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Rolls-Royce Engines and Unleaded Fuel

The debate has been going on for a number of years, but a note of urgency has crept in now that the Australian Federal Government has reduced the lead content of leaded petrol and set a timetable for further reductions over the next few years until it is removed completely by the end of the century.

Suddenly, owners are worrying about the valves and valve seats in their veteran, vintage and early post-war cars. Will we be faced with increasing wear in these cars, with a loss of performance and eventually with horrendous overhaul bills because of lead being phased out?

It is the uncertainty of the situation, the lack of factual information which is fuelling (no pun intended) the anxiety.

Whilst at Crewe recently Tony Ward (NSW Branch Technical Officer) had a discussion with a Rolls-Royce Motor Cars Ltd executive on the subject of unleaded fuels and was handed a paper by the retired Chief Engineer, Power Train, of the Company, Mr.K.E.Lee. This addresses most owners' questions.

Article: Unleaded fuels for Rolls Royce and Bentley engines?

From a paper by K.E.Lee C.Eng., B.Sc. Hons., M.I.Mech.E., M.I.Mar.E., Chief Engineer, Power Train, Rolls-Royce Motor Cars Ltd (retired).

Lead in fuel has been used world wide as a means of reducing the tendency of engines to `knock' or `pink' while increasing the useful portion available for power of the heat value of fuel. This results in an engine's ability to the best fuel efficiency at the highest compression ratio without the incidence of spontaneous ignition before the proper flame front has arrived at that part of the unburnt mixture. Spark knock is a phenomenon experienced by all spark ignition gasoline engines and the sensitivity of each engine to this phenomenon is the most important limiting parameter in the relationship of fuel quality versus engine design.

This is very much an `empirical' relationship and is controlled between the oil and motor industries by means of deriving a specification using a standard series of tests. This is designated the `octane number' and there are two definitions used in the industries. The most widely recognised in Europe is the Research Octane Number (RON).

The octane number of a commercial fuel is rated against certain reference fuels on the scale of 0 to 100 numbers. 0 reference fuel is a heptane fuel and 100 scale fuel is an iso-octane fuel, neither of which need concern the motorist in their detailed definition as they only, but importantly, form the basis of a reference test on a single-cylinder research engine which is a standard for the industries.

However, the important point is how commercial fuels obtain their octane rating capability for use in commercial engines. It is interesting to note that various constituents of a cracking process of a crude oil have basically an inherent octane rating for the various hydrocarbon groups that can be used in gasoline blends. These are as follows:

Hydrocarbon Octane RON
Paraffins Generally low 0 - 60
Iso-paraffins Generally medium to high 50 - 100
Cyclo-paraffins Medium to high 75 - 100
Olefins Fairly high but variable around 90 90 - 100
Aromatics High 100 - 120

Motor gasolines (petrol) are a blend of the above fractions stabilised during storage and combustion by various additives which have changed with the continued development of the fuel and engine designs over the history of the motor car and commercial engines.

During the 1920s it was found that an octane improver utilised with the low basic hydrocarbon groups was the addition of tetra-ethyl lead (TEL), which has stabilised the quality of the fuel at higher octane numbers and was widely adopted.

Additives in general are very potent and are added in very small quantities up to 0.2% by volume. To be wholly acceptable an additive must not only be effective in its particular duty but must also be stable and fuel soluble with no undesirable side effects on performance or engine condition or on fuel storage stability. In the period 1920-1970 lead alkyls satisfied most of these criteria. In these compounds when added to gasoline the lead atom is bound in a fuel soluble form to give tetra-ethyl lead (TEL). During burning in the cylinder the lead is freed and forms a dioxide, where it absorbs the active molecules of the combustion mixture `end gases' that remain in the cylinder after combustion has commenced. If this reaction did not take place these end gases would ignite spontaneously to give uncontrolled combustion and `knock'.

