THAT KNOCKING.

Sun (Auckland), Volume 1, Issue 52, 24 May 1927, Page 11

THAT KNOCKING.

HOW FUEL AFFECTS IT.

FEED THE ENGINE WISELY

Motorists are beginning to make headway in the quarter-century struggle against carbon and its particular knock. Better engines, and now better motor fuels, are lending support in the car-owner’s favour, bat of outstanding importance is the growing determination to strip the carbon knock of its weapons of self-defence by turning on the searchlight of knowledge. To thousands of modern drivers the knock is no longer a puzzle, hut rather a simple result of equally simple conditions. Incidentally, it is well lr» point out that carbon knocks do not develop in the way many motorists believe. To the .inexperienced, carbon is pictured as a lot of black substance which chokes an engine to a point where its valves and pistons are thrown out of gear. A bad knock can arise from an engme having perfect operating valves and pistons, so long as there is jusi a little too much carbon deposited somewhere within the firing chamber. Ibis carbon simply takes up space that should be occupied by compressed gas, which is the same as saying that the more carbon there is in the firing chamber, the higher the compression ratio. The principle of the typical “noknock” fuel is that it permits quiet and easy operation of an engine whose firing chambers have been reduced in size through an accumulation of carbon. Gases tend to fire, or “go off,” oi their own accord under compression, sc it is readily appreciated that, if ordinary gas is compressed to a point highei than that originally intended for the engine, it is bound to offer what the engineers call “detonation,” which is the technical term to describe the “ping” of a burning gas m too rapid combustion. STUDYING CARBON ACTION. Motorists are reaching a better understanding of what happens in engine cylinders, and, in America, the introduction of many new kinds of fuels and fuel dope is offering an excellent opportunity to study the subject of carbon in its relation to engine action. Many are discovering why it is that the same engine, operating with the same kind of fuel, will knock worse one time than another. They have found the answer as well to the apparent conundrum as to why a newer, better engine will often knock worse than an old one in which there is an even greater accumulation of carbon. If the spark is not set too far ahead, and acceleration is not too iinveasonable, a cold engine may “ping” quite a bit when stepping on the gas, provided it has too much carbon in it. Another engine with more carbon may not “ping” at all under similar .onditions. Why the knocking in one case and not in the other?

The answer is simple enough when individual conditions are considered. One engine may be new and tight fitting, with the result that its norn.a’ compression is high, even when cold The carbon within its firing chamber? raises compression to a point where j large volume of gas, as in accelerating will break into detonations, whet he the engine be cold or hot. In the othe case there is considerable leakage o. normal compression when the engine i cold, with the result that the carbon actually serves to make the engine operate better in starting by raising the compression to a good normal. This older, or perhaps inferior, engine will commence to knock as the parts warm up and compression is held against leakage. If the newer, better engine is properly adjusted it should not knock much more after being in operation a short time unless it commences to heat up on a hill. Then the carbon within its cylinders becomes incandescent, thereby acting as au advance spark plug to pre-igmte the incoming gases before they are fully compressed. This causes a hack action on the piston, and the most familiar type of ca-b<*n knock. KNOCK ING PECULIARITIES.

Motorists are »lso finding that even the hot engine oTers various avenues of possibility in the knock due II exhaust valves to be provided with too little tappet clearance they may not jhu*t folly when supposed to be "closdsi, wUto the result that compression wifi !»• lost on a hill. An engine that 's ib otherwise good condition and UmU has been running cool may' «urpr2»c its owner by knocking less umi*r such conditions that when cool. „ . „ ... Many old engines do not ping like newer ones because their pistons and rings leak badly and because there is sufficient oil pumping to keep carbon deposits softer, and, therefore, less prone to pre-ignite. Of course, this is more or less of a temporary condition and nothing to be desired, since the great loss of power through such inefficiency renders it necessary to ha\e the engine overhauled. From such apparent inefficiency, however, engineers have developed a type

of accessory 7 that will keep carbon from being such an annoying imitator of the spark plugs. There are devices to permit the engine to suck in an oil or water vapour. These are designed to give that “rainy day” action of the engine and to keep carbon deposits soft. The best plan in fighting carbon is to use a no-knock fuel, such as a mixture of petrol and benzol. Carbon is the result of the carbonising of unburned and unexhausted hyalro-carbons in petroleum fuel, plus dirt and the residue from burned engine oil that pumps past the pistons. The hydrocarbons that are not combustible present the big nuisance, and this condition is greatly increased when the gas pre-ignites. Starting with an anti-knock tuel, therefore, postpones the time when there will be pre-ignition through excessive compression or the incandescent conditions of the carbon deposits and, in addition, continued use of an anti-knock fuel serves to provide a gas which will operate without detonating In cylinders choked with carbon. The anti-knock fuel simply changes the rate of combustion to meet the abnormal engine conditions. Of all sad words A man to jar: “Another note Due on your car.”