Lead-replacement petrol is slowly disappearing from the forecourts. Should you be concerned?

Green or red? It's the option that the owners of petrol-engined vehicles face each time they fill up. Most choose the unleaded option but many drivers, especially those of older vehicles, request lead-replacement petrol (LRP). The reasons for this decision vary from "to protect my engine" to "red petrol offers better fuel consumption".

For those red-petrol stalwarts, however, their tipple of choice is about to dry up because many fuel stations are scrapping the LRP option from their forecourts…

History

Unleaded petrol was first introduced here in 1996 and, since 1 January 2006, lead has no longer been used in petrol production. Health reasons was the main motivation for removing lead – the fuel's exhaust fumes are toxic – but leaded petrol also results in so-called "catalyst poisoning", where the fuel's exhaust by-product effectively clogs up the catalyst in new vehicles' exhaust systems. Increasingly stringent American and European emissions legislation also required the fitment of catalysts to minimise harmful tailpipe emissions. Catalysts are incompatible with leaded fuel because lead "poisoning" severely affects their operation.

Why was lead used in the first place?

Lead is a very effective (and cheap) octane booster for petrol. Octane is not an energy-content measure, but rather an indicator of the fuel's resistance to auto-ignition. This is important in a spark-ignition petrol engine where controlled ignition and combustion is required owing to initiation via a spark from the spark plug.

Auto ignition (or knock), on the other hand, is an uncontrolled ignition not initiated by the spark plug and this phenomenon can cause severe engine damage and, as such, must be avoided. A side benefit (unknown at the time) of using lead is its lubrication properties at the valve seats of cylinder heads. The lead deposit on the valve seat acts as a dry lubricant, which prevents valve-seat wear in engines with cast-iron (a relatively soft metal) cylinder heads.

With the introduction of unleaded fuel, this lubrication property became lost, giving rise to many studies into valve-seat recession (VSR) and the eventual development of LRP for engines with these cast-iron cylinder heads.

The basics of valve-seat recession

Adjunct Professor Andrew Yates, mechanical engineer and a specialist in internal-combustion engines, conducted one such study in 2001, titled An investigation into the mechanism of valve seat recession that focused on determining the causes of VSR when using unleaded fuel. The study revealed there is a strong relationship between valve rotation (during the opening and closure of the valve) and VSR. It is the hot exhaust valve that is prone to VSR because, in the absence of a valve-seat lubricant, the seat material can "weld" to the valve in microgranules. The rotating action of valves then turns them into grinding discs that wear the seat.

The study also indicated that severe VSR occurs if:
• The valve-seat material is “soft” enough to permit "micro-welding" to occur under sufficiently arduous conditions.
• The fuel formulation does not provide sufficient lubrication under the boundary-film conditions of valve seating.
• The engine speed is such that valve rotation is induced.
• The engine is operated for a significant length of time under the aforementioned conditions in order for existing protective deposits to be removed and for VSR to commence.

In his tests, Yates used a Ford 1600 Kent engine (with a cast-iron cylinder head) that was set up on an engine-test bed and connected to a dynamometer in a test cell. As valve rotation plays such a critical role during VSR, Yates first created a mathematical model of a single exhaust-valve kinematic system to predict the rotational speed of the valves as a function of engine speed. The result is shown in the graph to the right and shows non-linear behaviour that is closely linked to the natural frequency of the valvetrain. Testing validated the mathematical model and pointed out the greatest valve rotation rate was found at 4 000 r/min and at the redline of 6 000 r/min, leading to increased VSR.

Which engines are at risk of VSR?

Following the explanation of VSR, it's clear the only way it can take place is if the engine's valve seats are made of a soft material such as cast iron. The reason why aluminium-alloy heads (an even "softer" material) do not suffer from VSR-related problems is that special valve-seat inserts are used to provide adequate properties at the valve seat. Therefore, if your vehicle's cylinder head is made of aluminium-alloy, LRP is generally not required. This may even include older vehicles such as Volkswagen Beetles that have aluminium-alloy cylinder heads in their air-cooled engines.

What to do if you still need LRP

A report compiled by the department of energy, Overview of petrol and diesel market in South Africa between 2002 and 2013, showed that, in 2013, LRP accounted for 15% of the 113 million kilolitres of fuel sold. That's in contrast to the need for LRP in our market, which is below 1% according to a study conducted by a joint SAPIA and government working group.

This is problematic for the oil companies, as it is expensive to manufacture, store and distribute an extra fuel specification such as LRP if most vehicles on our roads don't need it. Phasing out LRP will remove much cost and complexity from the petrol industry. If the cylinder head is made of cast iron and requires LRP, the solution is an aftermarket additive to unleaded fuel each time you fill up. You can buy these additives at spares outlets, as well as at many fuel stations, and it will afford the same valve-seat protection.

Does LRP offer better fuel consumption?

The claim that LRP improves fuel consumption is a myth. Choosing the red nozzle has no advantages in the form of fuel consumption or performance over unleaded. On the contrary, as highlighted, if your vehicle is fitted with an exhaust catalyst, the additive carried in LRP (for example, manganese) can damage the catalyst in the same way lead did.

How to find out if your car needs LRP

If your engine was manufactured after 1995, it should be able to run on unleaded fuel. The reason is that cast-iron cylinder heads were phased out by 1995.
• Investigate further by consulting the Naamsa fuel-compatibility database.
• If you're still unsure, contact the manufacturer.
• Lastly, if you're running a classic car and are unsure about the material of the cylinder head, use an aftermarket additive when filling with unleaded fuel to restore the valve-seat-lubrication properties.

Author: Nicol Louw