PETROL

Having studied the situation and experimented with various fuels over the last 5 years, I think I can shed some light on the subject in simple terms.

Modem fuels have two main differences from the petrol our cars were designed to cope with. They are much more volatile and they have a much higher octane which, although it sounds contradictory means that the charge bums at a slow-er rate than low octane fuels.There is also the contentious issue of lead or no lead. Tetra-ethyl lead was used as an octane booster from about 1929 onwards, although cheaper grades of petrol were not universally leaded until after the war. The issues are:

1) Volatility. This is the "evaporativeness" of the petrol and is related to the amount of spirit (light grades or "aromatics") in the fuel. Our cars were designed to run on 20's and 30's petrol which was not so volatile as modem fuel and felt much greasier.

Problem: Highly volatile modem fuel can evaporate in the fuel pipes causing vapour locks. This in tum causes complete engine failure or at best rough run-ning. This problem is usually apparent on a hot day after the car is stopped with a hot engine. The petrol in my Chummy carb will actually boil on a hot day -the carb stops working when this happens!

Solutions: a) Lower the volatility of the fuel: under a special dispensation we may now add paraffin to petrol as long as it is for a car first used pre 1/1/47 and is less than 10%. I don't know what effect this has on the octane rating (see below).

b) Try to isolate the carb and fuel pipes from the heat. Route the pipes away from the exhaust. Ruby and Box rear tanks: try to isolate the main fuel pipe by wrapping it in heat proof tape as it runs down the chassis. If you can, make a heat shield for the carburettor.

c) Use the lowest possible octane fuel you can find. For reasons we'll see in a minute, this helps lower the underbonnet temperature. Advancing the ignition will also help get the temperature down - if necessary replace the rear main to get the engine advanced!

2) Octane Rating Modem fuel has a much higher octane rating than our cars expect or need. Remember the higher the octane rating the slower the fuel bums. This is to cope with the modem engines (relatively) huge valve overlap and higher compression ratio.

Problem: Using high octane petrol means that the fuel bums more slowly in  the cylinder. Our engines have the fuel in the cylinder and buming for a much shorter time than a modern. Net result? The charge is still buming when it goes out of the exhaust pipe! The exhaust gets very hot and the exhaust valves bum out in double quick time. (Mmm ever noticed how short a time Austin Seven exhaust valves last these days?). Also the underbonnet temperature sky-rockets and we make problem I above even worse.

Solutions

a) Again, use low octane petrol. You will make the problem worse by using super unleaded or 4 star. (Ever used an A7 on French "Essence" ? They run beautifully because Essence is only 89 octane or so.)

b) Again advance the ignition, the earlier you light the mixture the sooner it will go out!

c) The real answer of course is the wretched modem fuel bums for too long and needs speeding up. Maybe paraffin would do this but I haven't tried it yet. Perhaps we should import large quantities of French petrol

3) Leaded vs Unleaded. Having established that the Seven would thrive on a low octane fuel, why do we need to use leaded petrol when lead is used as an octane booster'? In the thirties both leaded and unleaded fuels were available. Economy cars like the Seven were able to run on leaded fuel but were designed for "straight" fuel. After the war almost all fuel was leaded and the motor man-ufacturers were able to take advantage of a side effect of the lead in that it acts as a lubricant to the valve gear. The manufacturers were able to use softer grades of cast - iron in blocks and heads and softer valve guides. By the 1950's and 60's, the average engine was fully dependant on tetra-ethyl lead and use of unleaded fuel in these engines will cause disaster.

Modem unleaded fuel has octane improvers other than lead (e.g. Benzene) and performs no upper cylinder lubrication at all. This has little consequence in its own right in a pre-war cheap car, I have found that my Sevens have covered many thousands of miles on premium unleaded with no valve problems at all (except that the wretched exhaust valves last all of five minutes - due to the mixture still being alight as it whistles by.)

The main problem is that even premium unleaded has too high an octane rat-ing and premium unleaded is lower octane than Super. The main point is don't use 4 star or supergreen. Its far too high octane and this will ultimately cause you problems in the overheating/vaporisation area and probably bum out your exhaust valves even faster.

Any one for some good 68 Octane from Khazakstan?

TIM REYNOLDS

SuperTip No 4

I think we have all heard, and frequently experienced, the problem of pre-evaporation on a hot summers day's in traffic (scuttle tank owners excepted) - the petrol vaporising in the pump and pipes before reaching the carburettor - and the advice to have a piece of towelling and a bottle of water avail-able for such an event. But, have you ever tried to soak the towelling without wasting a stack of valuable water in the gut-ter? HINT! Take the towelling in a polythene bag - pour the water into the polythene bag!!

