Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: chris on 27/03/2014 21:00:37

Title: What was the role of lead in petrol?
Post by: chris on 27/03/2014 21:00:37

Why was lead originally added to petrol? What purpose did it serve, and how, chemically?

Title: Re: What was the role of lead in petrol?
Post by: chiralSPO on 28/03/2014 00:38:04

Tetraethyl lead was (and still is, surprisingly!) used as an anti-knock agent. It increases the temperature required for gasoline to combust, preventing it from going too early in the cycle. I'm not entirely sure what the exact chemical mechanism is for this effect, though. Other organometallic compounds, like ferrocene, are also used for this purpose.

Title: Re: What was the role of lead in petrol?
Post by: CliffordK on 28/03/2014 04:53:57

Knocking is predetonation. 

I thought that one of the main issues with lead in gasoline was cushioning the valve closure (although it is hard to imagine how much).

Anyway, it is recommended to install hardened valve seats in older (made for lead) car engines.

Title: Re: What was the role of lead in petrol?
Post by: chris on 07/04/2014 08:20:49

Thanks both of you; the reason for asking this question is because I thought I had heard both answers previously, but without any supporting "evidence". I've had a further poke around and according to this page on wikipedia: http://en.wikipedia.org/wiki/Lead_Replacement_Petrol - both answers are true; the lead coats the valves and it serves to elevate octane rating, making pre-ignition less likely.

Chris