Converting a Diesel Engine to Dual-Fuel Using Natural Gas
As we have seen from the previous section, the use of natural gas in a diesel engine as a dual fuel requires the engine to be modified as although natural gas can be compressed it will not ignite without a spark plug or addition of diesel to the gas
Natural gas is being used along with diesel oil as a dual-fuel in most transport sectors, including ships engines.
Millions of vehicles worldwide have been converted to run on gas along with diesel, with India being a world leader in this field.
One of the biggest problems with using natural gas is its very low density. This means that a large tank is required to contain the gas.
Nonetheless, CNG is a much safer fuel than gasoline or diesel. Any gas leaks at filling stations or due to road accidents quickly dissipate in the atmosphere, rather than exploding or catching fire. The gas is available at numerous outlets across the states, being supplied as compressed natural gas (CNG).
The safety measures in place at filling stations include a sealed filling system at the pump nozzle and automatic cut-off if the hose should burst.
The cylindrical gas tank is designed to withstand the 3000lbs/square inch pressure of the CNG and is usually located in the boot/trunk of the vehicle, with the diesel fuel oil tank being kept in its original location.
Dual fuel conversion then consists of the installation of a cylindrical pressure tank and high pressure piping from there to the engine diesel/gas control unit.
In addition to solenoid valves, diesel modulator, high/low pressure gas filters and other small components, the following major engine components are required.
- Dual-Fuel Electronic Control Unit – the ECU
The ECU sends high speed wide pulse modulated signals to the natural gas and diesel injectors. The component bases this on constant measurements of the air manifold pressure and temperature along with gas pressure and temperature.
The unit controls the supply of dual fuel to the engine ensuring optimum fuel control, which is usually 8% diesel combined with 92% natural gas. This ensures maximum fuel efficiency along with lowest possible emissions of CO2.
Most modern ECU’s can also control the “knock" from dual fuel engines by automatically altering the combustion air/gas ratio. In the event of gas supply malfunction, the ECU will shut the gas system down and revert to 100% diesel supply.
- Turbo-Charger Air Bypass (TAB)
The TAB unit is required to control the amount of air provided by the turbo-charger. It achieves this through a bypass valve, usually of butterfly design.