Its potency as a knock controller lies in its ability to re-oxidise and repeat this chain-inhibiting reaction many times during the combustion process of one cylinder firing. In fact this molecular process, if unrestricted, then leaves lead oxides deposited on the combustion chamber walls and can cause plug fouling and insulator tracking, leading to misfiring, while also having a hot corrosion effect on exhaust valves. Further additives of dibromoethane and dichloroethane are therefore added to scavenge these compounds and evacuate them in the exhaust gas.

Thus it can be seen that though these `octane improvers' based on lead alkyls are added in minute quantities to gasoline they are by design emitted from the engine and out through the exhaust to prevent engine damage.

Relatively recently it has been said that these lead pollutants are harmful in high concentrations and hence the world wide movement is for so-called lead free fuels.

Incidentally, modern catalyst equipped cars must run on unleaded fuel, so that the lead is not deposited in the fine catalyst substrate that carries the chemical compositions that further reduce hydrocarbons and nitrous oxides. Catalysts become quickly poisoned when using leaded fuels and cease to be chemically active.

Lead free, or more precisely lead trace fuels, are now widely available where emission controls are in force which require the use of catalysts have been introduce in Europe, although in most European countries leaded fuels will remain available for the foreseeable future.

These trace lead fuels are manufactured by changes in the refinery blends and process, which together with more modern additives give similar octane number fuels to their leaded counterparts. However, the octane rating of automotive fuels has reduced from 100-97 RON range available at the height of the developments of TEL as an octane promoter.

Perhaps it is of interest to review the general trends of octane number since the discovery of TEL technology in 1921 and its first application in the 1930s as shown in the table. As octane numbers were not recognised until the early 1930s, early numbers are a calculation based on fuel quality at the time.

U.K. History of highest available RON Year RON Comment
Pre-1917 50
1922 56
1928 57 First introduction of TEL in U.K.
1931 78 More widespread but leaded fuel more expensive
1932 76 Prices equalised
1935 81 Leaded fuel more widespread still
1939 81 Almost all leaded fuel
1939-46 74 Second World War
1953 92
1954 93
1955-57 95
1958-59 97
1960 98
1961-70 99
1971-73 100
1974 99
1975-89 97
1989-present 95 Unleaded must be available

Thus, two aspects of operation for Rolls-Royce and Bentley cars need to be reviewed against fuels available in the past, present and future. These are

  1. Any effects of (TEL) lead deletion on engine operation with respect to any of the company's products; and
  2. The octane rating and its effects on operation against the original tunes by year of manufacture.

It is said by many that the deletion of TEL from fuels will cause serious valve seat/valve wear and hence as tappet clearances close will lead to valve burning. Many engines may well so suffer but two aspects need clarification:

First, engines fitted with aluminium heads which have valve inserts in high grade steel or irons will not suffer from this problem as this is the method by which cast iron heads are improved in terms of valve life if they suffer from valve breakage.

Second, valve seat wear of non-inserted iron heads Is extremely sensitive to combustion and operating metal temperatures and hence on large swept volume engines such as ours if such a sensitivity were present at all on older engines it will only occur at high speeds and load running. Load factors on large engines are generally lower than those on medium size and small family cars.

If we review the situation specifically, we have:

  1. Ghost engine: Low rated low compression. No problems envisaged as it was developed to run on unleaded fuels in the first place.
  2. Phantom I, post-AL series; Phantom II; Phantom III; All aluminium headed with valve inserts. No problems envisaged.
  3. Phantom I. pre-AL series; 20 h.p., 20/25, 25/30; Bentley 3.5 and 4.25 Litre; Wraith and Bentley Mk.V: An iron head developed as a combustion system on unleaded fuels with the Rolls-Royce 20 and Phantom I. No problems are envisaged but when using unleaded fuels check inlet valve clearances more frequently if the vehicle is driven hard.
  4. All post-war six-cylinder overhead inlet/side exhaust valve engines up to S1 series cars and any eight-cylinder engines built in this period: Aluminium head seat inserted. No problems envisaged.
  5. All V-8 engines from S2 onwards to current day: developed for operation on both leaded and unleaded fuels with aluminium heads with valve seat inserts. Absolutely no problems.

