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Natural Gas Fired Reciprocating Engine - Fuel System

Natural Gas Fired Reciprocating Engine - Fuel System

(OP)
I'm curious as to how the fuel system works on natural gas fired otto cycle engines. We have seen instances where ngas generators have sucked the main fuel line dry upon startup when it otherwise should have been properly sized. In a gasoline engine, I understand that a constant DP across injectors is usually maintained by either a regulating valve and return, or by variable pumping. However, with Ngas, the fuel pressure will most likely need to be increased from the utility to overcome cylinder pressures. I'm assuming there is some type of gas compressor that is part of this system. Any input in this area would be appreciated. Thank you.

RE: Natural Gas Fired Reciprocating Engine - Fuel System

Well, it depends.......

For the common "mixture charge" engine, where the fuel gas and air are mixed (in a mixer) before the turbocharger, then the fuel gas pressure into the mixer is zero, relative to atmosphere going into the mixer. The fuel gas is controlled in a gas train, comprising gas shut off valves and a zero pressure regulator. There will be a pressure drop across these devices and thus the pressure into the gas train will be above 20 milli bar, but could be over 100mbar.

A higher pressure is preferable, as the gas train will be smaller, as will the pipe diameter, and cost!

All pipework and any other devices (filters etc.) MUST be sized for the fuel gas flow at full engine power, this will prevent any pressure fluctuations during operation, which could lead to engine instability. As you say "ngas generators have sucked the main fuel line dry upon startup when it otherwise should have been properly sized". As someone once said to me "when all else fails, you have to engineer it!"

For natural gas fuelled engines, there may be need for a gas compressor to boost the pressure. In many cases, the utility pressure will be adequate.

For countries like the UK, the original gas supply was from coke works, and the supply pressure was low (20 mbar or so). When natural gas was introduced, the same distribution pressure was used. In some areas, higer utility gas pressures are available.

For biogas fuels, where are produced at low pressures from landfill sites or anaerobic digesters, a gas compressor is usually present.



There are other configurations of gas engine, such as those with direct fuel injection into the cylinders at several hundered bar, for these gas compression is almost always involved, as the utilities cannot supply at this pressure locally.



Each case has to be considered in its merits.


RE: Natural Gas Fired Reciprocating Engine - Fuel System

The (few) natural gas engines with which I have worked had gas carburetors, and no gas compressors, and no injectors. They are normal Otto cycle engines, with a vacuum in the intake manifold at all operating conditions.

Mike Halloran
Pembroke Pines, FL, USA

RE: Natural Gas Fired Reciprocating Engine - Fuel System

All the above is for stationary engines which I suppose is the OP's intent since he mentions utilities. I would just like to add that mobile solutions are completely different since the gas has to be hauled around with the vehicle and is either highly pressurized or liquefied. Most of the fuel injection systems work with a high enough pressure differential across the injectors to ensure critical (sonic) flow and thus the flow rate is strictly a function of upstream density and independent of manifold pressure and/or boost.

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RE: Natural Gas Fired Reciprocating Engine - Fuel System

I think you are 10 years back, from many years in cars the installations for NG works exactly like petrol, the difference is only in a pressure reducing valve from 300 bar (from tank) to 10 bar, which feeds electrical injectors (like petrol injectors) usually 4-6 inch before the cylinder head, directly in the intake manifold.
Once there was heavy steel tanks (for 300 bar, 4300 psi)but now, new cars have aluminium or carbon tanks.

RE: Natural Gas Fired Reciprocating Engine - Fuel System

(OP)
Informative posts. Thanks all.

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