Fuel delivery

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Fuel delivery

Postby turboshaft » Sun Oct 17, 2010 1:28 pm

Hello everybody , I have just discovered model engineering , and i am working on designing a ~1cc two stroke single engine compression ignited diesel engine.

I need some help creating the fuel delivery system and i was wondering if someone could share a basic venturi and needle valve design along with its principles of operation. How come there doesn't need to be a fuel pump?

Cheers and looking forward to hear from you guys.
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Postby chiefss » Sun Oct 17, 2010 3:24 pm

It depends on how you are going to induct fuel. Front induction, rear induction, reed valve, side port, etc. Take a look at any model airplane engine to see what most venturis look like.

They all operate the same way on Bernoulli's Principal. As a liquid (air) enters a smaller diameter pipe or some kind of restriction it must increase its speed. As velocity increases, pressure decreases and atmospheric pressure forces the fuel into the venturi through a small hole.
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Postby turboshaft » Mon Oct 18, 2010 12:59 pm

sorry i forgot to mention that , it's a sideport intake.

from what i understood, the needle valve and venturi uses the same principle as a carb , but i still can't figure it out how it will work without a fuel pump


unfortunately i don't have any model engine to look at at this time

cheers
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Postby dedaddy007 » Mon Oct 18, 2010 2:04 pm

On small diesel aircraft engines of this size there is no fuel pump. I think you may be looking at larger diesel engines that have fuel injection. On these size engines the fuel, oil and air are mixed together in the venturi and is ignited by the heat of compression in the cylinder. The fuel tank is kept at the same level as the venturi since it is venturi action that causes the fuel draw.

Hope this helps.
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Postby chiefss » Mon Oct 18, 2010 3:06 pm

Like I said, it works on Bernoulli's principal. As velocity increases through the venturi, the pressure decreases allowing atmospheric pressure to push the fuel into the venturi. Also called the venturi effect. A carburettor works the same way as it is a venturi.

here try this. It's just one a dozens online

http://en.wikipedia.org/wiki/Bernoulli%27s_principle

or this:

http://www.spiritus-temporis.com/venturi-effect/

It's a very simple concept.
Last edited by chiefss on Mon Oct 18, 2010 3:11 pm, edited 1 time in total.
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Postby chiefss » Mon Oct 18, 2010 3:07 pm

The first gasoline engines didn't have fuel pumps. They either worked with gravity feed of the venturi effect.
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Postby turboshaft » Mon Oct 18, 2010 3:51 pm

thank you very much guys.
now there's a couple more things i need to know

on a crankcase compression diesel engine is the diesel alone enough for lubrication ? will the absence of a fuel pump cause engine starvation during more aggressive maneuvers?

and does it make any sense to modify cylinder crown in order to improve cross scavenging on such a small engine ?
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Postby Frank Klenk » Mon Oct 18, 2010 5:29 pm

turbo

Yes, the fuel will provide the lubrication the engine needs. Typically on the smaller engines we use a mixture of equal parts of castor oil, fuel and ether.
The engine will run just fine without a fuel pump. Remember there is a venturi effect taking place inside the carburetor or if non carbureted just below the needle valve. Typically inlet openings are smaller for a diesel as it uses less fuel and hence better fuel draw.
Use schnuerle scavenging.

Frank
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Postby chiefss » Mon Oct 18, 2010 8:34 pm

There are no model diesel engines that use a fuel pump. The venturi effect is more than enough.

Only a few Glow engines use a pump and that's because the venturi is so big fuel draw becomes a problem. One version of the OS 120 Surpass even had a Root's blower supercharger on it but most have plain old venturi suction and work well. We all use muffler pressure now but it's really not needed.

When I flew C/L combat as a kid we would open the venturi a huge amount then use crankcase pressure to assist the venturi effect. Bigger and bigger venturis and we went to baby pacifiers and pen bladders to supply the pressure.
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Postby turboshaft » Tue Oct 19, 2010 10:38 am

I'm not quite familiar with schnuerle scavenging. Is it the same as the loop scavenging? and can it be done with a flat piston?

