# Hydrogen combustion engine

12-cylinder hydrogen combustion engine of the BMW Hydrogen 7
Hydrogen filler neck of a BMW

A hydrogen combustion engine ( hydrogen engine for short ) is an internal combustion engine that runs on hydrogen as fuel . It converts chemical energy into mechanical work and heat. The basis is the oxyhydrogen reaction (two parts hydrogen with one part oxygen) in a reciprocating or rotary piston internal combustion engine. Mostly reciprocating piston engines are used that work according to the Otto principle, but there are also hydrogen combustion engines that work according to the diesel principle. The efficiency of the hydrogen engine is between that of the conventional gasoline-powered Otto engine and that of the diesel engine. The hydrogen combustion engine is not to be confused with the drive system hydrogen-oxygen fuel cell and electric motor , it competes with them.

## history

Hippomobile
BMW E38 750hL

A hydrogen reciprocating engine for an automobile was first used in 1807 by the French officer François Isaac de Rivaz. He applied for a patent for this motor. In this engine, the hydrogen was carried in a balloon and blown into the side of the cylinder, where the hydrogen-air mixture was ignited with an ignition spark. The cylinder was quite long; During ignition, the piston was thrown upwards, at top dead center the piston fell back and a toothed rack attached to the piston engaged a toothed wheel that was connected to the wheels by a belt. The piston weight drove the wheels. Étienne Lenoir built the Hippomobile in 1860 , an automobile powered by a Lenoir engine that could use hydrogen produced by electrolysis as fuel. The not very reliable motor developed an output of 700 W at 80 min −1 and had an efficiency of around 3%. In 1938, Rudolf Erren converted some gasoline and diesel engines to run on hydrogen. In 1996 MAN tried to equip some city buses with hydrogen combustion engines. These engines worked according to the Otto process and had external mixture formation; later engines with internal mixture formation were also used. In 2000 BMW built the hydrogen vehicle E38 750hL with a reciprocating piston engine in small numbers of 15 units ; In 2007, the successor E68 was presented, 100 of which were built.

## Hydrogen as a fuel

### Properties of hydrogen

Hydrogen has a high energy content in relation to its mass, but since its density of 0.089882 kg · m −3 is not very high under normal conditions, the energy content in relation to its volume is very low. Therefore, the calorific value of a hydrogen-air mixture depends on how hydrogen and air are mixed with one another - the calorific value can be above or below that of a conventional gasoline-air mixture. Hydrogen has very large ignition limits , ranging from 4 vol% hydrogen ( air ratio ) to 75.6 vol% hydrogen (air ratio ). Theoretically, therefore, a hydrogen engine can still be operated with a homogeneous fuel-air mixture and the load can be controlled in the entire operating range of the engine by changing the quality . However, because of the wide ignition limits, it is also possible to use operation with an inhomogeneous, overstoichiometric mixture. As with other fuels, more ignition energy has to be used with a higher air ratio, but the necessary ignition energy for a hydrogen-air mixture is around 0.017 mJ (minimum value), only a tenth of that of a conventional gasoline-air mixture. At 858 K (585 ° C), the auto-ignition temperature of hydrogen is far higher than that of gasoline or diesel fuel, so hydrogen has a high knock resistance . However, this makes it more difficult to operate an engine that works according to the diesel process, which is why the charge temperature in a hydrogen combustion engine that works according to the diesel process must be increased by measures such as high compression in order to safely initiate spontaneous ignition. Since the laminar flame speed of a hydrogen internal combustion engine is very high with a maximum of 3 m s −1 , even with overstoichiometric mixtures, a short burning time can be achieved, which has a favorable effect on the efficiency, however, due to the rapid pressure increase in the combustion chamber, a greater load on the Engine means. ${\ displaystyle \ lambda = 10 {,} 1}$${\ displaystyle \ lambda = 0 {,} 13}$${\ displaystyle \ lambda \ approx 10}$

