Thursday, January 11, 2007

Powerhouse to barnyard - The gas engine

In my earlier post ‘The Powerhouse’ I covered a little about stationary steam engines and the technology behind them. If you’ve already read that, or if you know a few things about steam engines, you’ll probably find that you understand a great deal about antique gas engines as well.

Gas engines (or more appropriately, combustion engines) were basically the next evolutionary step in the development of machinery and machine operation. They were often made in a format very similar to that of the existing steam power of the day. A cylinder was mounted (often horizontally) on a frame, it held a piston connected to a pushrod that turned a crankshaft, flywheel, and pulley.

The general appearance of steam and gas engines is often quite similar, but the method of obtaining power is somewhat different. Naturally, while a steam engine’s power is supplied by a boiler, a gas engine relies upon some combustible liquid or vapor being ignited in the cylinder. Unlike most steam engines, the power is supplied on one side of the piston only. In fact, while a steam engine can be powered through almost all of it’s stroke (except the very end) a typical gas engine ran on a 4 cycle system where power could only be provided about a quarter of the time. Like many steam engines, this energy was used to turn a flywheel which smoothed out the flow of power from the engine.

There were other differences as well. Combustion can produce high pressures from relatively small amounts of fuel and air, so the huge pistons and valves seen in steam engines were not necessary. Of course the power stroke of a combustion engine can be rather violent so sturdy bearings, and crankshafts were necessary.

The control methods for these engines also had strong ties to their older steam engine counterparts. Typically a centrifugal governor was mounted somewhere on the engine either by taking power from the crankshaft, or by putting governor weights directly on the flywheel. Here’s an example.

On this engine, made by the Abenaque Machine Works, a small driveshaft is geared to the main crankshaft (behind the flywheel) This shaft has two weights (painted red) which are pinned in place so that they can pivot away from the shaft when they’re spinning quickly or be pulled inward by a spring at slower speeds. This is the same principle as in the steam engine governor from my ‘Powerhouse’ post. The way this principle is used, however, is noticeably different.

Achieving reliable and controlled combustion requires a consistent fuel supply and the ability to thoroughly mix the fuel with air. Fuel injection was unheard of when engines like this were first being built. Even good adjustable carburetors weren’t available for the early years of these one cylinder machines, so creating a reliable and controllable fuel supply for every stroke was not really an option. Even if the designers of the first gas engines had been able to provide a precise throttle and carburetor system, it’s questionable it would have been a success. Many engines were expected to run on multiple fuels, including gasoline, various grades of oil, or natural gasses. This no doubt created too many variables for early gas engine technology to compensate.

The solution found was the hit and miss style of operation. On a 4 cycle engine of any kind, it’s possible to have a power stroke on every second complete revolution of the crankshaft, but what’s the point of having a power stroke when you still haven’t used the energy from the last one? When a gas engine provides more power than is needed, it’s simply stored in the flywheel, so that the engine keeps running regardless of whether or not another power stroke is made. Because of this, all you have to do is keep the exhaust valve open to relieve pressure in the cylinder, and the engine will coast for a while after making a power stroke.

Here’s how it happens. An engine is running at the speed set by the governor. A load is applied to the engine (usually by way of a belt to some other machine.) This slows the engine down, so the weights of the governor are pulled inward by a spring or counterweight and this triggers some type of cam or linkage. The engine’s exhaust valve closes, the intake valve opens briefly to allow fuel and air to be drawn into the cylinder, and the piston compresses the mixture in the cylinder. A spark from the magneto and spark plug ignites the fuel and the piston completes its stroke under power, causing the crankshaft and flywheel to accelerate. The acceleration causes the weights on the governor to be pulled outward. The exhaust valve opens to allow the spent fumes to escape, and it is held open because the governor is now moving fast enough that it is not necessary to make another power stroke. And the cycle begins again.

