Please read the following for difference.
EFI & CNG Technology
On EFI-vehicle, there are 2 main technologies that are being applied, that is (1) EFI Fumigation and (2) Sequential Gas Injection. Here is the breakdown on the comparative technologies:
EFI & Fumigation
On the EFI-vehicle, you may see a Mixer attached between the airfilter hose and the Throttle Body Opening. This vehicle is using a Fumigation Technology, which is actually a GAS carburetion. Electronics like Injector Emulator and Lambda Emulator are meant to fool or deceive the EFI-ECU, sending you NO warning light or 'Check Engine' Alert on your speedometer.
In Petrol-mode, your vehicle is operating on FULL Petrol Injection (intelligent Fuel Management with metered petrol-liquid injection, directly into the combustion chamber). But when you operate in CNG-mode, you have downgraded your Fuel system to a Gas Carburetor.
The Fumigation CNG mixer acts as your traditional Carburetor Unit, pre-mixing your air with CNG before the Throtte Body. The Injector Emulator prevents the Petrol injector to open, by introducing current resistance or Ohm. The lambda (oxygen) sensor is fooled by the Lambda Emulator to provide the 'correct' stoichemetric reading - although it is now incorrect at CNG-mode.
With EFI-Mixer-Fumigation technology, you have a Carburetor-Engine on CNG-mode, and an EFI-Engine Management System for petrol-mode. In conclusion, EFI-mixer conversion is in fact a CARBURETOR on CNG.
Incidentally, the Throttle Body's buttervalve has become the Buttervalve for your newly-introduced 'CNG Mixer Carburetor Unit'. The entire Throttle Body is converted into a Big & Long Intake Manifold. This technology can lead to backfires.
Thus, EFI-CNG Fumigation technology is actually Gas carburetor system. On CNG mode, your vehicle is No longer intelligent.
EFI & Sequential Gas Injection
To prevent Gas Carburetor-mode, your vehicle can be installed with Sequential Gas Injection. Holes will be drilled nearest to your engine Intake Valve at an angle, and injector giggler (tips) will be inserted. The gigglers are used to spray CNG directly into your combustion chambers.
In fact, the CNG Sequential Gas Injector operates like your Petrol Injectors, but it sprays purely CNG. Each injector spray a metered volume of CNG on what we can as 'pulse width' or duration of the injector remains open to spray in gas. The longer it remans open, more CNG will be injected in at 40psi or 3 bar.
Programming the CNG Injector pulse width is very important, and you need to understand Fuel Mapping vs Ignition Mapping. This EFI-Sequential Gas Injection is like Petrol Injection Technology.