It can be seen therefore that all our engine's sensitivity to running on unleaded fuel only varies from minor to nil, and frankly apart from keeping a watch on tappet clearances on iron headed engines if driven very hard, very frequently no other action is required.

Turning now to the second aspect which is most important in terms of `knock' or `pinking', where sustained heavy knock can be dangerous to engine reliability, care should be taken with cars whose manufacture date means that they have engines which were tuned and matched against high-octane fuels on some post-war cars, this being 100 RON. For example, cars made in the late 1960s/early 1970s where 99-100 RON fuel was available, and includes S3s and early Shadows, these cars should have their static timing retarded to reflect the current availability of fuels within the country or area where they operate. However, infrequent filling with a lower grade fuel than that for which the engine is set normally results in slight `pinking' or `knock' and is not detrimental. If, however, this is continued on a long-term basis the engine should be adjusted accordingly. In round terms static advance should be retarded from the original maker's settings for our engines by 1 degree per octane number reduction. Thus cars tuned for 99 RON which now run on the 97 RON currently available should run approximately 2 degrees retarded from their original settings.

Finally, pre-1940 engines which were designed for octane numbers less than 90 should run on the low grade of fuel available in their sphere of operation today as super grades will not be rewarding in terms of increases in performance as these engines were properly tuned for the lower octane rating.

It is hoped that the above information gives some brief insight into the changes that have occurred over the past decades and will serve to allay the fears of our owners who still successfully operate older models of Rolls-Royce and Bentley motor cars.

This article is intended as information only and should not be taken as a recommendation by the author or the Club of any particular course of action.

(Reproduced with permission)

Tony has written an article clarifying this information in an Australian context.

Article: Unleaded fuels in Australia

Tony Ward - Technical Officer, Rolls-Royce Owners Club (NSW Branch)

To elaborate a little on what Mr.Lee has written I discussed the subject of valve seat recession with a number of motor engineers. I was told that the principal causes are sustained high r.p.m., i.e. 4,500 and over, high compression with steep camshaft profiles and heavy valve springs causing valves to snap shut quickly with heavy impact. Lead not only raised the octane rating but cushioned the impact of the valve upon the seat, which is particularly valuable at the higher temperatures associated with exhaust valves.

Now, let's look at how these factors affect Rolls-Royce and Bentley cars. To my mind, not one model from the earliest Silver Ghosts to the end of the six-cylinder engine in the Silver Cloud I and Bentley S1 series meets these conditions in any way! Rolls-Royce engines were robustly built from the best materials available in their period. They were under-stressed, low in r.p.m. and low in compression ratio.

It is true that though the Silver Cloud started off at 6.6:1 compression ratio it was raised to 8:1 on the later series of that model. However, Rolls-Royce paid particular attention to valve and valve seat material quality and at 25 m.p.h per 1,000 r.p.m. in top gear you would only be doing 3,000 r.p.m. at 75 m.p.h. (120 km/h), which is above the maximum speed limit! In a Bentley Mk.VI at 22 m.p.h per 1,000 r.p.m. you would only be running your engine at 3,400 r.p.m. at 75 m.p.h!

The other key factor in the debate is mileage. Just how many miles per annum are we likely to do in these cars? Not much in the case of pre-war cars, I would think, but perhaps somewhat more for the post-war six-cylinder models.

However, a couple of experienced technical members of the R.R.E.C. sounded a note of caution in conversation with me at Althorp last year, saying that they would like to see a test carried out in which a Rolls-Royce model was run for a large mileage on unleaded petrol. The valve seats would need to be checked before and after the test to see if any recession had occurred.

