*5.2. IC Engine*

One of the main industries contributing to GHG emissions worldwide is the transport industry. The search for the right alternative energy source to reduce fossil fuel addiction has been a long, challenging journey. Ammonia-fuelled vehicles, which have received a lot of publicity recently, are one of the solutions to reduce GHG emissions and fossil fuel dependency. Ammonia can be used as fuel in both spark ignition (SI) and compression ignition (CI). Numerous companies and research groups have tested these engines in the last decades. However, ammonia poses undesirable combustion properties, which require further study of its combustion properties. Table 11 compares the characteristics of ammonia with other IC engine fuels.



Note: L (liquid); C (compressed); AFT (adiabatic flame temperature); HCR (heat capacity ratio).

Ammonia has low flame velocity, very high auto-ignition temperatures, narrow flammability limits and high vaporisation heat compared to other fuels [217]. Narrow flammability limits and high auto-ignition temperatures create problems for NH3 to be used in the engine [221]. Although NH3 can be used as single fuel in the CI engine, extremely higher CR is required to auto-ignite the fuel [222,223]. In addition, high latent heat of vaporisation at the time of injection decreases the gas temperature in the engine, which further complicates it [221]. In the SI engine, the use of ammonia is restricted by low flame velocity and narrow explosion limits resulting in incomplete combustion [224]. Ammonia combustion in the SI-engine can be improved by providing stronger igniters such as plasma jet igniters, smaller combustion chambers to overcome the resistance of NH3 combustion [223]. Supercharging can also achieve improved combustion [225].

In addition to the problems mentioned above, ammonia also shows low flame speed and specific energy in combination with high ignition energies and high auto-ignition temperatures, resulting in a relatively low propagation rate from the combustion [16]. Although ammonia has been successfully used as mono-fuel both in SI and CI engines, such a low ammonia combustion rate induces inconsistency in combustion under conditions of low engine load or high engine speed [225]. Thereby, it is essential to mix with secondary fuel to overcome its disadvantages as a fuel. In addition to ammonia, potential fuels to be used in SI engines are hydrogen, methanol, ethanol, ethane and gasoline. For CI engines, fuels with higher cetane numbers are preferred as a combustion promoter due to the better ignition characteristics [223]. However, these approaches require some special features [16].

On a dual fuel CI engine, ammonia could be used up to 95% of the fuel energy basis with diesel as a combustion promoter. An optimal mixture, however, is 60% of ammonia on an energy basis because a smaller amount of ammonia would limit the flammability of ammonia [226]. Other studies suggested that an optimal content of ammonia is between 60–80% based on the mass basis [225]. A demonstration of biodiesel as a combustion promoter by Kong et al. [227] revealed that the fuel performed similar engine performance characteristics with ammonia/diesel blends. The operating characteristics are, however, different when Dimethyl Ether (DME) is used as the ammonia fuel combustion promoter. The study shows that ammonia could be used up to 80% only when DME is used as a combustion promoter [217]. Moreover, the study also revealed that the fuel mix of ammonia and DME has a competitive energy cost with current diesel fuel. Even the addition of NH3 in the blend has been shown to significantly raise emissions of CO, HC and NOx [228].

On the SI engine, gasoline is used as a combustion promoter in most of the studies. A compression ratio of 10:1 is required to get optimal operation of the engine with ammonia content 70% [229]. Gaseous fuels are chosen for SI-engines due to the same phase with ammonia gas, while anhydrous ammonia would lower the temperature of the in-cylinder, thus adversely affecting subsequent turbulence triggering deteriorated combustion and misfire [228]. In the SI engine powered by ammonia/gasoline, gasoline is port-injected while ammonia gas is direct-injected. Direct injection of ammonia gas substantially lowers the cylinder temperature due to the high latent heat of ammonia. Thus, creating turbulence in the combustion chamber will enhance the combustion of the fuel. However, too small swirls do not affect the combustion, while too large swirls have a negative effect on the combustion by blowing out the flames due to the slow propagation of the ammonia flames [223].

Another alternative is by using hydrogen as a secondary fuel by installing an on-board reformer to split ammonia into hydrogen and nitrogen [230]. Morch et al. [219] give a complete database of SI engine performance with ammonia/hydrogen mixture as a fuel. Ammonia and hydrogen have, in this investigation, been incorporated into the CFR engine intake manifold. A series of studies were undertaken with different excess air ratios and hydrogen ammonia ratios. The results revealed that a fuel mixture with 10 vol.% hydrogens has the best performance in terms of efficiency and power. In a comparison study with gasoline, there is a high possibility that an increase in efficiency and power is caused by a greater compression ratio. The analysis of the system has also shown that most of the heat

required can be covered by the exhaust heat. The diagram showing the fuel system setup for the experiments is given in Figure 9.

**Figure 9.** Diagram showing the fuel system setup for the experiments [219].

Much farther trials of ammonia/hydrogen-fuelled engines were also applied to more commercial applications in Italy, where a prototype for electric vehicles with a 15 kW engine was built. The study found that the optimal performance in full load is achieved with 7% of hydrogen content while only 5% in half-load [219]. More recently, Ezzat and Dincer [231] proposed, thermodynamically analysed and compared two different integrated systems of an ammonia/hydrogen-fuelled engine. The first system is made up of hydrogen production. In the second system, an ammonia fuel cell is added to complement the IC engine. The study shows that the first system has higher energy and exergy efficiency with 61.89% and 63.34%, compared to the second system with 34.73% and 38.44%, respectively. The study also shows that when compared with pure ammonia injection, the use of hydrogen from cracked ammonia is extremely beneficial. In addition to road transportations, ammonia is also a favourable fuel for marine industries [232,233]. Unlike the automotive industry, marine systems are not space-constrained, so that catalytic equipment can be deployed for NOx reduction solutions. In the recent development, MAN Energy Solutions replaces the 3000 B&W double-fuel engines operating in the field of LPG and diesel engines [234].
