How do Rotary engines(Gnome engines) work ?

Rotary Gnome engines were widely used in aviation before & during WW I ! Besides aviation, Gnome engines were also used in some early motorcycles & aircrafts. So lets see what are & how do rotary engines(gnome engines) work ?

If you are looking for pistonless rotary engine, click this link – Parts, working & advantages/disadvantages of WANKEL ROTARY ENGINE !

About Gnome engines :-

Rotary/Gnome engines are a type of internal combustion engines usually designed with an odd number of cylinders in a radial configuration. Unlike the inline engines used in automobiles & motorcycles, the piston-cylinder arrangement in radial configuration are different. In this config. the piston in the cylinders reciprocate outward from the central crankcase.
You might have observed them in historical documentaries or movies in which a man first rotates the propeller by his hand & only then the engine starts ! 🙂

Starting of rotary engines require human power at first !

History :-

A man named Felix Millet patented the design of rotary engine in 1888. He showed a 5-cylinder rotary engine built into a bicycle wheel in 1889(What an idea…! Just pedal the cycle once/twice & then let the bicycle wheel & those cylinders do all the work !).
A machine powered by his engine took part in the Paris-Bordeaux-Paris race of 1895 and the system was put into production by Darracq(a French motor vehicle manufacturing company) in 1900.

An 1897 Felix Millet motorcycle ! source:- wikipedia

An 1897 Felix Millet motorcycle !
source:- wikipedia

Parts :-

Piston – The piston is used to transfer the expanding force of gases to mechanical rotation of crankshaft via a connecting rod. The piston is able to do this because it is secured tightly within cylinder using piston rings to minimize the clearance between cylinder and piston !
Crankshaft – A crankshaft is a part which is able to convert the reciprocating motion to rotational motion. Here, in rotary engines the crankshaft is fixed & the whole crankcase rotates !
Connecting rod – A connecting rod transfers motion from a piston to crankshaft which acts as a lever arm.
Inlet & Outlet valves – It allows to enter fresh air with fuel & to exit the spent air-fuel mixture from the cylinder.
Fixed crank – All the connecting rods are connected to the fixed crank. The crankcase rotates around one end of the crank while the connecting rod assembly rotates around the 2nd end.
Spark Plug – A spark plug delivers electric current to the combustion chamber which ignites the air-fuel mixture leading to abrupt expansion of gas.
Propeller – The propeller on the front side of the nose produces thrust.

Working of Gnome/Rotary engines :-

Animation of a rotary engines. Credits:- Michael Frey

Animation of a rotary/gnome engine. Credits:- Michael Frey

Seriously, they are pretty interesting… ?
None of the pistons will reciprocate inside the cylinder if the connecting rods are attached to the centre(Point A-forms one end of the crank). Instead they are connected to a point just above the centre which forms the crank(Point B-other end of crank) !
If you observe the animation, you will notice that either of the suction stroke OR power stroke starts from the top side & ends while it reaches the bottom side during rotation. While Compression OR exhaust stroke takes place when the cylinders starts rotating from bottom to the top side.

So how do the particular strokes takes place during the rotation ?

Consider any one piston in it’s cylinder from the gif. You will observe that a piston is at the TDC(Top Dead Centre) when it is in the top side during rotation & at BDC(Bottom Dead Centre) when the cylinder is facing down towards ground.
This happens because… the length of the crank is equal to stroke length(distance between the TDC & BDC).

Case 1:- When the piston is facing downwards, the distance between crankshaft & the piston is only the length of connecting rod. Hence the piston reaches upto BDC & we know piston moves towards BDC either during suction OR power stroke.
Case 2:- When the piston is facing upwards, the length of connecting rod adds up to length of crank which makes the piston upto the TDC. And we already know that the piston moves towards TDC either during compression stroke/exhaust stroke.

The figure explains length of crank = stroke length !

The figure explains length of crank = stroke length !

Length of the crank = AB
Length of the connecting rod = BP
Width of the piston = Wp
Distance of TDC from point B = l(tdc)
Distance of BDC from point B = l(bdc)

Now from fig.1,
AB + BP + Wp = l(tdc)    ….(1)
From fig.2,
BP + Wp = l(bdc)   ….(2)

Subtracting (2) from (1), we get,
AB = l(tdc) – l(bdc)
Since AB is the length of crank & stroke length is the distance travelled by the piston from BDC to TDC ,

Length of crank = Stroke length

Keep in mind :
Point A acts as the centre of rotation of the crankcase(cylinders) while the connecting rod assembly rotates around  the Point B !

How do rotary engines start ?

As we discussed above in the first gif, a torque is required at first to start the engine(which was given by hand). Imagine I’ve just rotated the propellers upto some extent; at this time, the cylinders(which are facing downwards) suck air inside. Now I have to again rotate the propellers until the same cylinder comes at the topside. As soon as it reaches the topside, the mixture gets compressed fully & a perfectly timed spark plug ignites the mixture & the engine starts !
(I wonder why didn’t they use starter motors)

Advantages :-

  • Improved cooling :- The rotation of the whole crankcase created it’s own fast-moving airflow which helped to maintain the temperature.
  • High power-to-weight ratio :- Many other engines needed a flywheel to reduce vibrations & for a smoother run. In rotary engines, the crankcase itself acted as a flywheel which reduced the weight significantly & thus lead to high power-to-weight ratio.

Dis-advantages :-

  • Controlling of aircraft :-
    The huge rotating crankcase created a incredible gyroscopic effect. Whenever a body rotates, it is acted upon by some inertial forces. In case of radial engines, due to the inertial forces – it was difficult for pilots to control the aircraft. Due to the direction of the engine’s rotation, left turns required effort and happened relatively slowly, combined with a tendency to nose up, while right turns were almost instantaneous, with a tendency for the nose to drop.
  • Improper & high oil consumption :-
    Well you might it a bit funny. Rotary engines made engine fumes heavily smoky from partially burnt oil(which was added to fuel. The unfortunate side-effect was that World War I pilots inhaled and swallowed a considerable amount of the oil during flight, leading to persistent diarrhoea ?. Flying clothing worn by rotary engine pilots were like literally soaked in oil.[Source]

Having more advantages than rotary engine, later radial engines were used during WWII !
In todays world, we use gas turbines/jet engines in aviation as they are significantly more efficient than the early ones.


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