Overhead Camshaft Research Articles

Camshaft and cam follower materials

In the past, essentially empirical approaches have led to the breakthroughs required for successful new designs and these have usually involved improved lubricant additives. There is now a general view that a fundamental understanding of the whole valve train tribology is necessary to produce solutions to problems which are expected to arise as engine technology develops over the next decade. Specific problems have recently been encountered with overhead camshaft (ohc) finger followers and the use of hard chromium plating on some components has presented a problem. This problem is similar to those that have also been encountered in piston ring/ liner situations where it has been found that smooth chromium surfaces are difficult to lubricate. Commercial processes have been developed which rely for this success on the production of surface porosity to enhance reten-

Paper 6: The Engines of the Smaller Fiat Cars

The paper deals with the story of the engines intended for the smaller Fiat cars, starting with the ‘500’ model produced in 1936: all engines with displacements in the neighbourhood of 500 cm3. With small cars, especially of the utility class, the adaptability of the piston engine to a variety of design and space requirements is used to best advantage. The size, weight and displacement must blend with the design of the car as a whole; in turn, the design must feature the highest degree of simplicity compatible with the qualities required. It is, therefore, important for the engine of utility cars to be designed as a function of the general architecture of the vehicle. At Fiat, after a period in which economical cars of conventional design were produced, work was started along this line with the ‘500’ (Topolino) model in 1933–1934. The paper illustrates the design of this model and describes the reasons behind some of its engineering details, as well as the subsequent developments which led to the final solution. Also outlined are the more important designs of experimental engines studied as substitutes for the ‘500’ model: a transversal four-cylinder engine, with overhead camshaft, built in 1945, for a front-wheel drive prototype; an air-cooled, two-cylinder in-line engine, mounted transversally, for a rear-wheel drive prototype; an opposed two-cylinder engine, with longitudinal axis, in replacement of the previous unit; a two-cylinder in-line engine with longitudinal axis, finally chosen for the ‘New 500’ model introduced in May 1957. Also described are the further developments of the latter engine for the version with horizontally arranged cylinders suitable for underfloor application. The paper highlights the more interesting aspects regarding both the design and the experiments made and explains the principal reasons leading to the choice of given solutions and to the modifications which followed.

Paper 8: The NSU Prinz Engine

As the automobile becomes an indispensable consumer article for all strata of society, its importance as the status symbol of a definite social position declines more and more. This natural process of development has obviously been accelerated by special traffic problems the origin of which must be looked for in the good economic circumstances of the past years. Only they could cause motorization to take such great steps forward. Amongst the many constructions which were created with the intention of combining the requirements of daily professional and town traffic with a comfort sufficient also for extensive holiday travel, the NSU Prinz, as designed in 1955 and then produced in series as Prinz I and II and since May 1961 in its present form, seems to meet the requirements of a large and steadily increasing circle of buyers. The maintainable speed of about 120 km/h, which is quite sufficient even for fast roads and motorways, has been obtained with a motor performance of 30 hp at an air resistance cw = 0·43 and a front area of 1·57 m2, which allows good elbow and head room to the passengers. A four-stroke motor of relatively expensive construction with hemispherical combustion chamber and overhead camshaft was chosen which enables the car to have a good fuel economy. In consideration of town traffic we have stressed the good combustion in the neutral position and the lower partial load range, in order to keep the CO content of the exhaust gases low. We succeeded, in designing the body, in keeping the change of the yawing moment in accordance with the angle of attack, within very modest limits. The dry weight of 537 kg is far below average and could only be attained with considerable effort and after extensive measurements had been carried out. In this manner the drive unit, inclusive of suction and exhaust devices, weighs 100 kg, the chassis (axles, wheels, suspensions and steering parts) 127 kg, the body (raw) 168 kg, glass 28 kg and seats and fittings 114 kg. The ratio of vehicle weight to permissible total weight of 1:1·86 naturally requires an exact study of the chassis and distribution of weight. The centre of gravity changes its position with the different loads in a longitudinal direction by less than 5 per cent from the centre of the wheel base to the front and to the back. This small deviation causes a type of steering behaviour which remains almost static with all loads. In every case the vehicle is slightly under-steered. In this connection we obtained the measurement values for μθ of 0·55 with one passenger and of 0·54 with four passengers. Thus we believe that the driving behaviour of this vehicle takes into account the circumstances of modern traffic and that the usable space offers sufficient comfort for four or five people. For how long the shape of body will correspond to the public taste is a matter of conjecture: however we hope and also believe that the choice which can today be observed, of the simple form which refuses any superfluous decoration, will last for rather a long time.

Paper 5: The Hillman 875 cm3 Engine

The paper covers the constructional considerations, design and development of the engine to be installed in a rear-engined vehicle. The need for minimum weight and size coupled with adequate power output resulted in the decision to use light alloy diecastings wherever possible. The various methods of diecasting are considered and the methods used for the various components are discussed with the reasons for their use. Problems with cast-in cylinder liners, machining considerations and the implications of laying the engine at an angle in the car are covered. With regard to performance, several factors are discussed: the reasons for adopting the four-stroke cycle, four cylinders, water cooling; determination of capacity from power required data; the use of a single overhead camshaft; combustion chamber shape and compression ratio; port shapes and determination from air flow research. Then follow the various stages of development work and endurance testing.

Deterioration in Automobile Cams and Tappets

A paper was presented to the Automobile Division of the Institution of Mechanical Engineers in London, on 8th January, by Dr. It. Cazaud, Dipl.Eng., M. Renout, and C. Daubertes, Dipl.Eng. (all at Simea, France) entitled “Contribution to the Study of Deterioration in Automobile Engine Cams and Tappets”. The cam‐tappet wear problem has become very important in the past few years because of the continuous increase in maximum rotational speed of engines and it would seem that the ideal solution lies in decreasing the inertia forces by giving up the rocker system, and using the arrangement of the overhead camshaft acting directly on the valve stem.

A Method of Measuring Scavenging Efficiency and its Application

The paper deals with a new method of measuring the scavenging efficiency in a loop-scavenged two-stroke cycle diesel engine and its assessment on a test engine. The two valves installed on the cylinder head are to be timely opened by the overhead cam shaft driven at half an engine speed, and to extract the gases of the same cycle before and after the scavenging, giving the possibility of determining either the scavenging efficiency when the engine is operated as fourstroke cycle, or the trapping efficiency when done as two-stroke cycle. The experments employing this apparatus revealed a great deal of valuable information for improving the scavenging system, and also indicated that the scavenging efficiency in a well-designed cylinder could reach over 90% at a delivery ratio of 1.3∼1.4, which does not seem to be inferior to that obtained in the uniflow scavenged system.

Contribution to the Study of Deterioration in Automobile Engine Cams and Tappets

During the last few years the cam-tappet problem has become very important because of the continuous increase in the maximum rotational speed of engines for the purpose of increasing power. This necessitates the overloading of the springs in order to offset the inertia effects of the distribution system as a whole; the result of this is to increase the specific pressures of the cam-tappet unit. As the present tendency is to make engines revolve at ever greater speed, it is to be expected that the problem of the behaviour of these components in service will be more and more difficult to solve. There is no doubt that the ideal solution lies in decreasing the inertia forces by giving up the rocker system, and using the arrangement of the overhead camshaft acting directly on the valve stem. For engines with the traditional system of valve action by rockers, the problem of the deterioration of cams and tappets has been studied by Simca on a specially designed testing machine. The laboratory tests thus made have been followed by tests carried out by the Shell Company on engines on the test bench. The present paper sums up the results obtained.