Different Times - Different Thinking

As present day owners of pre-war cars have expressed concerns that the unleaded petrol will have a detrimental affect on engine valves and valve seats, it came as something of a surprise to come across the following piece by Louis Montell in a 1944 issue of the Morris Owner at a time during the war when the only petrol that had been available was unleaded 'pool' petrol.
The reader can draw his or her own conclusions - Editor

Only Just Useable

Our standard 'Pool' fuel, with an octane somewhere in the late 60's, while not out of the petrolic 'top drawer', so to speak, was at least tolerable for our needs; but for the ever-growing fleet of American Service cars it evidently was not, and it was proposed, therefore, that in order to save the shipping complications through two grades of petrol being supplied to Britain, one only should suffice, vis Ethyl petrol; because, it was suggested, while many American cars could not run on our unleaded 'Pool', all of our cars could, on the other hand, run on their 'doped' fuel.

To this seemingly reasonable argument there appeared to be no objection, and Ethyl duly arrived in the proportion of 3 to 4 cc per gallon.

What we did not know in time, however, was that this additive attacked exhaust valves in the engines not modified to run on the fuel, and literally scores of thousands of exhaust valves were burnt out soon after its appearance!

Hard upon the heels of Ethyl there arrived technicians from the States to advise us as to the protective procedure; a visit which, while it had its use, and breathed friendly intentions, also indicated no little lack of imagination and forethought, proving as it did that our transatlantic friends were evidently already quite aware of what was in store for us; and in the circumstances we felt constrained to think that a little more timely warning was called for, especially as what we were eventually told regarding preventative measures was already well known in technical circles here. What one really required, in fact, was not belated instructions on what to do, but sufficient time in which to do it. However, this is now 'spilt milk', and as such we must try to forget it, with, albeit, the proviso that as more 'milk' is on the way, we had better improve our methods of handling it.

Why Ethyl?

To come down now, therefore, to hard facts; just what is Ethyl? What specially attractive virtues can it have to warrant its retention as an anti-knock measure, despite its apparent corrosive properties? In fact, why do we need either Ethyl or any other octane booster at all when, according to thermo-dynamic laws, all that should concern the engine are the calorific (or heat) values and the volatility of the fuel?

The only reason we need such things as Ethyl is because for efficiency we must always have the highest useful compression ratio. Also, although we employ the word 'explosion', it is actually ill-chosen to describe the inflammation of an air/petrol charge which must take the form of a rapid but strictly progressive combustion in order to push and not hammer down the piston. The former does useful work on the crankshaft below, but the latter only makes a noise and tends to injure the immediate target which receives the blow.

The flame always starts and up to a point progresses 'according to plan'; compressing before it and heating the as yet unburnt live charge in advance of the flame front; but if, for either this or any other participating reason, it gets too hot, there comes a point when the whole of the live remainder fires spontaneously, not progressively as up till then, but simultaneously at all points throughout its mass. A hammer blow, in short, which does little work but makes a great deal of noise.

This is what is commonly called 'pinking', 'knocking', or 'konking', according to the respective degree of intensity in which it occurs, but it is technically termed 'detonating'.

To describe the detailed mechanism of detonation would not be helpful here. All that must be stressed at the moment is that the temperature of the live residue is the critical factor; the hotter it burns the earlier does detonation take place and the more of the charge is so wasted.

It will, therefore, be clear that our attentions must be concentrated either on keeping the residue cool by physical means, or by finding a chemical method of making the charge stand a greater heat before it detonates. The former is a question of engine design, and the latter, which we now measure in what we call 'octane numbers', is a question of petroleum technology and chemistry.

The Burning Question

Having broadly defined the position, we will revert in their order to the leading questions above.
What is Tetra Ethyl Lead? It is often rather unfairly described as a 'corrosive anti-knock dope', but the adjective is an ill-deserved one, because a substance that is generally defined as a 'corrosive' is active at all ordinary atmospheric temperatures and pressures. Ethyl, however, only becomes so when strongly heated, which property it shares with many other bodies that could not possibly be so described. Remember, for example, that one can very thoroughly burn an exhaust valve by ordinary air/petrol combustion if the mixture is too lean and/or the spark too late.

Unfortunately for the reputation of Ethyl, it happens that its burning properties set in at a rather lower temperature than when unleaded petrol is used, but otherwise there is very little difference, and if we can keep the exhaust valves below that particular 'critical' it is less harmful.

The 'primary' - ie the early part of the combustion - is and must be as hot as possible because it is from the resulting expansion that we get our driving power. It does not burn the exhaust valve, however, because this is now firmly down on a relatively cool seating and no flame passes round it. It is later on during the 'secondary', or what the Americans call the 'after burning', when off its seating and bathed in a blast of outgoing flame moving at hundreds of feet per second that the burning takes place. The slower (and therefore later) the primary combustion, as caused by lean mixture, low compression, late spark, bad scavenging, etc, the hotter is this secondary outgoing flame, and of course vice versa; ie the earlier the spark and the higher the compression, the better and more complete the primary phase, and therefore the cooler the secondary combustion.

Apart from this unfortunately placed 'critical', it is an excellent anti-detonative agent in that it does not interfere at all with the primary burning, the volatility or the calorific qualities of the petrol, but only with the end-flame of the combustion, and as its punitive action on exhaust valves takes place only above a certain heat, the obvious cure is - keep them below it.

Safeguards

Here are now the special recommendations to that end:-

The exhaust valve guides can be shortened so that they do not project beyond the top of the valve guide boss. They can be cut off quite square, and the hole in the guide should not be chamfered or recessed. It is better for the guides to stand a little inside the boss rather than to stand proud. Regarding the valve tappet adjustment, when setting valve clearance it is best to err on the wide side; in other words, it is better to have.003 in. or .004 in. extra clearance than to set them.003 in. or.004 in. tight. Also, care should be taken not to run with retarded ignition. Two or three degrees beyond the optimum advance is preferable.

And now a point about carburation. Do not run on full throttle for long periods. Lean mixtures and so-called economy settings should be avoided. Less valve trouble is likely to be experienced if the mixture is slightly on the rich side.

Generally speaking, careful and more frequent attention to top overhauls is desirable, and the standard recommendations as laid down in the manuals should be scrupulously observed in performing this operation.

To sum up the position now, it will be evident that the Ethyl trouble is not so much due to Ethyl as to inadequate warning of its advent, because had we been advised we would have known what was necessary and had time to arm ourselves accordingly.

There are literally millions of poppet exhausts which are having a bad time. Promises of better days to them savour of the ancient dictum , 'Live horse and you'll get grass.' To their owners one can only reiterate: in whatever way they call contrive it, keep them cool to the best of their ability.

Bibliography credits
The Journal of the Morris Register, Spring 1991, Vol.13 No.5