Ever heard the term 'rotary engine' but aren’t really sure what it means? Read on to discover the inner workings of this quirky powerplant!
There are several different types of engine commonly called 'rotary' but the one we're interested is the one found in cars. Its correct name is the Wankel rotary engine, named after its inventor Felix Wankel. Don't confuse it with the rotary engine as used on early planes as that's completely different!
Before you get your head around the concept of the rotary engine, it's wise to grasp the basic workings of the normal piston-combustion engine.
All engines you'll find in a regular 4-cylinder petrol car work using the same basic principle. The engine is made up of two main components, the engine block, and the cylinder head. Within the engine block you'll find cylinders, and within each cylinder there is a piston.
The piston is connected to a crankshaft via a connecting rod. Where the cylinder head joins the block you'll find an area called the combustion chamber, and this is where the magic happens - It's where air and fuel are mixed and a spark plug ignites the mixture.
This causes an explosion which forces the piston downwards. The crankshaft is cleverly constructed as it turns the linear (up and down) motion of the piston into a rotational (circular) motion. This rotational motion then goes into the gearbox and ultimately that turns the wheels and off you go.
Now a rotary engine also uses the same principle of combustion - but in a completely different way. Instead of having cylinders it has rotor housings, and within the rotor housings are the actual rotors - normally two per engine.
The rotors are triangular, with bow-shaped flanks, and they fit into the oval-like housings (the correct name is epitrochoid, but think of it as an elongated oval with pinched-in sides).
The rotors are mounted on an eccentric shaft (which is the equivalent of the crankshaft) and as the shaft is eccentric it allows the rotor 'tips' to remain in contact with the walls of the housing at all times, and this creates voids between the rotor and the housing no matter where the rotor is during its cycle. Each void expands and contracts as the rotor spins.
The voids act in a similar way to a combustion chamber, but unlike in a piston engine where the chamber has to undertake the intake, compression, combustion, and exhaust cycles, in rotaries there are dedicated areas of the housing that carry out each task.
Intake
As the tip of the rotor passes the intake port (basically a hole in the housing through which metered amounts of fuel and air pass). The movement of the rotor 'sucks' the fuel and air mixture into the engine. As the following tip passes the intake port the fuel and air mixture is 'trapped' and compression begins…
Compression
As the rotor continues its motion around the housing the volume of the chamber reduces, compressing the fuel/air mix. As the chamber reaches its minimum volume we're on to the next step…
Combustion
With the fuel/air mix squeezed into a tiny area the spark plugs do their thing (usually two per rotor) and combust the mix. This forces the rotor to rotate, and creates the 'power' which drives the engine.
Exhaust
The 'spent' fuel/air mix is then forced out of the exhaust port (again, basically a hole in the housing.
The cool thing about the way the rotary engine works is that for one revolution of the rotor three combustion strokes take place. Because the output shaft spins three times for each revolution of the rotor there is one combustion stroke for each revolution of the output shaft.
Still following? It carries out the same process as a combustion engine but with a fraction of the components.
There are no valves, no timing gears, no con-rods, no pistons, no crankshaft and only three main moving parts compared to the myriad of parts involved in making a 'normal' engine work.
Rotary vs Piston
PROS
• The nature of the engine means that a much smaller displacement can produce considerably more power than a comparably sized piston engine - a Mazda RX-8 is technically a 1.3litre, yet produces around 230bhp.
• The engines are much smaller, lighter, and (in theory!) have many less moving parts to go wrong.
• Due to the nature of the engine they are internally balanced - the rotors act like spinning counterweights, phased to cancel each other out. This means there are less vibrations so the engine is smoother and more capable of higher revolutions (10k rpm is by no means unheard of)
CONS
• Rotary engines are less fuel efficient than piston-propelled equivalents as they are less thermally efficient.
• Emissions are poor, and struggle to reach current regulations.
• Oil consumption is high - and failure to maintain oil levels can have catastrophic consequences.
• Rotor tips are under a huge amount of stress, and are prone to failure - this was a huge issue on older Wankels, and has yet to be completely addressed in modern variants.
• Rotaries have had a tiny fraction of the development costs over the years, so in many ways the engines are still under-developed.
• High emissions, low fuel consumption, suspect long-term reliability and a general mistrust of rotaries from the buying public means that cars suffer from crippling depreciation.