Compression ignition

[Nerd1000]

Some interesting and useful points in this thread. On that seems to have been missed is the problem of ignition delay and controlled rate of combustion. This is affected by the fuel itself, the atomisation from the injector and the air swirl/turbulence.

Diesels first trial with the injection of petroleum (I translate this as paraffin or kerosene) resulted in an uncontrolled combustion and the destruction or the indicator mounted on the cylinder head. This was the source of the commentary about the engine exploding. Diesel tried many different atomisation systems and finally settled on the air blast to produce the first successful commercial diesel engine. There were many other versions of solid injection developed but it was the Bosch system that became commercially successful.

For the ideal diesel cycle the fuel should ignite almost instantaneously when it is injected into the compressed heated air and continue to burn in a controlled manner during the rest of the injection period. This is fairly easy to obtain in a large low speed marine or stationary engine. These typically have very limited air turbulence and rely on the atomisation from the injector nozzle. For smaller higher speed engines there is much less time available for intimate mixing of the fuel and air and a vigorous amount of swirl/turbulence is required. This was originally achieved using a pre or swirl chamber where the air was compressed through a small passage into a separate chamber with the fuel injector. This required less atomisation from the injector but had greater heat losses resulting in lower thermal efficiency and harder starting.

If some fuel is dropped on a surface heated above the self-ignition temperature it will vaporise and ignite, but probable not quickly enough for an engine.

A number of working model diesels have been made, some may achieve close to the diesel cycle, others may operate in a different cycle where the fuel is injected very early in the compressing stroke so it has time to vaporise before the ignition temperature is reached. It can be difficult to tell but if the injection timing is more than 10- 15° advance it is probably on the second cycle.

My first 20cc two stroke diesel did run but I think it was actually running on the fuel that was blown past the piston.

It's worth noting that modern automotive diesels are partly operating on the second cycle. Premixed combustion is preferred because it produces less pollution, but longer mixing times produce a more violent initial combustion so a trade-off has to be made. That's also why compression ratios of automotive diesels have trended downwards, Mazda makes one with a compression ratio of only 14:1. Lower compression ratio allows a greater proportion of the fuel to be vapourised before ignition, without creating huge pressure spikes that would be noisy and might damage the engine.

There's a third 'diesel' cycle too: the MAN M-system, where fuel is injected onto the surface of a deep bowl in the piston, and evaporates as rapidly swirling air passes over it. Combustion occurs in a 'sheet' covering the inside of the bowl. M-system engines can run smoothly on fuel with very long ignition delay (like petrol) because they mostly avoid having a premixing phase between injection and ignition, so there is no big pressure spike and injection can be timed very early. But efficiency and pollution are worse.

Describing how to make an injector or injection pump is quite difficult so I don’t think Find Hansen is being difficult. At an amateur level it is not possible to measure and tolerance the various dimensions. I think I am achieving clearances of a few microns on a 2mm bore but can’t confirm that. My judgment of when a pin gauge will just enter a bore may be different to someone else’s. The angle of the poppet/mushroom valve on Hansen's injector is also difficult to measure. I guess that he knows how to set up his machines to achieve a working system but it will be different for others.

I have made some working injectors of this type. The cone angle is important, if it is too acute it will self-lock and if too obtuse the spray will hit the combustion chamber walls. The angle is initially set with the top slide on the lathe but deflections will make the actual angle different. Polishing the cone after hardening will change the angle again. Minh has also made this type of injector and I am sure has his own way of setting the angles etc.

Something I'd like to explore at some point is methods for making injection pumps that don't require such minuscule clearances. I think the plunger could be sealed using a stuffing box packed with telfon rings, in which case the clearances could be pretty loose while still sealing well. However control of timing and duration would have to be like Find Hansen and Minh's pumps where the stroke is varied by changing the tappet clearances, which does have the unfortunate side effect that start of injection is later at lower fuel delivery quantities.

 

[GreenTwin]

I am thinking along the same lines as nerd.
I am sure I could build a true diesel with a high pressure injector, but not at some tiny scale.

I would not attempt a diesel build with anything less than a 2.5" diameter piston, just so I could get a reasonable size for the injector and pump.

The o-rings on the pump seem like a great idea.
Don't all hydraulic cylinders get sealed with o-rings, and that has to be a very high pressure ?

As far as varying the stroke of the pump, I have not pondered that yet, and I don't have a good feel for the kinematics of how Find's variable pump system is working. I can see physically how the pump throw is varied, but I just don't have a good feel for what that means with timing, etc.

I guess Find's duration is a function of pump piston travel ?
More pump travel, more duration ?

As far as timing varying with Find's ramp control, it would seem that the pump could be driven by an eccentric, and the eccentric could be a shifting one, like some steam engines used, so that shifting the ramp also shifted the eccentric that drives the pump.

