Developments in Petrol Injection
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Two C.A.V. Experts Review Progress Made to Date in a Paper Read to the Institution of Mechanical Engineers on Tuesday
APAPER of exceptional importance, on petrol-injection equipment development, by Mr. K. Brook,
A.M.I.Mech.E., and Mr. W. E. W. Nicholls, was read before the automobile division of the Institution of Mechanical Engineers last Tuesday. Although the British aviation world became interested in the petrol-injection engine during the war, because of the fairly large employment of such units by the Luftwaffe, the interest in this matter shown by transport engineers in this country has been spasmodic. The period under review covers the past ten years.
Mr. Brook is chief Diesel engineer and Mr. Nicholls development engineer of Ltd. When that company first considered developing equipment for petrol-injection engines, little was known here of the extent to which advantages prophesied for such units could be realized in practice, although it was known that research on the matter was proceeding on the Continent and in the U.S.A.
The greatest single improvement likely to result from the use of injection in non-supercharged engines is additional torque arising from increased volumetric efficiency, because of the removal of carburetter choke, freedom in manifold . design and the elimination of induction heating. This gain occurs chiefly at higher, speeds, where its value under road conditions may be debatable.
B.m.e.p. Raised by 9 per cent.
The authors state that they know of a particular engine in which improvement was also effected at lower speeds, amounting to an increase of b:m.e.p. from 128 to 136 lb. per sq. in. at 800 r.p.m. or some 6.25 per cent. The corresponding increase at a maximum speed of 2,500 r.p.m. was 9 per cent.—from 125 to 136 lb. per sq. in.
This general raising of the torque curve was achieved without sacrifice of fuel economy. By the use of an individual metering pump to each cylinder, close equalization of fuel charges to all cylinders is obtainable. To be of value, this must be accompanied by equal accuracy in fuel distribution, which should not be difficult to obtain, having regard to the greater freedom permissible in manifold design.
The optimum mixture-strength requirements of petrol engines vary little with changes of speed, but cl.vary with load changes. The carburetter meters fuel in proportion to mass air flow, and thus finds it difficult to differentiate between changes in load and speed. The injection pump, however, being engine-driven, is inherently "speed conscious," and can be made "load conscious" by an automatic device which proportions its cyclic output primarily to induction pressure; thus, under all conditions of running, the optimum mixture strength is obtainable.
An advantage of the injection system, not obvious on the test bench, can be realized under road operation, as it is possible to prevent fuel from being used when the vehicle is coasting in gear. Miring coasting with throttle closed, manifold depression is greater than under any other condition of operation. For example, on one occasion, the coasting depression amounted to 26 ins, of mercury, as against 22 ins, when idling. This difference of 4 ins, can be arranged to move the pump output control to zero delivery.
The earliest application with which the authors of this paper were concerned was to a single-cylindered engine designed for a maximum speed of 3,000 r.p.m. As this was a two-stroke, the maximum pump speed was considerably higher than anything they had encountered in Diesel practice, and inertia difficulties were expected in the reciprocating parts. The important factor, nevertheless, is not so much plunger speed as plunger acceleration. In a Diesel pump, injection periods are short, resulting in rapid acceleration, but with petrol injection much longer periods are permissible and even desirable. Thus, actual acceleration at these high rotation speeds is often less than in the comparable Diesel pump at its much lower speed.
A rather serious result of high operating speed was an inability to fill the pumping cylinder on each stroke, this being aggravated, with the increased period of injection, by
the shorter period remaining for filling. One point emerging from theoretical examination was confirmed in practice-the influence of varying feed pressure is small, as it affects the filling process only as the square root of the absolute value. Increased feed pressure is thus not an adequate substitute for deficient time area of porting. As a result of this investigation, an element design was evolved with three filling ports, followed by further designs, in which the number of ports was increased to as many as eight.
Because of the low viscosity of petrol, doubts were felt as to the ability of the pump to operate with this as the only plunger lubricant. With a clearer .understanding of factors affecting seizure, it is now recognized that wear in pumping elements is little influenced by the presence or absence of direct lubrication. The main reason for retaining it is to prevent petrol leakage.Plunger lubrication is effective only in preventing external leakage of petrol.
It has not proved necesary to decrease working clearance (normally in the order of 0.00004 in. on diameter) between plunger and barrel for petrol-injection pumps, but it is important that a close tolerance be maintained. The petrol engine is far more sensitive to rnixture variation than the oiler, and its performance is greatly influenced by inequalities of mixture between one cylinder and another. Wellbalanced delivery from petrol-injection pumps is important.
The most obvious difference between injection pumps designed for oil and for petrol-injection engines is the muchdecreased size and weight of the latter. This is largely due to the policy of designing each pump for the particular engine concerned. The oil-engine pump must, for economic reasons, be suited to a wide range of engines, and is rarely , called upon to operate at the limit of its capacity.
As with oil engines, the importance of fuel filtration cann be too highly stressed. The authors of this paper had mo..e trouble with swarf, dirt, etc., during their experiments than from any other single cause. By comparison with a Diesel injector, the requirements of a petrol injector may not appea.; exacting. The. precise point at which injection starts anc! finishes is not critical, pressures can be moderate.
Care must, however, be used in siting the injector to ensurer that the entire air charge is evenly saturated with fuel. In small cylinder heads the space occupied by valves and plug often makes it difficult to select the optimum position for the injector, and the problem is complicated by accessibility con. siderations, particularly in multi-cylindered engines.
Injection on Compression Undesirable
With direct injection into the cylinder, and to some extent with port or manifold injection, there is little time in which to complete the thorough mixing of air and fuel essential to high specific output and good fuel economy. For this reason, injection on compression stroke, as in the oil engine, is undesirable. It has frequently been found on test that better results are obtainable by injecting into the manifold at some distance from the inlet valve.
Pintle-type nozzles were used in most of the early tests; but poppet nozzles proved more satisfactory. Little success has attended the use of swirl nozzles. The authors express doubt whether swirl nozzles have anything to offer over the poppet for manifold and port injection, whilst for direct injection their use would seem to be limited to cases where it is necessary to employ a spray offset from the axis of the injector.
Unlike the oil engine, the power of which is controlled by fuel regulation alone, the petrol-injection engine normally requires that the fuel-air ratio be maintained within close limits, and it is therefore necessary to regulate both fuel and air according to a definite relationship.
Adaptation of the conventional jerk-pump system to the special needs of petrol-injection engines presents no insuperable problems. This equipment is, however, costly, and cannot, as in the oil engine, be offset by the elimination of an ignition system. Consequently, a departure from accepted jerk-pump design may be desirable in the interests of simplicity and low cost.