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A coloured automobile engine

The internal combustion engine is an engine in which the combustion of fuel and an oxidizer (typically air) occurs in a confined space called a combustion chamber. This exothermic reaction creates gases at high temperature and pressure, which are permitted to expand. The defining feature of an internal combustion engine is that useful work is performed by the expanding hot gases acting directly to cause movement of solid parts of the engine, by acting on pistons, rotors, or even by pressing on and moving the entire engine itself.

This contrasts with external combustion engines, such as steam engines and Stirling engines, which use an external combustion chamber to heat a separate working fluid, which then in turn does work, for example by moving a piston or a turbine.

The term Internal Combustion Engine (ICE) is almost always used to refer specifically to reciprocating piston engines, Wankel engines and similar designs in which combustion is intermittent. However, continuous combustion engines, such as jet engines, most rockets and many gas turbines are also internal combustion engines.

History

The first internal combustion engines did not have compression, but ran on an air/fuel mixture sucked or blown in during the first part of the intake stroke. The most significant distinction between modern internal combustion engines and the early designs is the use of compression and, in particular, in-cylinder compression.

• 1509: Leonardo da Vinci described a compressionless engine.
• 1673: Christiaan Huygens described a compressionless engine.
• 17th century: English inventor Sir Samuel Morland used gunpowder to drive water pumps, essentially creating the first rudimentary internal combustion engine.
• 1780's: Alessandro Volta built a toy electric pistol () in which an electric spark exploded a mixture of air and hydrogen, firing a cork from the end of the gun.
• 1794: Robert Street built a compressionless engine whose principle of operation would dominate for nearly a century.
• 1806: Swiss engineer François Isaac de Rivaz built an internal combustion engine powered by a mixture of hydrogen and oxygen.
• 1823: Samuel Brown patented the first internal combustion engine to be applied industrially. It was compressionless and based on what Hardenberg calls the "Leonardo cycle," which, as the name implies, was already out of date at that time.
• 1824: French physicist Sadi Carnot established the thermodynamic theory of idealized heat engines. This scientifically established the need for compression to increase the difference between the upper and lower working temperatures.
• 1826 April 1: The American Samuel Morey received a patent for a compressionless "Gas or Vapor Engine."
• 1838: a patent was granted to William Barnet (English). This was the first recorded suggestion of in-cylinder compression.
• 1854: The Italians Eugenio Barsanti and Felice Matteucci patented the first working efficient internal combustion engine in London (pt. Num. 1072) but did not go into production with it. It was similar in concept to the successful Otto Langen indirect engine, but wasn't so well worked out in detail.
• 1856: in Florence at Fonderia del Pignone (now Nuovo Pignone, a subsidiary of General Electric), Pietro Benini realized a working prototype of the Barsanti-Matteucci engine, supplying 5 HP. In subsequent years he developed more powerful engines—with one or two pistons—which served as steady power sources, replacing steam engines.
• 1860: Belgian Jean Joseph Etienne Lenoir (1822–1900) produced a gas-fired internal combustion engine similar in appearance to a horizontal double-acting steam beam engine, with cylinders, pistons, connecting rods, and flywheel in which the gas essentially took the place of the steam. This was the first internal combustion engine to be produced in numbers.
• 1862: German inventor Nikolaus Otto designed an indirect-acting free-piston compressionless engine whose greater efficiency won the support of Langen and then most of the market, which at that time was mostly for small stationary engines fueled by lighting gas.
• 1870: In Vienna, Siegfried Marcus put the first mobile gasoline engine on a handcart.
• 1876: Nikolaus Otto, working with Gottlieb Daimler and Wilhelm Maybach, developed a practical four-stroke cycle (Otto cycle) engine. The German courts, however, did not hold his patent to cover all in-cylinder compression engines or even the four-stroke cycle, and after this decision, in-cylinder compression became universal.

