written by: Willie Scott•edited by: Lamar Stonecypher•updated: 11/22/2011
Natural gas can be used as a dual-fuel in diesel engines following a conversion. The gas is compressed (CNG) and contained in a pressure tank usually in an automobiles boot or underneath the truck’s body. It is less toxic and safer to use, emitting less CO2, than a normal petrol or diesel engine.
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As governments worldwide seek methods of reducing their CO2 emissions from transport, natural gas can provide an answer. Refined natural gas (methane) has a chemical composition of 4 hydrogen atoms, but only 1 carbon atom. Therefore, on combustion in an engine it emits less CO2 than gasoline or diesel oil. The gas can be compressed and stored in the automobile or truck where it is a much safer and environmentally friendly option to diesel oil or gasoline fuels.
The following sections examine the use of natural gas as a dual fuel in a diesel engine. The first section provides an overview of the chemical composition differences between gasoline, diesel, and natural gas.
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Comparing Diesel, Gasoline, and Natural Gas
The three fuels used to propel our automobiles, RVs, and commercial trucks are all hydrocarbons or derivatives of them. As such they all contain various numbers of carbon atoms in hydrocarbon chains that when combusted produce CO2. It is these CO2 tailpipe emissions that our governments are trying to mitigate in the transport sector.
Listed below are some of the components that make up the composition of hydrocarbons used to fuel our transport;
1. Diesel Oil
Carbon Atoms - Diesel oil typically contains between 8-18 carbon atoms per molecule, depending on the grade of crude it is refined from.
Chemical Formula - C12H23
Density – 7 lb/US gallon
Energy Content – 128,700 BTU/US Gallon
CO2 Tail-pipe Emissions - 73.5 gms/MJ
Carbon Atoms - 6
Density - 6.217 lb/US gallon
Chemical formula- C6H14
Energy Content – 115,500 BTU/US Gallon
CO2 Tail-pipe Emissions - 73.4 gms/MJ
3. Natural Gas
Carbon Atoms - 1
Chemical Formula – CH4
Energy Content – 1,000 BTU/cu.ft
Density – 8 kg/m3 (0.07lbs/US Gallon)
Emission of CO2 – 15g/MJ
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Spark-ignition vs. Dual Fuel Gas/Diesel Engines
Converting a diesel engine to run on natural gas will reduce the amount of tailpipe emissions of CO2 dramatically; however the gas is not like diesel oil that fires under compression ignition.
It is necessary therefore to provide a means of igniting the gas within the cylinder, and there are currently two methods of achieving this.
Convert to spark-ignition.
Convert to dual-fuel compression combustion ignition.
Conversion of a Diesel Engine to Spark Ignition
This requires the following components to be replaced or upgraded
Cylinder head modification for inclusion of spark plugs.
Spark ignition system to add.
Modifications to cooling systems.
Air throttle required in the intake system.
Compression ratio requires to be lowered usually by fitting low compression pistons.
Normally the conversions are difficult to retrofit.
Using gas there is no fallback to alternative fuel as in dual fueled engines.
Note: Engines fuelled wholly by natural gas using spark ignition are available as new builds by various manufacturers.
Conversion to Dual Fuel requires the Following Modifications
No major modifications required to a standard diesel engine apart from air inlet control valves.
No ignition system required; engine will still fire on compression ignition.
The existing diesel fuel tank, pumps, and injectors will be retained.
High pressure gas tank required.
Fitting of a new electronic control module required to facilitate change-over from diesel to gas and vice versa.
All mods can be fitted retrospectively.
Diesel can be used on its own if the vehicle runs out of natural gas.
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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.
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Operation of a Diesel/Natural Gas Dual Fuel Engine
When a diesel engine is converted to run on natural gas as a dual fuel engine it operates in much the same manner as the original diesel engine, changing over to gas/diesel mix when optimum engine operating temperatures are reached.
The operation of a typical dual fuel 4-stroke diesel/natural gas engine is examined below;
1. Intake Stroke.
As the piston descends down the cylinder, a measured amount of gas is injected into the air inlet manifold and is sucked into the cylinder as a gas/air mixture.
2. Compression Stroke.
The piston continues on down until it reaches BDC; it then rises back up the cylinder, compressing the gas/air and raising its temperature. Just before TDC a measured amount of diesel oil (pilot diesel) is injected into the combustion chamber.
3. Power Stroke.
The small amount of diesel ignites into hundreds of little sparks due to compression combustion. This in turn sets off the combustion of natural gas that powers the piston back down the cylinder.
4. Exhaust Stroke
The piston returns again to BDC and, as it begins to rise up the cylinder, the exhaust valve opens expelling the exhaust gasses into the turbo-charger inlet turbine, thus completing the 4-stroke cycle.
The 4-stroke cycle of a dual fuel engine is shown below.
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Converting a diesel engine to a dual fuel engine incorporating natural gas is becoming more popular in the transport sector.
The gas will not self-combust and needs a small injection of pilot diesel to provide an ignition source through compression ignition.
A big drawback to conversion of the diesel engine to run on natural gas is the addition of a large high-pressure gas tank; this takes up most of the space in the automobile’s trunk.
Nonetheless, there are several benefits of dual fuel engines in that they are more efficient, produce less CO2 emissions, and run on 90% natural gas. Natural gas is much more abundant worldwide than crude oil from which diesel oil is refined.