Ignition Systems for Dedicated and Bi-Fuel CNG Engines
What you must know
CNG has a high octane rating (roughly 120–130 RON equivalent), so dedicated engines can run higher compression and advanced timing - but gas is harder to ignite than gasoline, so ignition systems are typically robust: coil-on-plug or distributorless waste-spark, platinum/iridium plugs, and wider gaps per OEM spec. Bi-fuel engines must compromise: they often use the gasoline compression ratio and a timing map that the ECM switches when fuel type changes. Know the knock sensor's role, the cam/crank sensor inputs for sequencing, and the fuel-type switch logic on bi-fuel units.
Traps
The exam likes plug gap: CNG often calls for a different (frequently smaller) gap than gasoline to reduce misfire under high cylinder pressure - always follow the OEM spec, not the gasoline default. Another trap is assuming a bi-fuel engine uses the same timing map for both fuels. Also, a misfire that appears only on CNG points to ignition demand, not fuel quality alone - high cylinder pressure stresses the secondary ignition.
Rule of thumb
Misfire only on CNG = inspect secondary ignition first (plugs, coils, boots), because CNG's higher compression raises voltage demand.
Worked example
A bi-fuel bus runs fine on gasoline but misfires under load on CNG. Plugs are gapped to the gasoline spec. Correct action: regap or replace plugs to the CNG specification and re-test - the wider demand under CNG load was breaking down the secondary ignition.