• 1879: Karl Benz, working independently, was granted a patent for his internal combustion engine, a reliable two-stroke gas engine, based on Nikolaus Otto's design of the four-stroke engine. Later, Benz designed and built his own four-stroke engine that was used in his automobiles, which became the first automobiles in production.
• 1882: James Atkinson invented the Atkinson cycle engine. Atkinson’s engine had one power phase per revolution together with different intake and expansion volumes, making it more efficient than the Otto cycle.
• 1891: Herbert Akroyd Stuart built his oil engine, leasing rights to Hornsby of England to build them. They built the first cold-start compression-ignition engines. In 1892, they installed the first ones in a water pumping station. In the same year, an experimental higher-pressure version produced self-sustaining ignition through compression alone.
• 1892: Rudolf Diesel developed his Carnot heat engine type motor burning powdered coal dust.
• 1893 February 23: Rudolf Diesel received a patent for the diesel engine.
• 1896: Karl Benz invented the boxer engine, also known as the horizontally opposed engine, in which the corresponding pistons reach top dead center at the same time, thus balancing each other in momentum.
• 1900: Rudolf Diesel demonstrated the diesel engine in the 1900 Exposition Universelle (World's Fair) using peanut oil (see biodiesel).
• 1900: Wilhelm Maybach designed an engine built at Daimler Motoren Gesellschaft—following the specifications of Emil Jellinek—who required the engine to be named Daimler-Mercedes after his daughter. In 1902 automobiles with that engine were put into production by DMG.
• 1908: New Zealand inventor, Ernest Godward started a motorcycle business in Invercargill and fitted the imported bikes with his own invention – a petrol economiser. His economisers worked as well in cars as they did in motorcycles. He invented 72 models of the economiser and by the 1930s was recognised as the world’s leading authority on the internal combustion engine.

Applications

Internal combustion engines are most commonly used for mobile propulsion in automobiles, equipment, and other portable machinery. In mobile equipment, internal combustion is advantageous, since it can provide high power-to-weight ratios together with excellent fuel energy-density. These engines have appeared in transport in almost all automobiles, trucks, motorcycles, boats, and in a wide variety of aircraft and locomotives, generally using petroleum (called All-Petroleum Internal Combustion Engine Vehicles or APICEVs). Where very high power is required, such as jet aircraft, helicopters and large ships, they appear mostly in the form of turbines.

They are also used for electric generators (i.e., 12V generators) and by industry.

Operation

All internal combustion engines depend on the exothermic chemical process of combustion: the reaction of a fuel, typically with the oxygen from the air, although other oxidizers such as nitrous oxide may be employed. Also see stoichiometry.

The most common modern fuels are made up of hydrocarbons and are derived mostly from petroleum. These include the fuels known as dieselfuel, gasoline and petroleum gas, and the rarer use of propane gas. Most internal combustion engines designed for gasoline can run on natural gas or liquefied petroleum gases without major modifications except for the fuel delivery components. Liquid and gaseous biofuels, such as ethanol and biodiesel (a form of diesel fuel that is produced from crops that yield triglycerides such as soybean oil) can also be used. Some can also run on hydrogen gas.

All internal combustion engines must achieve ignition in their cylinders to create combustion. Typically engines use either a spark ignition (SI) method or a compression ignition (CI) system. In the past, other methods using hot tubes or flames have been used.

Petroleum internal combustion engines

Main article: Petroleum

Gasoline Ignition Process

Electrical/gasoline-type ignition systems (that can also run on other fuels, as previously mentioned) generally rely on a combination of a lead-acid battery and an induction coil to provide a high-voltage electrical spark to ignite the air-fuel mix in the engine's cylinders. This battery can be recharged during operation using an electricity-generating device such as an alternator or generator driven by the engine. Gasoline engines take in a mixture of air and gasoline and compress to less than 185 psi and use a spark plug to ignite the mixture when it is compressed by the piston head in each cylinder.

