Atkinson Engine

The Atkinson engine is essentially an Otto-cycle engine with a different means of linking the piston to the crankshaft. It was originally designed to compete with the Otto engine, but without infringing on any of Otto's patents.
The clever arrangement of levers allows the Atkinson to cycle the piston through all four strokes in only one revolution of the main crankshaft, and allows the strokes to be different lengths -- the intake and exhaust strokes are longer than the compression and power strokes (In this illustration ... see below). 
This also obviates the need for a separate cam shaft. The intake (if used), exhaust, and ignition cams are located on the main crank shaft.  My illustration shows only an exhaust cam. 
Everything I know about the Atkinson engine came out of Building the Atkinson cycle Engine .   This illustration draws heavily from that excellent book. 
10/1/00 Update: 
A sharp-eyed, well informed visitor to this site has just discovered an important error
in one of the above statements, and in my illustration.  In Atkinson's original engine, the power stroke was made longer than the intake stroke.  Apparently, by allowing the mixture to expand to a larger volume than was drawn into the cylinder, more energy was extracted giving greater fuel efficiency than the Otto-cycle (everything else being equal).  Thank yous go to George, for pointing this out; and John for the additional information. 
My illustration draws from a book about building a model of the Atkinson engine.  I may have my measurements wrong, or perhaps even the model designer may not have taken the stroke lengths so carefully into account.  After all, he didn't have much to work with: "...Jim was able to build his engine using one simple drawing and a photo..." 2
In any case I still recommend the book, and still feel that I've correctly illustrated the fundamental operation of the Atkinson.  I would love to hear any additional comments about this delightful engine.
CO2
 Motor

This style engine could be powered by steam (I've heard of at least one) but is more commonly seen in small model airplane engines powered by compressed air or CO 2 (carbon dioxide) gas. The popular Air Hogs toy airplanes are propelled by this style motor.

At the top of the stroke, the pin on the cylinder presses the ball valve upward, admitting high pressure gas into the cylinder.

The gas expands, driving the piston downward.

when the piston advances past the exhaust port, the high-pressure gas is released.

 

Flywheel (or propeller) momentum carries the piston upward to complete the cycle.

This animation also illustrates the CO 2 reservoir, or "fuel tank." Compressed CO 2 is a liquid and becomes a gas as the pressure is released.  Another way to state this is that the liquid CO 2 boils at normal atmospheric temperature and pressure, so one might say this engine runs on "CO 2 steam."

 

 

Model engines of this type have been made to incredibly small dimensions.  Stefan Gasparin produces one with a displacement of only 3.2 cubic millimeters!

 

Coomber Rotary Engine

 

I first learned of this delightful engine while attending the PRIME show in Oregon.  A most prolific modeler, Marlyn Hadley, had one on display.  I referred to his excellent book 6 to create this illustration.
I have not illustrated the valve linkage, as I'm not exactly sure what it looks like!  It appears to be a rotary type, incorporated into the main drive shaft.  Steam would be admitted to one end of the cylinder at a time, just as in any other double-acting steam engine.
The inner dimension of the stationary ring is not circular, but is slightly elliptical.  The main bearing is offset from the center of this ellipse by a one half the stroke length.

 

Crank Substitute Engine

 

Like the Coomber, this engine also came from Marlyn Hadley's wonderful book. 6  I can do no better than to quote him:

 

                       This elaborate arrangement of gears and linkages enabled the builder to eliminate the crank as we know it.  While this engine required more labor to construct, it did make a compact engine which did not require a heavy crosshead as the connecting rod connection on the bar between the two gears moves but a very small amount.  This means the piston rod guides can be made of a lighter construction.
I do not know who invented it, when, or what prompted the inventor to think this arrangement is better than a crank...
                                                                                  -Marlyn Hadley

Four Stroke Engine

The four stroke engine was first demonstrated by Nikolaus Otto in 1876 1, hence it is also known as the Otto cycle . The technically correct term is actually four stroke cycle .  The four stroke engine is probably the most common engine type nowadays.  It powers almost all cars and trucks.
The four strokes of the cycle are intake, compression, power, and exhaust.  Each corresponds to one full stroke of the piston, therefore the complete cycle requires two revolutions of the crankshaft to complete.

open by the vacuum produced by the intake stroke.  Some early engines worked this way, however most modern engines incorporate an extra cam/lifter arrangement as seen on the exhaust valve.  The exhaust valve is held shut by a spring (not illustrated here).

