In usual aftermarket LPG conversions, a "mixer" is added to the airflow just before the airflow control valve (i.e. the throttle, also known as the "butterfly", mentioned above). A mixer is essentially a tube which the air flows through. It has a carefully designed internal profile though, such that the air initially flows through a medium diameter hole, which then expands to the maximum internal diameter of the tube as the airflow continues. Since air has momentum, this creates a partial vacuum at the expansion point. This vacuum is proportional to the airflow rate, and it is this that LPG systems use to meter the amount of gas joining the airflow.


Just at this expansion point, there are some small holes in the inside of the mixer. These pick up the partial vacuum and send this back along a pipe to a "vaporiser". The vaporiser has a large diaphragm in it which responds to the amount of vacuum in the mixer. As this vacuum (i.e. airflow rate) increases, the diaphram is pulled on (since the other side of it is referenced to normal atmospheric pressure), and this opens a progressive valve which is controlling how much LPG is allowed in, and so more LPG is expanded to gas. This gas goes back down the same tube, into the mixer, joins the airflow, and goes off into the engine to be burnt in the same way as petrol.