The sound of an engine like this is pretty unmistakable because you can hear it fire once and then it will usually ‘chuff’ softly several times as the piston cycles with the exhaust valve open.

The introduction of combustion engines created the need for another design change. The hottest part of a steam engine is steam that comes from the boiler. Even a boiler with a superheating system would never produce steam hot enough to melt steel. (If it did, it would melt the boiler.) Consequently, the steam cylinders, though hot to the touch, did not require a cooling system to maintain the integrity of the metal. Gas engines, however, had the ability to produce temperatures that would soften or melt most steels. This meant that new systems had to be invented to keep the gas engines cool as they ran.

A simple method of keeping a gas engine cool was to add cooling fins. Much like the computer cooling systems of today, this approach increased the surface area available to expel heat by means of heat radiation or convection. Such designs were often quite simple as in the case of this model gas engine, which has a set of fins around the outside of the cylinder.

Another method of cooling was commonly used on gas engines to keep them from overheating; the cooling tank. The basic explanation is that if you stick a pot of water on top of a gas engine’s cylinder, it won’t overheat because as soon as the water gets to 212F the water will start to boil away and take the excess heat with it.

The design of the tank varied from engine to engine, sometimes becoming a bit artistic, but typically it was just a rectangular tank mounted on top of a cylinder. Of course these engines were made with a water jacket around cylinder to cool it from all sides. On smaller engines, it was even possible to build the engine so that the tank was the only thing visible, and the cylinder simply ran through the tank. If extra cooling was needed, water could even be circulated through a radiator, much like on a car.

Of course I can't talk about cooling without mentioning this circa 1912 Aermotor. In this case, a tank with cooling fins was mounted vertically on top of the cylinder. This was likely intended to help increase the amount of convective heat loss, because the air which is heated by the fins will naturally tend to rise, creating more circulation along the length of the fins. Also, this design shed enough heat that you didn't continually need to add more water to the hopper (which was quite common in open hopper engines)

Of course, these fins also have the effect of making the engine look a bit outlandish.

Over all, gas engines tended to remain fairly simple in design for many years. This could likely be attributed to the fact that farms were the most common place to use a gas engine of this sort. If the engines were made too complex or require too much maintenance, they lose their usefulness in this environment. Eventually carbureted engines would become more common, as the improvements in design and manufacturing made it possible to control an engine by its fuel supply rather than simply using hit and miss operation. Much of this development was no doubt driven by the development of automobiles and tractors which required more powerful, smoother running engines. Still, these small engines were a familiar sight until electric motors became strong enough (and common enough) to be used in their place.

While your typical combustion engine was found on a farm, powering small pieces of equipment, there were other engines which were much larger and much more complex. Larger engines were needed in many applications. Factories, mills, power plants, ships, and other heavy equipment needed power too, and although they had relied upon steam in years past, many of them converted over to combustion engines.

These applications required plenty of power and long hours of operation, meaning that the engine had to require very little time for servicing and repairs. This is where diesel engines really shined.
Some of these engines still had the same cylinder and flywheel arrangements as their barnyard counterparts, but they they were usually noticeably larger, and it was fairly common to see them made with multiple cylinders.

These engines are not easy to come by, but when you get a chance to see one in operation it’s quite impressive. Here’s an example of a page engine which was originally used to power a drag line.

Today this beast resides in the powerhouse museum on the Mt. Pleasant fairgrounds. It’s a two cylinder diesel manufactured by Page Engineering Company (Chicago IL) where it has been restored to running condition. This engine produced 110 hp. When you consider the horsepower cranked out by today’s automotive engines, this may seem rather small, especially for an engine with 13” bore cylinders. Still it was quite powerful for an engine in its time.

The best part, though , is watching (and listening to) this engine run.

Next time, more on the use of stationary engines (gas and steam.) Stay tuned!



At 9:57 AM, Blogger jl said...

"Here's a simple example": after that point, I was lost.

: D

hugs to you, sir.


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