WHAT IS THIS PETROL “DOPE”?

Horowhenua Chronicle, 11 September 1934, Page 3

WHAT IS THIS PETROL “DOPE”?

LEAD AS A KNOCK-PREVENTIVE IDE HIGH COMPRESSION ENGINES

By the N.R.M. Engineers

“Doped” Petrol —spirit to which a stablizer has been added for the purpose of preventing detonation —was first introduced in America commercially. Several years later it was introduced in England, and it is now becoming widely used in Australia and New Zealand. Mention of this method of treating petrol was made by the N.R'.M.A. .engineers as far back as Ii)2o, but we are prompted to deal with the subject again in the light of developments. The thermal efficiency of an engine, or the useful work obtained from the heat liberated from the fuel, is dependent upon the compression. That is why the thermal efficiency of a compression ignition engine is greater than that of an internal combustion engine of the same size, and accounts for a falling off of power when an engine is allowed to lose compression through faulty valves, rings, etc.

It costs manufacturers no extra to build an engine •with a high compression, say up to 7 to 1, and ;t has always been their desire to do so, but the useful compression ratio that could be used has been limited by the-avail-able fuel. Knocking' would take" place in the cylinders when an ordinary fuel was used with only a •moderately high compression.

INCREASE IN POWER

The following figures show the in

crease in engine power and economy by raising the compression ratio: — Compression ratio, I to 1, increased to 5 to 1; increase in power and economy, 14 per cent. Compression ration o to 1, increased to 6 to 1, increase in power and economy 24 per cent. Compression ratio, 6 to 1, increased to 7 to 1, increase in power and economy 32 per cent. This gives approximately IS per cent more engine efficiency with the present day compressions, as compared with those of 1927. The increase in compression ratio has been made possible by improvement in fuel alone, and is regarded as a very meritorious accomplishment, for the limit has been reached in the design of power units with regard to their external dimensions, and other means must be found to give additional power output. OTHER EFFECTS.

From a purely thermodynamic consideration, it is obvious that a substantial increase in power output and a decrease in fuel consumption result from increase in compression ratio. On the road it will be noted that ’he increased compression ratio results :n an improved top gear performance and a greater acceleration ability of the ear in all speeds, but especially so at the higher speeds, due to the fact that the increased poAvcr made available by an increase in compression is a larger percentage of the net power available for acceleration.

It is generally thought that bearing loads are increased with higher compression. According, however, to a test conducted to bring’out the pressure on the bearings at various speeds with compresson at 5.5 to 1 up to compression of 7 to 1, we lind that as the engine is speeded up and reaches 3000 r.p.m. the bearing pressure is actually reduced below that of the inertia pressure. BEARING PRESSURE. That is to say, if the engine were rotated at a 'speed of 3000 r.p.m. ■without compression or .firing, the pressure on the bearings would be greater than during actual running at almost full throttle opening. At slow speed (1000 r.p.m.) the perssure due to compression and firing is greater than the inertia pressure. .

To Increase the power of an engine without large parts and -wtihout raising the overhead costs Was too great an advantage to be lost .sight of by the manufacturers,’ so the co-operation of fuel experts was obtained. These set themselves what proved to be a most difficult task in compounding a fuel that will burn without knocking with high compression and leave no injurious effects.

E&peiienced motorists know that Bengal added to petrol will improve thfe knock resisting properties of the i fuel, but. because it will detonate violently in an engine not provided’ with an effective cooling system, it was relegated, together with some 33,000 substances tried out by those who were responsible for the fuel that is daiiU£d to fulfil all requirements. TETBA ETHYL L&XB.

While RicaVd.o of Eh.glSk.nd was conducting expentoeht*, to discover a suitable, "dope” to suppress knocks, Midgley, and febyd, of General Motors, decided upofe Tetra Ethyl (pronounced eethile)- Lead as by far the most important commercial suppressor. All the other substances tested were unsuitable for one reason or another. In our notes on Tetra Ethyl Lead published in 1925 we mentioned that petrol so treated was not a •■eommer- ( cial success owing to the lead peroxide deposit, left in thej engine and accumulating in the spark plugs and valves/ pausing the plugs to short and valves to stick, but we suggested that this disadvantage would’'noj'doubt be overcome by adding a suitable : compound. ■ ”■ — : ' v -

This has now been successful accomplished, the unnamed compound being classed as Ethylene Dibromide, which converts the lead into a volatile halide ejected through the exhaust ports. WHAT IT IS. Tetra Ethyl Lead, or Load Ethide, is described as a colorless oily liquid with a sweetish odor, and having a specific gravity of about Ed. It is a powerful poison. The anti-knock power of Tetra Ethyl Load is stated to be effective when one molecule is added to 200,()()(.) molecues of fuel air mixture.

•When petrol treated with Tetra Ethyl Lead was first used commercially in America, it met with much opposition from the health authorities, until it was shown that employees of garages dispensing this petrol wore not affected to the same extent as employees or other industries using a lead compound. The same difficulty was met when this fuel was introduced into England, but when it wa.s found that its importation oi' use could not be prohibited under any existing law it was agreed by the medical profession that they would carefully note any trouble brought under their notice by poisoning as a direct and definite result of Tetra Ethyl Lead in petrol, and report to the Government after a 111 months’ trial. CLEAN BILL OF HEALTH. At the end of this period, there being nothing serious to report, Tetra Ethyl Lead petrol was given a clean bill of health.' This should be a sufficient assurance to members that leaded petrol may be used wtih confidence. They must not be deceived, however; the poison exists, but with ordinary care it is not injurious.

Nevertheless, teh presence of Tetra Ethyl Load is disclosed by a coloured tint in the petrol, and this should bo taken as a warning to use such as fuel, and not for any other purpose. We have had considerable first-hand experience with leaded petrol, and find it gives a very good performance in high compressioned engines without detonation, due to its higher octane value, and permits of the ignition being well advanced. PETROL REQUIREMENT.

Presence of gum in a fuel will set up trouble that is put down to the “dope,” but there is no doubt that the experience of the fuel experts is such that they can produce a petrol treated with Ethyl Lead to be satisfactory in every way when used as a fuel; a chemically pure petrol before adding the Tetra Ethyl Lead is all that is required. Of the white deposit found on the cylinders after use, we can only say that to date we have not found it harmful in any way. It is just a white deposit instead of a black one.