I am also looking at some methods of altering compression ratio without using a screw because I'm not happy with it hanging in the combustion chamber.

What do you mean by muffler pressure ?
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Postby chiefss » Tue Oct 19, 2010 6:09 pm

The contra piston does not HANG in the combustion chamber. It is the TOP of the combustion chamber.

Muffler pressure is just that. Pressure taken off the muffler via a small pressure fitting.

Again, look up schneurle porting online. There are many good sites with descriptions of it. Of course it uses a flat piston. That was the whole point.
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Postby Frank Klenk » Tue Oct 19, 2010 6:15 pm

Basically schneurle porting encourages flow in a loop, it is termed "loop scavenging" but that name was dropped.
Apart from large diesels with separate superchargers, two-stroke engines are generally piston-ported and use their crankcase beneath the piston for compression. The cylinder has a transfer port (inlet from crankcase to combustion chamber) and an exhaust port cut into it. These are opened together, as the piston moves downwards past them.
The simplest arrangement is a single transfer and single exhaust port, opposite each other. This "cross scavenging" performs poorly, as there is tendency for the flow to pass from the inlet directly to the exhaust, wasting some of the fuel mixture and also poorly scavenging the upper part of the chamber. Before Schnuerle porting, a deflector on top of the piston was used to direct the gas flow from the transfer port upwards, in a U-shapped loop around the combustion chamber roof and then down and out through the exhaust port. Apart from the gas flow never quite following this ideal path and tending to mix instead, this also gave a poorly shaped combustion chamber with long, thin flame paths.

In 1926, the German engineer Adolf Schnürle developed the system of ports that bears his name. The ports were relocated to both be on the same side of the cylinder, with the transfer port being split into two angled ports, one on either side of the exhaust port. A deflector piston was no longer required. The gas flow was now a circular loop, flowing in and across the piston crown from the transfer ports, up and around the combustion chamber and then out through the exhaust port.[1]

With Schnuerle porting, the piston crown may be of any shape, even bowl shaped. This permits a far better combustion chamber shape and flame path, giving better combustion, particularly at high speeds.

Your compression screw will not hang in the cylinder. Rather the screw will push a contra piston into the cylinder. It is a tight fitting disc at the top of the cylinder that can be moved up or down with the screw. This changes the compression as needed.
Muffler presure is achieved by installing a small fitting in the muffler. Connect a hose to it and run this to your fuel tank and it will pressurize the tank slightly. It works very well.

Frank
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Postby turboshaft » Wed Oct 20, 2010 10:38 am

Thank you so much guys , i'll start doing the CADs as soon as i have some spare time and keep you up to date.

Could you suggest some angles for the intake ports, how should they be oriented for the best flow ? Towards the centre of the combustion chamber maybe ? And is the exhaust port at the same level as the intake ports or it is higher like in regular designs ?



Also, I've seen that most two strokes exhaust manifolds have a plenum, what is an optimum plenum volume and exhaust manifold diameter for a 1cc engine ?
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Postby Jim Thomerson » Wed Oct 20, 2010 1:56 pm

Intake ports are generally just milled out of the cylinder wall, top and bottom parallel. The exhaust port isially opens before the intake port. Difference in timing is called "blowdown". I don't know what you mean by a plenum chamber. The performance of your engine is going to be limited by sideport induction timing. In the Indian Mills I have the intake port is simply a hole drilled in the cylinder wall.
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Postby chiefss » Wed Oct 20, 2010 5:39 pm

You are making this effort way to complicated.
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Postby turboshaft » Thu Oct 21, 2010 3:20 pm

Could any of you please share some plans ? The simpler the design the better.

