Model Diesel: 32mm bore, 38mm stroke, indirect injection

Hi everyone,

After Minh-Than built his diesel, I got interested in making my own. I've since gone full Nerd and gone down a very deep rabbit hole reading about diesel combustion systems, in particular the work Sir Harry Ricardo (who should be a personal hero of any model engine builder) did on getting small, high speed diesels to run well in the 1920s and 1930s. I'd like to run my engine on biodiesel or regular diesel fuel, and (potentially) use it to actually drive a load so we definitely need a good combustion system to avoid poor fuel consumption, excessive smoke, etc.

My plan to achieve this is to use a swirl chamber. This demands a high compression ratio: my design is planned to be around 23:1. I'll probably also need to preheat the induction air for cold starting (I can't fit both a glow plug and injector into the 10mm diameter swirl chamber). Fueling is planned to be by a bosch style jerk pump, which will be lubricated by the fuel itself so that we can tolerate some small leakage past the plunger.

Here's the current progress designing the guts of the engine.



Due to a goof-up on a customer job, I now possess a very large number of 20mm ID manganese bronze bearings, so they will be the mains. You can see the piston (three compression rings and one oil control) and conrod (fairly conventional) along with the heavy duty crankshaft and a stand-in for the camshaft and lifters. In the background is the injection pump housing.

Most of the work I've done so far was on the cylinder head. The counterflow intake design should help keep heat in the charge for smooth combustion and encourage swirl in the cylinder to aid the second phase of combustion outside of the swirl chamber.


The swirl chamber is formed by a hemispherical pocket in the head and a matching stainless steel insert that forms the lower part of the chamber and includes a tangential port connecting it to the cylinder. The Injector is of the inwards opening pintle type:

The barbed fitting on the top is a spill-over drain for fuel that escapes past the needle into the upper part of the injector. The needle seals to the bore via a delrin piston ring, and as rings have a gap there will always be some leakage.

I hope everyone's interested in the start of this big project! Much more design work remains to be done, along with prototyping fuel system components etc before starting the engine build itself. Wish me luck.

-Nerd

Nerd !!
A little bit of my experience
1 - When I tested my injector, it was the same as you designed it, it needed a greater amount of oil and pressure to compensate for the loss of oil and pressure through the injector body - this could also be due to I don't make injectors good enough - so consider this when you design your pump and stroke
2 - Make sure that the force of the needle spring is greater than the pressure in the cylinder, so you must measure the maximum pressure in the cylinder and the force required to open the injector (I try and fail , so it took longer)
Good luck !!!

My pump has a 4mm bore and 2mm effective stroke, so it should be able to supply a lot more fuel than we need to inject at full power. Hopefully this is enough to compensate for any leakage.

I've run some simulations in Lotus Engine Sim software to estimate the cylinder pressure. It should peak at around 100 bar during the power stroke. Rather high, but the nozzle is only 1mm in diameter so the force should not be too high. The injector spring is sized to have the needle lift at 100 bar on the 4mm needle diameter. Of course I'll need to do testing to make sure my maths isn't wrong haha.

Nerd,
its pretty easy to compute how much fuel it takes to combust all the air in the cylinder, and its a really small number, way smaller than 4mm diameter x 2mm stroke pump per cycle. Will you have a way to vary the pump. I've read about the Bosch type and its got an adjustable overflow port, kind of like a "sleeve valve" in some obsolete engines, which seems difficult to model.

may I ask out of total ignorance, why do you think you need a swirl chamber. I'm building a "Hansen" that doesn't have one.

yes, there is interest in what you're doing, keep us up to date !!!
Peter.

Nerd1000 said:

My pump has a 4mm bore and 2mm effective stroke, so it should be able to supply a lot more fuel than we need to inject at full power. Hopefully this is enough to compensate for any leakage.

I've run some simulations in Lotus Engine Sim software to estimate the cylinder pressure. It should peak at around 100 bar during the power stroke. Rather high, but the nozzle is only 1mm in diameter so the force should not be too high. The injector spring is sized to have the needle lift at 100 bar on the 4mm needle diameter. Of course I'll need to do testing to make sure my maths isn't wrong haha.

Click to expand...

Nerd1000...

