I sometimes sit and think about things I could turbocharge. Sportbike? Nah, I’d end up getting in trouble. Dirt bike? Nah, the thing is quite the handful already. But really, my dirt bike has a two-stroke engine. Is it possible to put a turbocharger on a two-stroke? Thinking about it, the intake and exhaust ports are exposed at the same time so it seems likely any extra air/fuel blown into the cylinder would just blow out the exhaust pipe decreasing efficiency. And it would likely throw the exhaust tuning off too, though that wouldn’t be a problem for a company with sufficient resources. So what’s the story on turbocharged two-strokes?
If you Google a bit you can find some one-off examples of folks fitting turbos to two-strokes. Google video has a Vespa with a turbo. You’re correct about the inherent problems of fitting a turbo to a two-stroke, but it appears to be possible.
Two stroke engines are more or less supercharged already. In motorcycle engines, the air/fuel mixture passes through the bottom end where it is pressurized to a few PSI when the piston drops. Smoother flywheels, solid or filled crankpins, tightly fitting crankcases, and/or faster moving reed valves all help increase the bottom end compression.
You might be able to increase power by adding a turbocharger, but I’d be careful not to add too much. The crankshaft seals are usually a press fit from the outside and could blow out. Cooling and detonation are also issues, more so than for four strokes. If heat isn’t well managed, the engine may tend to burn holes in pistons.
Large two stroke engines are normally supercharged rather than turbocharged.
A mechanically asperated engine does not normally need to use the piston to act as a air pump. So crank case seals are not a problem. The problem is in starting a turbo charged two cycle.
Adding a turbo or super charger to any two cycle engine is going to increase power at a cost of effency.
Every two stroke needs an induction pump of some sort. Simple ones use the crankcase, bigger and more efficient ones use roots blowers (blowers from detroit diesel twostrokes were the origional source for supercharging hotrods). And a few really big engines (trains and ship Diesels AFAIK have used turbocharers. The big problem is that you need somewhay to start it up and get the turbo working. Compressed air, and electric pre-spin motors are two ways of doing this.
To make forced induction work well in a two-stroke, you really need more complex exhaust valving than just the typical port uncovered by the piston. Reason being that such a port is still open when the bypass (or whatever the intake uses) port closes, so your intake pressure is only as high as the exhaust backpressure. (this is why lowering exhaust backpressure in normally aspirated two strokes normally reduces power) Two strokes with forced induction either use poppet exhaust valves in the head (which give good scavaging) or a rotary or other valve downstream of a typical exhaust port.
Another problem with turbocharging a simple two stroke is that there is no supply of pressuized oil waiting to be tapped.
The exhaust problem is turned to an advantage by a resonant exhaust system (expansion chamber) This is a form of forced induction and augmented scavaging based on accustic principals…works VERY well over narrowish rpm ranges.
This was very informative. Before making this thread, I searched the web for “turbocharged two-stroke gasoline” and similar strings but only turned up pages for two-stroke diesels. Afterwards, I got the idea to search for “turbocharged banshees” and found companies that offered kits. So what I’m gathering is it’s possible, but more difficult than with a four-stroke. If I wanted to do all the work myself, I’d be better off doing it on my sportbike or car.
Orbital have a system of seperate inlet and outlet valves, meaing that you don’t get the problem of having inlet and outlet open at the same time.
The trick on this engine is to have a pre-fuel air mixture/compressors, rather like a supercharger, some designs use a slave piston in its own cyclinder to achieve premix pressures - this is driven by the crank but no combustion takes place in it.
The premix is direct injected, but only once the outlet port is closed.This means that the piston is already on the up stroke so it has to be injected under pressure to overcome this.
These are very much more efficient engines, and produce very much less pollution, the crank does not sit in the fuel/lub oil mix either.
In many ways the orbital 2 stroke removes all the disadvantages of the 2 stroke, but delivers more power by weight than a 4 stroke.
Used on scooters made by Aprilia for the moment, this engine can scale up to over a litre and may find its way on to superbikes, its cleaner than 4 stroke, but it is not a simple as the conventional 2 stroke, this is a sort of halfway house between 2 and 4 stroke in terms of oil and injection systems.
