Toyota and Lexus hybrid on STAG LPG — when it runs on gas and when on petrol

In a Toyota and Lexus hybrid the petrol engine and the electric motors share the drive. An LPG STAG system connects only to the first of those. When the car runs on electricity the engine is stopped, the gas injectors are silent, and the HV battery charges from braking or from the engine running — not from the tank.

When a hybrid burns fuel at all

In town, with a warm engine and a charged battery, the petrol engine often stops. Pulling away from the lights, rolling up to a junction and a short stop happen on electricity. Gas appears on the bill only in the moments when petrol would have been burned anyway: acceleration, a climb, a trip, winter heating and charging a weak battery.

So the saving from LPG in a hybrid does not look like it does in a car that burns fuel all the time. In traffic the difference is often small, because the engine works little anyway. At high mileage and on longer trips the engine runs for hours — then the price of a litre of gas against petrol matters. How the Atkinson engine behaves on a trip is in a separate text on hybrid driving out of town.

How the STAG controller switches to gas

Every start is on petrol. The LPG reducer is heated by coolant from the engine. The STAG controller lets gas through only after the reducer reaches temperature. In an ordinary car the engine heats the coolant almost all the time. In a hybrid it stops, so in winter and on short sections the switch comes later, and some kilometres stay on petrol. That is not a fault in the system.

When the engine stops at the lights and starts again a moment later, the controller has to return to petrol and then — if the reducer is still warm — back to gas. A badly chosen map jerks at that changeover. It is easy to take that for a weak battery or a worn plug. A plug in a hybrid already works in short, cold starts; on gas a worn plug shows up sooner. The symptoms are in the article on plugs in a hybrid.

Port injection and direct injection

Older hybrids with port injection — Prius, Auris, Lexus CT200h — get a sequential STAG system. Gas goes into the manifold, and after the switch petrol is cut off.

Newer engines with direct injection, including in the Corolla, RAV4 and Camry, have petrol injectors in the combustion chamber. They must receive petrol in order to stay cool. A plain “gas only” sequence does not fit. The STAG controller is chosen for the injection type and leaves a petrol top-up. Before fitting, the specific engine matters, not just the Hybrid badge on the tailgate.

What stays the same

• Regenerative braking works the same — gas does not switch it off
• The HV battery still needs diagnostics; a weak pack makes the engine run longer and raises gas use
• The tank goes where that model has room. In some hybrids the pack occupies the boot, so the tank layout is not the same as in an ordinary car
• The liquid-phase and vapour-phase filters and the reducer have their own service, separate from the hybrid inspection

After fitting, the system goes into the registration document. Without that, the technical inspection will not pass it.

Summary

LPG in a hybrid does not charge the battery and does not drive instead of electric mode. It feeds the petrol engine when that engine is running, and only after the reducer is warm. The point of the system shows up at a mileage where the petrol engine really has work to do. If the car jerks after switching to gas, we check the map and the plugs first, not the HV pack at once. The battery offer is on the page for hybrid service.