The Use of the Engine as a Brake.
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By Henry Sturmey.
The report of the very interesting case upon this subject, which appears in THE COMMERCIAL MOTOR of 19th October, raises a point which, I fancy, is by no means as fully appreciated by many drivers of commercial vehicles as it should be, for, although, as stated in that report, in the case of Wilmot versus Southwell, the appellants failed to substantiate their claim to the use of a petrol engine as a brake, so far as complying with the requirements of the law is concerned, the fact remains that, not only does the engine and transmission form a very efficient brake on a petrol-driven vehicle, but it forms by far the most powerful and reliable brake which the driver has at his command, when used properly.
I take a particular interest in this question, because I was the first publicly to advocate the use of the engine as a brake, and to put the theory into practice in this country. This was some eight years ago, and it was put forward in connection with pleasure cars. At that time, the notion was scoffed at by the would-be cognoscenti, and many were the flat statements made to the effect that there was no braking power in the engine at all : long and learned dissertations were entered upon, with a view to proving, by theoretical reasoning, that such could riot be the case. But in the meantime. T had had practical demonstration in support of my views by the use of a little American car, which I imported and which was designed both to advance and retard its speed by throttle variation. In other words, the engine was used both to propel the car, and, to a large extent, to check it. I may mention, incidentally, that, at that period, throttle control was also scoffed at, and the idea gravely condemned as " no good," for asserted reasons of lack of economy in fuel consumption. This, too, was an idea of my own, which, I may mention, I patented as far back as 1895. Well, in the result, first one and then another maker tried it, and to-day, in touring-car practice, both throttle control and engine braking are recognized as, not only possible, but correct ; so much so, indeed, that the original system of cutting our the engine—in other words, putting out the clutch automatically when the brake is applied—has for some years ceased to be employed. In the utilization of the engine as a brake, consideration has to be given to the gear ratio employed at the time of use. Just as a car cannot be driven up a hill beyond a certain gradient without a change of gear, so, on the other hand, will the engine cease to be effective as a bake upon gradients beyond a certain steepness, until a lower gear ratio is brought into operation, for, just as in the propulsion of the car, the propelling power is obtained by the number of impulses per wheel revolution, so in retarding, the power of retardation varies according to the number of retarding compressions brought into play per wheel 'evolution. Hence, whilst a certain amount of retarding effect is secured by simply closing the throttle and running down a grade against compression, it is necessary, if steep grades are to be negotiated, to change down the gearing and, with a car running with the lowest gear ratio (first speed) in engagement, there are very few hills, indeed, which the car could not be absolutely depended upon to descend safely on the engine brake alone, the pace being regulated by opening or closing the throttle during the descent. As any good driver knows, however, it is rarely possible, with the sliding type of gear, to change down when on a down grade without first stopping the car, and, of course, if the brake power at any time proves insufficient to do that upon a bact grade, the driver is unable to avail himself of the great braking power of his engine, unless he has had the forethought and taken the precaution of putting the car on to its first speed before commencing the descent of the hill. This is one of the reasons why I am so strongly convinced of the unsuitability of the sliding gear, not . only for commercial-car use, but. for any car use ; it accounts for my well-known preference for gearing of the planetary order, which, being frictionally controlled, can be changed on to a lower speed regardless of the pace or the nature of the grade upon which the car is at the moment travelling.
No driver should attempt to descend bad hills without having first put his car on the first, speed. This is a point which has been well thrashed out in the touring-car papers, years ago, and it is to-day thoroughly recognized by touring-car drivers, but commercial-vehicle men have not had the matter brought home to them so forcibly, and it will be well if drivers are instructed that, if they are using a ear which is not built with a gearing which can be changed down to a lower ratio under any conditions of running, that they should make it a rule of their driving in descending all hills of a steepness greater than 1 in 10, or 1 in 12, according to the efficiency of their friction brakes, to stop their cars on the top of the hill and put in the low gear before commencing the descent, They may then be perfectly assured that they will be able to take their machine to the bottom in safety. At the same time, I think the legal authorities are quite right in excluding this power of the engine as one of the legal brakes of a petrol car. There are more cars built to-day with sliding gear than otherwise, and seeing that, as I have above set out, to be really effective the car must be put upon a low gear, it is manifest that, with sliding-gear cars, the engine cannot be sud denly, in case of emergency, used as a brake with any power two friction brakes, therefore, which can, of course, be instantly applied, are quite reasonable requirements in a car's equipment.