All the big, two stroke diesels I’ve worked with (all GM products in EMD locomotives) have used some form of forced induction. This is apparently a necessity for ‘scavenging’ the combustion chamber between the exhaust and intake parts of the combustion cycle. The pressurised intake air helps blow out any remaining exhaust gasses. Earlier models use Rootes blowers, which are mechanically driven (off the camshaft, I think), and so produce pressure in relation to engine speed. Later versions use turbo charging, for increased power output and efficiency. The turbo wouldn’t produce enough air pressure for scavenging at lower engine speeds, so it is mechanically driven by some sort of an overriding clutch arrangement, up to the point where the exhaust gasses are sufficient, after which it free wheels. So the engine is mechanically supercharged up to a point, and turbo supercharged thereafter, which I thought was quite clever.
The large, four stoke diesels that seen have all been turbocharged (none of this tricky clutchy assistance business).
That is interesting. Thanks. I knew of some ships that were turo charged and their sister ships were supercharged. The turbo charged ships were more problematic.
These were double acting direct drive crosshead engines.
I’m pretty sure the very largest piston engines are supercharged two stroke engines, such as in marine power. These have nonoscillating piston rods and crossheads, and the spaces above and below each piston are sealed. The crankcase is not part of the fuel cycle and has ordinary oil in it. These things are bigger than houses.
They are both 2 and 4 stroke. Supercharged and turbo charged. direct drive two cycles with turbo are problematic. Now Diesel electric would be different.
I found this on the wed
http://www.bloggingwv.com/the-most-powerful-diesel-engine-in-the-world/
It is a 2 stroke and turbo charged direct drive. Although the drawing looks single acting.
You hit the nail on the head. The two major issues are the reed valves and loss of pressure thru the exhaust will the piston is at BDC. Reed valves will never be able to keep up with the pressures required for postive aspiration… I’ve heard of it being done on snowmobiles and stuff, but I’m willing to bet they were 4 cycles. Diesles have managed to do it, but the use a poppet valve, which to me is not a true two cycle.
I do have a plan that can resolve these complications but I need some help in order to create a working prototype to be tested. My plan involves the same physics of a normal two cycle, but with a way increase compression without MOST of the pressure loss from the exhaust at BDC. If you know of anyone that might like to help, I would be most generous.
Do not know how you can not call them 2 cycles. On ignition piston goes down Near BDC exhaust port comes open the intake valve comes open, piston reaches BDC. One stroke. Piston starts up exhaust port closes intake valve closes compression begins. Near TDC injection of fuel and as pistion gets to TDC ignition begins. Second stroke.
the simple fact that they have one power stroke per crankshaft revolution defines them as two-stroke. it doesn’t matter if they’re blower-scavenged or not.
we already do this with resonance-tuned exhaust pipes, like the one on my hydroplane here:
http://home.comcast.net/~jz78817/stuff/stinger.jpg
that little 11cc engine is rated at almost 5 hp with proper exhaust tuning.
I totally understand that number of strokes per power cycle dictate that they are two cycles, however I guess nostalgically to me a two cycle should use the crankcase as the intake manifold. With port fuel injection, you could conceivably use an oil pump just like normal four cycles.
My point overrall was the efficiency and high rpm’s gained from two cycles was due to not using poppet valves and using ports in the chamber.
But yes, you are right, I only meant that they weren’t like the “traditional” two-cycles.
I’m not entirely sure about how those work, but I would think that it would have a narrow power band and not as much torque as horsepower (ie. high acceleration instead of pulling power)
Correct me if I’m wrong. I think two cycles are the most efficient engines and really wanna know all about them.
I meant intake AND exhaust manifold.
If you are talking about fuel efficiency, they aren’t. If you are talking energy density (pounds per hp) they are pretty good.
You sure? I’ve disassembled several dirt bike and weed eater engines and I’ve never seen a two-stroke engine that used the crankcase in any way as an exhaust manifold. The crankcase causes secondary compression on the intake, but I see no reason to pass the exhaust through the crankcase, especially in the same chamber where the intake is.