Quantcast

Mini diesel engine.

Help Support HMEM:

Steamchick

Well-Known Member
Joined
Apr 2, 2020
Messages
750
Reaction score
265
Location
Sunderland , UK
Interesting to hear these stories of injector work. Thankyou all!
Minh Thanh: Well done on making an injector. Watching your video clip I saw a single spray from the nozzle - looked pretty good to me: the classic cone spray. But I think that a "real" injector will be operating normally at much higher pressures than your plastic pipe can manage. (>30bar? >2000psi?). Steel pipe on "real" high-speed diesel engines is used to withstand the pressure (pulse) developed by the injector pump - which in turn opens the valve in the injector and allows the spray. The pressure in the cylinder is at anything up to 23 bar when the injector operates, so you have to overcome this pressure before the spray can happen. Check you design calculations to be sure your injector can withstand such pressure.
But (from my poor memory of 50 years ago) when as an apprentice I was shown the job and tested a box full of used injectors (yeah, made me an "ex-spurt"!), the "spray" was more of a conical cloud of diesel oil. I was using a Lucas CAV Hand-pump tester for diesel injectors. A very simple device comprising a hand pump, pressure gauge, steel pipe-work, reservoir for fuel oil, and mounting to connect a single injector.
See this for a modern version: Beta Diesel Injector Test and Calibrating Hand Pump - 960PMC
1611250175248.png

The idea (keeping hands, eyes and clothes away from the spray) being to check with a single pump stroke that the spray was conical against a plate with a cone drawn on it, no drip from the injector, and the injector opening pressure exceeded some value on the gauge. Maybe Alex 1952 can enlarge on details as I am sure from his 40 years experience he knows the kit I used for about 1 hour.... But in that hour I was told that if I put my finger over the injector and pumped, the oil would inject through the skin into my blood stream and kill me. Or at least I would be very ill and probably loose the finger as the oil would kill the flesh... So Alex is exactly right with his advice.
Take care. Wear Protective goggles, to keep splashes from your eyes, have a fire extinguisher within reach as Oil-mist clouds are highly inflammable, ensure no ignition sources exist, and wear appropriate hand protection as the oil can cause dermatitis (bad skin). There is a similar safety warning on this page, which should teach you anything the contributors above have not already mentioned: Diesel Tractor Injector Testing and Overhaul
Enjoy!
K2
 

Steamchick

Well-Known Member
Joined
Apr 2, 2020
Messages
750
Reaction score
265
Location
Sunderland , UK
Minh Thanh:
In your drawing the spray hole is relatively long - which will encourage a jet of oil, not the shear action at the pressure change that encourages the spray to form... (Please research the dynamics of fluids at jets to understand things that I don't, as I may be completely wrong?).
1611251631852.png

BUT, the drawings of injectors (from the web) show very short exit holes, where the needle/pintle extends through the hole - which I understand is to assist the formation of shear between the moving oil and stationary air/metal and encourages the spray formation? - So please re-check your design of the nozzle against the accepted theory?
1611251897055.png

Thanks for generating an interesting topic!
K2
 

ALEX1952

Well-Known Member
Joined
Jan 17, 2018
Messages
86
Reaction score
22
Location
SCOTLAND
In practice the spray pattern is not allways conical, it is sometimes shaped using different sized holes and angles, more so laterly trying to satisfy emissions, again a reason for going to electronic control, however if you do manage to get a good spray of fuel at the right time and pressure with no drip you will have achieved something that many have not or would not attempt, me included, I had enough problems with all the right gear. I'm ashamed to say that if we did have some high smoke pumps and the destination didn't have tight emission control they would probably go depending upon how busy we were. Injection pipes also get very hot and are noisy, why I don't know.
 

Mechanicboy

Well-Known Member
Joined
Jul 10, 2013
Messages
746
Reaction score
245
Minh Thanh..

There is difference type of the injector: 1. Tap outside the hole as picture showed by Steamchick are used in vortex chamber diesel engine , 2. very small hole who are near unvisible for the eye for car diesel engine or a bit larger for ship engine after the injection needle valve in direct injection diesel engine or 3. conical valve outside the hole in direction diesel engine (older type injection nozzle Burmeister & Wain), Find Hansen used this principle to run his model diesel engine.

The common rail diesel engine has constant high pressure diesel pump and the injector is controlled by ECU (Electronic computer unit) also same pressure in both ends of injection needle: no injection and the pressure is disappeared from upper part of injection needle valve when the injection valve is lifted up by pressure: injection is in working while the tradidional injector is activated by fuel injection pump (powered by hydraulic principe to lift the injection valve needle).

