TRANSMISSIONS
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by Paul Brockington, MIMechE
2. Matching the transmissions to the engine
HORSEPOWER is a product of torque and speed. If the speed of an engine is doubled at constant torque, the horsepower is doubled. Increasing the torque at constant speed gives a comparable increase in horsepower. In other words, torque is low-speed horsepower. It follows that increasing the speed of the engine gives more power at the top end of the output curve without increasing the torque .lower down.
It can normally be concluded that a high-torque unit is relatively efficient and has a favourable fuel consumption. But the weight per horsepower of a high-speed engine is better than that of a low-speed engine unless the latter is turbocharged. And the performance of a high-speed engine in terms of acceleration as well as maximum speed is as good as the performance of a low-speed counterpart if it is mated with a suitable multi-ratio gearbox. This would cost more than the type of gearbox with a lesser number of ratios that suits the low-speed engine, and depending on the route, the vehicle driver will have a more complicated job to do to get the best out of the engine. This can be the decisive factor.
Proposals of the EEC include a recommendation that torque as well as horsepower should be included in type approval. This would be approved by the majority of gearbox designers and vehicle makers whose targets include a better performance on the road with less bother.
Before dealing in a more practical way with the torque-versus-horsepower controversy, it is pertinent to give a typical example, showing how an operator can. be distracted by the need to cater for the type of driver he employs. A UK haulier who operates a number of fleets at home and overseas has a simple philosophy which he can't always put into practice because of the lack of suitable back axles and because the only gearboxes available with the necessary high torque rating are multi-ratio types.
This haulier would like his vehicles to be overgeared so that the driver could put his foot flat down for long periods without over-revving the engine, and he favours a wide-ratio box because it reduces the amount of messing about the driver has to do; his drivers prefer wide ratios on give-and-take roads and in traffic. But I makes an exception of motorways runnin for which, he says, a gearbox with' do ratios at the top end is a godsend.
The driver can be trusted on motorwa: to get into the most appropriate ge. because the gain in performance is obvioi and immediate, and gear changes a infrequent. A straight gearbox with do ratios at the top end and wider ratios at tl bottom end would meet requirements.
While the bhp /ton regulations and tl prospect of higher power-to-weight rah( are currently a dominating factor in vehic makers' design policies, a growir proportion of vehicles are carrying bu loads, and empty or light-load runnir probably accounts for at least 80 per cent heavy-vehicle traffic currrently operating c the highways. Raising the power-to-weig] ratio will increase the percentage.
Even if a vehicle is nominal' "underpowered" it may be "overpowerec for much of its working life, and if it equipped with a transmission giving a hig top gear (with or without overdrive) i speed potential may, and often is, misused.
Overgearing While the haulier quoted earlier agrel that a high gear can encourage speeding t emphasizes that overgearing, as distill from high gearing, is by-and-large beneflci because it reduces the vehicle's maxim] speed except in favourable condition Many operators would disagree with th concept, which goes to show how compl( the psychological problems can be and ho difficult it is to please everyone. ,ut everyone is agreed that catering for average driver is often non-conducive to lloying a transmission that would aid lomy and give easier driving if it were Duly used. The only way round the culty would be to fit a road-speed ernor or to control vehicle operations the aid of a tachograph. Undoubtedly, :ter control by the majority of operators 'educating" drivers to drive by the book ild encourage makers to rethink ine /transmission matching problems and [mission designers to evolve units that 'Id increase productivity.
slthough the range of transmissions now the market covers types of unit that Id cater for most applications, it is ropriate to reiterate that expediency ninates the scene, that the first sideration is generally torque rating and the second is price. As mentioned in the article in this series ringing the changes et what the operator wants is often a iible exercise—so long as the operator ws what he wants. Many operators need eating too.
lecause an engine with a high torque k-up gives more power at lower speeds majority of vehicle makers consider that bhp /ton regulations should be changed a gradient climbing requirement, which ild favour the high-torque engine. But, in DoE view, substituting gradientability bhp/ton could well be unpopular with Kers in the long run because a new icle that failed a gradient test would have be rated at a lower gross load or msively modified. And this could be very
tiy.
Ile bhp /ton regulations are blamed for :ouraging the use of derated engines giving a constant-horsepower characteristic at the upper end of the power curve. But while the acceptability of such units will continue to be prejudiced by the regulations until there is power to spare, an examination of the benefits that can be derived from their use, notably with regard to simplification of the transmission and reducing the frequency of gear changing, will provide a means of assessing some of the merits of good matching.
Petrol-engine advantages The petrol engine has a big advantage over the diesel with regard to transmission matching. While the rpm of a petrol engine can be increased above the speed at which maximum power is developed, the maximum speed of a diesel has to be limited to an rpm that is lower than the maximum-power speed to obviate excessive smoking.