===Diesel Engine Ignition Proc Sabrina james ignition systems, such as the diesel engine and HCCI engines, rely solely on heat and pressure created by the engine in its compression process for ignition. The compression that occurs is usually more than three times higher than a gasoline engine. Diesel engines will take in air only, and shortly before peak compression, a small quantity of diesel fuel is sprayed into the cylinder via a fuel injector that allows the fuel to instantly ignite. HCCI type engines will take in both air and fuel but continue to rely on an unaided auto-combustion process due to higher pressures and heat. This is also why diesel and HCCI engines are also more susceptible to cold starting issues, though they will run just as well in cold weather once started. Most diesels also have battery and charging systems; however, this system is secondary and is added by manufacturers as luxury for ease of starting, turning fuel on and off (which can also be done via a switch or mechanical apparatus), and for running auxiliary electrical components and accessories. Most old engines, however, rely on electrical systems that also control the combustion process to increase efficiency and reduce emissions.

Energy and pollution

Once ignited and burnt, the combustion products—hot gases—have more available energy than the original compressed fuel/air mixture (which had higher chemical energy). The available energy is manifested as high temperature and pressure which can be translated into work by the engine. In a reciprocating engine, the high-pressure gases inside the cylinders drive the engine's pistons.

Once the available energy has been removed, the remaining hot gases are vented (often by opening a valve or exposing the exhaust outlet) and this allows the piston to return to its previous position (top dead center, or TDC). The piston can then proceed to the next phase of its cycle, which varies between engines. Any heat not translated into work is normally considered a waste product and is removed from the engine either by an air or liquid cooling system.

Engine Efficiency

The efficiency of various types of internal combustion engines varies, but it is nearly always lower than electric motor energy efficiency. Most gasoline-fueled internal combustion engines, even when aided with turbochargers and stock efficiency aids, have a mechanical efficiency of about 20% . The efficiency may be as high as 37% at the optimum operating point. Most internal combustion engines waste about 36% of the energy in gasoline as heat lost to the cooling system and another 38% through the exhaust. The rest, about 6%, is lost to friction. Rocket engines can approach 70% efficiency at some parts of a flight; made possible by the very high combustion temperature and lower exhaust temperatures, but while the average efficiency depends on the mission, for a launch vehicle to reach Low Earth Orbit the overall efficiency is only around 10%.

Hydrogen Fuel Injection, or HFI, is an engine add-on system that improves the fuel economy of internal combustion engines by injecting hydrogen as a combustion enhancement into the intake manifold. Fuel economy gains of 15% to 50% have been claimed. A small amount of hydrogen added to the intake air-fuel charge increases the octane rating of the combined fuel charge and enhances the flame velocity, thus permitting the engine to operate with more advanced ignition timing, a higher compression ratio, and a leaner air-to-fuel mixture than otherwise possible]. The result is lower pollution with more power and increased efficiency. Some HFI systems use an on board electrolyzer to generate the small amount of hydrogen needed in the system, around 5% of total BTU. A small tank of pressurized hydrogen can also be used, but this method necessitates refilling. Hydrogen in liquid form is seldom used because it is difficult to store.

There has also been discussion of other types of internal combustion engines, such as the Split Cycle Engine, that utilize high compression pressures in excess of 2000 psi and combust after top dead center (the highest & most compressed point in an internal combustion piston stroke). The claimed efficiency of this engine, by calculation, is 42%. This has yet to be demonstrated as of March 2007.

Air and noise pollution

Internal combustion engines—particularly reciprocating internal combustion engines—produce air pollution emissions, due to incomplete combustion of carbonaceous fuel, leading to carbon monoxide and some soot along with oxides of nitrogen and sulfur and some uncombusted hydrocarbons, depending on the operating conditions and the fuel/air ratio. The primary causes of this are the need to operate near the stoichiometric ratio for gasoline engines in order to achieve combustion (the fuel would burn more completely in excess air) and the "quench" of the flame by the relatively cool cylinder walls. Quenching is commonly observed in diesel (compression ignition) engines that run on natural gas, when running at lower speed. It reduces the efficiency and increases knocking and sometimes causes the engine to stall.

