Two cycle turbo diesel.

[barnesrickw]

If building a two cycle turbo diesle, to find the amount of air needed for charge can you take the displacement times the RPM to get the required CFM.


Sent from my iPad using Model Engines

 

[lohring]

Two strokes need around 1.5 times their displacement for scavenging. This isn't a problem with turbo diesels since fuel doesn't tend to go out the exhaust. I don't know of any turbochargers suitable for model sized engines. There have been some experiments with 50 cc spark ignition engines, but so far tuned pipes (a no moving parts turbocharger) are a better solution in small engines.

Lohring Miller

 

[barnesrickw]

Yes, a turbo charger would have to be made. Beyond my scope at this time, but I think a three cylinder Detroit Diesel would make a neat model for the more talented.


Sent from my iPad using Model Engines

 

[WOB]

Even if a turbocharger could be found, there would still be a need for a compressed air source to provide scavenge air for starting the engine.

Large, stationary 2-stroke diesels use conventional shaft driven Roots blowers to get started. After coming up to speed they use turbo chargers to provide the scavenge air and the Roots units are disengaged. The benefit is better fuel economy.

WOB

 

[rcfreak177]

One of these may work.

[ame]http://www.youtube.com/watch?v=2sIOlwQtlMo[/ame]

 

[barnesrickw]

I've looked at those. People are trying them on sealed crankcase two cycles. Not sure they work there, but directly into the cylinder. But then fuel injection may be needed. Just ideas il kicking around.


Sent from my iPad using Model Engines

 

[lohring]

Those superchargers are useless according to modelers who have tried them. A roots blower is not really a supercharger because it doesn't have any internal compression. A screw type compressor has internal compression and would work much better, However designing and cutting the helical lobes on these compressors to the required accuracy isn't trivial. I was involved in the design of roughing cutters for these compressors built bu Ingersoll Rand in the 1970s. The final profile was ground on their oil-less compressors.

Piston compressors would work better than any others at a small scale and are easier to make. The compressor piston can balance the working piston. DKW investigated a lot of these possibilities before World War II. Some are pictured below. The tuned pipe turned out to be a better solution since it used exhaust energy to both lower the cylinder pressure for scavenging and raise the pressure just before the exhaust port closes for supercharging. See the graph below. Modern engines with tuned pipes have very low crankcase compression. The crankcase pump only starts and idles the engine. The pipe provides the pressures above that. This system has given a 125 cc engine 54 hp.

Lohring Miller

 

[Mechanicboy]

If you are planning to increase horsepower out of the two stroke engine then the turbocharger is unuseful in the two stroke engine due the ports of inlet and exhaustport is still open when the turbocharger or blower is blowing the air out of exhaustport and a part into the cylinder. Then the inletport is closed by piston, the pressure in the cylinder is disappeared due the exhaustport is still open. Tuned pipe works best with gasoline- or methanol/nitropowered two stroke engine to increase horsepower. The tuned pipe will not work with the diesel two stroke engine, blower only. Also the turbocharger is a blower in the dieselengine such as in a large shipengine where the piston is not used as a compressor to act as a blower as a small two stroke will be.

 

[lohring]

If you are planning to increase horsepower out of the two stroke engine then the turbocharger is unuseful in the two stroke engine due the ports of inlet and exhaustport is still open when the turbocharger or blower is blowing the air out of exhaustport and a part into the cylinder. Then the inletport is closed by piston, the pressure in the cylinder is disappeared due the exhaustport is still open. Tuned pipe works best with gasoline- or methanol/nitropowered two stroke engine to increase horsepower. The tuned pipe will not work with the diesel two stroke engine, blower only. Also the turbocharger is a blower in the dieselengine such as in a large shipengine where the piston is not used as a compressor to act as a blower as a small two stroke will be.

That is definitely not true. Turbocharged simple two strokes raise the exhaust pressure along with the intake pressure. The four cylinder two stroke pictured below develops around 800 hp with turbocharging and nitrous oxide injection. The tuned pipe helps scavenging by varying the pressures as pictured before. The turbine restricts exhaust flow, raising the pressure that this pressure variation occurs at while the compressor raises the intake pressure even more. A very long time ago the Rolls-Royce Crecy used constant pressure supercharging with an exhaust power recovery turbine. The later Napier Nomad, a diesel, did the same thing. Both engines raised the exhaust back pressure with the turbine and the intake pressure with the compressor. Large marine two stroke diesels do the same thing today.

