How it Works: The Piston

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How It Works: Piston

Engines may have changed considerably over the years, but the piston, leastwise in its general configuration, has not. It is still recognisably similar to its counterpart of a century ago. Refinements in detail and in the methods and materials of manufacture have been many, but the retention of its basic shape and proportions is evidence of an equally basic function.

In its simplest terms, the pistons function is to reciprocate inside a cylinder and seal its bore so as to vary its volume, while offering a means of connection with the rotating crankshaft to which its reciprocations must be related. Other considerations such as the shape of the crown, the depth of the skirt, the provisions for sealing rings, oil channels, expansion control, internal and external contours, the carriage of the gudgeon pin, and so on, are really only there to improve the pistons efficiency.

Since the piston is the main reciprocating part of an engine, its movement creates an imbalance. This imbalance generally manifests itself as a vibration, which causes the engine to be perceivably harsh. The friction between the walls of the cylinder and the piston rings eventually results in wear, reducing the effective life of the mechanism. The sound generated by a reciprocating engine can be intolerable and as a result, many reciprocating engines rely on heavy noise suppression equipment to diminish droning and loudness. To transmit the energy of the piston to the crank, the piston is connected to a connecting rod which is in turn connected to the crank.

Because the linear movement of the piston must be converted to a rotational movement of the crank, mechanical loss is experienced as a consequence. Overall, this leads to a decrease in the overall efficiency of the combustion process. The motion of the crank shaft is not smooth, since energy supplied by the piston is not continuous and it is impulsive in nature. To address this, manufacturers fit heavy flywheels which supply constant inertia to the crank.

Balance shafts are also fitted to some engines, and diminish the instability generated by the pistons movement. To supply the fuel and remove the exhaust fumes from the cylinder there is a need for valves and camshafts. During opening and closing of the valves, mechanical noise and vibrations may be encountered. A two-stroke engine does not require valves, meaning it doesn't need a camshaft, making these engines faster and more powerful.

In assessing a pistons efficiency, it is important to remember that the piston engine is not merely a mechanical device but is also a heat engine. When the piston rises on its compression stroke, it acquires heat as the air of the fresh charge experiences a rise in temperature, in accordance with that thermodynamic law which states that work is heat. When the compressed and heated charge is ignited and burns, then the heat liberated by combustion is translated into work done, in return, on the piston: the rapid heat rise is translated into pressure which acts on the piston crown to force it down.

As it goes down on its expansion stroke, the piston continues to soak up heat from the burning mixture, for the exchange of heat into work is not very efficient, and if the piston could not accept and shed this heat it would not last long. Some of its heat is given up to the next fresh charge on the downward induction stroke, but most of it has to find its way to the cylinder wall and there to the coolant, be it water or air. If the piston were a perfect fit in the bore, the problems of heat transmission would be slight, but the piston would be unable to move. Some clearance is essential, not only to permit the penetration of a lubricating oil film between the mating surfaces but also to allow for the expansion of the piston when it grows hot, and for the contraction of the cylinder when it grows cool.

Even if the piston and the cylinder were made of the same material (an idea that for good reasons has rarely been tried and even more seldom been successful) they would still expand and contract at different rates and times, and some clearance would still be necessary. The greater the clearance, the greater the clatter and the poorer the sealing. Pistons of cast iron, once the rule, would expand at much the same rate as the cylinder or its liner (almost always ferrous, allowing clearance to be minimised). Iron pistons, or even the thin-walled steel ones that soon replaced them, were too heavy for high-speed engines: their mass created enormous inertia loads as they accelerated up or down the bore.

S. M. Viale

Maximum piston acceleration is given by N2S/K (1 + 1/2R), where N is revolutions per minute, S the stroke, R the ratio of connecting-rod length to stroke, and K is a constant reconciling the chosen units of measurement. Therefore applied to the eight-ounce piston of a 1.3 litre enigne, it reveals that at 6000 rpm the piston momentarily exerts a force of one ton on its gudgeon pin. Experiments with aluminium-alloy pistons began in 1909 guided by engineer S. M. Viale at the Aquila-Italiana factory. Bentley made them work; Hispano-Suiza made them reliable enough to be viable; and by the end of the 1920s they were the rule. At first they needed lots of clearance, but silicon-aluminium alloys (such as Y alloy) of relatively modest thermal expansion soon proved suitable.

