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HOW THE ENGINE ACTS AS A BRAKE.
POINTS TO REMEMBER WHEN DESCENDING HILLS...
The action of the engine in re tarding a car when the throttle is closed is a very well-known pheno- menoa, but just why a braking action is produced is not by any mens fully understood. Some people, for example, will tell you that the compression in the cylin- ders tonds to prevent the pistons from moving, and therefore brakes the car as a whole, but others will refute this assertion by pointing out that, although work is done iz compressing the gases, an equal amount of energy is returned to the piston on the following stroke when the gases expand.
Before commencing to describe what is happening to the various working parts when the car is driy- to point out that there are several ing the engine, it will be as well different ways of using the power unit as a brake. First of all there is the common practice, when it is desired to slow down, of removing the foot from the accelerator pedal, so that bat little gas can enter the cylinders and the explosions be come very weak. The engine stili
is being fires regularly, but it forced to turn faster than its idling speed by the momentum of the car.
Switching OA,
that the car alowed down simply under the influence of friction in the wheel bearings and transmis son,
In each case measurements were taken showing the distance which was covered from the commence- ment of the test, at m.p.h.. to the instant at which the speed bad decreased to 10 m.ph. All tests were tande over one and the same stretch of level road and in the same dirction on each occasion; there was no wind.
CAT
With the clutch out, the coasted 390 yards before the speed felt to 10 mp.h., whereas, with the clutch in, the distance measured was 30 yards. The difference in these two distances was, of course, effect of the idling engine. In the entirely caused by the braking next test, with the engine switched off, the distance was reduced stil further (to 200 yards), and, finally, by opening the throttle wide and switching off, the maximum braking effect was obtained and the distance decreased to 100 yards.
Inside The Engine. side the engine, let
Turning to what is going on in
first consider the conditions when the car is coasting with the engine switched The next case to consider is re-off and the throttle nearly closed. presented by the course adopted by On the upward stroke of onch pis some drivers when descending long ton gases are compressed and a hills-e, closing the throttle and definite braking effect is obtained, switching off the ignition. Lastly, but on the following stroke the there is the method of switching off gases expand and help to drive the and opening the throttle wide to piston downwards. increase the compression pressure; alternatively, if on extra-air valve is fitted, the throttle is closed and the valve, opened wide, which has a very similar effect.
However, it is incorrect to say that the energy returned on the second stroke is exactly equal to the work expended or the gases during compression, because there In order to show, the degree of is slight difference, due to the braking obtained in these various beat which is lost through the On the exhaust cir umstances, some tests were made cylinder walls. with a well-known 10 h.p. light car stroke there will be a slight back under the three conditions describ-pressure tending to retard the pis ed above, and also, as a compari- ton and, on the following suction son, with the clutch disengaged so stroke, the considerable vacuum
THE
BEST-KNOWN
THE HONG KONG DAILY PRESS, WEDNESDAY, JANUARY 23rd, 1929.
MOTOR NOTES
formed in the cylinder will also have a braking effect.
If the throttle is opened wide, with the engine switched off, the result is to allow a bigger quantity of gas to enter the cylinder, so that B somewhat greater beat loss oc- curs during compression and the hack pressure од the exhaust Од etroke is alightly increased. the other hand, the vacuum during the induction stroke is not so great, "so that there is but little difference in braking efect, taking the cycle as a whole.
Pistol Friction.
Far more important than the passage of the gases through the cylinders, however, is the friction set up by working parts of the engine, such as the pistons and bearings. Of these the pistons are the more important, because con- ditions are not favourable to lubri cation.
Whenever there is a load on the piston and the connecting rod is at an angle to the nine of stroke, there is a tendency for the piston to be thrust sideways against the cylinder walls, thereby increasing the friction. Consequently, by opening the throttle, the piston load and side thrust are increased, so that a greater frictional effect is obtained. The difference is, how. evor, relatively small.
EL
However, there is a very good reason for opening the throttle (or. preferably, employing an extra-air valve), quite apart from the slight improvement in braking effect. On
difference of as much as 7 lb. per the suction stroke, when the throt tle is nearly closed, pressure 4 in. may exist between the upper and ower surfaces of the piston.. for oil to be pushed up past the sp that there is a great tendency rings into the combustion space. To is quite a common experience to have, trouble with oiled-up plugs if one coasts down a long bill with the engine turning at a high speed and with the throttle nearly closed, the reason for this phenomenon being found in the passage of oil post the pistons.
