Natural gas as a transportation fuel is not a newidea; however large finds of natural gas, and the technology to recover thisfuel at reasonable costs, have spurred increased national interest in CNG.Although NG has been used as a fuel in IC engines for a number of years, muchdevelopment and optimization are still possible, both for the case of dedicatedand bi-fuel engines. World-wide emphasison CO2 emissions reduction and fuel economy improvements suggests that it maybe worthwhile to make a small investment to understand what can be achievedwith a CNG (dedicated or bi-fuel) engine in passenger car applications as wellas in stationary engines. Not all automobile engines are suitable for usewith CNG as a fuel. CNG provides less upper cylinder lubrication than petrol ordiesel, so CNG-fueled engines are more prone to valve wear if they are notsuitably modified. To benefit from the usage of CNG in IC engines, it is essentialto know its combustion under the suitable conditions and to study the influenceof several parameters on it.
Methane, the primary component of natural gas, iscomposed of one carbon atom and four hydrogen atoms. This H/C ratio of 4:1 isadvantageous to an engine’s emissions as compared to gasoline which has an H/Cratio of about 1.85 4-methane.
The reason being that during combustion, heatenergy and oxygen mix with the methane to break the molecular bonds andre-combine them. This ideally turns carbon into CO2 and hydrogen into H2O.Thus, if there is less carbon and more hydrogen in the reactants, there should beless CO2 and H2O. This is indeed the case for CNG as compared to gasoline asnatural gas observes a CO2 emissions reduction of about 20% 5-methane. Thisis the reason that natural gas is typically regarded as a ‘greener’ fuel thanits petroleum based brethren.
Being a gas at normal temperature and pressure CNGmixes readily with air in any proportion. The ability of an engine to pump air termedas the volumetric efficiency (VE), which if reduced has an impact on themaximum power output. Liquid fuel (Gasoline) when atomized, generally consumesvery small space in the intake system and thus do not affect volumetricefficiency significantly. Gaseous fuels require about 4 to 15% of intake passagevolume. Space occupied by the fuel reduces the amount of air entering theengine; hence the power output of the engine is reduced. Due to vaporization CNGof in manifold as they mix with air in the engine’s intake to enter thecylinders, it absorbs energy and cools the fuel/air mixture. Cooler the mixture, higher is the engine VE.
Most gaseous fuels, on the other hand, are already in a vapor form and do notprovide any cooling of air/fuel mixture. This loss of cooling constitutes anadditional power loss of gas-fueled engine as compared to liquid fueled engine.Theoretically, loss in power output for CNG is around 4-10%. Octane rating of a fuel indicates how latelythe fuel will ignite and how well the fuel will resist pre-ignition before thespark plug fires. Higher compression ratios can be used due to the higherauto-ignition temperature and octane rating that avoids the mixture topre-ignite and makes the engine more knock-resistant. CNG has high octanerating great than 110 that allows CR to be higher than gasoline engines.
Thehigher the CR of an engine, the more efficient and powerful is the engineoutput. Utilization of CNG at its efficient operation involves differentcombustion techniques which is dealt in the following section.