncore]

I also find it interesting from an engine strategy viewpoint.

I believe the reason why two-stroke carburettor engines with turbos are not built is that it is more economic to design a motor with an appropriate compression ratio for the specified fuel and power. It seems the logic that justifies turbocharging a two stroke engine goes as follows.

You are operating in a class where engine capacity is regulated, but fuel is unspecified.

You start with a two stroke engine designed for low octane fuel, with an appropriately low compression ratio. You then run it on higher octane fuel.

You could increase the power marginally by increasing the compression ratio, say by modifying the head, gasket or piston crown. But that would not increase the mass of air and fuel inducted.

To get more power you need to increase the mass of air and fuel passing through the fixed volume crankcase pump. That requires a higher density charge be inducted to provide the increased air and fuel required. Both the inlet pressure and the outlet pressure must be raised, to the point just short of the onset of detonation.

You must forgo power lost due to the raised output pressure and temperature.
The bearings will be significantly overloaded.

I assumed you would tune the exhaust to block through-flow of fresh charge. But for multiple cylinders, or a broad range of RPM you must offer a reasonably constant exhaust back pressure.

I was considering what gives while controlling the engine. If the fuel charge from the carburettor is drawn through the compressor you cannot use a waste gate, unless you recirculate the air-fuel mix around the compressor only. At the same time you would need to maintain a controlled exhaust pressure.

Alternatively, if exhaust by-passing the turbine could be controlled, so as to maintain a set absolute exhaust manifold pressure, then the power would be less responsive to changes, as it would be regulated by turbo RPM.

It would be neat if you could use an engine block from a higher capacity engine, (to get bigger bearings and a greater crankcase pump displacement), with a standard piston and cylinder. But the geometry of the crankcase, connecting rod and the engine balance then conspire to require a complete redesign.

Before adding a turbo to a two-stroke carburettor engine, you need to work out how you might avoid it, or why it is necessary.

 

[bluechipx]

There is another plus when turbo'ing a two stroke. Before I installed the turbo, I put a small reed valve on the outside of the crankcase under a small cavity. This is the way outboard motors used to pressurize the fuel tanks to get around needing a fuel pump. I was curious how much crankcase pressuse a N/A motor had. The max pressure I saw was at full throttle and was 7 1/2 psi. On the dash of my hydro that was using the turbo, I had three pressure guages, boost after tubo, internal crankcase, and exhaust. At twelve pounds of turbo boost, you would expect the crankcase pressure to be 7 1/2 psi higher or 19 1/2 psi, but oddly the needle on my 25 psi gauge would be pegged. Because crankcase pressure wasn't a real concern, I never installed a gauge that could accurately check it.
Now here's something to give thought to, on a N/A two stroke, after the piston reaches BDC and begans it trip upwards, no vacuum can start until the piston clears the intake or transfer port and then slowly forms a vacuum under the piston. With a turbo I'm thinking the pressurizing of the crankcase can began much earlier, even before the piston reaches BDC? I will be testing an engine, the one pictured above, an engine that was built strongly that will be capable of much more boost soon and I will have the correct guages and I will explore the relationship of pressures in all three areas. The overly high crankcase pressure probably makes choosing the correct turbo a little less critical, maybe?

 

[bluechipx]

Baluncore, most engines that have been proven over time are easily capable of much more hp than they produce in stock form. My Mercury would run almost forever at stock 40 hp. I have seen over 100 and the failure was detonation which could have been remedied by a slightly higher octane fuel.

 

[bluechipx]

Luth asked;

you had problems to start the engine in the draw through config?
At first I was using a rope start on the outboard and due to the long path from carb to crankcase, it took a little experience to start at times. Going to electric start eliminated all starting issues.

 

[Baluncore]

I agree that for an arbitrary low compression engine, the power output can be increased by a turbo with a greater mass of higher octane fuel. But if an engine was designed from scratch, with an optimum capacity and compression ratio, then would it not produce more power from the same fuel as the turbo version, since lower exhaust pressure and temperature increases efficiency.