I wish I had time to do a deep-dive into a diesel build, but for me, the Ball Hopper Monitor is next in line.
I am just drooling over some of those nice 2-stroke diesel designs, and I am pretty sure I would build a 2-stroke diesel with typical high-pressure injector if I built a diesel.

I don't recall where I got these photos, but I think they are all 2-stroke diesels, perhaps all "Ellwe" design.

.
Edit:
The beauty of having an injector like Find's is that it is hydraulically actuated by the pump, unlike a Detroit Diesel injector, which I think is camshaft/pushrod/rocker-arm actuated.

.

 

[GreenTwin]

An interesting injector video.

 

[Nerd1000]

I am thinking along the same lines as nerd.
I am sure I could build a true diesel with a high pressure injector, but not at some tiny scale.

I would not attempt a diesel build with anything less than a 2.5" diameter piston, just so I could get a reasonable size for the injector and pump.

The o-rings on the pump seem like a great idea.
Don't all hydraulic cylinders get sealed with o-rings, and that has to be a very high pressure ?

As far as varying the stroke of the pump, I have not pondered that yet, and I don't have a good feel for the kinematics of how Find's variable pump system is working. I can see physically how the pump throw is varied, but I just don't have a good feel for what that means with timing, etc.

I guess Find's duration is a function of pump piston travel ?
More pump travel, more duration ?

As far as timing varying with Find's ramp control, it would seem that the pump could be driven by an eccentric, and the eccentric could be a shifting one, like some steam engines used, so that shifting the ramp also shifted the eccentric that drives the pump.

I wish I had time to do a deep-dive into a diesel build, but for me, the Ball Hopper Monitor is next in line.
I am just drooling over some of those nice 2-stroke diesel designs, and I am pretty sure I would build a 2-stroke diesel with typical high-pressure injector if I built a diesel.

I don't recall where I got these photos, but I think they are all 2-stroke diesels, perhaps all "Ellwe" design.

.
Edit:
The beauty of having an injector like Find's is that it is hydraulically actuated by the pump, unlike a Detroit Diesel injector, which I think is camshaft/pushrod/rocker-arm actuated.

Almost all diesel injectors are hydraulically actuated. The detroit ones just locate the pump on top of the injector, rather than having a pipe connecting the two.

 

[NapierDeltic]

Of course, I know a solenoid actuated injector, not being mechanical, is less fun; but have you thought at one?
Thus, the size of pump could be reasonably higher. And think a simple microcontroller could be able to drive it.

Later edit: Yes, an electronic control of a unit like that presented by GreenTwin above looks like hell....

 

[GreenTwin]

Of course, I know a solenoid actuated injector, not being mechanical, is less fun; but have you thought at one?
Thus, the size of pump could be reasonably higher. And think a simple microcontroller could be able to drive it.

Later edit: Yes, an electronic control of a unit like that presented by GreenTwin above looks like hell....

I am an old-school builder, so no electronics on my diesel.
It would work better, no doublt.

.

 

[GreenTwin]

Almost all diesel injectors are hydraulically actuated. The detroit ones just locate the pump on top of the injector, rather than having a pipe connecting the two.

I am told the 671 Detroit Diesel was one of the most reliable engines every built.
My family has been running one in a houseboat for 50 years.

They say "you can lose an injector, or even more than one, and the engine will continue to run at reduced power".
Ideal for a single-engine boat where you really don't want the engine to ever quit.

.

 

[Roger B]

I have looked at various possibilities for making very small injection pumps. Some key points are:

1) It must be possible to bleed all the air out of the system. A small bubble trapped somewhere in the pump will cause inconsistent or no injection.

2) The start of injection and the injection volume should be separately controllable.

3) Avoid plastics/elastomerics. These will tend to compress at the high pressures involved causing similar problems to trapped air.

4) Don’t underestimate the forces involved. For my first experiments the tappet was a miniature ball race, 6x2x2.5, running on a case hardened steel cam. The pump plunger was 2mm diameter. Several of these races shattered. I then tried a bronze (RG7) roller this started to spread after some trials. I am now using a hardened steel roller.

5) If you are using ball valves keep the movement to a minimum. The movement of the ball can displace more fuel that the injection quantity.

@ GreenTwin- The top engine picture is a Petter Atomic engine that I used as a prototype for my 20cc two stroke diesel.

 

[The Ignoble Troll]

It's worth noting that modern automotive diesels are partly operating on the second cycle. Premixed combustion is preferred because it produces less pollution, but longer mixing times produce a more violent initial combustion so a trade-off has to be made. That's also why compression ratios of automotive diesels have trended downwards, Mazda makes one with a compression ratio of only 14:1. Lower compression ratio allows a greater proportion of the fuel to be vapourised before ignition, without creating huge pressure spikes that would be noisy and might damage the engine.

