Turbocharging Aids Multi-fuel Units
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Thornycroft Research on Multi-fuel Engines Described to Institution of Mechanical Engineers
"HE ,multi-fuel engine of the future would appear to be a direct injection turbocharged unit having a high stroke-to-bore ratio, and embodying a very compact combustion chamber. The swirl ratio may be lowered in order to reduce beat losses and thus help to initiate the ignition process. Every effort will be made to maintain the combustion chamber at a high temperature during idle and light-load running conditions."
Mr. G. K. Martlew, experimental engineer and joint inspection manager of Transport Equipment (Thornycroft), Ltd., drew these conclusions from research work carried out by his company, on variants of their standard engines, which • he. described to the Institution of Mechanical Engineers in London, on Wednesday.
Of the four Thornycroft engines used for multi-fuel experiments, the turbocharged KRN6/S six-cylindered 11.3-litre unit had given the most satisfactory results. The turbocharger raised the pressure and temperature of the air charge. This assisted the burning of fuels such as the petrels, which had poor ignition characteristics under compression only.
As a result, the power outputs of the KRN6JS on dery and MT 74 (74-octane) petrol were almost identical, although maximum b.m.e.p. increased by about 5 p.s.i. at 1,000 r.p.m. when running on petrol. Specific fuel consumption of dery was higher than of petrol. This was the case with all multi-fuel engines, and inevitable because of the 15 per cent. lower specific gravity of petrol, making it necessary to inject more fuel into the cylinder to provide the same energy.
Until fuel was bought by weight rather than volume • (and this was unlikely), oils such as dery with a high specific gravity, would be more economical.
No Modification
The KRN6/S, like the other units employed for tests, had required no structural modification to enable it to operate on a range of fuels, other than positivc lubrication of the injection pump and additional venting of the fuel-delivery system.
British engine manufacturers who were carrying out multi-fuel research had accepted derv, aircraft gas-turbine fuel (A.V.T.A.G.) and MT 74, MT 80. regular and premium grades of petrol as the fuels on which their engines should run.
Mr. Martlew suggested that military thought in the U.S.A. might lead to the adoption of a fuel akin to A.V.T.A.G. for all gas-turbine and piston engines. It could he burned by certain types of British direct-injection engine without modification as its Wane rating was little lower than that of derv.
Whilst A.V.T.A.G, produced few complications in suitable direct-injection engines other than vapour locking at high temperatures, the patrols with cetane ratings below 20 required an engine with particularly good combustion characteristics. They also made modifications to fuel-feed and injection equipment essential.
This entailed positive lubrication of all parts of the injection pump, coupled with seal, ing arrangements for the pump plungers to prevent the fuel leaking into the pump body to dilute the lubricant.
Jerk-type injection pumps with this facility were now available and these overcame the problem. Extra venting of the injection-pump gallery was essential to prevent vapour locking with the volatile fuels. This was particularly important when the pump was fitted in an inclined position and had also been incorporated in the pumps now offered for multi-fuel engines. To burn petrol satisfactorily, the combustion chamber of a multi-fuel engine should have as small a surface area as possible, and be kept at a high operating temperature even under light load conditions. This should be combined with a low rate of heat loss during compression and the subsequent ignition period. The long-stroke direct-injection engine with a fairly high compression ratio met such a stipulation quite well.
The KRN6 engine, which had also been tested in naturally aspirated form, had originally been designed to maintain a high cylinder-block temperature of about 185 F. to reduce wear. This characteristic, combined with the compact combustion chamber and 1.37-to-1 stroke-to-bore ratio, made it suitable for multi-fuel operation without the necessity of modification.
In naturally aspirated form, the engine would start satisfactorily on petrol at temperatures down to 0° C. Starting aids were required for colder conditions. Once started, the engine tended to knock for a few seconds, hut would quickly settle down at a noise level slightly lower than when running on derv.
Light-grey exhaust smoke occurred when starting from cold, but this disappeared.as the engine warmed up and was entirely absent during normal operation. • This also applied to the turbocharged version, and it was apparent that the combustion efficiency of both units was not seriously affected at any point in the speed range by operation on petrol.
The blown engine using petrol made extremely little combustion noise. The power unit sounded like a spark-ignition petrol engine.
This appeared to be associated with the higher compression pressure and temperature of the air charge in the turbocharged engine.