Diesel engines produce a range of pollutants, including aerosols of (PM10) that are known to penetrate deeply into human lungs. Engines running on liquified petroleum gas (LPG) are low in emissions as LPG burns with low emissions and does not contain sulfur or lead.

• Many fuels contain sulfur, leading to sulfur oxides (SOx) in the exhaust, promoting acid rain.
• The high temperature of combustion creates greater proportions of nitrogen oxides (NOx), demonstrated to be hazardous to both plant and animal health.
• Net carbon dioxide production is not a necessary feature of engines, but since most engines combust fossil fuels, this usually occurs. If engines are run from biomass, then no net carbon dioxide is produced, since the vegetation absorbs as much or more carbon dioxide while growing.
• Hydrogen engines need only produce water; but when air is used as the oxidizer, nitrogen oxides are also produced.

In addition, significant contributions to noise pollution are made by internal combustion engines. Most of this noise produced is due to automobile and truck traffic operating on highways and street systems.

Parts

For a four-stroke engine, key parts of the engine include the crankshaft (purple), one or more camshafts (red and blue), and valves. For a two-stroke engine, there may simply be an exhaust outlet and fuel inlet instead of a valve system. In both types of engines, there are one or more cylinders (grey and green), and for each cylinder, there is a spark plug (darker-grey), a piston (yellow), and a crank (purple). A single sweep of the cylinder by the piston in an upward or downward motion is known as a stroke. The downward stroke that occurs directly after the air/fuel mix passes from the carburetor or fuel injector to the cylinder where it is ignited is known as a power stroke.

A Wankel engine has a triangular rotor that orbits in an epitrochoidal (figure 8 shape) chamber around an eccentric shaft. The four phases of operation (intake, compression, power, exhaust) take place in what is effectively a moving, variable-volume chamber.

A Bourke Engine uses a pair of pistons integrated to a Scotch Yoke that transmits reciprocating force through a specially designed bearing assembly to turn a crank mechanism. Intake, compression, power, and exhaust occur in each stroke.

Classification

The fundamental difference between an engine and a motor is that a motor converts electricity into mechanical energy, whereas an engine converts thermal energy into mechanical energy. At one time, the word "engine" (from Latin, via Old French, ingenium, "ability") meant any piece of machinery — a sense that persists in expressions such as siege engine. A "motor" (from Latin motor, "mover") is any machine that produces mechanical power. Traditionally, electric motors are not referred to as "engines," but combustion engines are often referred to as "motors." (An electric engine refers to locomotive operated by electricity).

However, many people consider engines as those things which generate their power from within, and motors as requiring an outside source of energy to perform their work.

Principles of operation

Reciprocating:

• Crude oil engine
• Two-stroke cycle
• Four-stroke cycle
• Six stroke engine
• Hot bulb engine
• Diesel engine
• Poppet valves
• Sleeve valve
• Atkinson cycle
• Free-piston engine

• Proposed
  • Bourke engine
• Improvements
• Controlled Combustion Engine

Rotary:

• Demonstrated:
  • Wankel engine
  • Nutating disc engine
• Proposed:
  • Orbital engine
  • Quasiturbine
  • Rotary Atkinson cycle engine
  • Toroidal engine
  • Trochilic engine

Continuous combustion:

• Gas turbine
• Jet engine
• Rocket engine

Engine cycle

Two-stroke

Main article: Two-stroke cycle

Engines based on the two-stroke cycle use two strokes (one up, one down) for every power stroke. Since there are no dedicated intake or exhaust strokes, alternative methods must be used to scavenge the cylinders. The most common method in spark-ignition two-strokes is to use the downward motion of the piston to pressurize fresh charge in the crankcase, which is then blown through the cylinder through ports in the cylinder walls.

