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Speedy Scania hits the as...

8th February 2001
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How it works

The first phase of Scania's UK alternative-fuel programme gives a new lease of life to one specific model, the 113M 4x2 tractor with its 11-litre Euro-1 engine. The re-engineering process (Scania studiously avoiding the word "conversion") will be carried out by the respected Elland, West Yorkshire, operation of Feather Diesel Services.

Engine internals

The most fundamental change is to the cylinder head, where the injector housing is modified to accept the spark plug. This is a normal 14mm car-type item, although of a specific grade to cope reliably with the smaller than usual air gap of 0.35mm. New inlet valves and guides are fitted, the guides being modified to allow the use of valve stem oil seals. This gives extra insurance against oil leakage increasing hydrocarbon (HC) emission levels.

As a spark ignition engine wouldn't live very long with the diesel's compression ratio of 17:1, the other major task is to reduce this to 12:1. This is done by machining away the central peak of the piston's combustion chamber to leave a more bowl-like shape. A military originated technique is used, which allows the pistons to be machined in situ, two at a time, and with total removal of swarf. This is obviously quicker, with all six pistons taking around an hour, but has the added advantage of preserving the precious carbon build-up which further aids Oil control.

Plumbing

Getting the air and fuel into the engine involves some new hardware. Air flows through the filtration, turbocharging and charge-cooling processes as on the original diesel system, although the turbo is replaced by a Holset item, with waste-gate and water-cooled bearing, producing a modest 0.8bar boost at this 350hp rating.

However, from the normal entry point of the inlet manifold, everything changes. The original manifold is mounted upsidedown, so that its entry is now at the rear of the engine. This feature apparently came about by accident, when project leader Brian McMurray witnessed a Swedish Scania apprentice getting a rollocking for incorrectly fitting a manifold.

A new chamber sits above and parallel to the existing manifold, and is joined to it by a U-bend which acts to increase turbulence. One important effect of this set-up is that only right-hand-drive trucks can be modified, thanks to clearance problems.

Electrics

To gain the maximum potential from gas power, it's not enough to just dribble gas into the inlet manifold. State-of-the-art electronics are applied in the form of the him Eclipse engine management system from America. Among its features is an automatic limp home" mode which takes effect if any fault causes a potentially damaging increase in exhaust gas temperature. The engines are "ultra lean burn" which means they operate at an air/fuel ratio up 1.64 times the stoichiometric ratio of a conventional petrol engine.

The final crucial element is ignition timing, and this is achieved by mounting a timing unit in place of the redundant fuel pump. This contains no less than three timing triggers, any one of which can perform the task on its own.

Gas storage and supply

The two different gases require different storage methods. Ambient temperature CNG is stored in six tanks made of multilayers of plastics and carbon fibre, which operate at 250bar, while LNG is stored at a much lower pressure but at a temperature of -162°C in a pair of tanks similar to those used in industry for liquid oxygen and nitrogen. Scania is fully confident of the impact resistance of both types.

The target vehicle range in both cases was 350 miles but in testing 400 miles has been exceeded comfortably. Compared with a diesel truck offering the same range, the LNG system has a weight penalty of 250kg, and CNG a penalty of 350kg.

Emissions

A catalytic converter specially designed for gas engines is sourced from Kemira of Finland. To achieve the highest possible operating temperature, and hence efficiency, the converter is mounted directly to the turbocharger's outlet (see table).

Driving impressions

Scania provided a pair each of LNG and o CNG vehicles for a brief evaluation off

road, together with a diesel-powered a tractor for comparison. All five are from 2 Safeway's distribution fleet and are on N 2,, and P registrations.

We started by driving the diesel to -a establish a baseline, and it has to be said cc that we were thoroughly impressed by

8 the condition of a truck that has covered

eL nearly 650,000km in under four years. Apart from a chattering gear lever bush, the whole thing felt nearly as good as new, with no slop or wear evident in the mechanicais or controls.

Apart from a little inevitable grubbiness, the cab likewise failed to reflect the mileage. The comments apply to all the samples we tried, some of which had done even more work.

The first impressions of driving with gas was the much lower levels of both noise and vibration. All of the vehicles were loaded to 32 tonnes, but direct performance comparisons were not truly valid, as the base diesel produces its original 320hp while the gas engines are giving a "healthy" 350hp. Maximum power, and the same 1,550Nm of torque, are produced at the same 2,000 and 1,100rpm with both types of engine. That said, the needle seemed to spin around the rev counter at a much greater rate than expected under gas power. The only objective measurement at the test venue was an uphill slope which dragged the 320 diesel down to fifth or sixth, while the gas engine propelled the same weight easily in seventh. Strangely, the gas truck didn't have the feeling of working hard under load that the diesel did, leading to the false temptation to change down, but faith pays off. With the minimum of familiarisation, any driver will soon be able to get the best out of the gas engine, while reaping the personal benefits of a much more pleasant working environment. Apart from the changed characteristics, the only difference the driver will notice is that the engine stop control has been replaced by a big red button which shuts the engine down before the key can be turned off.

External noise levels at Idle are reduced by around 10dB(A), at least a halving of the perceived level, and a factor which has obvious benefits for nocturnal urban operations.

In its first foray into alternative fuels, Scania has produced a product that will certainly appeal to a narrow but significant sector of the industry. The question seems to be not will it sell the reengineered trucks, but can it produce them fast enough to match the demand?

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