Turning to, the case in which the switch is left"on" but the throt tle is closed, the main difference between the internal conditions which then exist and those which obtain when the engine is idling is found on the expansion stroke, Here, instead of the compressed gases expanding gently as the pis ten falls, a weak explosion occurs which assists in driving the piston downwards to an extent partially dependent upon the speed of the engine.
It will, however, be noticed that the difference in the distance re- corded with the switch on "off" was only 15 per cent. Inci
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dentally, the possibility of oil pass OIL DETERIORATION. ing the pistons is just is great when the awitch is on," because the
conditions on the suction stroke are IMPURITIES WHICH RENDER practically unchanged.
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Another point is that if the en- gine be allowed to run switched off without using an extra-air valve, an explosive mixture is apt to be stored in the exhaust pipe and silencer, which may explode and damend that the oil be drained considerable damage when the periodically from the engine sump gine commences to re again and
and replaced with a fresh supply, hot exhaust gases are emitted. Loads Due To Inaștia. -
and as a rule motorists faithfully So far, we have only considered carry out the instructions. While the loads on the piston and con- realising that the reason for this in contact, with, and is absorbed necting rod occasioned by the gases procedure is the deterioration of in the cylinder, but it does not do the lubricant, many owners, how..
by, the film of oil on the cylinder ta forget the effects of inertia. At ever, probably do not appreciate alle. On the downward combus each end of the stroke the weight the exact manner in which the oil tion stroke of the piston much of represented by the piston, gudgeon is rendered unsuitable for further this oil is exposed to the best of the burning gases, and the dis- pin and upper part of the connect-
use, and
thought that these ing rod has to be brought to restorief notes may be of interest. The solved petrol in it is volatilized and re-started in the apposite direc-impurities present in waste lubriand burnt. A small portion of the tion, and this necessitates the ap-cating oil from car engine sumps phention of considerable force at may conveniently be grouped under high speeds. Indeed, in the major two beadings: (1) In solution, and is of engines the inertia load is (2) in suspension; is is proposed to apt to become almost as great as deal with them in this order. the explosive load at speeds in excess of 9,500 r.p.m.
it
Impurities La Solution.
Oil which has been in use for Consequently, if the driver's foot is suddenly lifted from the accelera- even a short time invariably con- tor pedal, when the car is travelling tains dissolved petrol, which re- at a high speed, the inertia loads duces the viscosity of the lubricant on the pistons create side thrusts, to such an extent that it ceases to which, in turn, produce considerable be suitable for further service. frictional resistance. This falls off Normally, the pollution occurs dur rapidly, however, as the speed deing the induction stroke, when creases, until finally, only the loads some of the fuel, which is in the form of a very fine spray, comes due to the gases are operative.
contaminated lubricant, however, is returned to the crankcase by the scraping action of the piston rings, and thus does not come in contact with the high-temperature gases, with the result that it retaina a small quantity of petrol. As the oil in the sump circulates it con- tinues to take up more and more fuel in this manner, and although undoubtedly some of the lighter constituents are volatilized by the heat, the total percentage of dis- solved spirit gradually increases. At the same time a small amount of light spirit is produced by the cracking" of the engine oil it-
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DURO MOTOR CO., LTD. Kowloos. TEL K. 216.
self, and this, combined with the absorbed fuel, even in comparative- ly small quantities, reduces the
body" ar viscosity of the lubri cant to an alarming extent.
In the case of the particular oil dealt with the presence of only Per cent. of petrol reduced the vis- at 60 degrees. cosity, measured Fahrenheit, from 98 to less than 50, while 10 per cent. of dissolved spiri: resulted in a drop to appre- ximately
It is obvious that if an oil with a viscosity of 88 is the most suitable for use in a particular engine, one- having a considerably lower figure cannot be expected to provide effi- cient "lulitication, and it follows that, if the car owner fails to drain the crankcase regularly and re plenish with fresh oil, trouble, in the form of overbeating and, badly worn bearings will be encountered. (Continued on page 9),
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