Rolls-Royce Owners' Club
of Australia

Unleaded fuels for Rolls Royce and Bentley engines: what should you do?

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:

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 90 100
Aromatics 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)

Back to Unleaded Fuel Page
RROC(A) Online Library
RROC(A) Main Page