It's a way of life. Once in a while I catch myself doing something and think, "I probably couldn't be anything other than an engineer." The latest incident came when I decided it was time to organize my closet. I took some measurments, started thinking about options and before I knew it I had it planned out in AutoCAD
Yep, planned complete with projected views. It was overkill but it did make me stop and think for a moment.
Engineers in my generation have little to no hand drafting experience. In fact by the time I was in school 2D CAD systems were passed over in favor of 3D modeling, at least in the world of mechanical engineering.
These are very effective tools that give us options nobody would have imagined just 50 years ago including the ability to generate ridiculously complex geometry that can be taken directly to CNC equipment.
The only problem I have with this is that when a computer program can do the heavy work for you it becomes easy to generate designs without having a thorough understanding of the geometry involved. Maybe I just worry too much about these things, but I still think there's something nice about being able to conjur projected views from just a concept and a few construction lines.
Friday, November 11, 2011
Saturday, November 05, 2011
Last time I opened these books I shared some pages on basic 4 stroke engines. This week I’d like show you the section on 2 stroke engines.
Perhaps the biggest advantage of two stroke engines is the simplicity of the design. By using ports in the cylinder wall it is possible to build an engine with one valve or even none at all. The other, perhaps more obvious, advantage is that unlike the 4 stroke Otto cycle design you get a power stroke every revolution, so power tends to be quite good compared to heavy and more complex engines. This is however one of the great weaknesses of this engine types as well, because by condensing the intake, compression, power, and exhaust into half the number of strokes, you end up shortening the power and compression as well as overlapping the intake and exhaust. These changes really cut into your efficiency as high compression and a long power stroke are needed to thoroughly burn fuel. The overlapping of the exhaust and intake also cost some efficiency by allowing fuel to escape through the exhaust port. Add to that the constant flow of engine lubricant through the crankcase to the cylinder and you’ll always have plenty of unburnt oil products blowing out the tailpipe.
Considering all that, it isn’t surprising that two strokes aren’t common in the automotive world. Where they tend to excel is in small handheld tools (blowers, weed whackers, chainsaws, etc) where weight and power are the primary concerns.
The fact that the two stroke appeared so prominently in this text is testament to the fact that in the 1920s people were still very unclear on where the automotive industry was headed. Of course, the American Technical Society was not shy about including extra information in this text. In fact they actually put in a section on aircraft engines.
Brigid mentioned the Antoinette engine in the comments so I did a little looking. I had no success in my old texts, but I did come across a real aerial oddity while I was looking. So, find your white scarf and we’ll take off on a tangent.
This is a Frederickson 70 hp rotary engine. Actually, I should say that again. This is a ROTARY engine, meaning the crankshaft is fixed and the cylinders rotate, unlike a radial engine where the crankshaft turns instead. Having the engine rotate may seem a bit odd, but during the early years of aircraft development this configuration did hold some advantages compared with some of the other options available. This approach could produce a reasonably lightweight design without the need for flywheels or cooling systems. Of course I wouldn’t want to ignore the fact that you were mounting a rather large spinning mass on the front of a light weight machine that has only its wings to provide opposing torque. That’s not necessarily a problem if you only need to turn in one direction, but it’s certainly not conducive to straight and level flight.
Still, rotary engines like the Gnome Monosoupape (single valve) kept much of the early air power aloft.
The Frederickson design is apparently an oddity even for a period rife with
Frederickson took a fairly simple approach and used a sleeve attached to the connecting rod by a swivel.
The lateral motion of the rod (relative to the cylinder) would move this sleeve back and forth, opening a port to admit the fuel air mix when the piston on the compression stroke and seals it on the power stroke thereby compressing the mixture prior to the intake phase. It’s actually quite a neat solution even if it might not be the best for maintenance.
By the time this design was developed, rotary engines were starting to fall behind their radial and inline counterparts. Rapidly improving design and manufacture meant that engines were becoming more reliable and able to rev to higher speeds, while rotary engines encountered dramatically greater wind resistance and higher structural stresses as the rev speeds went up.
Engines like the V8 Hispano Suiza became increasingly popular as did various flavors of radial engines. While the Gnome Monosoupape engine (probably the most common rotary design during the war years) topped out near 150hp, designs like the Hispano Suiza 8 cylinder entered production at 150 horsepower and quickly evolved to models producing well over 200 horses. These provided power for planes like the SE5a and various generations of Spad designs which had a significant impact on the balance of air power in 1917 and 1918.
I’ll start digging for more good tech manual material and see if I can come up with anything interesting for next time.
at 10:27 PM