Natural gas has a high octane rating and excellent properties for spark-ignited internal combustion engines. Due to its high octane rating (about 120–130 octane), it can be operated at higher compression ratios than gasoline. A high compression ratio is desirable because it allows an engine to extract more mechanical energy (power) from a given mass of air-fuel mixture due to its higher thermal efficiency. High compression ratios place the available oxygen and fuel molecules into a reduced space along with the adiabatic heat of compression - causing better mixing of the fuel and air. Thus they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree. This is the reason high performance race engines use high octane fuels and high compression ratios.
When CNG is used in an engine that is tuned for gasoline, about 10% power is lost because the gasoline compression ratio is lower. But when that engine has been optimized for CNG (with higher compression ratio) there is no power loss. Diesel engines have also been converted to CNG by adjusting the compression ratio and adding spark plugs. Since diesel engines operate at a higher compression ratio than CNG, the engines experience longer life using CNG due to lower operating pressures and the cleaner burning fuel.
CNG burns so cleanly that it releases virtually no by-products of combustion (soot or carbon particulates and acids) into the motor oil. Because the oil stays cleaner longer, it’s reasonable to expect longer intervals between oil changes— up to 25,000 miles. Furthermore, the engines themselves often have a longer service life and reduced maintenance costs. When gasoline is injected into the combustion cylinder, it tends to wash away the lubricant on the cylinder wall thus causing greater friction between the moving parts. In a CNG vehicle, the fuel enters the combustion cylinder as a gas and has little effect on the lubricant on the cylinder walls, thus extending the life of the engine.
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