It is interesting to note that lead was not originally added as a valve seat lubricant, but rather for the reasons explained by Mr.Lee. The lubricating property was an added bonus.

In discussions with an Ampol engineer I was told that the octane ratings of petrol in Australia at the present time are as follows:

Petrol RON
Super (leaded) 96
Regular (unleaded) 91
Premium unleaded 95

Shell have recently introduced a reduced-lead fuel which they call "Half Lead". However, the octane rating has been maintained at 97 by changes in the production process and *not* by adding benzene. It is ironic that the lethal qualities of benzene, which is a carcinogen, are now causing concern. It is currently present in all grades of petrol marketed in Australia and may well prove, as research progresses, to be even more of a health danger than lead.

In the USA, a lead substitute for valve seat lubrication purposes, called Valve Master, was developed by DuPont. This is poured into the tank with every fill of unleaded petrol. [Information about commercial source removed for posting. ChrisG] An item in a recent R.R.E.C. Bulletin states that tests are being conducted in Britain on an anti-valve seat recession additive which has been used extensively in the USA

Although some of the valve seats in our older cars may not be of the hardest material, eg. the bronze seats in the Phantom II alloy head, the key factor I think is usage. Are we really going to do the large mileages for any deterioration to be noticed? In any case, an additive as a substitute for lead may well be the answer to our problems - if indeed there are any problems!

This article is intended as information only and should not be taken as a recommendation by the author or the Club of any particular course of action.

(Reproduced with permission)

A response to the above article(s) by Robert Chapman was published in Pr�clarum in 1996. Robert takes a different view of the situation.

Response: Unleaded Petrol for all Rolls-Royce Cars?

R.A.Chapman M.I.A.M.E. M.SAE

I recently read an article in PR�CLARVM , written by a retired Rolls-Royce engineer that gave advice to owners regarding the use of unleaded petrol in engines originally designed for leaded petrol. He covered the two main areas of concern when considering using unleaded petrol, These being valve seat recession and compression ratio.

As a motor Engineer myself, I would like to express a contrary opinion drawing on my own experience in the field. Whilst giving my opinion I would like to explain the practical and technical reasons behind them.

In the article it is suggested that the overhead inlet side exhaust valve engined cars will at most need more regular checks of inlet valve clearances. Rather, it is known in practice that in fact it is the exhaust valves that need regular monitoring due to the fact that the exhaust valve seats suffer from valve recession when not protected by the lubricating properties of lead, when these seats are cut directly into the cast iron of either the cylinder head or the block.

When the exhaust valve seat is not lubricated by lead it becomes overheated and each time the valve opens, molten particles of the seat are removed by the valve head and the rotating action of the valve produces abrasive wear on the seat and effectively lowers the valve head into the seat. This action rapidly reduces valve clearance.

The results of this reduction in valve clearances will cause the exhaust valve opening time to be advanced exposing the exhaust valve head to the high temperature heat flash of the still expanding combustion stroke. It will also decrease the time the exhaust valve head is in contact with the seat which is the main area of heat dissipation for the valve to the cooling system. Both the early opening of the exhaust valve and the decreased time that the valve remains on the seat dramatically increase the temperature of the exhaust valve which in turn accelerates valve recession problem.

This cycle continues until there is no valve clearance and the valve head can no longer discharge its heat to the cooling system. This results in a burnt valve and a total loss of compression. It is very likely that engines that have travelled many thousands of miles on leaded petrol will be afforded some protection because the valve seat will be impregnated with lead, but if a valve regrind job becomes necessary and the valve seat is recut this protection will be removed.

My advice, if you are going to use unleaded petrol in this type of engine, would be to check exhaust valve clearances every 3000 miles and when a valve job is required, fit hardened exhaust seats as per S1/Cloud 1 engines.

The second area of concern is advice given regarding compression ratio. It is suggested in the article that it is possible to run all V8 engines on unleaded petrol as long as the ignition timing is adjusted to suit the lower octane rating of the unleaded petrol. The procedure advised is to retard the ignition timing by 1degree per octane number reduction.

