Bi-fuel vehicle
Bi-fuel vehicles are vehicles with multifuel engines capable of running on two fuels. The two fuels are stored in separate tanks and the engine is able to run on one fuel at a time. On internal combustion engines, a bi-fuel engine typically burns gasoline and a volatile alternate fuel such as natural gas (CNG), LPG, or hydrogen. Bi-fuel vehicles have the capability to switch back and forth from the gasoline to the other fuel, manually or automatically. A related concept is the duel-fuel vehicle which must burn both fuels in combination. Diesel engines  converted to use gaseous fuels fall into this class due to the different ignition system. The most common technology and alternate fuel available in the market for bi-fuel gasoline cars is Autogas (LPG), followed by natural gas (CNG), and it is used mainly in Europe. Poland, the Netherlands and the Baltic states have many cars running with LPG. Italy currently has the largest number of CNG vehicles, followed by Sweden. They are also used in South America, where these vehicles are mainly used as taxicabs in main cities of Brazil and Argentina. Normally, standard gasoline vehicles are retrofitted in  specialized shops, which involve installing the gas cylinder in the trunk and the LPG or CNG injection system and electronics. The conversion is possible because the gases can use the spark-ignition of a gasoline engine. Diesel conversions. A Diesel engine is a compression ignition engine and does not have a spark plug. To operate a diesel engine with an alternate combustible fuel source such as Natural gas, a Dual-Fuel system used with Natural gas as the main fuel while diesel fuel is used for the ignition of the gas/air mixture inside the cylinder. In other words, a  portion of diesel is injected at the end of the compression stroke, thereby maintaining the original diesel operation principle. (Running gas-only is possible, but requires more extensive modification.) Dual fuel operation in this case means the engine uses two fuels (gas and diesel) at the same time, as opposed to Bi Fuel which would mean the engine could have the option of using either fuel separately. There usually two type of conversions - low speed (below 1000 RPM) and high speed (between 1200 and 1800 RPM). Low and middle speed conversion. Gas is injected into the cylinder inlet manifold by  individual gas electromagnetic valves installed as close to the intake valves as possible. The valves are separately timed and controlled by injection control unit. This system interrupts the gas supply to the cylinder during the long overlap of the intake and exhaust valves (just typical for slow-speed and medium-speed engines – within the valve overlap cylinder scavenging is performed). This avoids substantial gas losses and prevents dangerous gas flow to the exhaust manifold. High speed conversion. Gas is mixed with air by a common mixer installed before turbocharger(s). Gas flow is controlled by a throttle valve, which is electronically  operated by the special control system according to the required engine output and speed. In order to avoid knocking of the engine, knocking detector/controller is installed, thus enabling engine operation at the most efficient gas/diesel ratio. Conversion features and benefits. Benefits relating to conversion to bi-fuel vehicles include savings on operation costs, little to no engine modification of the existing vehicle and non-dated output power. Since the vehicle will be utilising two fuel sources, there is also less workload being applied to the single engine, resulting in a longer expected engine life span, longer service period  and maintenance intervals. Other benefits also include potential emissions reduction and fuel flexibility. Gas types used. It is common to use CNG (Compressed Natural Gas) or LNG (Liquid Natural Gas) for bi-fuel operations. Both are also mostly used for Generator sets conversions, because the engine does not lose the output power. In recent years biogas is being used. The biogas composition and calorific value must be known in order to evaluate if the particular biogas type is suitable. Calorific value may be an issue as biogas is derived from different sources and there is low calorific value in many  cases. You can imagine you have to inject sufficient volume of gas into the cylinder to substitute diesel oil (or, better to say, substitute energy delivered by diesel oil). If the calorific value (energy) of the biogas was very low, there is a need to inject really big volume of biogas into the cylinder, which might be technically impossible. Additionally, the composition of the biogas has to lean towards ignitable gases and be filtered as much as possible of non-combustible compounds such as carbon dioxide. Associated gas is the last type of gas which is commonly used for bi -fuel conversions of generator sets. Associated gas is a natural gas found in association with oil, either dissolved in the oil or as a cap of free gas above the oil. It means it has almost the same quality as CNG or LNG. Diesel/gas ratio. It depends on the technical state of the engine, especially of the injection system. The typical Diesel / Gas ratio is 40/60% for the high-speed engines. If the operating output of the engine is constant and between 70-80% of nominal output, than it is possible to reach up to 30/70% ratio. If the  operating output is lower (for example 50% of the nominal output) or if there are variations, the rate is about 45/55% (more of diesel is used). For Low Speed conversions it is possible to reach the Diesel/gas ratio up to 10/90%. Generally, it is not possible to guarantee an exact diesel/gas ratio without a test being done after the conversion. Vehicles. Aftermarket 'bi-fuel' and even 'tri-fuel' conversions are also available.