Manufacturers learnt to make pistons elliptical, not circular, in section: expansion was greater along the axis of the gudgeon pin than perpendicular to it, and so clearance could be reduced even more. DAF even built struts of Invar (a nickel alloy of very low thermal expansion) into their Daffodil pistons to control expansion even more strictly. Sealing rings remained necessary, springy 'piston rings' of cast-alloy iron (later versions often coated with chromium or molybdenum disulphide for improved bearing characteristics against the cylinder bore) that offered the only direct route whereby heat could escape from the piston to the cylinder.

The path of heat flow is critical: the metal of the crown must be shaped so as to offer direct paths from the hottest areas (usually beneath the exhaust valves) to all the rings, not merely to the overworked top ring as is often the case. This is difficult to achieve with a steeply domed or deeply dished piston crown: in the latter case (presented by diesels and by Heron-type petrol engines with combustion chambers of the bowl-in-piston kind), it demanded that the rings be set well below the top edge of the piston, was inconsistent with the requirements of certain sleeve-valve or two-strokes with piston-controlled ports.

The Piston
The Piston.
Some designers relied on copious sprays or squirts of oil to help cool the piston from below, although it caused bad deposits and accelerated deterioration of the oil. Where oil was abundant, special means had to be provided for preventing its accumulation beneath the rings, where it could be pumped (by the rings' reciprocating action in their grooves) into the upper part of the cylinder to the detriment of consumption and combustion alike. The lowest of the rings may be a multiplex 'oil-control ring' which scrapes excess oil off the bore and passes it through drillings from the ring groove to the inside of the piston wall.

Sir Harry Ricardo

Oil-scraper rings used to be fitted low on the piston skirt (they also helped to deaden 'piston slap', a noise made by a cold high-clearance piston rocking in the bore around its gudgeon pin) but a knife-edge bevelling of the skirt edge is simpler and nearly as effective. The skirt is not always complete. Efforts to lighten the piston and to reduce frictional losses led designers of high-performance engines to create different designs - drilled skirts were popular in the 1920s, and even ball bearings were tried as a substitute for skirts. It was the Sir Harry Ricardo who found the best solution, the slipper piston, of which most of the skirt is cut away, leaving it intact only in the plane of the connecting rod's swing, where it has to resist side thrust.

Skirts were shortened, too, for the same reasons, but it could be overdone, making the piston wobbly in its bore. As a rough guide, the skirt should extend as far below the gudgeon pin as the rings are above it. Racing and aero engines have relatively short-squat pistons, touring cars have longer ones, and two-stroke or diesel pistons are often very long. With all these differences, it is surprising how little variation there is in gudgeon-pin sizes. Gudgeon-pin diameter should be judged in relation to the cylinder bore: a ratio of 1:5 shows weakness, 1:3 uncommon sturdiness. With such heavy inertia loads as those described, alternating at high frequency, the pin is subjected to heavy bending loads: should it bend appreciably, it will distort and weaken or even break the piston although - the bigger the pin, the less room is left for the bosses that support it.

Later versions had the skirt split by shaped slots which extended all the way up to the ring belt. The edges and ends of these slots could, if imperfectly designed and finished, act as stress-raisers from which cracks could originate. However, the split skirt was often thought worth the effort of developing it, as it offered the best of all practical means of reducing clearance and noise. The pistons shape is governed by external considerations, based on the cylinder-head design, and has little to do with specialised piston technology. It is essentially a controller of gas flows, of combustion, and of compression ratio, but there are instances in history where it did perform the unusual. One such case was in the 1937 GP Mercedes-Benz, whose designers were fearful of the consequences of valves sticking open - so each piston crown sported four little pimples that, at top dead centre, would just touch any tardy valveheads and give them a nudge so that they could shut again.
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