Spark-ignition two-strokes are small and light for their power output and mechanically very simple; however, they are also generally less efficient and more polluting than their four-stroke counterparts. However, in single-cylinder small motor applications, cc for cc, a two-stroke engine produces much more power than equivalent 4 strokes, due to the enormous advantage of having 1 power stroke for every 360 degrees of crankshaft rotation (compared to 720 degrees in a 4 stroke motor).

Small displacement, crankcase-scavenged two-stroke engines have been less fuel-efficient than other types of engines when the fuel is mixed with the air prior to scavenging, allowing some of it to escape out of the exhaust port. Modern designs (Sarich and Paggio) use air-assisted fuel injection, which avoids this loss, and are more efficient than comparably sized four-stroke engines. Fuel injection is essential for a modern two-stroke engine in order to meet ever stringent emission standards.

Research continues into improving many aspects of two-stroke motors, including direct fuel injection, amongst other things. Initial results have produced motors that are much cleaner burning than their traditional counterparts.

Two-stroke engines are widely used in snowmobiles, lawnmowers, weed-whackers, chain saws, jet skis, mopeds, outboard motors, and many motorcycles.

The largest compression-ignition engines are two-strokes and are used in some locomotives and large ships. These engines use forced induction to scavenge the cylinders. An example of this type of motor is the Wartsila-Sulzer turbocharged 2 stroke diesel as used in large container ships. It is the most efficient and powerful engine in the world, with over 50% thermal efficiency. For comparison, the most efficient small 4-stroke motors are around 43% thermal efficiency (SAE 900648), and size is an advantage for efficiency due to the increase in the ratio of volume to area.

Four-stroke

Main article: Four-stroke cycle

Engines based on the four-stroke or Otto cycle have one power stroke for every four strokes (up-down-up-down) and are used in cars, larger boats, and many light aircraft. They are generally quieter, more efficient, and larger than their two-stroke counterparts. There are a number of variations of these cycles, most notably the Atkinson and Miller cycles. Most truck and automotive diesel engines use a four-stroke cycle, but with a compression heating ignition system. This variation is called the diesel cycle. The steps involved here are:

1. Intake stroke: Air and vaporized fuel are drawn in.
2. Compression stroke: Fuel vapor and air are compressed and ignited.
3. Combustion stroke: Fuel combusts and piston is pushed downwards.
4. Exhaust stroke: Exhaust is driven out.

Five-stroke

Engines based on the five-stroke cycle are a variant of the four-stroke cycle. Normally the four cycles are intake, compression, combustion, and exhaust. The fifth cycle added by Delautour is refrigeration. Engines running on a five-stroke cycle are claimed to be up to 30 percent more efficient than equivalent four-stroke engines.

Six-stroke

The six stroke engine captures the wasted heat from the 4-stroke Otto cycle and creates steam, which simultaneously cools the engine while providing a free power stroke. This removes the need for a cooling system, making the engine lighter while giving 40% increased efficiency over the Otto Cycle.

Beare Head Technology combines a four-stroke engine bottom end with a ported cylinder, which closely resembles that of a two-stroke: thus, 4+2 = six-stroke. It has an opposing piston that acts in unison with auxiliary low pressure reed and rotary valves, allowing variable compression and a range of tuning options.

Bourke engine

Main article: Bourke engine

In this engine, two diametrically opposed cylinders are linked to the crank by the crank pin that floats on a "triple slipper bearing" (a type of hydrodynamic tilting-pad fluid bearing) that goes through the common Scotch yoke. Unlike the common two-stroke engine, the burnt gases and the incoming fresh air do not mix in the cylinders, contributing to a cleaner, more efficient operation. The Scotch yoke mechanism also prevents side thrust, preventing any piston slap, allowing operation as a detonation or "explosion" engine. This also greatly reduces friction between pistons and cylinder walls. The Bourke engine's combustion phase more closely approximates constant volume combustion than either four-stroke or two-stroke cycles do. It also uses fewer moving parts and has to overcome less friction than conventional crank and slider engines with poppet valves. In addition, its greater expansion ratio means more of the heat from its combustion phase is utilized than in conventional spark ignition engines.

