METHANE AS VEHICLE FUEL.
A fuel in expansion
The methane gas extracted
from Lake Kivu could also be an excellent fuel for road vehicles. Methane as a
fuel is known in France under the name GNV : vehicle natural gas. It has to be
compressed at 200 bars in special reservoirs. It is stored and used in the form
of a gas. The amount of gas in 1 m3 for one bar of pressure and at 20°C corresponds
to about a litre of deisel fuel. The calorific energy of the gas (is) 8.9 to 12.8
KwH/m3 depending on the amount of other gases, particularly CO2, mixed with the
Because of the high pressure for storing a reasonable reservoir,
size limits methane-powered vehicles to short distance usage. Methane offers an
attractive alternative to deisel and petrol for buses, taxis, vans for artisans,
delivery, civil service etc...
The combustion of natural
gas gives off less toxic pollutants. No sulphur, lead or benzene. No measurable
particles. No smell or black smoke from the exhaust.
Possibility of reducing
global emission of CO2 because this carburant contains the least carbon in its
Chemical equation of the combustion of methane in air : CH4 +2(02
+ 3.76 N2) = 2 H2O + CO2 + 2 53.76 N2)+ Energy
Chemical equation of the
combustion of deisel in air : C12 H26 + 37/2(02+3.76N2) = 12 CO2 + 13 H2O + 37/2
(3.76 N2) + Energy
better ratio C/H and so less CO2 emission
Methane gas is very high octane (120 - 130) which allows
it to function with high output in spark ignition engines. To compare, the octane
level of ordinary petrol in France is 95. The octane level measures the resistance
to spontaneous ignition when petrol is compressed and heated. The higher the level,
the more utilisation is possible in high output engines.
which are powered by natural gas have petrol engines equipped with gas injection.
So that they can work with both carburants the volumetric ratio is adapted to
the petrol. But this does not allow for the exploitation of the gas at its octane
level - consumption could be less with an engine specifically for gas.
vehicles are equipped with deisel engines transformed into spark ignition engines
: lowering of the volumetric ratio, specific systems for starting and gas injection
(the engine no longer works on a deisel cycle but on the spark ignition cycle).
In this case the potential of the gas is reached but double carburation is impossible.
engine transformed for GNV. Note injectors upstream from intake valves and the
spark plugs inserted instead of the original deisel engine's injectors.
lorry equipped for GNV
of gas injectors on a car (Citroen Berlingo)
The engine retains a petrol injection
system for functioning with double carburation
Gas storage in vehicles
for French urban buses are grouped by 7 (9 as an option) for a total volume of
882 litres (single tank volume 126 litres) at 200 bars, this giving the equivalent
of 210m3 of gas at atmospheric pressure. This volume is equivalent to 210 litres
of deisel fuel and gives the bus an autonomy of 300 to 400 km. These reservoirs
in carbon fibre and epoxy resin (or aluminium) have a maximum service pressure
of 288 bars and a minimum working pressure of 20 bars. Test pressure is 600 bars.
vehicles have a reservoir mounted behind the front seats. Some have the reservoirs
mounted under the floor, thus retaining useful space (see van at top of page).
in a light goods vehicle.
bus, the tanks mounted on the roof.
bus gas tanks
of methane carburant
A natural gas service station is more complex than
its traditional cousin since the gas needs to be compressed.
There are two
possible types of service station :
Simple compression (200 bars). Filling
is thus rather slow (several hours). Parking places have to be created for the
purpose of recharging.
Supercharging, followed by storing at high pressure
(250 bars) allows for rapid refilling on dedicated lanes. This solution is obviously
more expensive than the previous one;
A high capacity station (500 vehicles)
costs around 500K€. A medium capacity station (50 vehicles) costs 80 K€.
station for light vehicles
up a bus
This small scale apparatus permits the filling of
a private car's tank in a few hours (about 5 hours). It is designed for small
fleets of vehicles. In France it costs about 5000 €.
up at home
of its composition, natural gas is non toxic and lighter than air (0.55 density)
and thus rises very quickly (0.8 m/s) in the atmosphere, this avoiding all accumulation
at ground level in case of a leak. Do not confuse it with GPL (liquified petroleum
gas : a mixture of butane and propane) which has a density of 2.15 and which spreads
at ground level if there is a leak.
of fire or explosion
In order to have combustion with natural gas there
must be :
- A mixture of air and gas (between 5% and 15% of gas)
spontaneous ignition temperature of 540°C
- A confined space
mixture of air and natural gas has a very small range of ignition and the temperature
needed for spontaneous ignition is very high. Inflammation is highly unlikely
with a vehicle.
Tests show that methane gas is one of the safest fuels.
Natural gas is not toxic
- 80/95% methane
- no carbon monoxide
If there is a leak, the
natural gas can take the place of the air and thus deprive a person in an enclosed
space of oxygen.
Because of the speed with which natural gas rises this
risk is minimal in properly-ventilated areas and even less in vehicle users in
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