Is Petrol Injection Worth While ?
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A LTHOUGH the petrol-injection I", system possessed certain advantages over the normal aspirated engine using a carburetter, it was questionable whether the extra cost of equipment made the system worth the adoption. This was the view of R. Barrington, M.Sc., A.141.I.Mech.E., and E. W. Downing, research engineers of Joseph Lucas, Ltd., who read their paper on petrol-injection for road-vehicle engines at the Institution of Mechanical Engineers (Automobile Division) on Tuesday.
Fuel Economy
If the engine concerned was capable of burning efficiently a wide range of weak mixtures, it was unlikely that petrol injection would give any marked improvement over the part-throttle range. Nevertheless, an improvement in fuel economy of 10 to 15 per cent. had been obtained in an engine not specifically designed to run with very weak mixtures. This was made possible by the better distribution and the ability to operate at weaker average mixture strengths.
The authors thought that petrol injection would give increased power with increased economy because of (a) the better filling of the cylinders;
(b) the reduced requirements in respect of octane value; (c) a cooler combustion chamber which permitted the use of weaker mixtures at full load without over-heating; and (d) the use of a higher ,compression ratio as an alternative to, or in conjunction with, (b) and (c).
c20 During tests which they conducted, a 19 per cent, increase in power had been obtained in one engine, with an increase in specific economy of 12 per cent. A petrol-injection-engined vehicle was much more pleasant to drive because of the smoothness resulting from the more equalized torque impulses and the persistence of effort at full throttle on gradients where a carburetter unit lost power and stalled. The idling characteristics of the petrol-injection engine, too, were much better than those of a carburetted engine.
"The Commercial Motor," which was the first journal to conduct a road test of a petrol-injection-engined vehicle, can fully endorse these views. Starting at low temperatures was not, said the authors, appreciably better. the degree of over-fuelling required being about the same as with a carburetter.
The first tests were carried out with a four-cylindered 1,400 c.c. o.h.v. engine having a compression ratio of 6.5 to I. Cylinder-head injection was first tried,. but the results, so far as economy was concerned, were disappointing. It appeared that penetration of the spray was excessive for the small cylinders and that fuel was lost by contact with the cylinder walls Better results were obtained with inlet-port admission, but on light load, " missing " occurred. The best overall performance of the engine was obtained when the injection point was arranged on the manifold at about 21 ins, from the respective inlet valves.
Injection timing on the bench was set at 45 degrees after top dead centre, and on road _tests. this was altered to 90 degrees after top dead centre. Coupled to a dynamometer, the engine could be idled at 150 r.p.m. and running free, at about 300 r.p.m.
The power gain with hand-controlled mixture increased over the speed range up to a maximum of 8 per cent. at 4,000 r.p.m. Specific consumption showed little difference above 2,500 r.p.m., but below this speed petrol injection showed advantages of nearly 12 per cent. at 1,000 r.p.m.
Better Performance
With the engine installed in a vehicle it was found that the road performance, as judged by maximum speed and ability to overtake, was much better than with the carburetted 12 h.p. engine normally fitted in the vehicle. A notable feature was the complete absence of "flat spots."
On journeys of over 50 miles the fuel consumption of the petrol-injection vehicle varied with the conditions, but was usually at a rate between 35 m.p.g. and 38 m.p.g. A comparison between petrol injection and a carburetter set for economy showed that the former gave 38 m.p.g. and the latter 33 m.p.g. As to the question of reliability, 20,000 miles were covered in 18 months and the only troubles experienced with the petrol-injection system were associated with the lubrication of the engine-driven sleeve of the metering distributor_ which seized twice.