SMOOTH RUNNING.

Auckland Star, Volume LXIII, Issue 139, 14 June 1932, Page 14

SMOOTH RUNNING.

PETROLS DIFFER. HEAVY GRADES FOR HEAVY CARS. VOLATILITY ESSENTIAL. Petrols differ considerably in their individual characteristics, either, in the first place, between first and second grades, or, in the second place, between the various brands. Roughly speaking, first grade spiritsare usually much more volatile and afford very easy starting, whereas second grades are heavier, but generally give a much greater mileage in a heavy oar, so are more suitable for American and all heavier type car and lorry engines. A first grade spirit is essentia! in a highly tuned car in which there must be produced the highest degree of efficiency. It is the carburettor system, however, which plays the most important part in the choice of a suitable spirit. The "mixture" that enters the combustion chamber on the induction stroke is by no means a gas, but is merely a vapour, which has been produced by means of the spraying effect of the jet in the carburettor. Should the carburettor (beca\ise of a small choke tube and induc-

tion pipe tract, late inlet valve opening or a long stroke 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 & first grade spirit in order to obtain the best results. On the other hand, a carburettor that has a low spraying velocity (having a Hirge choke tube and induction pipe ♦fact, early inlet valve opening, or a short stroke motor) generally produces a vapour that is not nearly so line, and many contain a large proportion of unatomised petrol which is not burnt in the explosion, but which dilutee the oil on the cylinder walls, forms carbon, or passes 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 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 dependent on two causes; gas turbulence in the combustion chamber and a characteristic quality in the petrol itself which varies with the different brartds. 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 means 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 cayses "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 developed. Retarding the spark to a position where the motor is almost on the point of pinking under load gives the best results. It is therefore prudent to choose a grade or brand of petrol which will atomise completely and be entirely burnt during the combustion process. Very easy starting is not 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 bestrunning when the motor is thoioughly warm—-but the spirit must be volatile enough to vapourisc at low temperatures and not cause oil dilution.