SUPERTIP FROM ERIC JONES

COMBUSTION

(Dec 95)

The science of combustion is very complex; difficult for an engineer to perceive and completely out of sight of even the most enthusiastic motorist. It is hoped however that the following will bring some perspec-tive to the subject and dispel some of the popular myths and misconcep-tions which abound.

Firstly, there is no penalty for having a higher octane rated fuel than is necessary. It will not burn hotter or slower. The higher the octane rating the higher the compression ratio can be accommodated by the fuel before detonation. Higher compression ratio in an engine will give more power, a higher octane rated fuel will not. The lead additives do not increase the overall burn time of the complete charge, they decompose at high temper-ature and then work as an extinguisher, controlling the initial rate of the flame front. In practical terms the charge will take a similar overall time to burn and any speed variation in the flame front can be measured in mil-liseconds.

A little clarification of 'detonation' may be beneficial here. When the spark ignites the fuel/air mixture a flame front moves outwards, like the ripple in a pond, to pass through all the compressed charge within the chamber. However, part of the compressed charge trapped in a corner of the combustion chamber may be compressed further by the expansion of the burning gasses away from the spark plug, until it ignites spontaneous-ly before the flame front reaches it. Shock waves are generated by the two flame fronts clashing giving the characteristic 'pinking' noise and stressing the engine components.

The time taken for in air/fuel charge to burn is more dependent on com-bustion chamber shape, mixture strength, turbulence and compression ratio than fuel quality. The chemically correct or stoichiometric mixture ideal for complete combustion of all the fuel in the charge is about 14.75 air to 1 part of fuel, by weight, but it is possible to gain some more power if enriched to 12: 1.

However, excessively rich or very weak mixtures slow the flame so much that the engine will run badly and the pollutants in the exhaust increase.

The controlled conditions of combustion today can result in a fuel burn time of less than 1 hundredth of a second and less than 1 thousandth of a second in racing motor engines. An Austin Seven running at 3,000 rpm has 8.33 hundredths of a second available burn time between ignition and the exhaust valve opening, so the fuel will be completely consumed long before the exhaust valve opens. In fact, the speed at which the burn takes place gets quicker as the engine revs rise due to increased turbulence caused by higher piston speeds and manifold velocities. For this reason there is no need for any more ignition advance at 5,000 rpm than there may be at 3,000 rpm.

On the question of ignition timing the correct setting will give maximum combustion pressure at a point when the connecting rod and crankshaft web are sufficiently offset to turn the piston thrust into leverage. If the ignition timing is over advanced maximum pressure will occur sooner and when the connecting rod and crankshaft web are nearer in line the result in a reluctance for these components to accelerate (remember your cycling days - stamp on the pedal when its forward the wheel flies round, stamp on the pedal with it directly up and you hurt your foot!). This situation leads to excessive shock loads on all the associated components, manifested in harsh running. Increasing the ignition advance will shorten engine life and will not make it run cooler unless its already over retarded.

The question of leaded verses unleaded fuel keeps coming around. Tetra-Ethyl Lead was only added to petrol to enhance its ability to resist 'pinking'. It does not increase the 'power' of the fuel, just enables it to be used with higher compression ratios, as stated before. (The more the mixture is compressed before ignition the greater the expansion power when ignited).

It just so happens that the presence of lead in combustion deposits a coating which has a significantly beneficial effect in prolonging the life of valve seats in cast iron engines. Without it the cast iron valve seat contact area can be eroded by microscopic parts being picked off by the opening valve. This action causes the valve to settle further into the block, the tap-pet clearance closes up, the valve cannot seal correctly so. due to its prox-imity to the initial combustion, is subjected to burning gases escaping past during the whole power stroke and the valve 'burns'.

You have three choices

1, Use unleaded petrol and service your valves/seats as frequently as in the dark old days,

2. use unleaded petrol with hardened valve seat inserts fitted or

3,use leaded petrol while you can and extend the life of the valve seats and ultimately the engine block as long as possible.

FUEL 

(Feb 95)

By Malcolm Tucker

Much has been written and spoken of late about the problem of using unleaded fuel in the type of cars beloved by members of the Austin Seven Motor Clubs.