I also need some help figuring out what the spray bar does.
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Postby chiefss » Thu Oct 21, 2010 4:02 pm

It sprays fuel into the low pressure created in the venturi. These are very basic questions. I think you should bone up on the operation of model engines which themselves are very simple machines. It appears you know nothing about them. That being so, making one is going to be impossible.
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Postby turboshaft » Thu Oct 21, 2010 4:23 pm

Well , that's why i am asking for help in the first place. If you could recommend some books and maybe a cheap engine i could buy and have a look at it i would be most grateful
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Postby Jim Thomerson » Thu Oct 21, 2010 5:47 pm

You need the Motor Boys plan book. http://modelenginenews.org/mbi/mbi_book.html

That will tell you everything you need to know. The Motor Boys are a group of about ten experts who make running replicas of various wierd old engines. They make them for their own enjoyment. Some projects are cooperative and some are individual. I've visted with one of them on several occasions and am always blown away by the engines he has made. He has a full machine shop and a foundry for making castings.
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Postby chiefss » Thu Oct 21, 2010 11:10 pm

To learn about model engines try Any of Clarence Lee's books or Dave Gierke's book on the two cycle model airplane engine. Harry Higley has some good books on the subject.

A two cycle model airplane engine is about as simple as it gets. A typical glow engine has about 4 moving parts, The crankshaft, piston, connecting rod, and wrist pin. There isn't a whole lot more to one. The crankcase, head, back cover, and the drive washer and prop nut and washer. A a bunch of machine screws.

If you want a cheap engine, just go to eBay and look for any of the old McCoy red head engines. They go for anywhere from 5 bucks up to around 40 for a nib 35. There are zillions of other engines on Ebay cheap.

I'll sell you a used one for 25 bucks. Guaranteed to run.
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Postby turboshaft » Mon Oct 25, 2010 4:47 pm

I've just started the CAD, I hope I'll manage to build the engine by the end of the year. It's just a rough design , some dimensions still need to be sorted. Tomorrow I will do the internals and fuel delivery.


Image

here's a section view for a better look at the ports and one of the transfer passages.


Image
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Postby turboshaft » Tue Nov 09, 2010 8:03 pm

I've been a bit lazy with the updates , anyway here are the latest CADs , I still need to sort the compression screw and the fuel delivery. I think I am going for a two needle rotating barrel carb since it allows quite precise tuning.

Image

Image


edit:

Is there a chance that the contra piston won't stay in place ? I am thinking that both during the compression and power strokes it will be pushed against the screw by the pressure. Correct me if I'm wrong.
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Postby chiefss » Wed Nov 10, 2010 12:07 pm

It's supposed to be against the screw. That's how you adjust the compression. I really think you should read up on how these things work.
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Postby turboshaft » Wed Nov 10, 2010 1:43 pm

I think you misunderstood , I was asking if it is possible for it to slide away from the screw.
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Postby chiefss » Wed Nov 10, 2010 11:05 pm

If made correctly, no.
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Postby Frank Klenk » Thu Nov 11, 2010 5:43 pm

Turbo

To expand on Bruce's comment. It needs to be a tight fit. Not a push fit but rather a tap it in fit. I had one that was a easy slide fit and it hammered like crazy when it ran.
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Postby turboshaft » Thu Nov 11, 2010 6:21 pm

Won't it be too hard to change the compression ratio then ? Can you still turn the screw by hand?
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Postby chiefss » Thu Nov 11, 2010 7:57 pm

I really think you should read up on the subject. Try Davis Diesel Development. They make diesel conversions for most modern glow engines. Very good description on how they work. Most of the new ones have a tight fitting O-ring on the contra piston.
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Postby Jim Thomerson » Fri Nov 12, 2010 8:38 am

I'm all for an O ring on the contrapiston. If the contrapiston is lapped, getting it just right is not easy. There has to be no leakage. The contrapiston has to not "freeze up" once it warms up. etc. etc. I've had several diesels with too tight contrpiston and carefully worked them with 600 sand paper until they fit right. Crude, but effective, if you take your time and be careful
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Postby turboshaft » Sat Nov 13, 2010 6:34 am

Thank you , i was thinking the same so I will go for an o ring as well.

One more thing now, how do you prevent increasing the compression ratio too much, i was thinking some sort of mechanical block for the tommy bar?

When the engine is warmed up and you start turning the compression screw , is there a way you can tell you've went to far ?(hopefully before ruining the engine)
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