Putting diesel fuel into a petrol (gasoline) car will cause significant and often immediate damage because the two engines operate on fundamentally different principles. A petrol engine uses spark plugs to ignite a mixture of fuel and air, while a diesel engine relies on compression ignition, where air is compressed to such a high pressure that it heats up and ignites the fuel spontaneously. Diesel fuel is thicker, oilier, and acts as a lubricant for the high-precision components in a diesel fuel system. In a petrol car, it clogs the fuel injectors, fails to vaporize properly, and provides zero lubrication, leading to catastrophic engine failure. The repair bill can easily run into thousands of dollars.
The core issue is the difference in fuel properties and ignition systems. Petrol is a volatile solvent, and diesel is a heavy oil. When diesel is introduced into a petrol engine's fuel system, it cannot be ignited by the spark plugs. This leads to misfiring, black smoke from the exhaust, and the engine stalling completely. If the engine does run, the diesel will not lubricate the high-pressure fuel pump and injectors; instead, it will cause them to grind against each other without proper lubrication, leading to irreversible wear.
The financial impact is severe. You'll need a complete fuel system flush, which involves draining the tank, replacing the fuel filter, and cleaning or replacing the fuel injectors, lines, and pump. If the engine was run for an extended period, internal damage to the pistons or cylinders from improper combustion is possible, necessitating an engine rebuild or replacement.
| Factor | Petrol Engine | Diesel Engine | Consequence of Misfuelling |
|---|
| Ignition Method | Spark Ignition | Compression Ignition | Diesel won't ignite via spark; engine misfires/stalls. |
| Fuel Lubricity | Low; fuel is a solvent | High; fuel lubricates components | Diesel in petrol system causes pump/injector wear. |
| Fuel Viscosity | Low (thin) | High (thick) | Diesel clogs petrol injectors designed for finer spray. |
| Octane/Cetane Rating | Measured in Octane (resistance to knock) | Measured in Cetane (ignitability) | Incorrect rating disrupts entire combustion timing. |
| Typical Repair Cost | N/A | N/A | $1,500 - $5,000+ for full system cleaning/repairs. |
If you realize the mistake before starting the car, do not turn on the ignition. This prevents the fuel pump from circulating the diesel. The only safe solution is to have the vehicle towed to a professional mechanic who can drain and purge the entire fuel system.