At the vehicle's maximum road speed in any of the gears the output of a petrol engine is considerably less than the peak output. And when the load on the vehicle is increased by a gradient or additional air drag, road speed is reduced but engine output is increased. While power falls off as road speed is reduced below the rpm at which maximum power is developed, the range of road speeds between maximum mph and the lower speed at which the same output is developed may cover up to 30 per cent or more of the overall speed range.
To obtain this characteristic in the case of a diesel engine a normally aspirated unit has to be derated by restricting the fuel above a certain rpm or equipped with a turbocharger to increase the torque back-up without raising the power output correspondingly. The turbocharged engine is also derated compared with the peak power it would have produced if the potential of the turbocharger were fully exploited.
Improving torque back-up by employing a turbocharger in this way involves an insignificant payload penalty, compared with powering the vehicle with the same engine in naturally aspirated form. Using a derated naturally aspirated engine in place of a smaller normally rated unit of the same type involves a higher payload penalty, the difference in the power-to-weight ratios of the vehicles in each case partly depending on whether a lighter gearbox of suitable design is available for the derated engine.
In an extreme case a 10 /13-speed gearbox might be the most suitable for a higher-powered engine while a six-speed transmission would be more than adequate for a high-torque engine. This should reduce weight and cost penalties but as -indicated earlier some vehicles are equipped with a multi-ratio gearbox in place of a fiveor six-speed box because a suitable wide-ratio type with an adequate torque rating is not available.
Fast trunking runs Possibly the most important advantage of what might be called the semi-constant horsepower engine is that it eases the lot of the driver on fast trunking runs by enabling him to reduce journey times on hilly sections with less physical and mental effort.
The time may well come when these factors will play a much greater part in the choice of an engine/transmission system than they do now. And the choice could well be between a derated high-torque engine coupled to, say, a six-speed box with or without overdrive and a fully automatic transmission with eight or more speeds. If the vehicle were mainly used on motorways, the high-torque engine could be preferred by the driver because it would eliminate changing for a greater part of his running time. Changes that are made automatically can still be a worry. The system would be considerably cheaper and probably Lighter than a normally rated engine coupled to an automatic. Even the drivers' unions could be a force to be reckoned with one of these days in the choice of a vehicle transmission.
The Detroit Diesel Engine division of the General Corporation has produced graphs to show how the application of "power control" can be used to provide a substantially constant horsepower characteristic over 25 per cent or more of the power curve at the " upper end, without reducing the peak rpm, by restricting the fuel input.
In the accompanying graph, curve Al is the wheel horsepower produced by the Detroit Diesel 12V-71N two-stroke diesel in its normally rated form, while A2 is the curve of the same engine to which power control has been applied to reduce the wheel power developed from 350 hp to 290 hp at the peak speed of 2100 rpm. It will be noted that maximum power (about 310 hp) is produced around 1850 rpm and that the output at peak rpm is about the same as it is at 1575 rpm.
It would be essential to fit an engine with the higher rating if the vehicle power-required curve were as shown by the upper dotted line. And normally a fully rated engine giving a peak wheel power of 290 hp would be used to cater for a vehicle with a power-required curve shown by the lower dotted line, the reserve power available for acceleration and hill climbing being indicated by the dark shaded area. Employing a power-controlled engine for this application increases the extra power in reserve by an amount indicated by the light shaded area which is maximum at about 1600 rpm. Power reserve is more than doubled from well below 1200 rpm to just below the peak speed of 2100 rpm.
It should be noted that there would be little or no advantage in changing down to a lower ratio when driving a power-controlled vehicle at any engine speed between about 1500 rpm and .peak rpm. And employing ratio steps of less than about 40 per cent would not be necessary, the steps of some very close-ratio gearboxes being around 15/17 per cent.
The value of employing a "non-uprated" turbocharged engine is borne out by the road test report of an Atkinson /York 42-ton-gross five-axle artic (CM, February 21, 1969) which was powered by a Cummins NAC-TC turbocharged Custom Torque diesel having the same peak output as the naturally aspirated NAK250 of 240 bhp at 2100 rpm.
Turbocharging increases the maximum torque output from 660 lb ft to 900 lb ft and reduces the rpm at which it is developed from 1500 rpm to 1200 rpm. If the engine were normally rated it could be expected to produce a horsepower output well in excess of 300.
Although the engine was coupled to a Fuller Roadranger 10-speed twincountershaft range-change gearbox, the claim that a constant horsepower characteristic reduces the need for multi-ratios is borne out by the observation in the report that: "All 10 ratios were rarely needed except on steeper gradients; it was more convenient to make what Fuller calls step changes missing out alternate ratios."
