After my post explaining how internal combustion engines work a number of friends were quick to point out that I hadn’t discussed some popular variants of the standard ICE design. To satisfy them and my own curiosity, here is a discussion of several less common adaptations of the internal combustion engine.

Note: to avoid excessive wordiness I will refer to the most common gasoline-powered internal combustion engine as an ICE, even though technically every engine below is an ICE.

Diesel Engine

Diesel engines and ICEs are pretty similar. The biggest difference is that, while an ICE draws the fuel-air mixture into a cylinder bore all at once, this process occurs in two stages in a diesel engine. Air is drawn into the cylinder bore during intake and fuel is injected near the end of compression. Diesel engines have no spark plugs; instead, ignition is achieved because diesels compress the fuel-air mixture more than an ICE, resulting in temperatures high enough to start combustion.

As you may have guessed this has consequences for the kind of fuel a diesel engine uses. Diesel differs from gasoline in that it is heavier, denser, and less flammable, but also has more energy per gallon. Diesels are often more fuel efficient than ICEs, but they are more expensive and can be harder to start, making them less optimal in colder climates.

Rotary Engine

A rotary engine follows the same four-part cycle of an ICE but is designed such that each part occurs simultaneously. The equivalent of a piston in a rotary engine is a rotor, which resembles a triangle with rounded corners and sides. It sits within a rotor housing that contains intake ports, exhaust ports, and spark plugs.

This housing is not cylindrical or spherical, but rather has a somewhat irregular shape like a circle with its north and south poles compressed slightly. The result is that a given rotor side is able to form larger and smaller spaces as it spins. When the rotor passes over the intake valve it forms a larger space which draws in fuel-air mixture. It continues to spin and compresses the fuel-air mixture into a much smaller space, where a spark plug causes combustion. This drives the rotor and causes it to form another open space, with the exhaust generated by combustion being driven out of the exhaust port as that space closes.

When one side of the rotor is entering the compression stage, another side is finishing combustion and still another is beginning intake. There is also usually a second rotor in the same housing which is offset from the first by 180 degrees, meaning combustion is almost always happening.

Each rotor has a hole in the middle into which fits an eccentric shaft. The rotors are attached to the eccentric shaft at 180 degrees to each other, creating balance, stability, and low amounts of vibration. As in an ICE, the eccentric shaft is what transforms the rotor’s rotational motion into the vehicle’s propulsive force.

Compared to an ICE rotary engines have far fewer moving parts and generate a lot of power with relatively little size and weight. Their design does give rise to serious challenges with respect to preventing fuel and oil from leaking into places where it doesn’t belong. These challenges are met with an intricate series of seals both between a rotor and the housing and between rotors within the same housing.

Duke Engine

In an ICE pistons are attached at right angles to a crankshaft by connecting rods and arranged in a V or inline configuration. Igniting a compressed fuel-air mixture in the cylinder bore generates the power required to move the vehicle. An axial engine — of which the Duke Engine is but one modern incarnation — has its cylinders in a ring and attached to the crankshaft via a star-shaped component called a reciprocator. Each piston’s connecting rod is attached to one arm of the star, and the collective motion of all the pistons causes the reciprocator to spin. The reciprocator in turn is attached to the crankshaft in such a way that as it rotates the crankshaft spins in the opposite direction, with the result being that engine vibration is radically diminished.

Spark plugs and intake/exhaust ports are located on a stationary head ring positioned opposite the reciprocator. A spinning piston just starting its four-stroke cycle draws fuel-air mixture into its cylinder bore as it passes over an intake port, compresses it, passes over a spark plug which ignites the fuel-air mixture, expels the exhaust as it passes over an exhaust port, and then begins again.

This piston arrangement allows for the Duke Engine to be much smaller and lighter than an equally-powerful ICE, and with a greater fuel efficiency to boot. It also opens up myriad possibilities for experimenting with applications where the bulk and weight of a traditional engine have been problematic.