|THE MINI MAG. Volume 2 No.4|
Vol.2 Home Page | Index Page
| In the past several years, the push to remove all lead from petrol has increased notably. The World Bank, a strong advocate of lead removal, has pushed to implement programs in many countries to completely phase out leaded petrol in the next 5 – 10 years. In Australia, the demise of lead began in the mid-1980s, when unleaded petrol was introduced for new cars, all of which have catalytic converters and other emissions control technologies. Catalytic converters and oxygen sensors are poisoned by lead and rendered inactive.|
If your car is pre-1920, when petrol did not have lead added to it, the engine will have exhaust valve seats of hardened steel, and will run on unleaded petrol. But during the 1920's the fuel industry discovered that putting lead (tetra alkyl lead) in petrol increased its octane number. The increase in octane number allowed increased engine compression and spark advance, thereby providing more power and efficiency. The lead also provided a lubricating deposit on the exhaust valve seats. This protected them from valve seat recession (VSR), so the seats were then made from non-hardened steel.
Another benefit of lead was the lubrication of the bottom of the valve guide. The movement of the valve stem in the valve guide is normally very effectively lubricated by small amounts of engine oil passing down the guides. Whilst some wear of the valve and guide does occur at high temperatures, the lead provides a back stop solid lubricant film which protects against increased wear and seizure or valve burn, except under very severe conditions.
The amount of lead used by refiners has decreased greatly over the last decade. Leaded petrol now accounts for only 20% of total Australian petrol sales, and demand is decreasing. Also, the concentration of lead in leaded petrol (96 octane number) is now much lower, with refiners limiting lead to a maximum of 2000 milligrams per litre. Of course, both grades of unleaded petrol (91 octane number and 95-96 premium) contain essentially zero lead. For unleaded petrol, there is no intentionally added lead, and great care is taken to prevent contamination from leaded petrol. The amount of lead needed in petrol to lay down the lubricating deposit is very small. Peter Sloan, from the British Petrol Industry Association, says 500 milligrams per litre would be enough.
So, your vehicle was designed to run on leaded high octane petrol. However, if you use only “regular” unleaded, there are two possible consequences. One, the engine will “ping” because the octane number is lower. Two, your unprotected valve seats could suffer valve seat recession (VSR). Concerns about valve seat recession and valve stem wear, are generally over-stated, and under normal driving conditions, most vehicles with soft valve seats will not be at risk. However, the risk of engine damage is much increased when a vehicle is driven continuously under high speed / high load conditions.
Lead replacement petrol (LRP) will be marketed instead of leaded petrol. In February this year, the Queensland government announced that all Queensland petrol will be lead-free by March next year. LRP is already available in Western Australia, South Australia and Victoria, and may well appear in Queensland before the government deadline of March 2001. Lead replacement petrol will have the same octane number rating as leaded petrol, so switching will not cause pinging. LRP will contain an additive to prevent VSR. Oil companies will select an effective anti-VSR additive and add it at a concentration which ensures protection against valve seat wear.
Comparing the performance of the various additives which provide protection against valve seat recession is tricky. Manufacturers often have a tendency to favour test procedures, which magnify the performance of their own product, or overstate the need for anti-VSR additives. Nevertheless it is interesting to consider some of the published information. The results of the Barker (1) study are compared below in Table 1. The table shows that lead is better than phosphorus, which in turn is better than sodium.
Octel also conducted some comparative testing of lead, phosphorus and potassium, reported in ELT 94/11, dated Aug 1994, using a 1.3 litre Austin Maestro under moderate to severe conditions in cyclic engine conditions on a chassis dynamometer.
The preparation for the test entailed:
Fitting a reconditioned cylinder head with no lead or additive memory.
Replacement of oil and oil filter.
Adjustment of carburetor and ignition timing to manufacturer's specifications.
Testing of each additive was then performed after low speed cycles of 50/60/70 km/h to allow the valves to bed in.
100 cycles of :
5 min, 80 km/h, 3000 rpm
20 min, 100 km/h, 3750 rpm
10 min, 120 km/h, 4500 rpm
10 min, 80 km/h, 3000 rpm
20 min, 100 km/h, 3750 rpm
Tappet clearances were measured every four hours.