This means that for a 9 to 1 compression engine that was designed to run on 100 octane petrol and now to be run on 96 octane leaded petrol should have the ignition timing retarded by 4 degrees to a setting at idle speed of 1 degree Before TDC from its standard setting of 5 degrees Before TDC, and if the same engine was to be run on unleaded petrol of 92 octane the ignition timing would have to be retarded by 8 degrees, resulting in it being set at 3 degrees After TDC. Setting the ignition timing to this formula would result in substantial decrease in engine performance, idle quality, engine overheating and possible mechanical damage. It could also be illegal, as exhaust emission would be dramatically increased.

Engines with 8 to 1 compressions are on the border line for use with 92 octane unleaded petrol and if in good mechanical condition may just tolerate unleaded petrol.

It is generally accepted by most motor engineers, that retarding ignition timing is not an acceptable method of tuning an engine to run on low octane unleaded petrol. In my opinion, the major flaw in the advice to retard ignition timing to suit low octane petrol, is that the assumption is made that the octane requirements of an engine that has been in service for a number of years will be the same as it was when it left the factory. In practice this is definitely not the case and in fact it is likely to have increased.

There are many things that can occur during the life of an engine that will increase its octane requirement, these are compression increase due to carbon build up, cylinder head surfacing, cylinder block surfacing and cylinder over boring, inoperative exhaust gas recirculation equipment, increased inducted air temperature, lean petrol mixture, engine oil leaking into the combustion chamber and a worn distributor giving excessive mechanical or vacuum advance.

When a distributor is tested with an ignition oscilloscope it can be seen that a worn distributor will not only change the mechanical centrifugal advance curve (the rate of ignition advance relative to engine speed) but also the ignition timing of individual cylinders by several degrees. Since ignition timing is only checked on one or two cylinders, there is no guarantee that the ignition timing settings will be the same for the remaining cylinders. Added to this, it is quite common for compression pressure tests on individual cylinders to be ten p.s.i above the rest. It will be appreciated that the octane requirements of individual cylinders can be very different, much less individual engines.

Owners of carburetted turbo cars would definitely not be advised to follow the ignition retard advice as these engines already find it difficult to run on 96 octane petrol and would definitely suffer severe detonation if run on 92 octane unleaded petrol, although some may say that the compression ratio is only 8 to 1 and should tolerate unleaded petrol. It should be remembered that the compression ratio stated by the manufacturer is the theoretical ratio and is very different to the running or dynamic ratio. The theoretical ratio is calculated and assumes that the cylinder will be 100% filled at atmospheric pressure, but in practice a normally aspirated engine will only manage approx 75 to 80 per cent filling of the cylinder at maximum torque. Turbo charged or forced induction engines achieve far greater than 100% cylinder filling and therefore have much higher dynamic compression ratio when running under boost conditions and have a compression ratio of approx 11 to 1. Owners of these carburetted turbo cars would be advised to continue using leaded petrol while it's available and consider converting to lower compression pistons in the future.

My advice for owners of 9 to 1 engines would be to continue to use leaded petrol until an engine overhaul is required and then rebuild it using lower compression pistons.

Owners with 8 to 1 engines should firstly make sure that all systems are working correctly, clean combustion chamber deposits by using Redex upper cylinder lubricant or similar and have ignition distributor advance curve checked before changing to unleaded petrol, and when overhaul is required convert to lower compression pistons. Owners with 7.3 to 1 engines need only ensure that their engines are in good tune, and that all systems are working correctly.

The only other alternative to the octane requirement problem of all these engines is the use of L.P.G which has an average octane rating of 110, contains no lead and has much lower pollution levels than both unleaded and leaded petrols.

This article is intended as information only and should not be taken as a recommendation by the author or the Club of any particular course of action.

(Reproduced with permission)

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