Researchers from the Indian Institute of Technology (Banaras Hindu University) Varanasi, the Lakshmi Narain College of Technology Bhopal and other Indian engineering centers have converted the gasoline engine of the popular Maruti Alto compact car to compressed natural gas (CNG). The vehicle has a production 796 cc engine with a CNG-compatible kit, including new injectors, pressure regulators and an upgraded fuel delivery system. The scientists’ goal was to find a simple and inexpensive way to reduce emissions in urban areas without waiting for a complete transition to electric vehicles. In contrast to many factory-produced solutions that use both natural gas and gasoline, the engineers opted out of using gasoline as a pilot fuel and made the engine a completely natural gas-powered one.
In order to achieve stable operation with the new fuel, the researchers had to solve two key problems. The first was to ensure reliable ignition of the gas mixture, given that methane burns more slowly than gasoline and requires a more powerful spark. The second was to compensate for the lower energy density of the gas, which causes the engine to lose some power under the same conditions.
Seeking to solve the first problem, the scientists replaced conventional spark plugs with iridium ones that had gaps of 0.5 mm, 0.6 mm and 0.8 mm between the electrodes. Iridium proved preferable to copper due to its high hardness and ability to provide a stable spark even with lean gas mixtures.
The researchers solved the second problem by reconfiguring the injection system: they increased the injector opening time by 12–18% to compensate for the lower calorific value of the gas and advanced the ignition point by 6–8% compared to gasoline operation.
The tests were conducted on a computerized rig with an eddy-current brake, which made it possible to precisely set engine load and speed in the range from 1,500 rpm to 5,000 rpm. To ensure reliable results, each measurement was taken three times. The researchers found that the optimal mode was achieved with a plug gap of 0.6 mm: the spark is powerful enough to ignite a lean mixture, and the combustion process remains stable. With a smaller gap (0.5 mm), flame propagation was poorer at high rpm, and the likelihood of ignition failure increased with a larger gap (0.8 mm).
As a result, carbon monoxide and unburned hydrocarbon emissions dropped by about 42% and 38%, respectively, compared to gasoline. This means that a widespread conversion to natural gas could significantly improve air quality in urban areas. At the same time, engine power was reduced by just 4–5%. This is considered non-critical for urban use. While a slight increase in nitrogen oxide emissions was also observed at high loads due to higher combustion temperatures, the researchers managed to limit this effect by fine-tuning the ignition timing.
The researchers concluded that converting compact cars to pure methane is a technically feasible and economically viable solution that could serve as an intermediate step towards more eco-friendly transport.
At the next stage, the scientists plan to conduct long-term wear tests and check the performance of the system in various climatic conditions.
