Bi-fuel Operation: The conventional gasoline vehicles are converted to operate either on gasoline or natural gas.

Dedicated NG Operation: Vehicle operating only on natural gas using spark ignition.

Dual-fuel CI engine Operation: Natural gas usually is carburetted/ inducted along with intake air and gaseous fuel is ignited by diesel injection spray. The amount of diesel used may be as low as 10%.

As natural gas has very high octane number, dedicated natural gas engines can be built with a high compression ratio of around 11:1, which results in significant improvements in fuel efficiency and lower carbon dioxide emissions. Lean burn spark ignited, high compression ratio heavy duty engines can be built to give very low particulate emissions compared to diesel engines and a high-energy efficiency similar to IDI engines. The stoichiometric SI engines can utilize three-way catalysts and therefore, it provides the greatest emission reduction potential.

Effect of Natural Gas on Emissions

Light-duty SI, natural gas engines run at stoichiometric conditions and use 3 Way catalysts. With natural gas, mixture enrichment during cold starting which is needed with gasoline operation is not required as the fuel is already in gas phase. Hence, with natural gas operation lower unburned fuel emissions during cold starting and warm-up phase are obtained.  Similarly, as very little mixture enrichment is necessary during warm-up and transient engine operation, CO emissions are also lower. With the use of electronic engine and fuel management and emission control technology,  natural gas  vehicles complying with the US Tier 2 and ULEV standards can be built more easily compared to gasoline vehicles.  Emission results for two vehicles operating on gasoline and natural gas vehicles are compared in Table 8.16. Natural gas cars gave more than 50% lower non-methane hydrocarbons, 10 to 50% lower CO and 10 to 80% lower NO_x.