### Hydrogen storage

The hydrogen for the hydrogen engine is either liquefied (–253 ° C), highly compressed (300–700 bar) or stored in a chemical or physical compound (such as metal hydride or LOHC ). Metal hydride storage systems were still in the development stage in 2018. When storing in compressed or liquefied form, energy must be used to store the hydrogen; this corresponds to around 15% of the calorific value of the hydrogen. Liquid tanks outgas due to unavoidable insulation losses when not in use. With the BMW Hydrogen7, this process begins after 17 hours of idle time and can result in a half-full tank evaporating within 9 days. The disadvantage of metal hydride tanks is their high cost and low storage density of around 2 to 3% of the mass. Hydrogen can be stored and transported in liquid form without pressure through the hydrogenation of organic substances ( e.g. N-ethylcarbazole ). In mathematical terms, storage densities of 14–20 percent by mass are possible. Storage densities of 6–8 percent by mass were achieved in the laboratory.

## Features of the hydrogen combustion engine

Hydrogen engines can be divided according to different characteristics. Since the Otto process is usually used because the diesel process has not yet reached a near-series state for hydrogen operation, a distinction is made mainly according to the type of mixture formation: Essentially, the two methods of manifold injection (external mixture formation) and direct injection (internal mixture formation) are used that can also be combined. In addition to the mixture formation already mentioned, all distinguishing features are listed below:

• Functional principle (Otto, diesel or diesel pilot injection process)
• (Partial load) change in torque (change in quantity or change in quality )
• Mixture formation (external or internal mixture formation)
• Engine design ( reciprocating piston engine or rotary piston engine )
• Temperature level of the fuel (cryogenic or ambient air temperature)
• State of charge (free suction or charged)
• Charge distribution (homogeneous or stratified)

source

### Torque change

Because of the wide ignition limits, it is possible and, because of the better efficiency, also useful to adjust the torque in a hydrogen combustion engine only by changing the injected fuel mass and not to throttle the intake air (change in quality). In order to optimize the idling behavior and the burning time in the lower load range, however, it can be advantageous to throttle the intake air. If a three-way catalytic converter is used for exhaust gas purification, throttling can also be used. ${\ displaystyle \ lambda \ approx 1}$

### Mixture formation

#### External mixture formation (intake manifold injection)

The hydrogen stored under pressure is blown in gaseous form with a slight overpressure into the intake pipe in front of the inlet valves ( multi-point injection ). Central injection has proven to be unsuitable. The fuel is mixed with the air before it enters the combustion chamber. This mixture is ignited externally in the combustion chamber after compression. Since the hydrogen displaces some of the fresh air that is drawn in, the mixture heating value is lower than in a comparable petrol operation, which means that under the same conditions and with a stoichiometric mixture, the output is around 17% lower than in petrol operation. Alternatively, cryogenic (liquid) hydrogen can be injected. The comparatively warm intake air is cooled down, the gas mixture decreases in volume and the degree of filling improves. With kyrogenic intake manifold injection, the output is at the level of direct injection and around 15% higher than when operating with gasoline. However, with intake manifold injection, an increase in the engine load leads to re-ignition, since the mixture can ignite in hot spots, which in turn means a performance disadvantage.

#### Internal mixture formation (direct injection)

In this mixing process, gaseous hydrogen is blown directly into the combustion chamber under high pressure. The charge mixture is cooled and ignited with a spark plug. The filling compared to the suction tube injection is higher and the lower Carnot process temperature is lower. This increases the thermodynamic efficiency and the performance. In the case of direct injection, there are various mixture formation concepts that differ based on the number of injections per work cycle (single injection or multiple injection) and based on the injection time.