Fuel pump with big bore and small stroke is much difficult to regulate the amount of fuel by regulator and operator. Better to have small bore and the stroke is regulated by regulator depending on revolution and load. Find Hansen diesel engine has 2 mm bore and the stroke is regulated.

1 mm hole nozzle?? The hole in the injector is much smaller than 1 mm in the 2 litre direct injection diesel engine, also the hole is from 0,2 to 0,02 mm +- 0,005 mm depending on the number of holes per nozzle. More number of holes per nozzle = smaller holes. In the model engine is not practical to drill smaller than 0,02 mm hole. The practice of using electro-discharge machining (EDM) drilling of fuel injection nozzles is limited in terms of the hole size it can produce effectively and the length of time needed to drill. Fuel injector nozzles drilling with laser-based machines is attractive alternative to elder methods.

Better to use "Burmeister and Wain" injector with valve outside the injector as you find in the Find Hansen diesel engine. It's much easier to make injector than drilling such small holes in the nozzle.

Unnecessary to have swirl camber in model diesel engines as compression is high enough to ignite the fuel. With a swirl chamber, you may have trouble getting high enough heat of compression to ignite the fuel in a small engine. With swirl camber you need glowplug to ignite the fuel under starting the engine when the engine is cold.

Mechanicboy said:

Nerd1000...

Fuel pump with big bore and small stroke is much difficult to regulate the amount of fuel by regulator and operator. Better to have small bore and the stroke is regulated by regulator depending on revolution and load. Find Hansen diesel engine has 2 mm bore and the stroke is regulated.

1 mm hole nozzle?? The hole in the injector is much smaller than 1 mm in the 2 litre direct injection diesel engine, also the hole is from 0,2 to 0,02 mm +- 0,005 mm depending on the number of holes per nozzle. More number of holes per nozzle = smaller holes. In the model engine is not practical to drill smaller than 0,02 mm hole. The practice of using electro-discharge machining (EDM) drilling of fuel injection nozzles is limited in terms of the hole size it can produce effectively and the length of time needed to drill. Fuel injector nozzles drilling with laser-based machines is attractive alternative to elder methods.

Better to use "Burmeister and Wain" injector with valve outside the injector as you find in the Find Hansen diesel engine. It's much easier to make injector than drilling such small holes in the nozzle.

Unnecessary to have swirl camber in model diesel engines as compression is high enough to ignite the fuel. With a swirl chamber, you may have trouble getting high enough heat of compression to ignite the fuel in a small engine. With swirl camber you need glowplug to ignite the fuel under starting the engine when the engine is cold.

Click to expand...

Very interesting information!!!!!!!!!!!!!!

Thank you (even though I'm just following the thread!!).

Here is how the nozzle is drilled with laser..

Mechanicboy said:

Nerd1000...

Fuel pump with big bore and small stroke is much difficult to regulate the amount of fuel by regulator and operator. Better to have small bore and the stroke is regulated by regulator depending on revolution and load. Find Hansen diesel engine has 2 mm bore and the stroke is regulated.

1 mm hole nozzle?? The hole in the injector is much smaller than 1 mm in the 2 litre direct injection diesel engine, also the hole is from 0,2 to 0,02 mm +- 0,005 mm depending on the number of holes per nozzle. More number of holes per nozzle = smaller holes. In the model engine is not practical to drill smaller than 0,02 mm hole. The practice of using electro-discharge machining (EDM) drilling of fuel injection nozzles is limited in terms of the hole size it can produce effectively and the length of time needed to drill. Fuel injector nozzles drilling with laser-based machines is attractive alternative to elder methods.

Better to use "Burmeister and Wain" injector with valve outside the injector as you find in the Find Hansen diesel engine. It's much easier to make injector than drilling such small holes in the nozzle.

Unnecessary to have swirl camber in model diesel engines as compression is high enough to ignite the fuel. With a swirl chamber, you may have trouble getting high enough heat of compression to ignite the fuel in a small engine. With swirl camber you need glowplug to ignite the fuel under starting the engine when the engine is cold.

Click to expand...

I should have been more clear, the injector is a pintle type with a centerbody in the hole and a tapered valve seat, so while the hole is 1mm the actual area for fuel to flow through is much less.