I see no reason the same tuned pipe pressures wouldn't be developed regardless of the fuel. There are no small two stroke diesels that I know of since a spark ignition engine in chain saw or motorcycle sizes will be a lot less expensive and lighter for the same power. Turbochargers are more compact and work better than tuned pipes for the large engines. Model "diesels" (actually compression ignition engines running on an air/ether kerosene mixture) could easily use tuned pipes where their racing rules would allow it.

Lohring Miller

 

[WOB]

Someone please explain how a tuned exhaust down stream of a turbocharger works. It seems to me that the turbocharger turbine chops the exhaust flow and essentially filters out the exhaust pressure variations so that there is no pulse to tune for. If not, and it doesn't completely level out the pressure pulses, it certainly must reduce their magnitude. It seems self evident that the ideal situation is max exhaust pressure at the turbine inlet and lowest possible pressure at the turbine outlet. Therefore, it seems a large dia. exhaust/tail pipe is ideal.

WOB

 

[dman]

Someone please explain how a tuned exhaust down stream of a turbocharger works. It seems to me that the turbocharger turbine chops the exhaust flow and essentially filters out the exhaust pressure variations so that there is no pulse to tune for. If not, and it doesn't completely level out the pressure pulses, it certainly must reduce their magnitude. It seems self evident that the ideal situation is max exhaust pressure at the turbine inlet and lowest possible pressure at the turbine outlet. Therefore, it seems a large dia. exhaust/tail pipe is ideal.

WOB

those tuned pipes are upstream. even if that's what you meant a tuned pipe doesn't work like a common collector header on a 4 stroke that uses velocity and inertia and negative reflections from and increasing pipe diameter to scavenge the cylinder and help it blow down as much as possible, also giving the intake a solid yank so that the intake can give a little positive pressure. it uses resonance and positive reflections from a reducing pipe diameter to pressurize the charge and increase density. with a turbo you are doing the same thing. the back pressure on the turbine helps with that pressurization, the positive reflections from the tuned pipe also help keep you from over scavenging the engine due to the possibility of higher intake pressure than exhaust pressure.

a more dense charge gives a bigger bang, hence more power but also more exhaust energy and a bigger reflection thus a denser charge and a bigger bang. maybe not the most efficient pump but a pretty effective way to make a big bang.

as far as those model turbos are concerned. well it depends on the exhaust geometry and the ability of the bearing to support high shaft speeds. i mean like 1 million rpm!. pressure from a centrifugal fan is proportional to the square of tip velocity (also depends on geometry, but if something is scaled in every dimension you can use this rule to see the differences in pressure or required speed) so scale really matters with these things!!!. the belt driven units are pure BS no matter how much the manufacturers will side step the issues and talk about exhuast tuning to explain the issue of the exhaust timing being longer than the intake. the fact is there is no way to mechanically drive a fan fast enough to make positive pressure at these scales in the first place.. as far as the exhaust driven units, it's feasible that they can work in the right situation but maybe unlikely that the available units actually do.

 

[barnesrickw]

A long time ago I used to be able to calculate proper pipe diameter and length for four cycle engines. It was a problem on a physics exam. I've never studied for a two cycle much, but it sounds fun. What little I know is the key seems to be in the expansion chamber. On the other, I wasn't really looking for anything high performance, just wondering about ways to get good scavenging without a charged crankcase.


Sent from my iPad using Model Engines

 

[barnesrickw]

And if the little turbos blow more than the suck, they might cavitate. Whole lot of shaking going on.


Sent from my iPad using Model Engines

 

[lohring]

If you are serious about really learning about turbomachinery, there is a series of lectures available on youtube. Essentially the hot exhaust gas expands from a higher pressure through the turbine to drive the compressor. The turbine is located at the end of the tuned pipe and takes the place of the stinger. Both of these restrict flow out of the pipe to increase the average pressure in the pipe. The diverging section of the pipe expands the exhaust gas to create a low pressure wave and the converging section of the pipe creates a high pressure wave. These waves arrive at the exhaust port at different times to create the pressure trace shown above. A more complete description of what happens is Tuned Pipe Design Factors.

Lohring Miller

 

[barnesrickw]

Thank you for the link. I like to learn stuff like that just to exercise my brain. It needs it.


Sent from my iPad using Model Engines