TORQUE versus Horse-power
Page 34
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By BRYAN A. PARKER BRITISH oil-engine manufacturers and vehicle builders generally favour engines of high torqueoutput at a low engine speed. The maximum horse-power developed at high engine speed is usually considered to be less important, as it is the torque value that determines the maximum gradient climbable in any given gear. A high torque at low speed also implies less use of the lower gear ratios and this is usually cited as an inducement to lower fuel consumption and a lower rate of engine wear.
In America, a different approach is made. There it is argued that it is more important for the engine to operate at its most efficient speed and load range, which is usually between 50 per cent. and 20 per cent, below the peak horsepower. Ability to climb a hill quickly, even if several more gear changes are required, is also considered more desirable than climbing it slowly with the engine .running at low speed.
This may explain why American manufacturers have succeeded in selling their products in what could have been wholly British markets.
Oil Economy
Recent experiments in America with eight different makes of engine, all operating over the same road with nearly identical loads, proved that engines working within their most efficient operating range burned 18 per cent. less lubricating oil than those which were not. The life of these engines was nearly doubled before a major overhaul became necessary.
At low engine speed, pistons cannot recede fast enough from maximum pressure, the result being that extreme loads are imposed on pistons, pins and big-ends. Higher temperatures in combustion chambers, hotter valves, slower coolant and oil circulation, plus reduced oil flow, create a tendency to oxidize lubricating oils, with a subsequent breakdown of the oil film and accelerated wear of pistons, cylinders and rings. .
Considering that the estimated total life of an engine is Im. miles, the saving in overhaul costs can be considerable.
American tests also showed that the closer the engine operating range, the longer became the life of the engine.
If is obvious that to obtain near constant-speed operation, a multiplicity' of gears must be used. Two big ArneOzan manufacturers have turned ex their, entire efforts in this direction and. have produced gearboxes, as auxiliary installations, in 27 different ranges of ratio to suit all conditions.
In each case the four or five gears are run through, then a two-speed attachment in the box is brought into use, This type of gearbox is becoming increasingly popular, although it costs more than double that of a three-speed auxiliary gearbox, and repair costs are correspondingly high.
High torque enables a vehicle to be started fully loaded without excessive slippage of the clutch. The higher the torque, the less will be .the slippage. required to move the vehicle away from rest, whether on level ground or on a gradient. Therefore, high torque at low rp.m. is of prime importance, but by installing an extra-low gear, the engine torque is multiplied and thus an engine with low torque is able to handle the same loads as a unit of higher torque output under identical eonditions.
These gears are frequently so low as to make a change into the next gear on even the slightest gradient an impossibility, which immediately defeats their purpose., In order to be able to make a change from the lowest " crawler " gear into the first road gear, it is necessary to interpose an auxiliary somewhere in the transmission, such as a two-speed rear axle or a three-speed auxiliary gearbox.
A formula has been developed whereby the correct selection of main gearbox, auxiliary and rear-axle ratios can be quickly calculated, and using the formulm given later it is-possible to calculate the best ratios to suit almost any condition.
_A term, perhaps misleading to British operators, is grade-ability. This means the ability of a vehicle to move a load from rest on a gradient. A minimum grade-ability of 10 per cent. is considered necessary for stilling on level ground. For average highway operation, lowest-gear grade-ability should be at least 17 per cent.
A British lorry, fitted with a standard British oil engine of 125-130 b.h.p. with its high torque at low speed, would have a performance comparable with that of an American oil engine in the 150 b.h.p. class. In the case of the British engine, maximum -torque' would probably be attained at approximately 1,000-1,100' r.p.m., as compared with the 1,500-r.p.m. for American engines.
Missing Ratios
With the transmission arrangements mentioned, and with the highest torque available only at the higher speed of the American engine, it is not possible to use all ratios when changing down. From fourth underdrive it is necessary to miss third overdrive, and select third direct gear, which entails a two-handed change, using both levers at the same time. This also applies between third direct and the second-gear range.
Either one must select second overdrive from third direct, or second direct from third underdrive. Thus, a great portion of the theoretical efficiency from the use of multiple ratios is lost. With the higher torque produced by British engines at' low engine speeds, :t. would be possible to employ the skipped gear to good purpose and thus gain 1-5 m.p.h. road speed, with a saving in time and added fuel economy.
Is it not, then, sound business and good engineering policy to couple the higher-torque engine with a transmission of multiple geli ratios, and thus obtain the maximum performance and economy from the engine?
[An article dealing with the selection of axle ratios appeared in The Commercial Motor on November 5, 1954.1