The Evolution of Modern Small-Car Engines
Last year the Automobile Division of the Institution of Mechanical Engineers instituted a Symposium on the Design of Small Mass Produced Motor Car Engines, at which eight papers were presented. The following is a summary of these.
RENAULT. The fourth paper was read by R. Bachex, Chief Designer, Engine Section, Regie Nationale des Usines Renault of Billancourt. His problem was to design an engine for a 4-seater car capable of 130-135 k.p.h., weighing 700 kg. empty and rated in France at 5 h.p., which imposed a maximum of 960 c.c. for a petrol engine, 1,371 c.c. for a c.i. engine. Rear-wheel-drive had been decided upon. The power needed was 42 h.p., with air-filter, dynamo and fan but less exhaust silencer, which would absorb 3% power.
Two variants had to be easily evolved from the basic design, i.e., one for a sports car, another for a f.w.d. utility vehicle of 800 kg. effective load.
The diesel engine was discarded because of a double handicap. The m.e.p. is lower by about 18%. At equal power speed is lower by about 18% due to high compression pressure necessitating larger parts, a limit due to the combustion phenomenon itself, and increase in c.c. to obtain the required power. Then a diesel engine needs at least 1½ times the c.c. on private cars of this h.p., and this, the larger parts and the injection equipment means a more bulky engine, heavier and more expensive than a precarburation four-stroke. The injection equipment can double weight and considerably raise the price in the case of small engines. Finally, diesel engines are more noisy.
It was observed that the diesel engine had the advantage of some 10% lower fuel consumption at full load, 15% at partial load, and its fuel is cheaper than petrol. But M. Bachex said these advantages did not make the customer decide in favour of the diesel when offered as an optional choice, although this should be corrected if the manufacturer decided to bring out a diesel at private-car production rate.
He discarded the two-stroke because its greater simplicity of manufacture is offset by generally high fuel consumption and because of the experiments necessary to solve problems of scavenging, lubrication, tick-over and port fouling, for a questionable advantage.
He also turned his back on air-cooling, being of the opinion that in-line engines do not lend themselves to this form of heat dissipation, which necessitates a greater distance between cylinders to accommodate the fins. For 4-cylinder opposed-cylinder engines air-cooling is more suitable but if installed at the front of the vehicle is difficult because air circulation is cumbersome; a fan in front overcomes this but then the rear cylinders are not properly cooled and specific power has to be limited. Although air-cooling dispenses with all user supervision, high temperature of the block reduces induction capacity and necessitates lowering the c.r. Water-cooling, on the contrary, gives easy regulation of engine temperature, rapid temperature rise, ease of transmitting heat to radiator, carburetter, etc., and reduction of noise.
But the user should not have to bother with water-cooling, which means supplementing the conventional circuit with an expansion chamber located in a region colder than the radiator, closed by a valve adjusted to float at 600 g./cm². This also gives a gain of about 15% in cooling efficiency.
For a capacity of about 1-litre the 6-cylinder in-line engine can be eliminated as pointlessly expensive and with too low a unit cubic capacity, likewise the 2-cylinder engine, the driving torque of which would be too irregular for a unit cubic capacity of 0.5-litres. An air-cooled flat-twin would suffer thermally, the in-line twin from a balancing fault necessitating a balancing shaft or complicated engine mounts. Three-cylinder engines suffer from poor equilibrium and their unevenness also calls for a balancing shaft or complex mountings, and they require better insulation.
A vee-four calls for a balancing shaft, duplicate parts and complicates the various manifolds—an expensive solution. An aircooled flat-four suffers from the above disadvantages, so an in-line water-cooled four was chosen. Size of engine was determined by taxable h.p., a cubic capacity characteristic of this being used, and performance adjusted by the opportunities offered by C/D ratio. Fuel consumption is not directly affected by cubic capacity—it is always possible, said M. Bachex, to adapt the large capacity so that it does not consume any more than the small engine, provided that better overall performance is not required. It is high performance that governs the level of consumption.
