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Since last August heavy truck operators have had to cope with 961un/h (60mph) speed limiters. And from 1994 EC speed limiter laws will demand that heavies are restricted to just 90km/h (56mph). Limiting a truck's top speed is no problem; the trick is to comply with the law without increasing journey times or fuel consumption.
And despite all the recent advances in engine management systems, that trick might be a tough one. TNT's group chief engineer Alan Parker knows more than most about this problem. Faced with integrating limiters into tight express delivery schedules he asked the tractor manufacturers what changes would be needed.
Some have told him not to worry: journey times can be maintained without resorting to more powerful engines. Others are less optimistic. Even running at 32.5 tonnes over level ground, they warn, when the speed limit falls from 96 to 90km/h faster acceleration will be needed to compensate for the lower cruising speed. Their solution is to continue the trend towards more power. Some experts believe that a 230kW (310hp) fleet tractor would have to be replaced with a 373kW (500hp) flagship.
TNT's engineers agree that more power will be needed, but they see no need to go as high as 373kW to cancel out the effect of a speed limiter. Using an air management kit they believe the same hypothetical 32.5-tanner, running over a typical UK parcels trunking route, could get away with 239kVti (320hp) at 96km/h. For a limiter set at 90km/h they reckon you'd need to go to 261kW (350hp).
Increasing power to maintain journey times at speed-regulated limits would be coming full circle. Limiters have been forced upon us because of a feeling that many heavy trucks were going too fast. Manufacturers, it could be argued, made it easy for heavyfooted drivers to exceed the speed limit by vying with each other to offer more and more power. The manufacturers and hauliers might well reply that the extra power is simply designed to reduce fuel consumption and journey times within the legal limit, also allowing high cruising speeds to be combined with good gradeability.
They might add that in any case a conventional speed limiter won't stop a fullyfreighted 38-tonner from breaking the speed limit when running downhill. "Downhill" speed limiters which operate under all conditions, are now with us—Scania exhibited one in the form of its integratred retarder at the Brussels Show—but most "normal" limiters simply cut off the engine's fuel supply when the preset speed is reached.
In undulating terrain the driver can take advantage of this to go faster than the limiter setting on a downward run, building up momentum for the following rise. This saves time and can also save fuel. A combined speed limiter/retarder, like Scanias, could increase fuel consumption because more power would be needed to make up for downhill momentum lost to the automatic retarder.
Hauliers whose vehicles spend most of their time on easy terrain might seem to have nothing to worry about. A typical radial-shod 38-tonner has a drag coefficient in the region of 0.7. On level going it only needs around 149kW (200hp) to maintain 96km/h—and the 90km/h Euro-limit should be attainable with a mere 127kW (170hp) The lower limit, and power, also implies a fuel saving of over 8%.
With a legal minimum of 4.5kW (6hp)/tonne no 38-tonner has less than 171kW (228hp) on tap and few fleet machines have less than 239kW (320hp) so why worry about limiters?
The answer, of course, is hills. At 96km/h in top even al% (1-in-100) motorway climb demands some 257kW (345hp), falling to 228kW (306hp) at 90km/h. On the face of it a haulier running, say a veesix Mercedes 1834 could manage with a veesix 1831 when the speed limit comes down. However any operators who decide to specify lower-powered engines in line with the lower speed limit should not forget that drivers will then need to change down sooner on hills and fuel consumption will suffer accordingly. Dropping a gear could hike fuel consumption by more than 10%.
Even that 1% climb can increase fuel consumption by 40% at 96km/h—and in this case the 90km/h limit actually makes the loss bigger.
At the end of the day whether a speed limiter and a 90km/h limit cuts your fuel bill or extends your journey times will depend to a large extent on the power of your engine. And that, of course, is where we came in.
We followed TNT's lead and asked the vehicle manufacturers: "In the light of UK and EC speed limiter legislation, what power would you recommend to an operator who wants his fully freighted, speed limited 38tonner to maintain a steady 96km/h on all sections of the UK motorway network?"
"We're certainly seeing an increase in the demand for more power—at least 10',. more than two years ago—and limiters are part of that increase. We would probably recommend 375-410hp, particularly because of the high torque output and flexibility you would get with that kind of engine. We'd probably also recommend coupling it to the Eaton SAMT (semi-automated version of the Twin Splitter) in order to maximise the engine's performance and allow quick shifts."
