Model Engine Maker
Engines => Your Own Design => Topic started by: Bruce W-S on December 19, 2018, 03:17:53 AM
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For the last year or two, I have developed an interest in diesel engines and in particular air blast injection. These engines were quite common in respect to large stationary and marine engines up until the early thirties when solid injection was preferred.
Air blast injection requires an independent supply of high pressure air (approx. 800 psi) which is fed to the injection valve along with a metered supply of fuel. The amount of fuel is metered by the governor to regulate the engine speed. These engines are characterised by having an air compressor mounted alongside the main engine and driven off the main crankshaft. These engines were the first commercial engines built from the Rudolf Diesel design.
I have attached a photo of the original engine that is located in the Diesel House Museum in Copenhagen. The people there were very helpful once I explained what I was doing and gave me free access to the engine including providing a small set of steps to use.
Since then, I have completed most of the drawings to the point where I can use them to start construction. The scale I am using is 6:1 which makes for a reasonable sized model where adding detail is not a significant problem. One of the major problems with diesel models are the forces and pressures involved. While you can scale the model, you can't scale the physics so the same forces that apply to the original engine also apply to the model.
The other problem as with all models, adjustments have to be made to suit the availably of materials available locally and I don't use castings. That is, everything is fabricated.
For the main engine, the majority of the components have been made and I am now in the process of fitting them together. One of the problems I have is locating a small pressure gauge that will read 800 psi.
The air compressor will be a two stage arrangement with inter-cooling which is vital to ensure that the compressor does not try to run as a diesel using the lubricating oil. The air supply must at all times be well below the flash point of the fuel. As with the original engine, the cooling system will be external as with the starting air. The engine will be air starting using a lever to change the position of the injection rocker and the air start rocker. There are four valves in the head being inlet, exhaust, air start and fuel injection.
The attached photos include the original engine. Note there are approximately 25 engines in various museums etc mainly in Germany, UK , Aust. has one in Melbourne and I have located one in the US however I am sure there are more.
Photos 2 and 3 are of the main engine without the cam shaft which is mounted on the head via some brackets.
Photo 4 is of the flywheel which is just over 400 mm and has a weight of 10 kg
Photo 5 shows the crank webs before pressing onto the crankshaft.
Photo 6 are some of the rockers and cams etc
Photo 7 shows the main bearing housings being bored out on the mill.
Photo 8 shows the crank webs being surfaced ground to size.
If anyone has an interest in these engines, I have a wealth of information that I am happy to share.
Bruce W-S
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That's quite a challenge :praise2: :praise2: As you say the physics don't scale very well and the losses in the air compressor may be a significant problem. The air blast compressors on full size engines typically absorbed 10% of the engine's output.
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8) That is going to be a big model.
How are you planning on achieving the high air pressure to drive it?
Jo
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That is quite a project Bruce!! Looking forward to seeing it progress.
Bill
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Very interesting project Bruce! :ThumbsUp: :ThumbsUp: :ThumbsUp:
I'll be following along and I'm looking forward to see more. :popcorn: :popcorn: :popcorn:
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Jo
While the air pressure is high e.g. 800psi the injection volume is small. The two stage compressor cycles twice for every air injection while the injection time is in m/seconds. There is also a ballast cylinder between the output of the compressor and the injection valve.
In addition, two air cylinders were provided and connected to the compressor on the original engine. These were used for starting however, I will be using an external air supply for starting so they will provide a reserve of air.
I will need to charge both the aux air bottles as well as the ballast cylinder first using an external power source via the flywheel before attempting an engine start. That the theory anyway.
Bruce
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A model of maritime history and a proud Danish history (unfortunately sold to the Germans 1980 > MAN) - I will certainly follow with interest.
Oh and welcome Bruce.
Best wishes
Per