The injection time can be selected early, so that the intake stroke is injected with the inlet valve open, but also late, so that the injection in the compression stroke occurs with the inlet valve closed. There is no clear limit between the two concepts, whereby the injection pressure must be higher the later the injection takes place. Injection during the suction stroke results in a mixture with a homogeneous mixture composition, while injection in the compression stroke leads to a stratified charge composition due to the insufficient time for complete mixture homogenization. Furthermore, there can be no re-ignition or pre-ignition during a compression stroke injection, since no hydrogen can enter the intake tract through the inlet valve (which is closed in this case) and there is no fuel in the combustion chamber during the early compression phase. Another advantage of compression stroke injection with mixture stratification is a very short combustion time with high conversion rates, which is beneficial for the efficiency and stable engine running, but this results in a very steep pressure increase in the combustion chamber that can exceed that of a comparable diesel engine. In the case of a homogeneous mixture, however, the burning time depends on the air ratio and the engine load point. ${\ displaystyle \ lambda}$

### Emission behavior

Nitrogen oxide formation as a function of the air ratio and temperature when
the fuel is blown into the intake manifold and homogeneous operation

Since hydrogen does not contain carbon, the exhaust gas cannot theoretically contain any carbon-containing pollutants such as carbon monoxide (CO), carbon dioxide (CO 2 ) and hydrocarbons (HC). However, the carbon-containing oil that is needed for engine lubrication helps ensure that traces of these three pollutants are also contained in the exhaust gas. The only pollutants that are contained in larger concentrations in the exhaust gas of the hydrogen combustion engine are nitrogen oxides (NO x ).

With intake manifold injection, the formation of nitrogen oxide depends on the air ratio and the combustion temperature; as the air ratio increases, so does the temperature and thus the tendency to form nitrogen oxide. With a very high air ratio of , almost no nitrogen oxides are formed, while the formation of nitrogen oxides increases sharply as the air ratio falls. At a maximum is reached while with further increasing air ratio nitric oxide formation decreases again. ${\ displaystyle \ lambda}$${\ displaystyle \ lambda> 2 {,} 2}$${\ displaystyle \ lambda = 1 {,} 3}$

In the case of direct injection, late injection into the compression stroke can reduce the nitrogen oxide formation that occurs at high engine loads when operating with a globally stoichiometric mixture. The reason for this is the mixture stratification resulting from a late injection. This creates an over-rich area in addition to a lean area, which means that the air ratios at which nitrogen oxides would be generated are bypassed. At a low load, however, a late injection of nitrogen oxides would result in the formation of the fuel-rich zones of the overall lean mixture.

## Diesel process in the hydrogen combustion engine

Since hydrogen has very wide ignition limits, it is in principle suitable as a fuel for a diesel engine. However, the auto-ignition temperature is very high at 858 K (585 ° C). This means that the final compression temperature in the combustion chamber must be around 1100 K (827 ° C) in order to ensure sufficient heat input into the hydrogen. Even with an increased compression ratio, it is not possible to reach this temperature. The intake air must therefore be preheated, which, however, makes it impossible to use it in a motor vehicle, such as a passenger car, since the preheating of the air can only be regulated with difficulty. In order to still use the diesel process, the principle-related self-ignition does not have to be initiated by self-ignition of the hydrogen, but by that of an auxiliary fuel. The diesel pilot injection process was developed for this purpose. In addition to the hydrogen, a small amount of diesel fuel is injected, which ignites more easily than hydrogen. Once this diesel pilot jet burns, it acts as a pilot ignition for the hydrogen.

• The exhaust behavior is good. The products of combustion are water vapor and nitrogen oxides (NO x ); the latter can be well controlled with the air ratio . The consumption of lubricating oil causes traces of carbon dioxide , carbon monoxide and hydrocarbons .
• The indicated efficiency of hydrogen internal combustion engines can be around 35%, which makes a hydrogen internal combustion engine more efficient than a conventional gasoline engine operating according to the Otto process, the indicated efficiency of which is around 28%. In comparison, a comparable diesel engine has an indexed efficiency of around 40%.

