Engine Tale. Part 3: Abir Ghosh and the Cylinder SagaPosted: February 5, 2016
This is Abir Ghosh
Abir is in-charge of making sure thatUshtara Engineering Pvt. Ltd., produces good quality parts on time.
Like everybody else at Ushtara, even though his job profile sounds like a desk job, Abir is not above rolling up his sleeves and manning the machines, inspecting 2000 parts in one shot or removing burr from errant parts. He is good at data analysis, process design, CAD and CAM.
Abir’s efforts at streamlining production and HR have created the available time and resources which we are now re-allocating to R&D
Abir can be reached at firstname.lastname@example.org, +91 8431473438
Before we begin the narration…
A HISTORY LESSON
The first two-stroke engine was invented and patented by Scottish Engineer Sir Dugald Clerk in 1881, but the engine he invented had a separate charging pump to bring in the fresh air-fuel mixture. It was Englishman Joseph Day who realized that the crankcase of the engine could itself be used as a charging pump.
The kind of engine these guys used is called a cross flow scavenged engine.
The problem with this kind of engine is that it leads to a really weird piston shape and therefore a not very efficient combustion chamber geometry. The large mass on top of the piston also causes other problems like heat dissipation problems and difficult-to-balance engine.
This German dude Dr. Ing. Adolf Schnuerle realized that by having at least 2 transfer ports and by angling them just right, the shape of the piston could be optimized for better combustion chamber geometry. This type of engine is therefore called a Schnuerle ported or loop scavenged 2 stroke engine.
There’s a pretty cool animation on this page, about how a loop scavenged engine functions.
Almost every small 2-stroke engine being manufactured today, uses some form of loop scavenging. The two stroke engine has no valves and the flow of gases into and out of the combustion chamber is controlled by ports and their interaction with the piston. This makes the cylinder, arguably, the most complex and the most vital component of the whole engine.
WHAT SHALL WE MAKE THE CYLINDER OUT OF?
The earliest cylinders were made of cast iron. Many, especially in developing countries, still are. Cast iron is a high carbon alloy with good machining and wear properties. It is however heavy and more difficult to cast. It is also not as good as aluminum alloys at heat dissipation.
In the last three to four decades, engine designers have been gravitating towards the use of various aluminum alloys for cylinder manufacturing. Although aluminum is light weight, easier to cast and has better heat dissipation properties, it is not without flaws. The biggest drawback of aluminum is that being a soft material, it has really poor wear resistance.
Engineers have come up with lots of ways to overcome or circumvent the poor wear resistance properties of aluminum, including, having a cast iron sleeve, coating the bore with various materials like Nikasil and using high silicon content alloys which are are then etched.
We decided that we would make four types of cylinders and offer them all to customers. The market would then decide which we should keep and which we should discard.
- Cast iron cylinders
- Aluminum cylinders with inserted cast iron sleeves
- Aluminum cylinders with hard chrome plated bores
- Aluminum cylinders with Nikasil or similar plating on the bores
THE ART AND SCIENCE OF PATTERN MAKING
Once we had designed the cylinder, and verified its geometry, thermodynamics, fluid dynamics and strength,
We added machining stock and tapers and got some patterns and core boxes made by Bhaskar Patterns, Jalahalli. Pradeep the proprietor is a highly competent pattern maker and an imaginative artist in one.
For the uninitiated, patterns are objects in the shape of the part. You mold the sand to the shape of the pattern before pouring liquid metal into it.
Cores are sand forms in the shape of the Empty Spaces in the casting. When you pour liquid metal into the sand mold, the cores occupy the places that the metal shouldn’t flow into.
So a core box is a form whose cavities, when filled with sand, impart to the sand the shape of the hollows in the casting. Boy that was convoluted!
Anyway, that’s why a pattern maker has to be a bit of an artist because he has to imagine the shape of the negative spaces.
A HORRIBLE EXPERIENCE
The following Saturday, we took the patterns to New Sun Foundry, Yeshwantpur to get some cast iron castings poured. The manager assured us that the castings would be ready by the following Monday.
When we arrived at the foundry on Monday, we found that not only were the castings not ready, the patterns had been allowed to lie in the sun and rain outdoors over the weekend!!!
Because we were committed to getting the castings from them, and pursuant to their assurances, we accepted their word, that they would finish the castings the following day.
When we arrived on Tuesday after noon at the foundry, we found that they had been unable to make satisfactory sand molds. The fins in mold were only partially formed and the center core was misshapen.
They had also damaged our patterns due to careless handling and misplaced one of the core boxes!
It took them about 20 minutes to find the missing core box from the big messy heap of finished parts, patterns, raw materials and scrap that is their foundry.
We hastily took back our patterns and core boxes from New Sun Foundry, vowing never to return and took them to Pradeep for repairs.
Still recovering from our negative experience at New Sun Foundry, we decided to make an aluminum casting so we took the patterns and core boxes to Varsha Castings, Peenya and got them poured.
GOSHDARN AND DAGNABBIT!
One of the transfer cores had floated away with the molten aluminum
See how there’s a thin rib on one side of the bore, that’s not supposed to be there, see the rendering above.
We decided to learn even from this failure and section the casting;
Varsha Castings made another casting for us. This time without any core shifts.
But it was not with defects. There were small blow holes in the bore and other critical areas.
CAST IRON WOES AND LEARNINGS
Ashoka Bearing Housings Pvt. Ltd., is primarily a manufacturer of Cast Iron bearing housings, also known as plummer blocks or pillow blocks. Because the mass produce these Bearing Housings, they have an in-house foundry where they pour high grade cast iron.
My first impression of this foundry was one of cleanliness, competence and good old fashioned work ethic. What a difference from New Sun Foundry!!
Unfortunately, they were unable to make the sand mold, to the admittedly complex pattern. After two attempts, they gave up.
We asked the owner if he would pour the cast iron into a mold made at Varsha Castings and he said he would.
Bhaskara, the molding expert at Varsha casting made the mold in two halves.
They were assembled with the cores that the guys at Ashoka Bearing Housing made
The cast iron was then poured into the assembly and…
The the upper mold half, which would have worked perfectly with aluminum, had literally floated on the denser molten iron alloy.
We tried again. This time we brought Bhaskara from Varsha Casting over to Ashoka Bearing Housing, to make a bigger, heavier mold using their own flasks;
!@##$ COLD SHUTS AND &^%$#@ JOINT FLASH!!!
Cold shuts occur when some of the metal solidifies before the mold is completely filled, not allowing further molten metal to enter. This could happen because the runners and gates were not well designed or because the pouring rate was sub-optimal. Cold shuts cause the casting to lose integrity and break.
Joint flash occurs when mating faces of the two halves of the mold are not well matched, allowing some metal to leak between them.
Back to the drawing board.
We had to find a process which was not as dependent on the skill of the person making the mold or the one pouring. A process which would be repeatable.
So we made Dump Core Boxes
These dump core boxes already have well designed runners, riser, well and pouring basin built in.
We used the dump core boxes to make the molds and assembled them with the cores.
So what happened next?
Did we fail again?
Did we succeed in making a good casting?
Find out in Part 4