Re: four wheel drive is easy with steam
Posted by:
Ken (IP Logged)
Date: April 11, 2008 12:29PM
Hi Johnny:
When balancing an engine we balance for 1/2 the reciprocating mass plus all of the rotating mass. I wrote an article for "The Steam Automobile" a few years ago that went into detail as to why this is so and examines the fundamental concepts of engine balance. I can e-mail you a copy if you desire, it is currently in Microsoft Word though I could likely convert it to another format. It would certainly be more understandable than covering the topic in this venue if for no other reason than the illustrations.
The above statement is generally true except when balancing symmetrical cranks (such as a typical inline or boxer 4 cylinder or an inline 6) in which case the engine layout is such that the forces involved totally cancel themselves out if the crank itself is balanced to zero. It also doesn't hold true if we intend to use 2 balance shafts to cancel out primary unbalance forces, then we just balance for rotating forces.
In a single cylinder engine without active auxiliary balancing devices, the best we can do is reduce the peak primary unbalance by 50%, and we can't do anything about the secondary unbalance force. Figure we can, maybe, reduce the shaking by about 40% give or take a bid depending on stroke and conn rod length. Those little 5 HP one cylinder engines on lawnmowers are balanced about as well as they can be, and they shake like the devil. There are single cylinder engines built to test concepts in automotive engines, these may use either 2 dummy pistons or up to 4 balance shafts to let them run freely without shaking apart.
Realistically, I can't see a 500-700 rpm engine in a car. The tires on my truck aren't huge, but they are about 2 feet in diameter. That works out to about 840 revolutions per mile or 840 rpm at 60 mph. Figure an 80 mph top speed and that's about 1120 rpm. Of course, this will go down as tire size goes up. Still and all, even Saint Abner was running about 1,800 as I recall, way back when grandpa was still a pup. Admittedly, the unbalance goes up with the square of rpm, but the power goes down as the product of rpm. You'd need either really huge cylinders, massively inefficient long cutoff or extremely high pressure to move a car at highway speeds with a 600 rpm engine. While the low rpm would cut imbalance impressively, you'd need massive pistons and conn rods to get the power you need, so the improvement isn't what you'd expect. The bigger powerplant would add more vehicle weight which would necessitate a heavier structure soooo... Anyhow, realistically I think you need to consider some kind of gearing on each engine to let it wind up a bit so as to keep engine size down. Added advantages are that thermal losses and ring leakage is likely to be less as rpm goes up. Downside is that a stepdown gear on each engine is starting to get you closer and closer to a conventional drive system in parts count.
I'll admit old time steam cars were kind of flimsy, but they weren't terribly powerful either. A modern car will be beefier, but the bigger powerplant will also shake more. Anyhow, besides being a bit brute force, adding extra mass to dampen vibration detracts from vehicle desirability as far as economy, cost and handling go. Also, it isn't like the shaking goes away, you are just masking it. The odds that something or other is being stressed or shaken loose is still good.
Actually, most of the powertrain forces acting on a car aren't chaotic. The piston power pulses come at predictable intervals, as do any uncancelled unbalance forces. The crankshaft, being the heart of the engine, keeps these things in synch.
If I were to attack the idea of differential power to a split powertrain, I'd consider avoiding all the electronic gimmickery with some careful mechanical engineering. I'd try something on the order of a cam connected directly to the steering that changes compression on some springs proportional to the output desired for each engine. The springs could be integral to pressure regulator valves, ie, the spring tension is what controls the valve pressure setpoint. Perhaps a better solution would be to use a system similar to an old stationary steam engine governor whereby the governor controlled engine speed by adjusting cutoff. In this instance the steam chest pressure would act against the spring load to adjust cutoff in such a way to maintain the desired pressure going to that engine. It has the advantage of being easily diagnosed, you can look at a mechanism and see if the spring is broken or the cylinder is leaking, usually harder to figure out what those sneaky little electrons are doing inside a chip.
Mind you, I'm not against innovation. I have a hunch a lot of people would call me nuts if they saw the engine admission valve, burner, feed pump and condenser system I am working on for my own project. So feel free to just say "He's a whack job anyhow, what does he know?" and take a stab at your dream.
I REALLY like the go kart idea, makes solid sense to me. My own engine concept is a very large (4 inch bore and stroke) single cylinder engine, even with very short cutoff it would be too lively for all but the hottest go karts. I'm therefore looking at installing it in a sand rail or dune buggy, which is nothing more than an overgrown go kart. I like the low cost and open framework, one fits my budget and the other makes troubleshooting a breeze.
Regards,
Ken