Choice of C/D ratio is governed by our poor knowledge of bearing lubrication imposing a limit to engine speed, and by rocker-gear effects, whether in the form of oscillating speed of the rocker gear, the natural frequency of the rocker gear or the natural frequency of the valve springs. To extend the limits calls for bearings less sensitive to heating, lighter rotating parts, increase in oil-pump performance, better oil-cooling, increased rigidity of valve rocker gear, raising the camshaft to the top of the block or even into the head, use of double valve springs. All mean an increase in cost. Other disadvantages of a big bore for a constant cubic capacity are increase in engine length, increase in weight of pistons, liners, gudgeon-pins, bearings, cylinder head and block, and consequent cost increase. Once the c.c. has been determined, therefore, the smallest bore compatible with performance must be selected. [Ford argue conversely, but M. Bachex was arguing with low c.c. essential due to the tax on h.p.—ED.]
In designing a family of engines it is convenient to start with two different capacities. Varying the bore is preferable to varying the stroke, as it simplifies manufacture. The three engines envisaged were :
(a) Private car, of less than 960 c.c. and 42 h.p.
(b) Sports version, of less than 960 c.c., and 45 h.p., on premium fuel.
(c) Utility version, of 8 m./kg. torque, about 41 h.p.
The solution was :—
(a) 65 x 72 mm. (956 c.c.).
(b) 65 x 72 mm. (956 (c.c.).
(c) 70 x 72 mm. (1,108 c.c.).
The piston had a thin skirt machined in barrel form, and offset gudgeon-pin. It dispensed with a compensating device, carrying a cylindrical compression-ring of spheroidal graphite cast-iron with a chromium plating, a conical sealing ring of lamellar cast-iron and a U-flex type flexible scraper ring. Those rings enable oil consumption to be kept between 0.2 and 0.8 g./h.p. for an engine speed equal to ¾-maximum speed. The cast-iron compression-ring has a high breaking strength and the plating enables wear to be kept to acceptable figures for mileages exceeding 100,000 km. The gudgeon-pin is tightly gripped in the little end, enabling all play in the assembly to be reduced, and the con.-rod is of steel, with a rod/crank radius ratio of 3.56; a perpendicular split gives better performance from the big-ends.
One bearing per cylinder was selected, to give better strength up to a c.r. of 10 to 1, better rigidity, and improved bearing performance owing to less shaft distortion and better distribution of loads. By narrowing the centre and end bearings engine length is reduced. All the bearings have practically the same dimensions, crankshaft length being governed by liner size. The crankshaft is 5.46 times the bore, against 6 to 6.45 times in earlier engines. With filter, electrics and flywheel the engine weighs 87 kg.
The crankshaft is cast in spheroidal graphite cast-iron, which, for equal strength, is more accurate and saving in machining than a cast-steel shaft. The counterweights are unmachined. White metal bearings obviate the expense, necessity for hardening the shaft, and the oil filter which sintered copper-lead requires. For a thickness of 11 to 14/100 mm. a maximum pressure of 250 kg./cm² and an average of 90 kg./cm² at maximum engine speed for con.-rods and 85 kg./cm² for upper journal bearings is permissible.
The oil pump pinions are made by sintering. The supply pressure is 4.5 kg,/cm², only the upper bushes in the bearings have oil collecting grooves, to increase bearing capacity, and a single oil inlet to the rod ensures greater oil supply at maximum speed than using a hole drilled diametrically with two orifices. There is an additional diametrical hole in the gudgeon-pin. The block is of cast-iron to reduce cost but an aluminium block cast under pressure would reduce weight by 55% a saving of 11 kg. for the Renault 1,160-c.c. engine, and might eventually be adopted, “depending on the evolution of techniques and cost of material”. The use of centrifugally cast iron wet liners is justified by the simple block casting, reduction in foundry and machining rejects, and greater wear resistance; metal of liners can differ from that of block, and the structure of the metal is better, due to centrifugal casting. Also, worn liners can be easily replaced by new ones. The sealing problem is achieved by thin tracing paper gaskets. The design of the block facilitates its passage through the transfer machines; in machined form, with covers, it weighs 21 kg.
