WE HAVE argued in CM before that a high power
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to weight ratio does not necessarily entail a poor fuel consumption. This point of view now receives added weight from the results of our test of the DAF F1'2800 DKS tractive unit.
The DKS is specified with a highly turbocharged and charge cooled version of DAF's 11.6 litre (708cuin) diesel engine, which provides a 7kW/ tonne (9.6bhp per ton) power to gross weight ratio at 32.5 tonnes (32 tons) gcw. Running at an acceptable average speed over CM's ,1,1721un (728 mile) operational trial route, the DAF returned one of the best consumption's yet achieved in one of our road tests.
Designed for operation at gross weights of up to 56.9 tonnes (56 tons) gross weight, the DKS runs very comfortably and well within design capacity at 32.5 tonnes (32 tons) gcw. Apart from the good fuel consumption, one could expect the further advantages of greater vehicle life and less downtime than with a lesser powered vehicle.
The 38.7 litre / 100km (7.3mpg) overall fuel consumption returned by the DKS has been pipped only by a few vehicles, including the Volvo F86 and the Scania LB81 tractive units, which just top the minimum 4.4kW/tonne (6bhp per ton) power to gross weight ratio. An Atkinson powered by an 8LXB Gardner has also bettered this figure, but the lesser powered DAF FT2800 DKTD, at the ,same weight and over the same route, could not match it.
On motorway the DKS returned a very creditable 38.7 1itre/100km (7.3mpg) consumption and on the 40mph maximum speed stretch of Al between Neville's Cross and Darrington the DKS gave a remarkable 25.7 litres/1001cm (11mpg).
With its superbly appointed sleeper cab and high-power engine, the DKS is more suited to long-haul T1R and intercontinental operation rather than to domestic work. However, should there be an increase in the minimum power to weight ratio from the present 4.4kW/tonne (6bhp per ton) limit or should gross weight increases be granted, then the DKS has sufficient power in hand to cope. For instance, the DKS has sufficient power to operate at 38 tonnes (37.4 tons) gcw at a power to weight ratio of 6kW/ tonne (7.4bhp per ton).
As tested, the unladen unit tips the scales at 6.9 tonnes (6.8 tons) and coupling to a 12.2m (40ft) semi-trailer to keep the outfit overall length within the 15m (49ft 2.5in) legal maximum poses problems. Our vehicle was fitted with a sliding fifth wheel, and by shifting it forward and coupling to a 1.3m (4.3ft) kingpin trailer the outfit just scraped within the limit to leave sufficient cab/trailer clearance. A standard depth cab is available as an option for the 2800 range; it is £227 cheaper and 120kg (2.4cwt) lighter than the sleeper version, and coupling problems are eliminated.
Comparing the DKS with other sleeper cab models recently tested by CM, it is lighter than the Berliet TR280 and Ford HA4231, but heavier than the Volvo F88. The Ford, with a similar output engine, weighs around 7.5 tonnes (7.4 tons), the Volvo with 17bhp less engine output weighs 6.7 tonnes (6.6 tons).
The DKS is the most powerful model in DAF's 2800 range. Apart from being highly turbocharged, the 11.6 litre (708cuin) engine is also charge cooled, that is, before air from the turbocharger is drawn into the cylinders its temperature is reduced by about 50 C by using an air-to-air heat exchanger. Cooling the air increases its density and permits a greater mass of air to be introduced into the cylinder to improve output significantly. Drive from the engine is transmitted via a 420mm (16.5in) single-dry-plate clutch and 12speed with crawler Fuller gearbox to DAF's own 13 tonne rear axle.
As expected, the DAF, with its high power, was well-suited to motorway operation. Typically, the vehicle maintained speeds in the high fifties except in the busy traffic on the southern sections of M1 and M6 on the first leg and on gradients. On the flat it was necessary to keep a sharp eye on the speedo needle to keep the speed within the legal limit. Maximum vehicle speed is 104km/h (66mph).
The lowest motorway speeds were recorded on M6 Shap and on the Shepshed "slow lorries" gradient north of -Leicester Forest East services on Ml: M6 Shap reduced vehicle speed to 64km/h (40mph), Shepshed gradients to 45km/h (28mph). On A roads the DKS effortlessly maintained 64km/h (40 mph) with the engine turning at a very economical speed.
An indication of the improved performance available from the DKS over its stablemate the DKTD (CM July 5, '74), which is specified with a derated, non charge cooled, version of the same engine developing a maximum 184kW (248bhp), is given by the time to climb Carter Bar. The DKS was severely baulked at the base of the climb, but despite this early loss of speed the slower moving vehicle was overtaken and the summit reached one second faster than with the less powerful vehicle.
Second gear was the lowest selected over the hilly A68 section for •the sharp I in 5 gradients on Riding Mill and Castleside. In each case the engine was revving comfortably and there were still two gears in hand, first and crawler.
The MIRA tests were completed in pouring rain and even though the track was saturated the DAF produced acceptable stopping distances. From 64km/h (40mph) the vehicle stopped within 42in (137.4ft) with the outfit veering slightly to the offside on the greasy surface.
Surprisingly, the DKS was slower than the DKTD When accelerating through the •gears to reach 80km/h (50mph). The 4.5sec difference is all the more significant in that the DKTD was fitted with a six-speed with splitter gearbox and all of the 12 ratios were used in strict sequence. The DKS took 94.5sec to reach 50mph, missing the odd gear to get the fastest time.
Wheelspin prevented a restart on the 1 in 5 gradient, which was covered in water. The vehicle was not specified with a diff lock.
Apart from crawler, the 13 speeds of the Fuller box are arranged in a four-speed gate like that of a car. There are four speeds in each of the two ranges with a further split on each ratio in the high range.