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TS3 a Natural Multi-fuel Engine

16th January 1959
Page 50
Page 50, 16th January 1959 — TS3 a Natural Multi-fuel Engine
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Tests Show Standard Oil Engines with Slight Modifications Will Run also on Petrol and Other Fuels

ACOMMER TS3 oil engine has completed many, hours of satisfactory running on 80-octane petrol during experiments into the development of multi-fuel engines. This information was disclosed in a paper by Mr. H, L. Troughton and Mr. R. L. Fowler, presented to the Diesel Engineers and Users' Association in London yesterday.

Research into multi-fuel units had been conducted in this country mainly by Service departments who wished to enjoy the economies of the oil engine but did not want to tie vehicles to one fuel supply. Their basic requirement was for an engine that would run on any of five fuels-light diesel fuel, aviation gas-turbine gasoline, 74-octane petrol, 80-octane petrol and commercial premium-grade petrol.

Spark Ignition Unsuitable The spark-ignition engine was discounted as the basis for a successful multifuel unit because of its need for fuels of very definite properties. It was impossible even to design such an engine to run satisfactorily on all grades of petrol without incurring thermal inefficiency and an increase in specific fuel consumption on either highor low-octane fuel. Moreover, the need for a volatile fuel to ensure good fuel-air mixing in the inlet manifold ruled this unit out for the heavier fuels.

Thus the compression-ignition engine has been chosen for this development. Unlike the spark-ignition engine, it was not limited in compression ratio by the fuels used. Indeed, an increase in compression ratio was often required to achieve satisfactory operation on a range of fuels. Although this imposed increased mechanical loadings on the engine, it improved thermal efficiency.

The combustion problem which had to be solved when burning petrols in a compression-ignition engine arose because they had a much lower cetane rating than normal diesel fuel. This rating indicated the resistance of the fuel to spontaneous combustion; fuel with a low cetane number had greater resistance to spontaneous combustion than a fuel with a high rating.

Additives Not Specified •

Petrol, when ignited by compression, gave rise to a long delay after injection, increasing the violence of combustion and the noise produced. Although chemicals or lubricating oil could be added to petrol to improve its eetane rating, it was not thought desirable to specify additives for the fuels of a true multi-fuel engine.

In designing a suceessful four-stroke compression-ignition engine able to consume a variety of fuels it was essential to control rigidly the combustion process. This could be done either by regulating the rate of conflagration after the delay period or by reducing the delay period to the minimum and burning the fuel as it was injected.

When running on petrol, a long-stroke

di rect -injection comprcssion ignition cl4

engine was far more efficient than units having " squa re and " over-square " bore-to-stroke ratios.

As petrol had a lower specific gravity than diesel oil and possessed fewer B.Th.U.s per c.c., it was necessary to increase the injection-pump rack setting when running on petrol to ensure that the same amount of chemical energy was injected into the cylinder. This increased the combustion time and could, if carried too far. result in increased exhaust-gas temperature. The alternative was to increase the size of the injection-pump plungers, although to do so might lead to excessively high combustion pressures when operating on diesel fuel.

Tests on a long-stroke direct-injection engine with the injection pump adjusted, when using petrol, to give a similar power output to that achieved on diesel fuel had shown that with 80-octane petrol, maximum torque was achieved at 900 r.p.m., whereas when running on dery this occurred at 1,400 r.p.m.

A 3.5-litre compression-ignition engine which was not satisfactory on light fuels was modified by reducing the size of the mask on the inlet valve, which, with other changes, directed the incoming charge over the hot exhaust valve. A new shape of piston cavity was introduced and the compression ratio was raised from 16.5 to I to 18.75 to 1. These alterations produced an acceptable multifuel unit.

Large engines were more easily adapted to burn a number of fuels than

small ones. Generally swirl and precombustion-chamber systems on the smaller engines tended to give too high a rate of pressure rise for efficient combustion of light fuels, whilst the same system on a larger engine could be satisfactory.

German multi-fuel engines, however. were usually of the ante-chamber type and were derivatives of existing oil

engines. They employed compression ratios as great as 26 to 1, They were mainly quiet in operation and did not suffer apparently from bearing troubles.

The " M " system of multi-fuel combustion which originated also in Germany was mentioned. This design employs direct injection into a cavity in the top of the piston by an injector with two nozzles. Air swirl is induced in the cavity and a small initial injection is made into the centre of this bowl, whilst the main injection is made to the side of the cavity in the same direction as the moving air. This results in finely controlled combustion.

Although it had run well on the test bed, a two-stroke engine, used during the experiments, which had port-controlled inlet valves and poppet exhaust valves had been extremely difficult to start. This was thought to result from the low compression ratio (14 to 1). causing an insufficient compression temperature which was reduced further by the latent heat consumed in evaporation of the volatile fuel, The opposed-piston two-stroke engine was thought to be particularly suited to multi-fuel operation. The stroke-to-bore ratio was higher than in any other type of engine and at top dead centre the area of cooled combustion chamber was as low as could be achieved with any design.

By the use of tangential inlet ports, a high degree of swirl could be imparted to the incoming air. The use of a two-part piston employing a steel crown enabled the combustion chamber to be maintained at a .higher temperature than would be possible with a one-piece piston.

After many hours on the test bench and much operation on the road, it was found that this engine (the TS3), was a natural multi-fuel unit. Power outputs on 80-octane petrol were above those obtained with diesel fuel, whilst cold starting was easy at all times.

In addition to overcoming combustion difficulties with multi-fuel engines, it was also essential to modify injection equipment to cater for the lighter fuels. As these fuels do not lubricate the conventional in-line injection pump as diesel oil does, it was necessary to provide some means for positive lubrication.

When running compression ignition engines on petrol much trouble had been experienced through vaporization of the Fuel in the gallery of the injection pump. To relieve this it was essential to have a continuous flow through the gallery.

Regarding engine life, 10,000 miles had been completed with a two-stroke unit. Wear was no greater than with a similar engine using diesel fuel,


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