Although there are multiple ways of producing free hydrogen, those methods require converting combustible molecules into hydrogen or consuming electric energy. Unless that electricity is produced from a renewable source—and is not required for other purposes—hydrogen does not solve any energy crisis. In many situations the disadvantage of hydrogen, relative to carbon fuels, is its storage. Liquid hydrogen has extremely low density and requires extensive insulation—whilst gaseous hydrogen requires heavy tankage. Even when liquefied, hydrogen has a higher specific energy but the volumetric energetic storage is still roughly five times lower than gasoline. However, the energy density of hydrogen is considerably higher than that of electric batteries, making it a serious contender as an energy carrier to replace fossil fuels. The 'hydrogen on demand' process creates hydrogen as needed, but has other issues, such as the high price of the sodium borohydride that is the raw material.

These include sturman industries (whose all-ternative engine was recently profiled by ammoniaenergy); hydrofuel inc. Of canada; and song-charng kong’s program at iowa state university. Caterpillar’s patent implies the use of a secondary fuel that is not hydrogen, but is silent on which ignition method might be employed. Natural gas engines are often the result of converting existing diesel or gasoline engines. But engines specifically designed for natural gas are also being developed.

Jet engines use a number of rows of fan blades to compress air which then enters a combustor where it is mixed with fuel and then ignited. The burning of the fuel raises the temperature of the air which is then exhausted out of the engine creating thrust. Multiple cylinder engines have their valve train and crankshaft configured so that pistons are at different parts of their cycle. It is desirable to have the pistons' cycles uniformly spaced especially in forced induction engines; this reduces torque pulsations and makes inline engines with more than 3 cylinders statically balanced in its primary forces. However, some engine configurations require odd firing to achieve better balance than what is possible with even firing. With an even firing pattern, the pistons would move in unison and the associated forces would add.

This design is commonplace in ci engines, and has been occasionally used in si engines. The cylinder head has an intake manifold and an exhaust manifold attached to the corresponding ports.

The intake manifold connects to the air filter directly, or to a carburetor when one is present, which is then connected to the air filter. It distributes the air incoming from these devices to the individual cylinders. It collects the exhaust gases from the cylinders and drives it to the following component in the path. The exhaust system of an ice may also include a catalytic converter and muffler. In boating, an internal combustion engine that is installed in the hull is referred to as an engine, but the engines that sit on the transom are referred to as motors. Next there are the carbon-based dual-fuel players working with compression-ignition systems.

The necessary high voltage, typically 10,000 volts, is supplied by an induction coil or transformer. The induction coil is a fly-back system, using interruption of electrical primary system current through some type of synchronized interrupter. The problem with this type of ignition is that as rpm increases the availability of electrical energy decreases. This is especially a problem, since the amount of energy needed to ignite a more dense fuel mixture is higher. The spark-ignition engine was a refinement of the early Deutz Engine Parts Catalog which used hot tube ignition. When bosch developed the magneto it became the primary system for producing electricity to energize a spark plug. Small engines are started by hand cranking using a recoil starter or hand crank.

Prior to charles f. Kettering of delco's development of the automotive starter all gasoline engined automobiles used a hand crank. Using a separate blower avoids many of the shortcomings of crankcase scavenging, at the expense of increased complexity which means a higher cost and an increase in maintenance requirement. An engine of this type uses ports or valves for intake and valves for exhaust, except opposed piston engines, which may also use ports for exhaust. The blower is usually of the roots-type but other types have been used too.