The petrol engine is a "spark-ignition" engine. A spark plug is needed to create the spark.
Diesel engines are known as "compression ignition" engines with the heat for ignition/combustion provided by the compression of the engine - hence no spark plug.
good crack from all you peeps. it was a serious question. a couple of guys in the pub said their cars didnt have plugs, they were so convincing. i just had to ask.