The injector for hot bulb engine is difference from diesel engine: One way valve and the orifice similar Primus kerosene stove or adjustable spray nozzle to regulate from thin jet to atomized diesel fuel to control the heat of hot bulb to example full load the thin jet goes direct into the cylinder and under idling atomized diesel fuel to keep hot bulb warm.
 

minh-thanh

Well-Known Member
Joined
Dec 29, 2016
Messages
763
Reaction score
453
Location
Viet Nam
Hi All !
Thanks for the comments !

About the plastic tube: I just tested to see if my injector is okay ...
About the nozzle : when I made the injector, I just thought and tried to make it sealed, when the injector is ok ( i hope so ) I shortened the nozzle
About of injection pressure ... I thought I would have to design and build an engine with a cylinder diameter of about 20 mm, and check the pressure in the cylinder, that would give me a few parameters needed.
About the pump to check the injector, if possible, I think i will to change the hydraulic jack to a pump. ( My brother gave me an old hydraulic jack , cheap one ) maybe it will save money 😅
 

grahamgollar

Member
Joined
Jan 14, 2020
Messages
18
Reaction score
8
Quite right ALEX. I've got a collection of pintles/needles for use as centre pops or centre finders. You're also correct about today's throw away technology BUT if you're half way across the Pacific in a Force 9 hoolie and injectors malfunction (a polite description) there is still no alternative to the old fashioned approach. A Marine Engineer's tool bench always has a porthole glass and a pot of jeweller's rouge ready to hand.
 

Steamchick

Well-Known Member
Joined
Apr 2, 2020
Messages
750
Reaction score
265
Location
Sunderland , UK
Hi again :
I don't know all the calculations (just some), but the intention of compression for diesel engines is to heat the air "instantaneously" in the compression stroke so that when the fuel is injected it hits air above the flash point of the fuel - so "instantly" ignites and burns for the firing stroke. Of course, hot-bulb, glow-plug and other devices enable cold starting, and provide a heat source when running to assist ignition, thus allowing lower compression ratios to be used. I suggest you read some texts on this before deciding a minimum compression ratio that you should exceed when designing your engine.
See this on pre-combustion chambers: Diesel engine pre-combustion chamber - Yahoo Search Results Video Search Results
Mostly used on indirect engines for smaller diesels, such as cars - as the larger engines - for trucks, ships and power stations can manage without (and are more efficient!). They are effectively a development from the hot bulb - where the early engines used to inject/ignite the fuel spray.
Glow-plugs are initially heated by the electric current, but then remain hot (and glowing) heated by the running engine, thus ensuring reliable ignition of the fuel when injected. But on larger engines, at the highest compression, the ignition is simply from the heated air from the compression stroke.
I may be wrong, but the I understand the reason for maintaining compression below a certain point is to minimise the Nitrous Oxides in the exhaust, produced when the combustion temperature exceeds 920degrees C or thereabouts. Particularly in modern vehicle engines. Also a boosted (turbocharged or supercharged) engine uses a lower compression ratio, so the final pressure at injection is where it is most desirable for the optimum combustion. - but I doubt you are interested in that for your model!
At the smallest end, model aircraft diesel engines use ether in the fuel which has a very low flash point so ignites freely with the elevated temperature of the compressed air from the available heat of compression. Glow-plug engines have no need for ether. (Ether Fumes are horrid for people! - You fall asleep and wake with a hangover!).
Mention of diesel fuel pipes getting hot is (as I understand?) simply from the work done by the injection pump - the fuel is effectively the cooling fluid for the pump and carries away most of the heat developed in the pump. But as the fuel flow is small the temperature must be high. Actually, the heat is also developed as the pulsations in pressure in the fuel pipes stress (expand) and relax (contract) the steel tube and the pump is doing the work to stress (expand) the pipes, and when the injector opens the work absorbed in the stress of the steel pipe is released into the fuel - and some of this work is lost as heat... which adds to the heat from cooling the pump. (Enough of the physics!).
Enjoy your modelling!
K2
 

Mechanicboy

Well-Known Member
Joined
Jul 10, 2013
Messages
746
Reaction score
245
About the pump to check the injector, if possible, I think i will to change the hydraulic jack to a pump. ( My brother gave me an old hydraulic jack , cheap one ) maybe it will save money 😅
You can create the test instument with the fuel injection pump cylinder/piston from old diesel pump from diesel car engine + manometer to test the injector.
 

Mechanicboy

Well-Known Member
Joined
Jul 10, 2013
Messages
746
Reaction score
245
Glow-plugs are initially heated by the electric current, but then remain hot (and glowing) heated by the running engine, thus ensuring reliable ignition of the fuel when injected. But on larger engines, at the highest compression, the ignition is simply from the heated air from the compression stroke.
In the glow plug engines who are using the methanol as base fuel are glowing under running due catalytic reaction affected by methanol to ignite methanol/air mix.

In the car diesel engine the glow plug are to preheating the air before start up the cold engine, then the engine is running then the glow plug is not glowing. Ignited by heat of compression after the engine is started up.