Du Pont's phosphorus-based Valvemaster additive and an unidentified potassium additive were tested at different concentrations in comparison with lead at 0.05 g/l. The results of these tests showed that lead out performs phosphorus which is, in turn, better than the potassium additive.
In terms of valve seat protection there are no reports available which compare sodium directly with potassium but the indication is that the two alkali metals have a broadly similar anti-wear performance.
The US Environmental Protection Agency/US Dept. of Agriculture report on the testing of unleaded, low lead and non-lead additives, tested engines using duty cycles that represented normal operating conditions. The views of the independent consultants, which oversaw the project, were - Only Lubrizol's PowerShield when at four times the recommended concentration was effective in eliminating VSR. However, at high concentration, the build up of engine deposits was observed as well as increased contamination of the engine lubricating oil. The Du Pont additive (DMA-4) at the recommended concentration gave some protection, but also produced some deposits and lubeoil contamination.
Significantly, the joint report points to the higher valve guide wear with unleaded petrol. This suggests that unleaded petrol may increase valve stem wear. It also refers to the contribution of engine air/fuel ratio to higher exhaust temperatures, indicating that engine adjustments as well as engine speed or load can influence VSR. Note, not all valve seat inserts are hardened, and non-hardened inserts will under perform non-inserted seats due to the poor transfer of heat across the insert/cylinder head interface.
Tests on low lead petrol (0.026 g/L) still showed some VSR, but within acceptable limits for these engines when well maintained and performing similar duty cycles. The point is made that a improperly maintained engine might experience excessive VSR even when high concentrations of lead are in the petrol.
Louis Leviticus, considering the role of combustion chamber temperature in VSR, stresses that combustion chamber temperature may be a major contributor to VSR below a critical seat hardness value. He noted that high temperatures may occur for a variety of reasons not associated with vehicle or engine speed. High loads, inadequate cooling, lean fuel mixtures, ignition problems and combustion chamber deposits are all sited as factors that could affect engine temperature.
The relative performance of additives - Esso VSR work - Esso's investigation on VSR over twenty years with over thirty VSR sensitive vehicles. Great attention was paid to the setting up of the car engines such as removal and skimming of cylinder heads, alignment, the skimming of valve seats with new valves fitted and a full engine service. The cars were loaded with the equivalent of four persons, then low, normal and high speed driving regimes were tested. Sodium, potassium and phosphorus additives were used. Under normal speed conditions the additive performance, as measured by VSR, was varied but in no case as good as that of lead. Under high speed conditions all the additives were considerably worse than lead. Esso concluded that for sensitive vehicles, VSR was not seen using low lead petrol (0.050 g/L).
Relative performance of additives -Sainsbury's evaluation of LRG containing Sodium- The test conditions used by BICERI were determined in consultation with Rover Group. The LRG tested contained Lubrizol sodium-based additive PowerShield.
Three reports were produced:
On a Rover A Series engine using petrol designated LRG 2 which contained 10.2 ppm of sodium.
On the same Rover A Series engine with an overhauled head using low lead 4 star petrol.
On the same Rover A Series engine with an overhauled head using leaded 4 star petrol.
Conclusion from these tests are a PASS on Test 1 (equivalent to Rover specs). And FAIL on Test 2 and 3 (equivalent to Rover specs).
Salisbury's Evaluation of Lubrizol PowerShield in Bench Tests at BICERI
Test Period 1 2 3
Duration 30 Hours, 3800 rpm
60% Load 30 Hours, 4700 rpm
60% Load 30 Hours, 5500 rpm
Wide Open Throttle
Result of Test Pass Fail Fail
Equivalent Motoring on level road - driver only Acceleration in low gear or cruising in high gear Stronger acceleration in low gear or cruising in high gear at higher speed Maximum power acceleration and maximum vehicle speed
Typical road speeds for:
Maestro 1.3 litre with 5 speed gearbox 60 mph in 4th 76 mph in 5th 75 mph in 4th 94 mph in 5th 97 mph (limited by gearing)
Metro 1.3 litre 4 speed gearbox 70 mph 87 mph 95 mph (max vehicle speed)
Metro 1.0 litre 62 mph 76 mph 90 mph (max vehicle speed)
Relevance of anti-wear additive evaluation.