Compression. As the piston rises the poppet valve is forced shut by the increased cylinder pressure.  Flywheel momentum drives the piston upward, compressing the fuel/air mixture.

Power. At the top of the compression stroke the spark plug fires, igniting the compressed fuel.  As the fuel burns it expands, driving the piston downward

Exhaust. At the bottom of the power stroke, the exhaust valve is opened by the cam/lifter mechanism.  The upward stroke of the piston drives the exhausted fuel out of the cylinder.
This animation also illustrates a simple ignition system using breaker points, coil, condenser, and battery.

Larger four stroke engines usually include more than one cylinder, have various arrangements for the camshaft (dual, overhead, etc.), sometimes feature fuel injection, turbochargers, multiple valves, etc.  None of these enhancements changes the basic operation of the engine.

Gnome

 

The Gnome was one of several rotary engines popular on fighter planes during World War I.  In this type of engine, the crankshaft is mounted on the airplane, while the crankcase and cylinders rotate with the propeller.
The Gnome was unique in that the intake valves were located within the pistons.  Otherwise, this engine used the familiar Otto four stroke cycle.  At any given point, each of the cylinders is in a different phase of the cycle.  In the following discussion, follow the master cylinder with the green connecting rod.

During this portion of the stroke, a vacuum forms in the cylinder, forcing the intake
valve open and  drawing the fuel-air mixture in from the crankcase.

The mixture is compressed during this phase.  The spark plug fires toward the end of the compression stroke, slightly before top dead center.

The power stroke happens here.  Note that the exhaust valve opens early -- well before bottom dead center

This engine has a fairly long exhaust stroke.  In order to improve power or efficiency, engine valve timing often varies from what one might expect.

When I first learned how these engines worked, I thought the only person crazier than the engine designer was the one who paid money for it.  At first glance it seems ridiculously backwards .

Nonetheless, a number of engines were designed this way, including the Gnome, Gnome Monosoupape, LeRhone, Clerget, and Bentley to name a few.  It turns out there were some good reasons for the configuration:
                                               
                                                Balance. Note that the crankcase and cylinders  revolve in one circle, while the pistons revolve in another, offset circle.  Relative to the engine mounting point, there are no reciprocating parts. This means there's no need for a heavy counterbalance.

                                                  Air Cooling. Keeping an engine cool was an ongoing challenge for early engine designers.  Many resorted to heavy water cooling systems.  Air cooling was quite adequate on rotary engines, since the cylinders are always in motion.

                                                 No flywheel. The crankcase and cylinders provided more than adequate momentum to smooth out the power pulses, eliminating the need for a heavy flywheel.


All these factors gave rotary engines the best power-to-weight ratio of any configuration at the time, making them ideal for use in fighter planes.  Of course, there were disadvantages as well:


Gyroscopic effect. A heavy spinning object resists efforts to disturb its orientation (A toy gyroscope demonstrates the effect nicely).  This made the aircraft difficult to maneuver

Total Loss Oil system.  Centrifugal force throws lubricating oil out after its first trip through the engine.  It was usually castor oil that could be readily combined with the fuel. (The romantic-looking scarf the pilot wore was actually a towel used to wipe the slimy stuff off his goggles!)  

The aircraft's range was thus limited by the amount of oil it could carry as well as fuel.  Most conventional engines continuously re-circulate a relatively small supply of oil.

Jet Propulsion

I've grudgingly included this section by popular request.  Rocket and turbojet engines are fabulous technological achievements--But they're so simple the animations are boring!
...At least I think so.  You be the judge!