• Despite the higher efficiency, the performance of hydrogen combustion engines is lower than that of gasoline-powered Otto engines.
• The efficiency of a hydrogen combustion engine is worse than that of the conventional diesel engine and the traction battery and electric motor system.
• Hydrogen has very poor lubricating properties because it does not contain any carbon and at the same time it attacks the lubricating film. The lubricating film is attacked by the hydrogen in two ways: On the one hand by the hydrogen flame, which burns right up to the wall, and not, as is the case with gasoline, goes out when approaching the edge zone. On the other hand, through hydrogenation : hydrogen attacks the carbon-carbon bonds of the long-chain hydrocarbons in the lubricants, the fragments of which burn. A way out of this problem is a ceramic coating and the waiver of lubrication of the running surfaces at all, which is made possible by combining ceramic with ceramic as a running partner.
• The use of liquid hydrogen in a vehicle engine is associated with considerable effort because of its low boiling point (−253 ° C), both during the refueling process and on the vehicle itself, where special materials must be selected that can withstand such temperatures. In addition, the hydrogen heats up over time and therefore requires a larger volume. This means that the hydrogen has to be released into the environment over time and the tank empties. Furthermore, all components must have a very high level of impermeability, since hydrogen, due to its small molecules, diffuses heavily even through other dense substances and leads to embrittlement.
• Investments are required in order to be able to guarantee an infrastructure for a comprehensive supply of hydrogen. In 2011 there were only around 200 hydrogen filling stations worldwide .
• In Germany, hydrogen is predominantly (approx. 90%) obtained by steam reforming , i.e. from fossil, finite primary energies, primarily natural gas. The efficiency in the production of hydrogen is strongly dependent on the type of production; In the case of electrolysis, an efficiency of 38% is achieved in the EU electricity mix, while the production of diesel fuel from crude oil is more than twice as efficient with an efficiency of 85%.

## literature

• Helmut Eichlseder, Manfred Klell, Alexander Trattner: Hydrogen in vehicle technology - generation, storage, application. 4th edition. Springer, Wiesbaden 2018, ISBN 978-3-658-20446-4 .

## Remarks

1. Normal conditions mean a temperature of 273.15 K (0 ° C) and an air pressure of 101.325 kPa

## Individual evidence

1. Eichlseder, Klell, Trattner, p. 32f.
2. Eichlseder, Klell, Trattner, p. 34f.
3. Eichlseder, Klell, Trattner, p. 37
4. Eichlseder, Klell, Trattner, p. 225
5. Eichlseder, Klell, Trattner, p. 32f.
6. Eichlseder, Klell, Trattner, p. 54f.
7. Richard van Basshuysen (Ed.): Otto engine with direct injection and direct injection: Otto fuels, natural gas, methane, hydrogen , 4th edition, Springer, Wiesbaden, 2017. ISBN 978-3-658-12215-7 , pp. 520f.
8. Eichlseder, Klell, Trattner, p. 109
9. Eichlseder, Klell, Trattner, p. 113
10. a b heise online, November 22, 2006: On the way in the hydrogen 7 Series , inserted on February 8, 2012
11. Eichlseder, Klell, Trattner, pp. 138f.
12. a b c Eichlseder, Klell, Trattner, p. 208
13. Eichlseder, Klell, Trattner, pp. 206f.
14. Eichlseder, Klell, Trattner, p. 209
15. a b Eichlseder, Klell, Trattner, p. 211
16. a b c Eichlseder, Klell, Trattner, p. 210
17. Eichlseder, Klell, Trattner, p. 212f.
18. Eichlseder, Klell, Trattner, p. 213
19. Eichlseder, Klell, Trattner, p. 204
20. Eichlseder, Klell, Trattner, p. 214
21. Eichlseder, Klell, Trattner, p. 224
22. Eichlseder, Klell, Trattner, p. 220
23. Daimler and Linde want to build hydrogen filling stations (source: Handelsblatt, as of June 1, 2011)
24. Interview with Wolfgang Reizle (as of July 28, 2011 Source: Auto Motor und Sport)
25. http://www.h2stations.org/ Map of the hydrogen filling stations
26. Eichlseder, Klell, Trattner, p. 13f.