Hydrogen Use for Transportation
Hydrogen Internal Combustion Engines (HICE)
Hydrogen will operate easily with most internal combustion engines due to its wide range of flammability. The difficulty is in getting the engine to work efficiently and reliably. The wide range of flammability means that the engine can operate over a wide range of air to fuel ratios. This allows for a ‘lean’ hydrogen fuel supply to the engine leading to greater fuel economy as well as a more complete combustion of the fuel reducing emissions such as NOx [1]. Running the engine too lean however can result in greatly reduced power over equivalent capacity petrol or diesel engines
Another property of hydrogen that can improve fuel economy is its relatively low ignition energy requirements (approximately one order of magnitude less than is required for petrol ignition). However, this low ignition energy does mean that if ‘hot spots’ in the combustion cylinders develop (on the spark plugs or exhaust valve), pre-ignition can occur leading to reduced efficiency and reliability.
Hydrogen has a high auto-ignition temperature. During cylinder air compression, the temperature of the air rises as denoted in the thermodynamic equation below;
Where;
V1/V2 = compression ratio
T1 = Inlet Temperature
T2 = Final Temperature
Y (Gamma) = Ratio of specific heats
Large auto-ignition temperatures (585 degrees Celsius) mean the engine can operate at a high compression ratio without risk of igniting the fuel prematurely [2]. This is beneficial as it means the engine can operate at much higher compression ratios than that of hydrocarbon based fuels. By comparison, petrol and diesel only require 260-460 and 180-320 degrees Celsius. Higher compression ratios are advantageous since a more complete combustion can occur improving the thermal efficiency.
High flame speed and diffusivity of hydrogen in an engine are also advantageous from a performance and efficiency aspect. The high flame speed of hydrogen (3.46m/s for hydrogen whereas petrol is significantly slower at 0.42m/s) allows the engine to get closer to the theoretical thermodynamic efficiency [2]. Additionally, high diffusivity of hydrogen leads to a more complete mixing of the air and fuel in the cylinder thus improving the combustion. High diffusivity has the added safety benefit should a leak occur where the hydrogen gas would rapidly disperse into the atmosphere whereas a hydrocarbon based fuel would leak on the vehicle.
Since hydrogen has a very low density and is a gas at ambient conditions, it displaces a large volume of the air in a combustion chamber. Typically, a petrol engine will require 1-2% of the volumetric capacity of the combustion chamber whereas, for complete hydrogen combustion, 30% of the combustion chamber is displaced by the hydrogen gas. This results in a reduction in power between a petrol and hydrogen engine. This problem can be rectified by altering the fuel injection configuration. Most petrol engines currently use intake manifold fuel injection where the fuel is mixed with the air in the intake manifold before being injected into the combustion chambers. Petrol engines are now beginning to use direct injection for improved power and economy (diesel engines have used this for a while). Direct injection is particularly useful for hydrogen engines as it allows the cylinder volume to be completely occupied by air before the hydrogen is injected to the closed cylinder at high pressure. This can produce power outputs of around 20% higher than that of intake manifold injection petrol engines whilst further eliminating opportunity for pre-ignition.
Manufacturer BMW have focussed more on developing hydrogen ICE (internal combustion engine) technology rather than solely focussing on the fuel cell option. The advantage of this is a result of the similarities between conventionally fuelled engines and the hydrogen ICE as discussed earlier. BMW’s experimental hydrogen engine vehicle known as the ‘Hydrogen 7’ (based on a petrol engine BMW 7-series) uses a V12 engine similar in architecture to one of the firm’s petrol engines. This offers the advantage of being able to seamlessly switch between operating on petrol and hydrogen fuels even whilst in motion since the car has dual fuel tanks. However the hydrogen engine produces significantly less power than the petrol engine (260hp versus 438hp respectively) due to the need to run the engine on a lean fuel mixture to avoid pre-ignition [3].
A further disadvantage of the hydrogen ICE when compared to a fuel cell is that they can never reach zero emissions like the fuel cell. The relatively large quantities of NOx produced by the hydrogen engine necessitate the use of a filter in the exhaust system to keep the harmful emissions to an acceptable level.
References:
[1] http://www1.eere.energy.gov/hydrogenandfuelcells/tech_validation/pdfs/fcm03r0.pdf
[2] “Fundamentals and Use of Hydrogen as a Fuel”, Author: K. K. Pant and Ram B.
Gupta[3] http://www.autobloggreen.com/2006/09/12/bmw-officially-announces-the-bmw-hydrogen-7/