Rocket

The rocket engine is the simplest of this family, so I'll start with it.  

In order to work in outer space, rocket engines must carry their own supply of oxygen as well as fuel. The mixture is injected into the combustion chamber where it burns continuously.  The high-pressure gas escapes through the nozzle, causing thrust in the opposite direction.

 

 To illustrate the principle yourself, inflate a toy balloon and release it (without tying it off!).  ...rocket propulsion at its simplest.

Turbojet

 

The turbojet employs the same principle as the rocket.  It burns oxygen from the atmosphere instead of carrying a supply along.

Notice the similarities: Fuel continuously burns inside a combustion chamber just like the rocket.  The expanding gasses escape out the nozzle generating thrust in the opposite direction.

Now the differences: On its way out the nozzle, some of the gas pressure is used to drive a turbine .  A turbine is a series of  rotors or fans connected to a single shaft.  Between each pair of rotors is a stator -- something like a stationary fan.  The stators realign the gas flow to most effectively direct it toward the blades of the next rotor.


At the front of the engine, the turbine shaft drives a compressor .  The compressor works a lot like the turbine only in reverse.  Its purpose is to draw air into the engine and pressurize it.


Turbojet engines are most efficient at high altitudes, where the thin air renders propellers almost useless.

 

Turboprop

The turboprop is similar to the turbojet, except that most of the nozzle gas pressure drives the turbine shaft -- by the time the gas gets past the turbine, there's very little pressure left to create thrust.
Instead, the shaft is geared to a propeller which creates the majority of the thrust.  'Jet' helicopters work the same way, except that their engines are connected to the main rotor shaft instead of a propeller.

Turboprops are more fuel efficient than turbojets at low altitudes, where the thicker air gives a propeller a lot more 'traction.'  This makes them popular on planes used for short flights, where the time spent at low altitudes represents a greater percentage of the overall flight time.

Turbofan

The turbofan is something like a compromise between a pure turbojet and a turboprop.  It works like the turbojet, except that the turbine shaft also drives an external fan, usually located at the front of the engine.
The fan has more blades than a propeller and spins much faster.  It also features a shroud around its perimeter, which helps to capture and focus the air flowing through it. These features enable the fan to generate some thrust at high altitudes, where a propeller would be ineffective.

Much of the thrust still comes from the exhaust jet, but the addition of the fan makes the

engine more fuel efficient than a pure turbojet.  Most modern jetliners now feature turbofan engines.

As you can see all of these engines are conceptually very simple, and have very few moving parts, making them extremely reliable. They also have an excellent power-to-weight ratio, which is partly why they're so popular in aircraft

Like most of my illustrations, these are extremely simplified.  Turbine engines often employ more than one shaft and have other more complex features that I really don't understand and, frankly, don't care to investigate further.

 

Oscillating Steam Engine

 This style steam engine employs the cylinder as the steam valve.  It operates on the same principle as the locomotive steam engine.

Steam from the boiler enters the power manifold and is and is admitted to the top end of the cylinder when the cylinder port aligns with the manifold port.  The steam presses the piston downward, driving the flywheel around one half turn.

 

 

At the end of the stroke the cylinder shifts, exposing the top port to the exhaust manifold.  The expended steam is released.

 

At the same time, the bottom cylinder port, aligns with the power manifold, admitting steam to the bottom end of the cylinder.  This presses the piston upward, driving the flywheel around another half turn. 

 

At the end of the stroke, the bottom port aligns with the exhaust manifold, releasing the expended steam. 

Due to its exceedingly simple construction, this type of engine is popular in working toy steam engines, including one I had as a kid.  An even simpler type employs power in only one direction, relying on flywheel momentum to carry the piston around for the remainder of the cycle.  This is called a single acting engine.  The type illustrated here is a double acting engine.