While I am in no way qualified to formulate an answer or give advice on this problem I have researched and spoken to several leading experts in the field of fuel and engine requirements. What you can read below is my own interpretation of the facts as I see them, written in as plain a way as I can manage, without detracting from the scientific accuracy.

Before the advent of unleaded fuel we had the choice of two star, which had an octane rating of about 92 and four star with a rating of 97/98. Octane rating is simply a way of measuring the resistance to detonation or pinking' for a given fuel. Detonation is the uneven burning of the fuel over a longer time than engine design allows for. The tendency to detonate becomes more critical the higher the compression ratio of the engine. As the majority of our sort of cars were fitted with low compression engines, there was no need to buy the more expensive four star fuel.

The time arrived when it became politically desirable to try and do away with added lead in petrol for motor cars. It was explained that unleaded was suitable for all cars provided that ignition was retarded a tad and you kept an eye on tappet clearance. Unfortunately this advice turned out to be inaccurate and was withdrawn in quick order. The advice that followed was varied and contradictory. My own conclusions have thus been drawn from these two convoluted facts.

Lead was first added to petrol in the 1920's to help achieve higher octane ratings, a better controlled combustion, a degree of lubrication and the covering of internal surfaces within the combustion chamber. The first two points do not concern us as modern fuel technology is able to achieve these benefits by means other than using lead. The latter points do concern us, as to date, there are no alternative additives to lead that will perform these tasks.

Lets first look at what happens inside a cylinder as combustion takes place. The inlet valve opens and a mass of petrol and air is sucked into the cylinder, the piston moves up on the combustion stroke, with both inlet and exhaust valves closed. Ignition and combustion of the fuel/air vapour occurs and send the piston down on its power stroke. It returns upwards to force the spent gas out through the now open exhaust valve. Simple text book stuff, so lets look a little deeper into what happens.

In chemical terms, the petrol/air mixture does not burn evenly. The front of the flame, the area near the spark, burns fast and fiercely at perhaps 2,000 degress centigrade, the 'back flame' away from the spark only partially burns at 300/400 degrees C. This leaves a residual amount of unused oxygen within the charge. As it is expelled through the exhaust valve, the oxygen reacts within the iron of the valve seat and froms a microscopic amount of oxide. It adheres to the underside of the hardened valve. If lead is present in the fuel, it coats the underside of the valve and valve seat, thereby stopping the iron from oxidlising. In mechanical terms, the valve deforms as they slam shut and also relative to the valve seat. The iron oxide is the same stuff as jeweller's rouge, an abrasive material. Now, no prizes for guessing what happens when you place an abrasive between two surfaces and move them relative to one another under pressure. The softer of the two will begin to wear and that is exactly the problem with a soft cast iron valve seat, the type that many of the cars that interest us were fitted with. The rate of this wear can be excessive and is dependent on the angle and width of the seat and operating temperature of the engine.

Without lead, the inlet valve is also susceptible to increased wear as the lubricating effect of the lead is denied to the valve and seat surfaces. Again, operating temperatures and geometry are critical in the extent of wear.

We are obviously stuck with the geometrical problem of our own particular engine, but the problem of operating temperature can be minimised by the way in which the engine is used. It has been proven that the higher the engine speed  and load, the greater the valve seat recession. On a test engine, recession rate doubled when engine speed was increased from 3,800 to 4,400 RPM. Conversly a significant drop in wear rate was found at 2,000 RPM.

I draw to following conclusions from these facts. When using a lead free fuel there is an increased rate of wear upon both the inlet and exhaust valve seats. This rate of wear is not important if the engine has an aluminium head with (hardened) valve seat inserts. Some cast iron heads had hardened inserts, and some are capable of having them fitted. Many cast iron heads that rely on the parent metal for the valve seats are dangerously susceptible to valve seat recession. It is therefore better to use four star fuel with lead than two star without. The occasional tankful of unleaded will do no harm at all. If prolonged use of unleaded is unavoidable, then keep engine speeds and loading to a minimum.

There are to date, no fuel additives on the market that stand up to ~ independent scientific testing as a safe alternative to leaded fuel. The preceived effect of overheating when using four star in an engine designed to take two star is not correct.

The above information has been gleaned from conversations with and articles written by Dr Matthew Vincent of Shell Petroleum, Ken Lea, ex-Technical Director, Rolls-Royce Cars Ltd and the research team of the Federation of British Historical Vehicle Clubs.

With acknowledgements to Malcolm Tucker and the Solent A7C.