There's a third 'diesel' cycle too: the MAN M-system, where fuel is injected onto the surface of a deep bowl in the piston, and evaporates as rapidly swirling air passes over it. Combustion occurs in a 'sheet' covering the inside of the bowl. M-system engines can run smoothly on fuel with very long ignition delay (like petrol) because they mostly avoid having a premixing phase between injection and ignition, so there is no big pressure spike and injection can be timed very early. But efficiency and pollution are worse.



Something I'd like to explore at some point is methods for making injection pumps that don't require such minuscule clearances. I think the plunger could be sealed using a stuffing box packed with telfon rings, in which case the clearances could be pretty loose while still sealing well. However control of timing and duration would have to be like Find Hansen and Minh's pumps where the stroke is varied by changing the tappet clearances, which does have the unfortunate side effect that start of injection is later at lower fuel delivery quantities.

Iirc the Mazda with the 14:1 is gasoline not diesel, and uses innovativing timing/injection.

Part of the SKYactive family I think.

I could be mistaken.

 

[Klrskies]

I'm a bit confused, are modeled diesels timing the injection to well before tdc and compressing the mixture to ignition point or reaching full compression and then injecting into the highest combustion space temperature?

 

[Roger B]

Both. I think my 20cc four stroke is injecting into full compression and igniting as the injection start is 10-15° BTDC. In it's two stoke version it was not.

 

[Klrskies]

Both. I think my 20cc four stroke is injecting into full compression and igniting as the injection start is 10-15° BTDC. In it's two stoke version it was not.

1) If fuel injection begins 15 degrees before TDC and cylinder pressure is now able to provide the required heat to begin ignition of the smallest fuel particulates distributed about the composition space. So, we are reaching peak mechanical compression ratio and going beyond it as injected fuel is burning and expanding, increasing combustion space pressure.

2) In the next microseconds, as the crank rotates a few more to TDC, new fuel continues being injected, and as the piston rises to TDC and provides the lowest combustion space volume. Existing mixture, as well as any new, continuously injected fuel continues to be heated, ignited as the cylinder pressure increases exponentially.

3) As the piston begins dropping down, the injection phase begins to stop. The combustion space pressure is burning the last of its mixture and pressure reaches its peak as the piston is forced downward, the last of mixture burning and beginning to produce diminishing pressure in the combustion space as the piston is approaching its half way down the cylinder position.

I'm making generalized assumptions in that I don't know when the initial burn will begin and how long the injection will continue as the piston begins it's downward movement?

 

[Roger B]

There is a generally accepted delay between the commencement of injection and the commencement of combustion. Various attempts have been made to reduce/control this delay such as a small pilot injection to start combustion followed by the main injection. This obviously produces additional complications in the injection system. Modern electrically controlled injection systems may use multiple injections to control the pressure rise and maximum cylinder pressure (returning to Diesel’s original concept). Good atomization is important to minimize the delay and also to obtain good combustion.

I have not yet achieved a satisfactory level of atomization as can be seen by the puddle of unburnt fuel under the exhaust pipe. The engine has rum with both poppet/mushroom type injectors like Find Hansen’s as well as conventional needle types as in the video. I tried an open 0.2 mm hole backed up by a non-return valve bit got no real combustion (0.2 mm would be a typical hole size for a multi hole injector for an engine of 100mm bore or bigger.) Atomisation is difficult to quantify, I have built a test pump and either observe the patten produced on a sheet of paper or hold a small flame in front of the nozzle and see if the fuel cloud will ignite.

 

[Nerd1000]

Both. I think my 20cc four stroke is injecting into full compression and igniting as the injection start is 10-15° BTDC. In it's two stoke version it was not.

Have you tried advancing your injection timing? 20 degrees?

 

[Roger B]

It is quite sensitive to injection timing, too much advance or retard and it won't start.

 

[Nerd1000]

It is quite sensitive to injection timing, too much advance or retard and it won't start.

Curious that it won't start if too advanced. Is it kicking back in that case, like ignition is way too early?

Regarding the exhaust spooge, it kind of sounds like your engine is wet stacking, which can be a problem for full sized diesels too. Usually a symptom of the load being too light for the engine. Something else to try would be to drive a load and see if the extra heat makes it clear up. I'd suggest a fan or centrifugal pump, they increase their load rapidly when RPM rises (load proportional to RPM cubed if I recall correctly) which would make it easier to keep the engine running at a consistent speed given you don't seem to have a governor fitted in the video.

 

[Roger B]

I believe that if it is too far advanced the air temperture has not risen sufficiently to ignite the fuel.
I was told by a diesel fitter that the main purpose of the excess fuel device fitted to older diesel engines as a starting aid prolonged the injection period to give a chance of something igniting.