Mc Donald Model tractor

Roger thank you for the information on your injector system. I have redesigned my injector for the tractor and am now in the process of making it. I have managed to borrow an injector test pump for testing when I reach that stage. What is the bore and stroke of the pump on your engine ? As the pump and injector are both detachable I have decided to now paint the model and work on the injection system while the paint is drying.
Fred.

I designed my pump based on a 20 cc cylinder and a 14-1 Air Fuel ratio. This gave a fuel requirement of 2mm³. With a 2mm diameter plunger the calculated stroke for full power is 1.5mm.

The finished pumps have a bore of 2mm and a maximum working stroke of 2mm. The actual stroke is reduced either with a wedge or a helix spill to around 0.6mm for a light load.

If I remember correctly your engine is 80 cc so you would need to consider a maximum fuel delivery of 8mm³, so maybe a 3mm / 1/8” plunger.

I made my own simple test pump and use a spring balance to determine the approximate opening pressure of the injectors. It is also good practice at getting the required plunger fit to achieve the injection pressures.

No dramatic news to report as I have been working on the injector, now up to Mk4. The photo shows a drawing of the latest injector. I have made all of the body but have yet to make the internal part that controls the point of injection and hopefully produce a fine spray. I do have the loan of an injector test kit and will use this to set the spring pressure. Being a scale model I have attempted to make this part to match the rest of the model but it does mean drilling very small holes! I am going to remake the pump only this time with a 3.2mm but that is for next week. Thanks for the information RogerB and I hope my effort produces similar results to yours.

Fred.

What injector pressure are you looking for? I am using 90-100 bar (estimated) but you shouldn’t need anything like that.

I think that you need to reduce the length of your 0.35mm nozzle hole on two counts. Firstly the relatively long thin passage will tend to cause the atomised fuel to coalesce and secondly drilling a 0.35mm hole several millimetres long is a significant challenge. I would suggest the nozzle hole should not be much more than 0.7mm long (2 x d).

The long 1mm fuel passage will also be somewhat of a challenge to drill. I avoided this by having a long reduced diameter section on the needle but that makes whole thing longer and the fuel inlet somewhat clumsy.

Finally I moved on to the mushroom/poppet design which removes a number of the manufacturing challenges and avoids the fuel drain.

As an aside, the length of the hole (s ) in the nozzle will affect the spray angle.

Since a model will probably not be expected to run at Full Load Rated Speed for long periods, if at all, one can probably be a little more lax over needle lift and sac volumes,

For the prototype machine, i would have expected Injector breaking pressure to be of the order of 65 - 70 bar

It was only in the late 30s that pressures reached the 120 - 130 bar level (Atmospheres actually, at the time)

Eventually pressures crept up through 180 Ats to 210 as Injection pumps and injector manufacturing techniques improved. (We expected a Leyland 0600 to be marginally better with CAV injectors.than with Leyland ones, probably based on slightly better lapped fits, leakage rates and tighter tolerances.

Since leakage is subject to pressure and time, higher pressures will increase leakage, both in the pump and the injector nozzle (Leakage is one of the features which checked in full size products ) In a model it will be difficult to better the lapped fits of the conventional products, But with 90 years of manufacturing experience and development. modern materials and technique. may allow that.

To produce scale models of F I E which function is no mean feat anyway.

Good on yer, chaps!

Howard.

I couldn't find this video clip yesterday. I didn't realise how old it was This is a confirmation of the quality of atomisation with a needle injector like the one above. As precautions there is a damp towel just out of view and a 1kg dry powder extinguisher by the door

https://youtu.be/jpkbryz4h78

This is the Mk5 injector! I designed 5 but only made 3 of which this is the latest and I am pleased to say that it works and gives a good fine spray. I always realised that this part of the model would be a real challenge and take time. The piston is made from bronze and the body from steel and both are lapped to give a good fit. The lapping part of the process took far longer than expected and with the hope that as the injector heats up in use the fit should tighten up and help retain the pressure. I will carry out further experiments to determine the final spring pressure and try different size gas jets to see what effect these have. It’s scale is with in that of the model and is not to overpowering. I have had plenty of practice at drilling small holes (1.0mm dia) and pleased to report that no drills were broken in the process ( although one did disappear out of the chuck and out of the window due to centrifugal forces). The two main parts are sparingly silver soldered together to avoid blocking the fine holes and passages. Although it is difficult to see on the drawing there is a fuel hole near where the needle pass through the support to allow the fuel into the lower chamber.