 

Revolving Cylinder Engine

 



 

This is yet another of Marlyn Hadley's 6 model engines.  The inventor is not known.
The valve is not illustrated, but is apparently a rotary type, admitting steam to one end of the cylinder at a time. 

Wankel Engine

In the Wankel a triangular rotor incorporating a central ring gear is driven around a fixed pinion within an oblong chamber

 

The fuel/air mixture is drawn in the intake port during this phase of the rotation.

 

The mixture is compressed here.

 

The mixture burns here, driving the rotor around.

 

And the exhaust is expelled here.

The rotory motion is transferred to the drive shaft via an eccentric wheel (illustrated in blue) that rides in a matching bearing in the rotor.   The drive shaft rotates once during every power stroke instead of twice as in the Otto cycle.

 

The Wankel promised higher power output with fewer moving parts than the Otto cycle engine, however technical difficulties have apparently interfered with widespread adoption.  In spite of valiant efforts by Mazda, the four stroke engine remains much more popular.

 

Single Cylinder Stirling Engine

 

This type of Stirling engine, known as the beta configuration 3, features just one cylinder with a hot end and a cool end.  The working gas is transferred from one end of the cylinder to the other by a device called a displacer (here illustrated in blue).  The displacer resembles a large piston, except that it has a smaller diameter than the cylinder, thus its motion does not change the volume of gas in the cylinder - it merely transfers the gas around within the cylinder

 

The same four phases of the Stirling cycle are at work here:

 

Expansion. At this point, most of the gas in the system has just been driven to the hot end of the  cylinder.  The gas heats and expands driving the piston outward.

 

Transfer. At this point, the gas has expanded.  Most of the gas is still located in the hot end of the cylinder.  Flywheel momentum carries the crankshaft the next quarter turn.  The  bulk of the gas is transferred around the displacer to the cool end of the cylinder.

 

Contraction. Now the majority of the expanded gas has been shifted to the cool end.  It contracts, drawing the piston inward.

 

Transfer. The contracted gas is still located near the cool end of the cylinder.  Flywheel momentum carries the crank another quarter turn, moving the displacer and transferring the bulk of the gas back to the hot end of the cylinder.

 

Steam Locomotive Engine

 

 

Steam engines like this drove trains from the early 1800s to the 1950s. 1  Though the engines varied in size and complexity, their fundamental operation remained essentially as illustrated here.

 

In a steam engine, the boiler (fueled by wood, oil, or coal) continuously boils water in an enclosed chamber creating high-pressure steam.



Steam from the boiler enters the steam chest and is admitted to the front end of the cylinder by the valve slide (illustrated in blue).  The high pressure steam presses the piston backward, driving the engine wheels around one half turn.


At the end of the piston stroke the valve shifts, allowing the expended steam to escape through the exhaust port (underneath the blue valve slide).  The high pressure steam escapes in a quick burst giving the

engine its characteristic choo choo sound.  

At the same time, the valve slide begins admitting high pressure steam to the back end of the cylinder.  This presses the piston forward, pulling the engine wheels around another half turn

At the end of the forward stroke, the steam is released from the rear portion of the cylinder (another choo ).

The steam engine has a 'dead' spot at the extreme end of each stroke while the valve is transitioning from power to exhaust.  For this reason, most engines had a cylinder on each side of the engine, arranged 90 degrees out of phase, so the engine could start from any position.  This illustration only shows one side of the engine. 

Stirling Engine with Ross yoke

 

Andy Ross, a prominent Stirling engine experimenter, invented the linkage illustrated here. 3  The engine is identical in operation to the two cylinder Stirling. In this illustration, the left cylinder is the hot cylinder.

 

The linkage allows the engine to be more compact and reduces side loads on the pistons and connecting rods (since their travel is almost linear). 