Two valve arrangements are open to the designer—a hemispherical chamber with valves in V-formation or a wedge chamber with parallel valves. The former gives slightly better efficiency and breathing but is a pure loss on an engine whose maximum speed is restricted by considerations of a mechanical order. The wedge chamber has the advantage of simplicity, is less sensitive to knocking and particularly pre-ignition, and poor breathing is offset by a smaller stroke/bore ratio. So the wedge-shape was chosen by Renault.
An aluminium head was adopted because although a cast-iron head is of greater simplicity, its technical properties are not so good. Maximum c.r. is lower with an iron head by about 0.5, at which point there is a loss of efficiency of about 3%, it is necessary to maintain a film of water between inlet and exhaust channels—on a 70-mm. bore the loss is about 5 mm., or 7% of the bore—and there is less resistance to pre-ignition and re-ignition due to higher wall temperature.
A low-set camshaft limits engine speed, not only because of the additional valve gear weight but principally due to elasticity of the push-rods which introduces stray vibration into transmission of motion from camshaft to valves. So Renault use as high a position as possible, while keeping the push-rods in the block and without the drive housing coming above the upper surface of the block. A simple timing chain with hydraulic tensioner is used, pinions being cut from flat steel plate. The cast-iron camshaft has cam profiles hardened directly when cast and runs directly in the block. The law of motion is of the “continuous acceleration” type, a satisfactory compromise consisting of taking half of a sine wave as the shape for positive acceleration. The angular extent of this acceleration must be in a fixed relation to the natural frequency of the rocker gear and maximum engine speed. Such a law can be quickly determined by elementary calculations based on the double differences of the lifts, when the value at the limits of the lift and its first derivative are known.
Exhaust valves are of 21 4NS steel, known for its resistance to corrosion by lead oxide. The seats angled at 90° with a minimum width of bearing surface of 1.5 mm. for the exhaust valves, are of cast-iron, which has a good heat transfer coefficient. The valve springs are shot peened and varnished and have spirals with a progressive pitch to dampen vibration.
The carburetter, naturally single chamber in the basic version, has a minimum of metal connecting float chamber to carburetter chamber to reduce percolation effects, the butterfly valve is separate from the main body and insulated by a gasket, and the butterfly valve chamber is of aluminium to withstand distortion caused by heating when the car is stationary. Three compression-ratios are used – 7.5 to 1 for lower than 90-octane fuel, 8.5 to 1 for the 956-c.c. and 1,100-c.c engines running on 90-octane fuel, and 9.5 to 1 for the sports engine, using 96-octane petrol. Permanent mould casting produces the chamber direct from the foundry and different versions are obtained by altering the height of the head. Three versions of inlet manifolds match the three variants, each having its individual carburetter and camshaft. There is a competition version of the engine, with twin o.h. camshafts, above the basic cylinder block. The 956-c.c., Caravelle 956-c.c. and Estafette 1,108-c.c. engines all produce about 40/45 b.h.p. from 4,000 to 6,000 r.p.m. The sports 1,108-c.c. engine gives just over 70 b.h.p. at 6,000.r.p.m., the twin-cam 995-c.c. racing engine 90 b.h.p. at 7,000 r.p.m.
Great care is observed over engine sealing, to prevent ingress of dust. To collect air from a dust-free zone presents problems with a rear-mounted engine and an accurate study has to be made. The intake is before the radiator and in the centre-line of the car. A cellulose element type air filter was chosen, tests with silica (HK) of the Moulin des Prés, showing an efficiency of 98%, as soon as clogging reaches 10 g. Dust intake equal to 200 g, of this dust for a head loss of less than 150 mm. of water implies a capacity sufficient for more than 30,000 km without cleaning, the average dust intake being 1 g./1,000 km. This filter also serves as an inlet silencer, weighs only 1.45 kg., and is fixed directly on the carburetter by a central screw, three small columns serving as bearers. For dusty countries a cyclone is mounted directly on the air filter and a throw-away type oil-filter is supplied. With Renault’s sealing and filtering precautions, engine wear after about 100,000 km. can be kept within the following limits: average liner wear less than 0.012 mm., maximum liner wear less than 0.045 mm., wear at cut in chromium-plated rings is 0.3 mm., diametrical wear of crankshaft is less than 0.06 mm. and diametrical wear of camshaft bearings is less than 0.03 mm.—W..B.