BACK PRESSURE IN EXHAUST

Evening Star, Issue 21881, 19 November 1934, Page 15

BACK PRESSURE IN EXHAUST

ROBS ENGINE CF POWER Automobile engineers admit that the exhaust muffler is an inherently objectionable device, and one that has been litted to cars only under the stress ol necessity and in order to diminish the running noises of the engine. 'I he common muffler or silencer is either a plain expansion chamber or one provided with baffle plates or tubes, which produce a silencing effect, but to some degree cause back-pressure in the exhaust pipe. That in turn spells a falling off in engine efficiency, for if the escape of the exhaust gases is in any way hampered there is a tendency for the following charge in each cylinder to become contaminated with a proportion of unexpelled exhaust gas. Much depends on the type of engine and the lay-out of the exhaust system, and it is significant that the practice both in car racing and aviation is to dispense with any form of muffler and allow the gases to be expelled directly into the atmosphere. During recent years/Various vacuum mufflers have been tried, and it is understood that a type invented by an Australian resident has undergone very successful official trials on British aircraft. The underlying principle of the fitting is that the silencer is so built that the passage of the gases through it tends to build up a vacuum effect at the outlet, so that; instead-of fi positive back pressure being created, there is actually a reverse effect. An eminent French engineer has stated that if a propei supercharger is employed in an engine, together with a vacuum type muffler, the power output can be alraost - doubled; or, conversely, the same driving force obtained from a unit of half the capacity of the engine, which is not so equipped. _ Persons who complain of the noisiness of many motor cycles often overlook the technical obstacles to • thenbetter “ silencing.” In the case of a high-speed, air-cooled engine it is spcci-, ally necessary to avoid apy constriction of the exhaust passages, since not only is the developed power affected, but overheating might result. ImU problem is, however, being gradually solved, and many of the most recent models are remarkably-quiet in operation. Some experts think that car manufacturers might devote more attention to this aspect of'efficiency, and it lias been noted that numbers' ol models have outlets which seem to be very small in comparison to the size of the valve ports, and the diameter of the pipe between the mulfler and tije manifold. The car owner would not he well advised to attempt to alter the exhaust system of his car, except under expert advice, but he should not neglect to clean the muffler at least once a year. Carbon deposits build up within it, and where there aie baffle plates or tubes it is not easy to dislodge them. Some engineers advise that the muffler should be detached, heated with a blow lamp, and then tapped with a hammer; hut as a rule it will suffice if the muffler is hammered with a wooden mallet, at the end or a run, and when it is hot, and the engine is then accelerated so that the detached carbon will be blown out. The fact that the outlet of some exhausts is smaller in diameter than the inlet is not necessarily anomalous, for during their traversing of the muffler passages are cooled and t then volume reduced, but in the opinion of many engineers there is still wide scope for improvement.