Fred.

That looks good Are the gas jets screwed in so you can easily change them?

Yes the gas jets are screwed into the lower body ( 4.5 metric) and I have a range of four different sizes to choose from. This makes it easier to clean and change sizes. The number 5 design allowed me to ream the bore through and lap before assembly but did involve some tricky silver soldering.

Fred.

You could say practical progress has slowed down but the thinking and reading continues. I have redesigned the fuel filter system as I have discovered that small jets and any form of a solid do not go well together and will block the system. I am hoping that the new filtration system will cure this problem - it is not quite to scale but looks ok. I still have to strike a balance with the bore and stroke of the fuel pump. A larger bore gives lack of accurate speed control but would give more fuel in a short time at the injector. A longer stroke with small bore gives more time/ distance for control of speed - more experiments required! I am not alone with this problem as I have read else where that other modellers have experienced the same problem with injected Diesel engine models. Christmas is just round the corner and as a result work in the shed will slow down. I had hope to have the model running by the end of the year but didn’t say which year! Take care and stay safe.

Fred.

Have you managed to get it firing in hot bulb mode yet or are you still experimenting with the injection system?

Roger I am still experimenting with the injection system and have had to modify the test equipment and filtering system.

Fred.

With Christmas and New Year over its good to be back in the shed. I am not sure what the plural for a group of injectors but this is the sum total made so far. The drawing shows the next stage of the development and next weeks work in the shed. Some of the above have worked but I need to have a consistent performance. I have tested them to 1,600 psi and feel that should be sufficient for the low compression engine. One problem that I did encounter was blockage in the small passages and jets and to overcome this I have now fitted a three stage filter system in the fuel line. The first stage is a gauze filter inside the fuse tank, the second stage is the fuel filter on the side of the cylinder and the third stage is a micro filter in the fuel line to the pump. I did have to remake the original fuel filter slightly larger as I wanted to incorporate a filter from a lawn mower. The plumbing is now complete. From what I have read on other forums I am not alone in trying to solve the problems of injectors and pumps. Painting has been put on hold as the weather is very humid at this time of the year and it’s not a process I enjoy! I feel that the development stage will take longer than I anticipated. The drawing does not show the blocking off of the passage ways necessary to make it work.

Fred

That sounds about right A quick check suggests I have at least 9 injector designs plus five injection pumps. You will get there and it's great when you do

Absolutely awesome work Fred. I wonder if you have seen the YouTube videos of this Danish gentleman. Find Hansen YouTube Channel

Thank you for your comments Steve and yes I have seen the YouTube that you suggested. The quality of his work is wonderful. Although I have not added to this column for some time I have been working on the model and now have the engine running. I have make some changes to the method of the fuel injection and now use a spark plug for ignition just to make life a bit easier and also had to cure a small water leak. I now have the engine stripped for checking and make some modifications as access to the oil sump drain and water drain from the cooling system needed to be made easier to use. The full size tractor did use spark ignition to aid starting in cooler climates and then when warmed up the hot tube would take over.

Fred.

Glad to hear that you have had it running Did the governor system work or haven't you got that far?

Fred , your work is a remarkable combination of skill, determination and patience.This model is in my top ten wonderful models list of all time.

I am sure it's in my wish list as well! Congratulations 👏, very inspirational.

Kind regards, vic

Thank you Roger and Vic for the interest shown in my model. After 3 1/2 years work it was good to see and hear that there is life in the old girl ( tractor) and although it’s not finished the end is now much closer. I began to feel that I was going round in circles with the injector and pump and as the prime factor was to have a working model I had a rethink of the process. I think that I have cured the water leak into the engine. A small shrinkage crack in the transfer port was the culprit and this has since been sealed. To bring the engine to life I have added an ignition system driven by a Hall effect unit with the magnet housed in an aluminium holder in the steel flywheel. Call it a defeat if you like but I did need to have a working engine and this system was used on the full size tractor. While I had made a working injector ( one of six) I decided that I would take the easy way out and use indirect injection. Originally I had oil being injected into the reed valve but have now made this the entry point for the fuel injection shown in the photo. A thin brass pipe with the end blocked off and two .3mm holes drilled sprays fuel into the air stream when the reed valves open. I still have work to do but a short initial run looks promising. While I have the engine stripped down I have made minor alterations to allow easier access to the both the excess oil drain from the crankcase and water from the cooling system. I am looking forward to the next development session next week. Photo to follow.

Fred.