It was also noted in the report that a range-change box had the big advantage over a splitter type that every gear change was positive but that, if every gear were needed, there was more manual effort required. A Lipe Rollway air-assisted clutch was fitted to the vehicle and this was said to work "extremely well" with no delay or loss of feel.
It is known that a body of practical engineers is confident that low-weight diesels developing at least 80 per cent more power than established types are a viable proposition. A relatively short time ago the .prospect of an increase in bmep of turbocharged engines from the current maximum of around 150 psi to 300 psi or more was only taken seriously by a few dedicated backroom boys. But this prospect could become a reality in four or five years and probably will be a reality in less than 10 years.
The unit will almost certainly be a turbocharged /charge-cooled diesel with variable compression ratio pistons. In an article on turbocharging diesel engines in the February 1971 issue of The Journal of Automotive Engineering, Mr D. Q. Martin of Perkins said that variable-compression ratio will be used in future together with a high degree of charge cooling. And it is significant that the differential diesel engine (the DDE) was a Perkins project. Mr Martin forecast that engines with a power-to-weight ratio of 5.9 lb /bhp would be produced.
Power to spare
There is little doubt, therefore that in the fairly near future there could be more than enough power to spare to enable a low-speed engine to be used that provided a iconstant-horsepower characteristic over at least 60 per cent of the power curve. And this would be a challenge to transmission designers to produce a gearbox that enabled the engine to operate at its optimum efficiency over most of the load/speed range. This objective will be much easier to achieve than it is now. And the vehicle will be much nicer to drive.
The scope for providing a considerable improvement in operational efficiency when running light was indicated in "Technitopics, Ideology of Transmission Matching" (CM, January 29, 1971), in which it was shown that the specific fuel consumption of a 16-ton-gross platform vehicle powered by a 180 bhp diesel was some 20/30 per cent higher over a major part of the load/speed range when running empty than that of the same vehicle carrying a loaded container. And it was also shown that this increase could be recovered by employing a higher overall ratio giving a 40/45 per cent reduction in engine speed.
In the case of more powerful vehicles with low air drag, fuel wastage when running empty could be greater than the figure quoted. If a constant-horsepower engine were employed reducing the engine speed when running light by providing matching higher ratios would be a practical proposition because of the extra reserve of power available for acceleration and hill climbing.
Air drag of a van or a container-carrying vehicle can be reduced by 20/30 per cent by the relatively simple means of radiusing corners and so on, and simple streamlining will undoubtedly be applied to an increasing extent to vehicles operating at mare than 50 mph. And a heavy high-powered vehicle with low air drag poses a particular' difficult transmission problem. Two or moi overdrive ratios and a road-speed govern might well be required if the vehicle is operate efficiently.
It is perhaps fortuitous that the trer towards lower-speed turbocharged engin' for higher powers over the past three or foi years has coincided with a growir awareness on the part of vehicle make] that a low-drive-line rpm may be desirab in some applications as a means 1 obviating certain transmission problems the vehicle is to be operated at a sustaine high speed on motorways or the equivalent In theory a high-revving engine has ti advantage over a relatively slow-spec engine producing the same power that 0 torque transmitted through the drive line reduced, and it should be possible to tu lighter transmission units if they we' available. In practice high-drive-line spea can produce propeller shaft vibrations th; are destructive if a critical speed is sustaine over a long period.
Mismatching of the transmission an engine may, therefore, take the form employing a transmission system tlu allows the shaft to over-speed. And it is ai to quote a relevant example given by well-known vehicle maker who prefers employ diesels that operate at less tha 2300 rpm because they do not create s many transmission problems.
If the rated speed of an engine is much i excess of about 2100/2200 rpm it is n't normally expedient, according to th maker, to use a gearbox with an overdriv The relatively high rotational speed of tF propeller shaft when it is stepped up from higher output speed can create such sevei vibrations in the drive line that stresses ai increased unacceptably and bell-housin bolts and so on are loosened after relatively short time. In the cited exampl the engine developed its maximum power 2800 rpm and was used in conjunction wit a five-speed-and-overdrive box. Th vibration problem was overcome b employing a straight five-speed unit and a Eaton two-speed axle which, as mentione in the first article, provides an overdriv ratio (or high top) behind the crown who assembly and does not increase the speed c the propeller shaft.
A comment from David Brown Ge2 Industries on future developments may regarded as ambiguous, but ambiguitic abound when considering the future. "W are considering a concept of transmission, a technical spokesman of the company ha stated, "which will avoid the use multi-speed axles, that will give the drive adequate performance from the gearbo ratios and that will reduce driver fatigu involved with gear changing and above a that will add to vehicle safety."
Vehicle safety! An automatic c semi-automatic can promote safe driving b easing the lot of the driver—at a cos Could a cheap alternative to establishe forms of automatic be produced that woul give a favourable fuel consumption? If sue a transmission were immediately availabli this series could have been condensed into page or two.