 

 

Two Cylinder Stirling Engine

The Stirling engine is one of my favorites.  It was invented in 1816 by Rev. Robert Stirling of Scotland. The Stirling is a very simple engine, and was often billed as a safe alternative to steam (since there's no boiler to explode).  It enjoyed some success in industrial applications, and in small appliances like fans and water pumps, but was eclipsed by the advent of inexpensive electric motors. 3  Since it can run on any source of heat, it now holds promise for alternative fuel engines, solar power, geothermal power, etc.

 

 

 

Stirling engines feature a completely closed system in which the working gas (usually air but sometimes helium or hydrogen) is alternately heated and cooled by shifting the gas to different temperature locations within the system.  

In the two-cylinder or alpha configured 3 Stirling, one cylinder is kept hot while the other is kept cool.  In the illustration the lower-left cylinder is  heated by burning fuel.  The other cylinder is kept cool by an air cooled heat sink (a.k.a. cooling fins).

 

 

The Stirling cycle may be thought of as four different phases: expansion, transfer, contraction, and transfe

 Expansion. At this point, most of the gas in the system has just been driven into the hot cylinder.  The gas heats and expands driving both pistons inward.

Transfer. At this point, the gas has expanded (about 3 times in this example).  Most of the gas (about 2/3rds) is still located in the hot cylinder.  Flywheel momentum carries the crankshaft the next 90 degrees, transferring the bulk of the gas to the cool cylinder.
Contraction. Now the majority of the expanded gas has been shifted to the cool cylinder.  It cools and contracts, drawing both pistons outward.

Transfer.

The now contracted gas is still located in the cool cylinder.  Flywheel momentum carries the crank another 90 degrees, transferring the gas to back to the hot cylinder to complete the cycle.

This engine also features a regenerator , illustrated by the chamber containing the green hatch lines.  The regenerator is constructed of material that readily conducts heat and has a high surface area (a mesh of closely spaced thin metal plates for example).  When hot gas is transferred to the cool cylinder, it is first driven through the regenerator, where a portion of the heat is deposited.  When the cool gas is transferred back, this heat is reclaimed; thus the regenerator "pre heats" and "pre cools" the working gas, dramatically improving efficiency. 3

Two Stroke Engine

The two stroke engine employs the crankcase as well as the cylinder to achieve all the elements of the Otto cycle in only two strokes of the piston.

Intake.  The fuel/air mixture is first drawn into the crankcase by the vacuum created during the upward stroke of the piston.  The illustrated engine features a poppet intake valve, however many engines use a rotary value incorporated into the crankshaft.

During the downward stroke the poppet valve is forced closed by the increased crankcase pressure.  The fuel mixture is then compressed in the crankcase during the remainder of the stroke. 

Transfer/Exhaust. Toward the end of the stroke, the piston exposes the intake port, allowing the compressed fuel/air mixture in the crankcase to escape around the piston into the main cylinder.  This expels the exhaust gasses out the exhaust port, usually located on the opposite side of the cylinder.  Unfortunately, some of the fresh fuel mixture is usually expelled as well.

Compression. The piston then rises, driven by flywheel momentum, and compresses the fuel mixture.  (At the same time, another intake stroke is happening beneath the piston). 


Power. At the top of the stroke the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward, to complete the cycle.

 

 

Since the two stroke engine fires on every revolution of the crankshaft, a two stroke engine is usually more powerful than a four stroke engine of equivalent size.  This, coupled with their lighter, simpler construction, makes two stroke engines popular in chainsaws, line trimmers, outboard motors, snowmobiles, jet-skis, light motorcycles, and  model airplanes.  Unfortunately most two stroke engines are inefficient and are terrible polluters due to the amount of unspent fuel that escapes through the exhaust port.

 

 

Watt Beam Engine

 

This illustration shows the general arrangement of a typical beam engine.  Beam engines were used in many factories to drive machinery of all types and were sometimes built to enormous proportions.  I have omitted the valve gear, as it was substantially the same as the locomotive engine.

It was important to restrict the motion of the piston and rod to a straight line in order to reduce friction and wear on the upper cylinder seal. The Watt linkage illustrated here is one of many contrivances for accomplishing this linear motion.