h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I thought 'knocking' was pre-ignition - spontaneous combustion of the mixture in the cylinder. (In petrol engines spontaneous combustion is always bad - combustion is designed to be started by the spark plug. Diesel engines rely on it however - they have no spark plugs.) This is bad for the engine because with pre-ignition, the fuel starts to burn whilst the piston is still travelling upwards, which means it is working against the normal rotation of the engine, placing large stresses on it that it wasn't designed to cope with (and which of course massively reduce its efficiency - energy is going into slowing the engine down!).

To say that knocking is explosion is at best an oversimplification, and possibly entirely wrong - the distinction between an explosion and very rapid burning is, according to my dictionary, non-existent. (One meaning of explosion is very rapid burning; I'm not aware of special requirement something has to fulfil in order to qualify as an explosion, other than producing a very rapid expansion of gases.) So as I understand it, petrol *does* burn explosively in a petrol engine. Knocking only occurs if this happens prematurely.

Sorry, that was a generalization, I glossed over knocking. In reality knocking is caused when two wavefronts in the combustion chamber collide. On one hand you have the pre-ignition wavefront moving in and the other you have the newely formed spark-induced wavefront. They meet somewhere in the combustion chamber before the piston has peaked and the resulting prematurely heated gases exert an enormous pressure on the piston face before it reverses direction, causing a pinging or knocking sound. Explosion infers an intense, focused chemical reaction, whereas burning infers a slower reaction. I'd have to re-read some references to give a better definition.

Thank you for taking interest in the article. I've got more on the way.

So what other articles are you planning - more automotive stuff?

Some of it. I've got an article on Noodlin' about to get through the process, and an article on hot rodding in general. I've also got an article on Rochester Quadrajets and an article on supercharging and such. I also have some other articles on general stuff and I try to keep a journal. You might look at try page if you're interested, just click on Flyboy.