PETROLS DIFFER

Evening Post, Volume CXIII, Issue 31, 6 February 1932, Page 21

PETROLS DIFFER

That one brand of pietrolis Just as good as another is quite often ,Jieard from motorists, evon motorists who are men of long experience. Petrols do differ in thoir' individual characteristics, oither, in the. first place, between first and second grades or, in the second place, between the various brands; Boughly speaking, first-grade spirits are usually much more volatile and afford, very easy starting,, whereas second grades are heavier but generally give a much greater mileage in a heavy car, and are more suitable for. American and all heavier type ciir and lorry engines. A first-grade; spirit is essential in a highly tuned ear-in which there must be produced the highest degree of efficiency. , ..,, ..../ ....

However, it is the carburettor system wh"ich plajs the most important part in the choice of a suitable spiritl. The "mixture" that enters the combustion ' chamber on the induction stroko is by no means a gaa but ia merely a -vapour, w'lilcli nas been produced by means of the sprujiiig effect of the iot in the raibuifcttoi. Should

the carburettor (because of a small choke tube and induction pipe tract, late inlet valve opening or a.long Btroko motor) have a high spraying velocity the petrol vapour entering the combustion chamber will be a very fine spray or vapour. This is the reason why so many English cars require a first-grade spirit in order to obtain tho best results. .-■■,'-.■

On the other hand, a carburettor that has a low spraying velocity (having a largo choke tube and induction pipe tract, early inlet valve opening, or a short stroke motor) generally produces a vapour that is not nearly so fine and many contain a large proportion of unatomiscd petrol drops which are not burnt in the explosion but which dilute the oil on the cylinder walls, form carbon, or pass out through the exhaust pipe. ■

Each particular make of car requires a brand of petrol that will suit' the conditions found in that car, and it is foolish to imagine that any or every grade or brand of petrol is suitable. Some brands of petrol atomise more easily than others, some are more volatile, some' tend to cause oil dilution, while others cause either easy or difficult starting from cold, and so on:

Every motorist has noticed that on certain brands of spirit .his car may knock or "pink." This knocking or "pinking" is dependant on two causes: gas turbulence in the combustion chambor and a characteristic quality in the petrol itself which varies with the different brands. Designers now prefer a high compression ratio and a type of cylinder head which gives a very small combustion chamber and a high degree of gas turbulence—the latter mean's a higher flame rate and this may mean instantaneous combustion at several places in the combustion- chamber itself if the petrol is not quite suitable. This ultra-rapid combustion causes "pinking" under certain conditions and will be increased and accelerated by the use of certain grades' or brands of petrol. ■ . , :

Knocking or- "pinking" is a desirable thing in any motor, since it shows that maximum output is; being develop: cd. Betarding the spark to a position where thi motor is almost on the point of pinking: under load gives the best resultß.

It is therefore prudent to choose a grade or brand of ..petr6l which, irrespective of carburettor velocity, will atomise completely and be entirely burnt during the combustion \ process. Very easy starting is riot necessarily a sign of a volatile spirit,.but may probably be caused through too rich a carburettor setting. For all-round purposes the best" and most economical spirit is: that which gives the beet running when the motor is thoroughly warm—but tho spirit must be volatile enough .to vaporise at low " tempera-

turea and not cause oil dilution. It was recently stated that tlierd are 50,000.registered; vehicles in: New Zealand that are laid up by their owners because of tho bad times. Many owners indeed cannot afford to run their cars at' present,- but surely^ there are many who are merely.economising au'd could, if" necessary, put their cars on the '.road.'! . . ' '. Probably the average owner spends at least £5 on motoring per month, so that if all those 50,000 vehicles were again on the. road no loss than a quarter of a million." sterling would-be put into circulation per month. The best way to economise i$ not to lay the ear-up indefinitely, but to use it only for essential! purposes and for a little healthful motoring at the week-ends, and not for running to the office and back several times a day. Laying up means that the tires perish, the battery goes flat, the oil congeals, and the paintwork of the car deteriorates very quickly.—