Small injector

[Drei]

Hi,

Have anyone made an injector, like a diesel or petrol injector. I was thinking to fit one on a model engine but I thick it would be very difficult to built. I have an idea how a diesel injector wirks with a plunger etc. Maybe the plunger and bore wuldnt be so difficult but the injector itself is very complicated.

Andrei

 

[Jasonb]

This is a nice diesel model runs 150bar pump pressure with a solonoid activated injector, write up was in ME a while back

http://www.model-engineer.co.uk/albums/member_photo.asp?a=6778&p=123122

j

 

[agmachado]

Hello Guys,

I also have interest to in this subject... there is some literature about this ?

Thanks,

Alexandre

 

[LongRat]

After reading the only literature I have seen about model fuel injectors for internal combustion (the book by Hubert W. Schillings) I'm interested in this too. In his version, the injector is nothing but a nozzle with a check valve in the line behind it, fed by an engine-driven gear pump. The nozzles are open all the time as far as I can tell, and the fuel flow is simply a function of engine RPM. There is nothing in the book about the size and geometry of the nozzles. The basic method of constant, non-pulsed injection is similar to how I believe some early full-size mechanical fuel injection used to work.
One point here is that in a throttle-body injection system or intake port injection this would work fine, but injecting into the cylinder directly (as per diesel) would place a lot more thermal load on the injector and call for a different design I expect. Plus, the injector would have to be actuated and not flow 100% of the time. To me this sounds much more difficult!

 

[Stan]

There are two basic types of diesel injectors. One is just a nozzle which is fed by a high pressure injection pump. The other type, called unit injectors are a nozzle with a mechanical or electric driven piston behind the nozzle, and are fed by a low pressure pump

The nozzle type is relatively easy to make but the pump is difficult. Unit injectors are a complicated piece of work..

 

[agmachado]

Hi Guys,

My two cents...

In the inline injection pumps have a camshaft, which synchronized with the ignition point, moves a small pistons that pressurize the diesel into the cylinder at a pressure of 200 to 250 bar. Is this correct ?

So... I do not understand yet how these small pistons receive of the diesel, I know that there is a pump of low pressure and maybe this low pressure is sufficient to feed the high pressure pistons. The acceleration of the engine occurs controling the quantity of the diesel received in the high pressure pistons, the valve of the accelerator!

The nozzle needs to atomize the diesel fuel and support the compression ratio about 20:1.

Cheers,

Alexandre

 

[Stan]

Modern diesel engines have an injection pressure of 2000 bar or higher. The nozzle has many very small holes in very precise location to get optimum combustion.

Old style engines with unit injectors did have the pump operated by the camshaft. New style engines have the timing controlled by the computer so usually use electric operated injectors.

With a common rail supplying low pressure fuel to the injectors the volume of fuel is controlled by the position of the piston in the injector. I haven't seen the inside of the new electric injectors, but on the old style the piston was rotated so that more or less opening was in front of the inlet port. As a point of interest there is a high volume of fuel available at the inlet with excess used to cool the injector.

On a diesel engine the foot pedal only controls the engine governor or computer. It is up to the governor and the computer to inject enough fuel to maintain engine speed (as set by the foot pedal).

Building a model unit injector would be an enormously complicated project.

 

[cfellows]

There is a small amount of information about injectors and injection pumps on Find Hansen's web site. He builds model hot bulb engines which uses injectors. Nice engines, worth a look.

http://www.findsminimodelhotbulbengines.dk/Side1.html

Chuck

 

[mu38&Bg#]

I recently purchased the SIC set for "The Dux" four stroke injected diesel. The injector in this design is nothing more than a nozzle, with no valve at all. This is the designer's engine. http://goo.gl/Vn4Jm The pump design is simple and even allows for variable speed. I'd say it's very basic overall, but seems to have worked for him.

The real difficulty in a model size diesel is accurate pumping of minuscule amounts of fuel. A diesel may be 40:1 air:fuel at idle and approaches 15:1 at full load. This is by mass of course. So the system must be able to pump this tiny volume of fuel and the injector atomize it, in order for combustion to occur. The injector orifice must be very small. In the Dux, it's just a scratch between to mating parts, though the designer suggests it could be .07mm (.0028"). I think a hole of this size would be too large.

 

[Drei]

the modern diesel common rail operate at about 2000 bar as it was said. This will provide a very fine and atomized fuel to make sure that the injected fuel burns all and has an efficient explosion. the timing and pulsation needed is controlled by the ECU but the injector is opend and closed by means of a material called pizo electric material. This material reacts and moves when a voltage is applied and also vis versa when a pressure is applied a voltage is produce. This is the most advance thecnolgy of injecting fuel, which is also able to inject 6 times while a diesel piston is providing work (moving downwards after explosion) to make a more efficient burn and make sure that all the fuel is burnt rather then injecting all the fuel at once. This means that is very accurate and also extreamly fast.

Drei