THE HONGKONG - TELEGRAPH, MOTORING SUPPLEMENT. SATURDAY, JUNE 28, 1930.
THE MOTORCAR ENGINE AS A BRAKE.
Some Interesting Figures That Upset Ideas.
“NEGATIVELY LOADED" USAGE ON HILLS.
without the With the advent of free wheels to be negotiated It is only natural that there has personal fatigue involved in apply been recently a certain amount of ing the brakes and without any the engine risk of the shoes and drums getting discussion functioning as a brake. Whether overheated.
respect of this sort of circuma- tances that the argument general-
y arises Rather Is it concerned Fousey), car with the braking action of the - gine in more ordinary conditions.
Bix-cyl car
Bi-cyl car
Car
From 20
Fron
10
ED
*With clutch in
With clutch out.
It has always, in the past, been o.commonly accepted that to bruke
I do not suggest that these figures with the clutch "in" is to get a quicker stop than to bruke with the are the limit of accuracy. They a fallible clutch out which latter implies could not be that, as wear and tear In a thrust-race that human factor is involved. But they is often not, too. robust), that I was fare not far wrong, for the runs rather surprised to hear. the other made and the times taken were in- jevening a well-known racing motor numerable, and the figures given 1st assert that, except when an in- are the mean of them all. direct gear was engaged, the en- gine, if a very sudden reduction of speed was required, was not only "A not a brake, but its momentum it is desirable or no to let it act in. On the long declines of Alpine might easily be such as to impose
the Pyrenean passes
an unnecessarily high load upon the this capacity, no one could possibly and
engine is wheel brakes. deny that down a long hill the "negatively loaded"
very good So much engine is a most efficient and con- beyond question a venient sort of brake. With second brake Indeed, or third gear engaged, and with the that the free wheel device that
to
It is not.
so When the Engine is not a Brake. It is obvious that if the wheel
would be
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An Unexpected Result. The first thing that strikes me is that such difference as there is between them is to all intents and purposes negligible. At most it amounts, only to fractions of a second. Curiously enough, the six- cylinder car was always a trifle slower in pulling up with the clutch out at 30 m.p.h. than with clutch in which was contrary to anticipation
throttle set to "dling" position, the does not allow the drive to run brakes will, from 50 m.p.h., bring motor will check the speed of the solid when required car to a definite limit, and thus scarcely worth serious considera- the car to a standstill in z seconds, checked this result half a dozen however, in and if the engine, from the re-times, and found no legitimate volutions equivalent to 50 m.ph. means of getting away from it. It allow long, fairly steep gradients tion.
taken seconds to return to is just one of those funny little fidling speed, then the engine has no things that happen when least ex- inherent braking influence: It will pected. It does not, however, only have this if its time for slow-vitiate the argument. ing down over the range mentioned. Inext made a long series of tests is y seconds.
(it is surprisingly difficult to get It is always a waste of time to reliable results without the most quarrel about "facts" which are not elaborate instruments) to ascertain established, and, as in this discus-how long each engine took to come sion no definite facta were available from a given speed down to its I determined to produce some as idling condition, this latter being quickly as possible. Those that I taken as equal to a road speed on obtained, with two cars of distinct top gear of six miles an hour.... By different characteristics, are of
fered herewith. I make no pretence at analysing them in a more than superficial manner, but I think they are interesting,
THAT IS
1930
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Speed
Spirit
These again, are the means of numerous clockings,
They are, to my mind, singularly interesting, as they indicate that engines stop, or, rather, slow down, just about as quickly as a car under One point must, however, be men-full breaking stress. Evidently, if tioned at the outset. On first prin- the engine in such conditions doca siples it is evident that an engine act as a brake, it is only to a very And the advantage with a large flywheel will be less small extent. efficient as a brake in emergency of the six over the four is only a conditions than one with a smaller matter of a very few possible yards, ywheel. By a parity of reasoning Previous to these measurements I the six-cylinder motor will be a would, on general impressions, have
brake
the better
than
four-made more than a small bet that denl yclinder, and the eight than the a six, revving at not less than 2,500 ix. Of the two cars, of roughly r.p.m., would take a great similar weight and power, with more than 44 seconds to drop to its which my experiments were made idling speed of a little over one- one was a four and the other a six,tenth of that speed. This is the When The figures I give will indicate how kind of matter in which, however, amali is the real difference between one can be easily deceived. them.
As to the modus operandi, this was simple enough. On a stretch of road as nearly as possibly level, each car was driven at 20, 30 and 40 miles an hour. A passenger with a stop watch on receiving the two requisite signals measured the time required to bring the cars to a standstill, first with the clutch in and then with the clutch out.
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a good stop watch returns the same result time after time one hay to admit that one's personal ideas are utterly at fault. The following were the results in question:-
From
20 m...
From From
50
40 m.p.b. mp.h.
Four engine 2.15 secs. 1.75 secs, 5.0 socs. sterngine
2.5 cc. 3.5 sec. 4.5 sec.
As a further. check upon the figures I had obtained I ran each car at 40 m.ph and measured the time it took to Come down," with- out the use of the brakes and with the clutch Hin," to 8 m.p.h. (at any lower speed than this the engines were apt to be a little fractious). The mean time for the four- cyelinder was 35 seconds, that for. the six-cylinder was 30 seconds. I apologise for the fact that these figures are so "round," but neglect- ing immaterial fractions that is how they came out; and I guarantee that they are not faked.
Here again was something of a surprise, for half a minute and up- wards is a very long period of time. It suggests that in traffic negotia tion and in ordinary, touring work (aside from going down long de- clivities) the engine is not very much use 28 a brake by itself, for the simple reason that it is not a quick enough brake. Further ob servation has but served to confirm this general statement. I find, in fact, that in normal driving I use the brake about twice as often as I thought I did. It is so deplorably easy to forget an action that is al- most entirely instinctive.
If there is any deduction to be drawn from these facts it is simply that the old idea that the engine myth, is a valuable brake is a except under the special conditions alluded to. Therefore, to suggest that the free wheel means a great deal more wear and tear on brakes is false. It means a certain amount more wear and tear, but only a certain amount. And this, be it re- marked,
of upon. д piece mechanism deliberately designed to accept wear and tear,
Another minor deduction is that any depreciation upon the brake surfaces must be more than com- pensated by reduction in oil con- over-run motor sumption. The (ie., the engine when driven by the rear wheels), that takes haif minute to slow from 40 m.ph. to 8 m.p.h. is manifestly going to suck a good deal of oil into its combus- tion chamber... Far, better let it perform the change of speed in 5- seconds.
What the Smoke Puff Means. When you next follow a car down a long grade note, what happens when its driver re-opens his throt- tle at the bottom. Unless he has a free wheel, it is fifty to one that when the engine accepts its lend it will eject a puff of blue smoke from the exhaust pipe. The engine has acted as a brake but it is not quite such a perfect brake as it is some times represented to be.
That the engine ceased to be available as a normal-driving brake is one of the worst charges I have heard brought against the free- whcel principle. I am now satisfied that the objection is about equiva lent in value to the efforts of n man trying to bore a hole in 9 in. armourplating with a luke-warm tallow candle. The "Autocar."
**
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