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Is Power to-Weight Important?

2nd May 1958, Page 117
2nd May 1958
Page 117
Page 118
Page 119
Page 117, 2nd May 1958 — Is Power to-Weight Important?
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A Study of Power to Weight Ratios of Oil engined Vehicles Reveals Little : Gearing is the Significant Performance Factor By a Transport Engineer

F.iVEN in engineering circles it is sometimes assumed that the power-to-weight ratio of commercial-vehicle

chassis is the be-all and end-all of its performance. This is quite erroneous because the quoted power-to-weight ratio gives little indication of what any particular vehicle can be expected to do.

To begin with, the very term is vague because some manufacturers deem it better to quote the net power output of their engines (the power developed as installed in the chassis with all auxiliaries), whereas others prefer to quote the gross output, which is the peak power on a test bed with the engine stripped of its auxiliaries.

Many manufacturers, in fact, will not disclose which rating they quote, in which case it can usually be taken that it is the gross b.h.p. figure. This can be somewhat misleading, and is caused partly by the American practice of quoting gross figures. Naturally enough, British manufacturers selling in competitive markets overseas against American makers consider it advisable to quote their maximum output figures when the Americans are doing the same.

Wherever possible, the power figures quoted in The Commercial Motor are net, but, even so, the power-toweight ratios given are little indication of what can be expected of the vehicle. A fairly concrete rule is that the

brake horse-power merely affects maximum speed in most cases, whereas torque output decides acceleration rates and gradient and top-gear performance. A further factor with regard to horse-power is the speed at which it is developed. Assuming an engine power curve rises between 0 and 4,000 r.p.m., the overall usefulness of the engine is greater than that of a slower-" revving" engine with a similar maximum output rating. Apart from anything else, more complex gearing is required to give the same performance with a slower engine as is obtained from a faster engine with a relatively simple gearbox.

Assuming that high speed is regarded as all-important, as is often the case overseas, this can be obtained by using vehicles with high power-to-weight ratios, although it can also be secured over reasonably level routes by employing relatively low-speed, low-power units with overdrive or other auxiliary types of transmission. Even good acceleration is not entirely dependent upon high power—high b.h.p. not necessarily implying high torque—and much depends on engine and transmission characteristics.

Controlled tests conducted in America with a vehicle running at about tons gross, showed that a 25-per-cent.

increase in speed required a 73-per-cent. increase in power, whereas 26 per cent. higher weight demanded only 10 per cent. greater power.

Another set of tests conducted with a heavy vehicle at a fixed gross weight, the power-to-weight ratio of which was varied by adjusting the fuel-pump settings, showed that a 20-per-cent. increase in power reduced the fuel economy by 9.5 per cent. and improved overall journey time by only 6 per cent. Thus, even with the high labour costs current in America, the time saving did not make up for the additional fuel costs, with the net result of higher overall operating costs.

From this it would appear that the higher the powerto-weight ratio the lower would be the fuel economy, but this holds true only under certain conditions. For instance, it has been proved that a low power-to-weight ratio will give better economy when the vehicle is operated over level routes, but that a high power-to-weight ratio will afford greater economy when the vehicle is worked over hilly routes. This is mainly because, whereas on level running both vehicles would be operated in their highest gear, hills demand much more gear-changing with a low power-to-weight ratio than is required with a high powerto-weight ratio. This also means less fatigue on the part of the driver, assuming a non-automatic transmission.

Although since the war the power-to-weight ratios of British heavy vehicles have not changed greatly, other than those designed specifically for export markets, substantial increases in power are to be seen among the lighter classes, particularly delivery vans derived from cars. This is obviously because car designers are intent on giving greater power and, therefore, higher speed.

Taking two specific examples, a current British 10-cwt. van has a power-to-weight ratio of 27.7 b.h.p. per ton gross weight and a contemporary 6-7-cwt. van affords 29.6 b.h.p. per ton gross. The 10-cwt. vehicle was shown to be appreciably more economical when running non-stop and slightly more economical when making one stop per mile, the differences being 2.8 m.p.g. and 0.3 m.p.g. respectively. When making four stops per mile, however, the van with the higher power-to-weight ratio was better by 1.9 m.p.g. than the 10-cwt. vehicle. This virtually bears out the American findings, because the van with the higher power-to-weight ratio has less need for the gears when engaged on concentrated stop-start work than the lower powered vehicle.

Other examples can be quoted to show that it can generally be taken that with vehicles of this capacity a high power-to-weight ratio is advantageous when working in towns, but for non-stop operation greater economy will result from a less powerful engine. Even so, in terms of gross and payload ton-m.p.g., lower-powered vehicles usually show better figures than equivalent weights of vehicle with more powerful engines.

Taking medium-capacity vehicles, the reverse can be proved. For example, one manufacturer offers 4-ton and 6-ton chassis powered by a similar 80-b.h.p. oil engine. The 4-tonner gives 19.7 m.p.g., its power-to-weight ratio being 11.1 b.h.p. per ton gross. The 6-tanner yields 16.25 m.p.g. and its power-to-weight ratio is 8.5 b.h.p. per ton gross, The difference in gross running weights of these two vehicles is just under 2i tons. The other basic difference was confined to the axle ratios, the 4-tonner having a slightly higher ratio.