I'm not planning on regulating the plunger stroke, it will always be 3mm (the first mm is used covering the inlet port, so 2mm effective). Instead I want to rotate the plunger in the barrel, combined with an angled cut or 'helix' in the end of the plunger this should cause the inlet port to be uncovered again at a controllable point partway through the injection stroke, spilling the excess fuel back to the supply. This is how Bosch style jerk pumps work on full sized diesels. Of course if I can't make it work on this scale I'll have to switch to something else.

peterl95124 said:

Nerd,
its pretty easy to compute how much fuel it takes to combust all the air in the cylinder, and its a really small number, way smaller than 4mm diameter x 2mm stroke pump per cycle. Will you have a way to vary the pump. I've read about the Bosch type and its got an adjustable overflow port, kind of like a "sleeve valve" in some obsolete engines, which seems difficult to model.

may I ask out of total ignorance, why do you think you need a swirl chamber. I'm building a "Hansen" that doesn't have one.

yes, there is interest in what you're doing, keep us up to date !!!
Peter.

Click to expand...

Yes for my engine it is something like 2 microliters, far less than the pump should theoretically provide.

However, I have low expectations for the clearances in my pump, injector, etc. I expect them to leak, a lot! So I will need extra to compensate. This is also why I was planning on using the fuel to lubricate the pump.

Pintle injector is made for rocket engine,.. never in the diesel engine. After the fuel is injected, never fuel is dripping out of injector. The injector must be closed when the fuel is not injected into the cylinder.

Mechanicboy said:

Pintle injector is made for rocket engine,.. never in the diesel engine. After the fuel is injected, never fuel is dripping out of injector. The injector must be closed when the fuel is not injected into the cylinder.

Click to expand...

The Wiki page is incorrect. Pintle nozzles are common in IDI engines. Just google pintle type diesel injector and look around.

Nerd1000 said:

The Wiki page is incorrect. Pintle nozzles are common in IDI engines. Just google pintle type diesel injector and look around.

Click to expand...

Now, I remember, it's a very old construction to spread the fuel around inside the swirl camber. Allready gone out of production since the common rail diesel engine is most used in all diesel cars in Europe since 1995 as standard. (I'm car mechanic in 30 years work.) The other name on pintle injector is Pintaux injector with a extra hole in the other side of the hole.

The older engine has still pintle nozzles, but spare parts is difficult to buy today.

Nerd !!
1mm size worked with my engine so it should be fine with yours .

minh-thanh said:

Nerd !!
1mm size worked with my engine so it should be fine with yours .

Click to expand...

This is your injector with outside valve, true..?

I tried a lot, these are just the parts I keep, almost like Ithat put them in the trash


Remember one thing: I never say what I haven't done or tried,.
And I have never heard other people speak and repeat or rely on other people's experiences
Remember that

minh-thanh said:

I tried a lot, these are just the parts I keep, almost like Ithat put them in the trash
View attachment 139450

Remember one thing: I never say what I haven't done or tried,.
And I have never heard other people speak and repeat or rely on other people's experiences
Remember that

Click to expand...

Minh-Thanh,
My golly, I have so many collections just like that. Sad but true. It is soooo hard to throw that stuff out. Lots of failures before success. But something new is learned each time. Good work.
Lloyd

Hi Lloyd !

Lloyd-ss said:

Minh-Thanh,
Sad but true.
Lloyd

Click to expand...

There's nothing to be sad about
You and I make engines with what we have in hand, with machines, tools ... unprofessional (or rather too bad) compared to diesel companies.

so today I tried making a test-version of my injection pump barrel and plunger. The plunger helix came out well, as did the barrel with initial lapping. Sadly, I rushed ahead too far and began final lapping of the two parts together before getting the size close enough- the consequence was galling and seizure of the plunger in the barrel, ruining both. Lesson learned, and next time I may try hardening the barrel to minimise the potential for galling.

Nerd1000 said:

so today I tried making a test-version of my injection pump barrel and plunger. The plunger helix came out well, as did the barrel with initial lapping. Sadly, I rushed ahead too far and began final lapping of the two parts together before getting the size close enough- the consequence was galling and seizure of the plunger in the barrel, ruining both. Lesson learned, and next time I may try hardening the barrel to minimise the potential for seizing.

Click to expand...

Bummer. Its all a learning experience and sometimes the journey is the real prize of the project. How big is your reject box? Each item in there will have its own story.
Keep at it!