The 1.2 Kappa petrol is the latest engine from Hyundai which is currently available in its cars like i10 and i20. The Kappa engine uses new technologies that cut weight and friction to boost fuel economy. Kappa engine generates 80 PS at 5,200 rpm and 11.4 kgm of torque at 4,000 rpm in the 1.2 version.
On the other hand the 1.4 gamma engine was launched by Hyundai few years back, and it is available in its cars like Hyundai i20 1.4 L petrol. The 1.4 L version Gamma engine is capable of generating maximum power of 100 @ 5,500 (PS@rpm), along with a maximum torque of 13.9 @ 4200 (kgm@rpm).
So where the kappa engine is better for small cars, where the high fuel efficiency is the first requirement of the customer, the Gamma engine are used in slightly upper segment cars where the we give more concern to the power than fuel economy.
Initially, Hyundai Elite i20 Magna comes with a petrol and diesel engine but a CNG kit can be fitted with the one which has petrol engine. You will have to sacrifice with power to get more fuel economy and if you convert a petrol car into a CNG car, you may loose the warranty on the car and resale value of CNG cars is lesser than petrol cars. It is always recommended to get the CNG kit fitted from the authorized service center so that you can avail the warranty services as well. Moreover, the performance will depend on how the car is driven and maintained.
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How much fuel goes into a car varies on a lot of factors. Now you haven't mentioned how many km did you drive post the low fuel warning light as that could also determine how much fuel goes into the car. Even though the manual says that the tank capacity is 45 litres, the amount can vary by a small margin, give or take 2 or 3 litres. Also filling fuel in the hot day or in the night also makes a difference considering fuel expands during the hot day and contracts when the temperature is cooler. All said and done, make sure you fuel up within 3 to 4 km after the low fuel indicator light pops up.