Performance ranking for common anti-wear additives is that lead is better than phosphorus, which in turn is better than sodium or potassium.
All anti-wear additives are better than no anti-wear additive for soft valve seats.
Temperature is a critical parameter in determining the extent of wear.
All testing involved engine preparation prior to testing.
Three mechanisms which may contribute to varying degrees to damage:-
1. Incomplete valve to valve seat sealing when valve clearance is effected by VSR leading to escaping combustion gases.
2. Uneven valve seat wear as a result of valve guide wear leading to a break in the gas seal. Later valve guide developments have reduced this tendency.
3. Breakdown of the gas seal from spallation of the protective coating of alternative anti-wear additive residues.
The Octel report EL 87/28 shows the sodium-based additive Powershield in a 1.3 ltr Metro engine on a test bed coupled to a dynamometer. Three cylinder heads were tested, one with unleaded only and two with the same unleaded fuel to which 8.3 ppm sodium had been added. The unleaded fuel showed high VSR, while one head on the sodium additive showed severe VSR on ONE cylinder and much less VSR on others while the second head showed no measurable VSR.
On exhaust valve No.2 the indications were a transparent glaze over the larger part of the seating area; dark bluish/black spots on the seating surface; a large area of developing valve burn of the broad faced variety; at the edge of the burnt area, two channels in the glaze which would break the gas seal; and faint evidence of fork-like runnels originating at the lower edge of the seat. These conditions indicating Hot Corrosion. The conclusion is that the wear mechanism in the presence of the anti-wear residue is NOT that of classical unlubricated wear when welding of asperities and creation of wear debris helps to propagate the wear process. This process, while not covered in this document, is indicative of the Swedish experience with turbo-charger corrosion from alkali metal additives.
Merely to measure VSR as most studies have done provides a rather narrow focus on a complex phenomenon. As the table shows, 1 out of 4 valves having a problem, other factors are at work.
Valvemaster - a phosphorus-based additive – was tested by Melbourne University as part of New Zealand’s abolition of leaded petrol in 1996. A Holden Commodore with 30 thou over sized pistons was tested with premium grade unleaded petrol, one test with and one without Valvemaster added. The test (Australian Standard AS4430.1) calls for measurements at 1, 25, 50 hours into the test. The standard specifies that normal wear is less than 0.25 mm in 50 hours of operation under the prescribed duty cycle. The test showed no VSR to inlet valves and the exhaust valves were 1=0.00mm, 2=0.18mm, 3=0.23mm, 4=0.08mm, 5=0.03mm and 6=0.09mm of VSR.
MMT is manganese-based additive that has been used as an octane improver in virtually all Canadian unleaded petrol for many years. While its octane benefits have been known for many years, recognition of its performance as an effective anti-valve seat recession additive is more recent.
What's down the road? With the Swedish experience of various additives on the market giving way to the formation of low melting point eutectic formation, and turbo charger damage due to alkali metal additives, it would be prudent that only one type of additive be available. The Retail Motor Industry in the UK proposes that because the anti-wear properties of phosphate coatings have been well established for a range of alloys, and phosphates are well known within the fuels and lubricants field, that phosphorous-based additives are the only ones to use. Nevertheless, MMT is approved as an anti-VSR additive for British lead replacement petrol.
There are several steps that the motorist should take to protect their engine:
1. Have the engine properly serviced to ensure tappets are adjusted to manufacturer's spec's.
2. Ensure that the ignition timing and fuel/air mixture ratio are correctly set.
3. Ensure the cooling system is efficient. Quality coolant added, not tap water. Radiator surfaces are clean and bug free. Thermostat & pump operating properly.
4. Do not mix LRP with leaded petrol.
Reference material from:-
Caltex Australia (for petroleum statistics)
Dr. Laurie Palmer Retail Motor Industry Federation (UK) (engine damage, 18 Nov 1998)
Associated Octel (on Valvemaster)
US Environmental Protection Agency/US Dept. of Agriculture Mechanical & Manufacturing Engineering, University of Melbourne.