If that is the case, and you build a new two-stroke carburettor engine to operate with a turbo, you would do better by building a different engine and eliminating the turbo. Which suggests that only an engine capacity regulation can fundamentally justify a turbo.

 

[bluechipx]

I agree that for an arbitrary low compression engine, the power output can be increased by a turbo with a greater mass of higher octane fuel. But if an engine was designed from scratch, with an optimum capacity and compression ratio, then would it not produce more power from the same fuel as the turbo version, since lower exhaust pressure and temperature increases efficiency.

If that is the case, and you build a new two-stroke carburettor engine to operate with a turbo, you would do better by building a different engine and eliminating the turbo. Which suggests that only an engine capacity regulation can fundamentally justify a turbo.

I'm assuming you are talking about efficiency such as gas mileage? In the performance world getting the max hp out of a certain cu in engine is the bottom line. The ultimate example could be a top fueler that consumes ten gallons of fuel in a quarter mile. My 44 cu in engine at 14.7 lbs of boost behaves like a N/A 88 cu in. one.

 

[Baluncore]

My 44 cu in engine at 14.7 lbs of boost behaves like a N/A 88 cu in. one.

You seem to have confirmed that the turbo on a two-stroke carburettor engine is justified by the arbitrary engine capacity restriction.

 

[bluechipx]

You seem to have confirmed that the turbo on a two-stroke carburettor engine is justified by the arbitrary engine capacity restriction.

With increased engine capacity comes increased weight, I guess my experiences have been getting the maximum performance from the smallest, lightest engines and the concept of fuel efficiency has never been important.

 

[Luth]

Hi, please remain on topic, we speak about turbocharging 2 stroke gas engine carbureted. The primary question is if the picture set up can really do more hp than the naturally aspirated. Gas mileage is not important we speak only about hp gain between the same set up with and without a turbo.

 

[bluechipx]

Hi, please remain on topic, we speak about turbocharging 2 stroke gas engine carbureted. The primary question is if the picture set up can really do more hp than the naturally aspirated. Gas mileage is not important we speak only about hp gain between the same set up with and without a turbo. View attachment 269436

I'm with you all the way on this one, I was wondering where things were starting to go myself.

 

[Luth]

Online I have found a research(?) conducted by some Indian students on the 2 stroke turbo but there aren't many details.
In this research carb is blow trough and there is not an expansion chamber.
Is not the set up of the previous image I have attached but can help us to understand something.

Here the link of the research:

https://www.academia.edu/37091140/Turbocharged_2_Stroke_Single_Cylinder_98_2cc_Si_Engine

(but we're is the turbo oiling system in this research? )

IS this research valid?

 

[Luth]

I'm with you all the way on this one, I was wondering where things were starting to go myself.

Yes the question simple

For example:
Turbocharged 2 stroke (reed valve, or piston port) carbureted: 5hp

Turbocharged 2 stroke (reed valve, or piston port) carbureted: hp?? More or not??

Same engine same parts not turbo and with turbo (just carburetion setting properly for each set up)

 

[Luth]

We can understand this, this thread is really costruptive and interesting!Luth

 

[Tom.G]

...costruptive...

And obviously rather creative!

 

[Luth]

Anyone have seen the Indian research I've posted? What do you think about it?

 

[bluechipx]

Anyone have seen the Indian research I've posted? What do you think about it?

I read it and it looks like they are blowing through the original carbs. The picture wasn't clear enough to see if they had a way to pressurize the float bowls with boost pressure. If they weren't they would get bad results.

 

[Baluncore]

Anyone have seen the Indian research I've posted? What do you think about it?

Couldn't download it without giving access to my address book. So no go.

 

[Luth]

Couldn't download it without giving access to my address book. So no go.

Don't download it you can just watch it online. Download it is required only if you want do it!