Hydrogen Fueled Internal Combustion Engines

In addition to its indirect use in fuel cells, hydrogen can be burnt directly in air within an internal combustion engine, with remarkable advantages over gasoline engines, such as in the case of the BMW Hydrogen 5th generation vehicles equipped with a hydrogen tank (Figure 4.5).9

Curious as it may seem, the reciprocating ICE operated in the Otto- or Diesel mode that came to market in the late 19th century is still the dominating power-train technology in 2012. However, an Otto cycle

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image153

Figure 4.5 The Hydrogen BMW 5 was unveiled at the Expo 2000 Exhibition. (Reproduced from Ref. 7, with kind permission.)

internal combustion engine running on hydrogen has a maximum effi­ciency of about 38%, 8% higher than the gasoline ICE. In addition, hydrogen engine conversion technology is more economical than fuel cells,10 and the ICE is familiar to engineers and craftsmen in the car industry as well as in repair shops. In general, the reciprocating hydrogen internal combustion DI engine has high power density with regard to volume and weight, is highly efficient and nearly emission free.

The idea is that, in order to be successful on the market, the char­acteristics of daily operation and performance of a hydrogen vehicle should be comparable to those of a conventional gasoline or diesel vehicle. After 25 years of research in the field, which started during the 1970s oil crisis, BMW is aware that this requirement can only be met by the installation of a hydrogen ICE (Figure 4.6) which uses energy dense liquid hydrogen (Figure 4.7).

The properties of gaseous hydrogen are significantly different from those of gasoline (Table 4.1). Liquefied hydrogen has lower energy density by volume than gasoline, by approximately a factor of 4, because of the low density of liquid hydrogen. Obviously, gaseous hydrogen has a very low density, which entails a lower density of the air-fuel mixture, Pg, and thus an 18% lower mixture calorific value, HG, in the external mixture hydrogen mode.

However, in the internal mixture mode (Figure 4.8) the low density of hydrogen is not relevant because the pressurized hydrogen is fed to the cylinder by a direct injection system that affords a 17% higher mixture calorific value for hydrogen when compared with gasoline.

Table 4.1 also shows a typical property of hydrogen, namely its capability for ignition within a wide range of air: hydrogen ratios, which

■ *•/ T *1

image154
Подпись: 2000
Подпись: 2500
Подпись: 500
Подпись: 1000 Подпись: 1500

image160image161Pressure [bar]

Figure 4.7 Power/mass characteristics of vehicles with conventional and alternative drive train systems. At room temperature and pressure, the density of hydrogen is so low that it contains less than 1: 300 the energy in an equivalent volume of gasoline.

(Reproduced from Ref. 7, with kind permission.)

image162
allows a remarkable increase in efficiency because of leaner combustion. The high flame propagation velocity of air-hydrogen mixtures entails outstanding combustion properties with significantly shorter combus­tion periods in the full-load range compared with gasoline engines.

Overall, owing to the ability to realize ideal combustion control with high compression ratios, scientists at BMW aim to increase the effective efficiency of a hydrogen DI internal combustion engine to 50%, com­pared with the current 37% (Figure 4.8).

Finally, prolonged experimentation has shown that NOx tailpipe emissions can be minimized to a few ppm by concomitant employment of a simple reduction catalyst for converting the NOx and unburned H2

image163 Подпись: lean combustion (no catalyst) Подпись: Chapter 4

image166engine operation

(reduction

catalyst)

engine out emissions

tailpipe emissions

Relative air/fuei ratio X –

Figure 4.9 Hydrogen direct injection engine out and tailpipe emissions.

(Reproduced from Ref. 7, with kind permission.)

to N2 and H2O in the tailpipe at full loads on a hydrogen engine (with l = 1, stoichiometric mixture), while at part load conditions, a lean engine operation mode (1> 2.2) is chosen electronically that prevents NOx formation in the cylinders, allowing almost emission-free operation throughout the entire engine load and speed range (Figure 4.9).

For comparison, the first generation hydrogen vehicle, a 4-cylinder BMW Sedan, went into operation in 1979. It had 60 kW maximum power and a top speed of 160 km h-1 with a range of operation around 400 km. The 5th generation vehicles, launched in 2000, had 150 kW maximum power and could be operated in a dual mode (hydrogen and gasoline), allowing them to extend their range of operation from about 300 km in the hydrogen mode to a total of about 900 km. For further development, future hydrogen ICEs will have higher exergy efficiencies as a result of reducing inherent irreversibilities, through utilizing the huge amounts of waste heat in the cooling system and the exhaust.

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