Compression ratio in the pre camber diesel engine: 20:1-25:1
Compression ratio in the direct injection diesel engine: 14:1-18:1
Compression ratio in the hot bulb engine/semidiesel engine : 6:1- 8:1 With torch lamp in continous use. 12:1-14:1 torch lamp for heat up before start up the engine and torch lamp not in use under running (some had glowplug to start up the engine and running with the hot bulb heated by combustion to example in these norwegian semidiesel engines Rap , Brunvoll and Wichman semidiesel socalled "cold cylinder head" start up with glow plug and run with hot bulb who are inside the cylinder head).

In the Find Hansen model diesel engine who has direct injection the compression ratio is 20:1 due the engine is so small that it need higher compression ratio to heat up the air before the fuel is injected 45 degree before BDC and the engine has 20 mm bore/40 mm stroke. In the large direct injection diesel has the compression ratio between 14:1-18:1 depending on bore/stroke ratio and the injection timing can be between 10-20 degree before BDC depending on rpm and heat of compression.

Norwegian Brunvoll semidiesel started up with glowplug and running with hot bulb who are inside the cylinder head socalled "cold cylinder head" as here the movie.

 

Steamchick

Well-Known Member
Joined
Apr 2, 2020
Messages
750
Reaction score
265
Location
Sunderland , UK
Thanks Jens, very interesting stuff!
Just a comment - and I may have got it wrong... - from what I was taught at work by the "diesel engineer" about the current design of car diesel engines: Glow plugs take 1 or 2 seconds to cool down, after the electric current is turned off. But they are in the hot combustion zone of the engine so remain hot - up near the combustion temperature - as the flames at around 900degrees C keep them hot. The design of the glow plug involves making them so they don't burn away with being in the combustion chamber, so they have similar requirements to the central electrodes of spark plugs as regards heat flow.
Also, to avoid carbon build-up on "cold surfaces", I understand the end of the glow-plug remains above 750~800 degrees C ? - I think surfaces below 700degrees C gather carbon on the surface? I'm sure I have seen glow-plugs when removed from engines and the end centimeter or so is clean of carbon? - But maybe I am wrong, as I have very little experience of this - and it was more than 30 years ago.... I am not an expert, just an "ex-spurt".
Interesting to hear about the various designs that you obviously understand well.
K2
 

Mechanicboy

Well-Known Member
Joined
Jul 10, 2013
Messages
746
Reaction score
245
Thanks Jens, very interesting stuff!
Just a comment - and I may have got it wrong... - from what I was taught at work by the "diesel engineer" about the current design of car diesel engines: Glow plugs take 1 or 2 seconds to cool down, after the electric current is turned off. But they are in the hot combustion zone of the engine so remain hot - up near the combustion temperature - as the flames at around 900degrees C keep them hot. The design of the glow plug involves making them so they don't burn away with being in the combustion chamber, so they have similar requirements to the central electrodes of spark plugs as regards heat flow.
Also, to avoid carbon build-up on "cold surfaces", I understand the end of the glow-plug remains above 750~800 degrees C ? - I think surfaces below 700degrees C gather carbon on the surface? I'm sure I have seen glow-plugs when removed from engines and the end centimeter or so is clean of carbon? - But maybe I am wrong, as I have very little experience of this - and it was more than 30 years ago.... I am not an expert, just an "ex-spurt".
Interesting to hear about the various designs that you obviously understand well.
K2
In the glow plug is not glowing in the whole time after the engine is started up due the glow plug is cooled down via cylinder head, but remain hot enough to avoid carbon build up, normally between 500 and 800 ° C (932 and 1472 ° F). This is important because it determines the efficiency of self-cleaning of the plug and is determined by a number of factors, but first and foremost the actual temperature in the combustion chamber. If in case, the diesel engine can't be a real diesel engine when the fuel mix is ignited by glow plug instead heat of compression same manner as semidiesel engine. In fact the diesel engine with glow plug can start up without use the glow plug if outside air is not cold.
 
Last edited:

minh-thanh

Well-Known Member
Joined
Dec 29, 2016
Messages
763
Reaction score
453
Location
Viet Nam
You can create the test instument with the fuel injection pump cylinder/piston from old diesel pump from diesel car engine + manometer to test the injector.
Mechanicboy !
Did you say pump as in the picture ?
1.jpg


Lots of useful information .
Thanks guys !
 

Mechanicboy

Well-Known Member
Joined
Jul 10, 2013
Messages
746
Reaction score
245
Thanh, this is feeding pump for diesel pump in the picture you marked with red circle. It's the pump element for diesel engine you need to create own testpump for injector. And keep pump clean with fuel filter between the fuel container and diesel pump.

diesel pumpe.jpg
 

Latest posts

Top