Another set of figures returned by a different manufacturer showed that a vehicle running at 7 tons gross weight (12.7 b.h.p. per ton) gave 20.4 m.p.g. and a similar engine in a vehicle running at 9 tons gross (9.5 b.h.p. per ton) gave 18.1 m.p.g. The vehicle with the higher power-toweight ratio thus proved to be more economical.

A simple example of this is of course, given by any 5-tanner, which might return 18 m.p.g. with a full payload and 22 m.p.g. with half payload, the power-to-weight ratio being higher under the latter set of conditions. Even this does not prove anything, however, because the figure obtained with the half-laden 5-tonner might still not be as good as that given by a 24-tanner with a smaller engine and lower power-to-weight ratio.

For instance, an eight-wheeler running at 24 tons gross with 125 b.h.p. engine would give about 8,1 m.p.g. when laden, its power-to-weight ratio being 5.25 b.h.p. per ton gross. Assuming that it has not an overdrive, it would yield about 12+ m.p.g. unladen (7+ tons), the power-toweight ratio then being 16.6 to 1. Now 7+ tons is nearly the weight at which a 5-ton vehicle would operate and its power-to-weight ratio would be about 12 to 1. Such a vehicle would give in the region of 19 m.p.g. In this example the vehicle with the lower power-to-weight ratio shows some 50 per cent. greater fuel economy.

Three examples of 5-tanners with different power-toweight ratios can be quoted to show that the vehicle with the highest is not only more economical than the others, but also has better acceleration. This particular vehicle has a power-to-weight ratio of 11.6 b.h.p. per ton gross and on test gave 19.7 m.p.g. and accelerated from 0 to 30 m.p.h. in 21.2 seconds.

Another 5-tonner, this time with a power-to-weight ratio of 10.7 hh.p. per ton gross, yielded 18.3 m.p.g. and accelerated to 30 m.p.h. in 33 seconds, whilst the vehicle with the lowest power-to-weight ratio-9.7 b.h.p. per ton— gave 16.5 m.p.g., but was slightly faster on acceleration than the second example.

A final example to show the danger of basing performance on power-to-weight ratio is given by the case of four vehicles all powered by a similar basic engine, but having varying payload capacities.

An articulated outfit had the lowest power-to-weight ratio, 6.3 b.h.p. per ton gross, and gave 13.9 m.p.g. on a level route. A four-wheeled chassis with a power-to-weight ratio of 7.7 to 1 ran at 14 m.p.g. over a similar route. Used in a lighter four-wheeler, the power-to-weight ratio of which was 9.2 b.h.p. per ton gross, the engine gave a fuel-consumption rate of 16 m.p.g. over a hilly route. But in another four-wheeler of about the same weight and power-to-weight ratio, the return was 12.9 m.p.g. over fairly flat country.

These four sets of figures show little definite co-relation between power-to-weight ratios and fuel consumption. The vehicle with the lowest ratio was almost as economical as that with the next higher ratio, whilst the two fourwheelers giving greater power to weight provided the lowest and highest fuel-consumption rates respectively.

Graphs relating to power, weight, fuel consumption and acceleration can be drawn, but none of them provides any conclusive evidence that performance can be directly associated with the power-to-weight ratio under every condition. So much depends upon the gearing of a particular vehicle that, taken by itself, power-to-weight ratio is virtually meaningless.

With multiple gear trains the performance of a vehicle having low power to weight can be matched in many respects to that of a vehicle with a higher power-to-weight ratio. Indeed, many manufacturers prefer to multiply their gear trains rather than to go to the expense of designing higher-powered engines, which, in any case, are bulkier and heavier.

Within certain limits there is nothing the matter with this system and there are, for example, several types of 7-tonner in production which employ 5-tormer power units, the lower power-to-weight ratio being compensated for by additional gearing, such as two-speed axles. When it comes to maximum-load four-, sixand eight-wheelers, however, the problem becomes somewhat more involved. But, even so, it has in the past been found more expedient to introduce auxiliary transmissions rather than new engines.

A possible solution, however, is to employ turbochargers, by which means the power and torque outputs of an engine can be raised by as much as 30 per cent. Without any consequential increase in the weight.or size of the power unit. This would not necessarily entailleavier transmission units if auxiliary gearing had been used with the unblown engine.

There is no hard-and-fast rule as to the best power-toweight ratio for any particular purpose, as so much depends on individual engine characteristics, overall gear ratios and the type of service envisaged. It does seem, however, that there is something to be gained from the use of relatively low-powered chassis, although, naturally, more work is involved for the driver, whilst it is fairly certain that operators running in hilly areas will need a higher powerto-weight ratio. In other words, select the power-to-weight ratio with the object of reducing the number of gear changes per mile and keeping up the road speed.

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