"In general operators are clearly moving up the power band. The Euro-1 Cummins Li 03 25 develops 1 ,1501 bft of torque which is an improvement of 20011ch over the nonEuro-1 Li 0, and that will help offset any restriction of a limiter at the bottom end of the market. The advent of the Perkins 335Tx and the 340 and 380 14-litre Cummins, together with the L10-350 CELECT and 3751x, provides us with no less than six engines between 325-380hp which allows an operator to choose the exact power rating to match his mission."
"375hp! Of course it depends on a number of major Factors: for instance whether the haulier is on an east/west or a north/south motorway run; how much time the rig runs laden; how often the driver wants to change gear, and so on. But the principle is that power will be even more important in a speed-limited regime than a non-limited one. We're definitely seeing the effect of limiters and that's being reflected in our vehicle specs. Customers ore asking 'should I buy 340 or 375?' If they're on long haul, laden, time-sensitive work, running with a limiter, we'll say 375 And when we say more power we mean more torque:the ability to get back up to cruising speed after roadworks or climbing a hill is becoming more important."
"In reality what's happened is that the manufacturers have talked up power limits, but operators are now beginning to listen to what we've been saying. Having originally asked for a 320hp engine, operators are now aware of the effect of limiters on journey times and are more open to looking at a 360hp engine. While power is rising, it's the greater torque that maintains a truck's speed up a hill. When we talk about power to hauliers what we're really suggesting is the adoption of more useable torque—usually around 10% more—at current speeds."
'We're seeing on increase in the level of power being demanded by operators—or at least an increase in the level of power they think they'll need for the near future. Speed limiters are the main reason, compounded by increasing congestion which will only get worse. In terms of power needed for the UK motorway network it is, of course, torque that is the all important factor. But as we all understand BHP better, most people we've spoken to are looking at an extra 30-50hp, moving from 320hp to 370hp."
"Looking at the recent figures for trucks sold over 350hp there's been little or no change, but there has been a big increase in the 300-350hp market at the expense of the 250-300hp market. Our previous 1729 tractor buyer will now be offered an 1831, or more likely the 1834, which has the extra power crd torque to overcome any performance limitation imposed by a speed limiter. The next logical step would be to 380hp when you'd get a torque increase of 18%—reflected in a lop-gear gradeability gain of arounc 0.4% This will obviously be more important as the lower EC speed limit of 90km/h comes into force when the need to maintain o consistant crusing speed will be even more significant."
"If you want to maintain a constant speed on the motorway you need to look at the entire driveline, not just the engine. Operators are clearly anticipating the effects of limiters and more and more are specifying slightly slower axle ratios. These give better gradeability in every gear, faster times up hills and better acceleration away from congestion. Our 385hp engine produces a lot of torque, but even with 1,38511oft at 1,200rpm on tap, you need to get the driveline right. In our case with the B18 box and its 0.65:1 overdrive top ratio you'll need a 4.919:1 rear axle. This combination gets you cruising at 96km/h at about I ,60Orpm where there's still around 1,150Ibft of torque; enough to get you over most motorway hills while keeping the engine comfortably in the green and keeping the fuel bills down."
"The operator should be looking for 360400hp to ensure optimum journey times combined with economic reality. The arrival of the EDC RI 3-380 is a reflection of the continued demand from operators for more power, and limiter legislation has helped Fuel that demand. To maintain such a high cruising speed on gradients requires high power and high torque, which mutually available on diesel Reducing the drag co-efficient fror 0.50 at this speed equates to requirement saving of 35hp, or a ti gradeability improvement of 0.2', anticipate an increase in the der the Scania Streamline package."
"We don't recommend any power limiters. If you look at what our c are asking for across the country w living with 10-litre 325hp engine they feel that's sufficient. The n( might be to 350hp, although up till iters have had little efect our on buying patterns. While we do c and 410 Cummins engines their ul been low. However, the new 380 creating quite a bit of interest an, like the way some people are mov 'The artic in our range that's far c the most popular with operators is rated at 320hp. That appears to than adequate for most users. Or fleet work that rating is still the mo. and we're not seeing any real ev speed limiters changing that. Ho\ one of those things that individual will have to decide on, and some I to go to 360hp for a specific open L by Alan Bunting
and Brian Weatherley