Controlled Combustion Engine

Main article: Controlled Combustion Engine

These are also cylinder-based engines, which may be either single- or two-stroke but use, instead of a crankshaft and piston rods, two gear-connected, counterrotating concentric cams to convert reciprocating motion into rotary movement. These cams practically cancel out sideward forces that would otherwise be exerted on the cylinders by the pistons, greatly improving mechanical efficiency. The number of lobes of the cams (always an odd number not less than 3) determines the piston travel versus the torque delivered. In this engine, there are two cylinders that are 180 degrees apart for each pair of counterrotating cams. For single-stroke versions, there are as many cycles per cylinder pair as there are lobes on each cam, and twice as many for two-stroke engines.

Wankel

Main article: Wankel engine

The Wankel engine (rotary engine) does not have piston strokes, so is more properly called a four-phase, rather than a four-stroke, engine. It operates with the same separation of phases as the four-stroke engine, with the phases taking place in separate locations in the engine. This engine provides three power 'strokes' per revolution per rotor (while it is true that 3 power strokes occur per ROTOR revolution, due to the 3/1 revolution ratio of the rotor to the eccentric shaft, only 1 power stroke per shaft revolution actually occurs), typically giving it a greater power-to-weight ratio than piston engines. This type of engine is most notably used in the current Mazda RX-8, the earlier RX-7, and other models.

Gas turbine

Main article: Gas turbine

Gas turbines cycles (notably jet engines) do not use the same system to both compress and then expand the gases; instead, separate compression and expansion turbines are employed, giving continuous power. Essentially, the intake gas (normally air) is compressed and then combusted with a fuel, which greatly raises the temperature and volume. The larger volume of hot gas from the combustion chamber is then fed through the gas turbine, which is then able to power the compressor. The exhaust gas may be used to provide thrust, supplying only sufficient power to the turbine to compress incoming air (jet engine); or as much energy as possible can be supplied to the shaft (gas turbine proper).

Disused methods

In some old noncompressing internal combustion engines: In the first part of the piston downstroke, a fuel/air mixture was sucked or blown in. In the rest of the piston downstroke, the inlet valve closed and the fuel/air mixture fired. In the piston upstroke, the exhaust valve was open. This was an attempt at imitating the way a piston steam engine works. Since the explosive mixture was not compressed, the heat and pressure generated by combustion was much less, causing lower overall efficiency.

Fuels and oxidizers

Nowadays, fuels used include:

• Petroleum:
  • Petroleum spirit (North American term: gasoline, British term: petrol)
  • Petroleum diesel.
  • Autogas (liquified petroleum gas).
  • Compressed natural gas.
  • Jet fuel (aviation fuel)
• Coal:
  • Most methanol is made from coal.
  • Gasoline-like fuels can be made from coal.
• Biofuels and vegoils:
  • Peanut oil and other vegoils.
  • Biofuels:
    • Biobutanol (replaces gasoline).
    • Biodiesel (replaces petrodiesel).
    • Bioethanol and Biomethanol (wood alcohol) and other biofuels (see Flexible-fuel vehicle).
    • Biogas
• Hydrogen

Even fluidized metal powders and explosives have seen some use. Engines that use gases for fuel are called gas engines, and those that use liquid hydrocarbons are called oil engines. However, gasoline engines are also often colloquially referred to as 'gas engines.'

The main limitations on fuels are that it must be easily transportable through the fuel system to the combustion chamber and that the fuel releases sufficient energy in the form of heat upon combustion to make use of the engine practical.

Diesel engines are generally heavier, noisier, and more powerful at lower speeds than gasoline engines. They are also more fuel-efficient in most circumstances, and are used in heavy road vehicles, some automobiles (increasingly so for their increased fuel efficiency over gasoline engines), ships, railway locomotives, and light aircraft. Gasoline engines are used in most other road vehicles, including most cars, motorcycles and mopeds. Note that in Europe, sophisticated diesel-engined cars have taken over about 40% of the market since the 1990s. There are also engines that run on hydrogen, methanol, ethanol, liquefied petroleum gas (LPG) and biodiesel. Paraffin and tractor vaporizing oil (TVO) engines are no longer seen.

Oxidizers

Since air is plentiful at the surface of the earth, the oxidizer is typically atmospheric oxygen, which has the advantage of not being stored within the vehicle, increasing the power-to-weight and power to volume ratios. There are other materials that are used for special purposes, often to increase power output or to allow operation under water or in space.

• Compressed air has been commonly used in torpedoes.
• Compressed oxygen, as well as some compressed air, was used in the Japanese Type 93 torpedo. Some submarines are designed to carry pure oxygen.
• Nitromethane is added to some racing and model fuels to increase power and control combustion.
• Nitrous oxide has been used, with extra gasoline, in tactical aircraft and in specially equipped cars, to allow short bursts of added power from engines that otherwise run on gasoline and air. (It is also used in the Burt Rutan rocket spacecraft).
• Hydrogen peroxide power was under development for German World War II submarines and may have been used in some non-nuclear submarines.
• Black or smokeless gunpowder has been used in diesel engine starters, to deploy or jettison equipment remotely, and by Alphonse Pénaud in pioneering model aircraft.
• Other chemicals such as chlorine or fluorine have been used experimentally, but have not been found to be practical.

Hydrogen engine

Some have theorized that in the future, hydrogen might replace such fuels. Furthermore, with the introduction of hydrogen fuel cell technology, the use of internal combustion engines may be phased out. The advantage of hydrogen is that its combustion produces only water. This is unlike the combustion of fossil fuels, which produce carbon dioxide, carbon monoxide resulting from incomplete combustion; and other local and atmospheric pollutants such as sulphur dioxide and nitrogen oxides that lead to urban respiratory problems, acid rain, and ozone gas problems. However, free hydrogen for fuel does not occur naturally, and oxidizing it liberates less energy than it takes to produce hydrogen in the first place, due to the second law of thermodynamics. Note also, that if the atmosphere is used as the oxidizer in high temperature combustion, the resultant nitrogen oxide byproducts must be reduced by an appropriate catalytic converter.

Another problem with hydrogen as a fuel in a conventional four-stroke poppet valve engine is a tendency to preignite, due to the presence of a hot exhaust valve. Certain engine types such as the Wankel rotary engine and various uniflow reciprocating types do not have this inherent problem. A recently developed nutating disc engine also appears to offer an alternative solution to this problem.

Being a thermodynamic process, the overall efficiency will likely be substantially less than if the hydrogen were converted to electricity in a fuel cell and stored in batteries or supercapacitors for high-demand portions of a vehicle's operating cycle.

Although there are multiple ways of producing free hydrogen, those require converting combustible molecules into hydrogen or consuming electric energy, so hydrogen does not solve any energy crisis. Moreover, it only addresses the issue of portability and some pollution issues. The disadvantage of hydrogen in many situations is its storage. Liquid hydrogen has extremely low density (14 times lower than water) and requires extensive insulation, whilst gaseous hydrogen requires heavy tankage. Although hydrogen has a higher specific energy, the volumetric energetic storage is still roughly five times lower than petrol, even when liquefied. The 'Hydrogen on Demand' process (see direct borohydride fuel cell), designed by Steven Amendola, creates hydrogen as it is needed, but has other issues, such as the high price of the sodium borohydride, the raw material. Sodium borohydride is renewable and could become cheaper if more widely produced.