Audi 100/A6

1990-1997 of release

Repair and car operation

14.8.6. Pistons

THE GENERAL DATA

The car engine sometimes compare to heart of the person. Really, it works constantly while the car moves. However, such comparison is not quite correct. After all heart, as well as any live organism, is continuous самовосстанавливается: in it constantly there are processes of dying off of old cages and their replacement new, young. That you will not tell in any way about the lifeless mechanism – about the automobile engine. It, despite all our diligence, wears out is almost irreversible. However intensity of such deterioration, a resource of the engine before major repairs, as well as durability of all car in whole, in many respects depend on that, is how much qualitative it is made and competently maintained.

The main details of the engine – pistons with piston rings, rods and cylinders are especially subject to deterioration. Work of pistons of the engine most impresses. After all, moving it vozvratno-is forward between the top and bottom dead points, they cover huge distance. So, at frequency of rotation of a cranked shaft of 5000 rpm and a piston stroke in 75 mm the total way of the piston to minute makes 375 m. For a business hour it is distance there will be already 2 km of 250 m, and for a month of operation for 8 hours per day, excluding days off, the piston will move on 460 km. At a car hard work for 5 years (namely such duration of operation of the car before capital repairs is confirmed with statistics) the piston will cover distance in 24 000 km!

So, deterioration of the piston and the details interfaced to it is inevitable. However sizes of deterioration of piston groups (pistons – piston rings) before major repairs of engines of various firms rather differ from each other. So, limiting deterioration of pistons and piston rings of engines Mercedes-Benz, Volkswagen, BMW, the majority of the American and Japanese firms comes after run about 300 000 km. At the same time engines of other, say, less perfect models, require replacement of pistons and piston rings already after 50 000 km of run (almost in 10 times less!).

In what here the reason? And how the durability of these details depends on service conditions? For the answer to these questions we will consider two typical designs of piston groups of the petrol engine and a diesel engine. We will remind, first of all, that pressure of gases in cylinders of these engines in the beginning of a working course differs approximately twice. In the carburettor engine or in the engine with direct injection of gasoline it makes 4055 kg/sm², in a diesel engine is 70–80 kg/sm². Therefore and pistons of petrol and diesel engines differ one from another though the main constructive decisions at them are identical.

The typical piston of the carburettor engine is cast from an aluminium alloy and covered outside by a layer of tin for improvement прирабатываемости to a cylinder mirror. Its top part – a head – has smaller on 0,1 mm diameter, than internal diameter of the cylinder. It is made for prevention of jamming of a head in the cylinder at a warming up. In ring flutes two are placed компрессионных and one маслосъемное a ring. The bottom part of the piston – a skirt – in cross-section section is oval, and on height has the conic form: in the top part – smaller diameter, than in bottom. Besides, in lugs of the piston with apertures under a piston finger вплавлены two steel thermoregulating inserts. All it is made for prevention of increase in a friction between a skirt and a cylinder mirror at piston heating. Having smaller, than at aluminium, factor of thermal expansion, these inserts pull together a skirt in a direction, perpendicular axes of a piston finger.

Aperture under a piston finger in modern engines usually displace from a symmetry axis in the right party of the engine. For correct assemblage of the piston with a rod and their installations in the cylinder of the engine about a lug aperture there is a label which should be turned towards a forward part of the engine. Such displacement do for reduction of a lateral making pressure force of the gases pressing the piston to one of the parties of the cylinder on a step "a working course".

The rod also should be correctly сориентирован in the engine. On its forward party is available apertures for the directed stream of oil on the loaded party of a mirror of the cylinder (in some engines this aperture is absent). Loose leaves and a cover of the bottom head of a rod also have corresponding labels for correct assemblage. Its further working capacity and durability essentially depends on accuracy of manufacturing of the piston and its true selection to a cylinder aperture. Conducting автомобилестроительные firms apply today system according to which pistons on external diameter are broken usually into five or six classes through 0,01 mm. Besides, they разелены on 3–4 categories through 0,004 mm according to diameter of an aperture under a piston finger. Engine cylinders also have similar division into five classes. Such system allows to pick up most precisely the piston of a corresponding class to any, even to the worn out cylinder, and a finger of the necessary category to an aperture in lugs and to a rod. For major repairs of the engines, consisting usually in расточке cylinders, firms let out repair pistons of the increased sizes.

The piston of a modern diesel engine is calculated on perception more high pressures, therefore it has the big thickness of the bottom and lugs. Besides, the design of the piston of a diesel engine differs from considered earlier a little. The main difference is placing of the chamber of combustion directly in a piston head. As combustion occurs at a finding of the piston near to the top dead point, hot gases heat up a piston head more strongly, and walls of the top part of the cylinder heat up rather less, than in petrol engines. For reliable consolidation of the piston in the cylinder on its external surface five flutes under piston rings are made. In three top flutes are established компрессионные rings. In the bottom flutes two are placed маслосъемных rings. Many firms make компрессионные rings of rectangular section, practically nothing different from rings of petrol engines. However more progressive though also, the design with конусной the top working surface is more expensive. An angle of slope forming a cone at such rings do usually 10 °. Application конусных rings provides some increase in their durability as on a step "a working course" a component of a pressure force of gases on конусную the ring surface in addition presses it to a cylinder mirror. Feature of service and repair of pistons with конусными компрессионными rings is the exact control of backlashes. Backlashes between a flute and маслосъемными rings supervise the same as and in petrol engines.

Forces of a friction between surfaces of a skirt of the piston and a mirror of the cylinder at diesel engines above, than in petrol engines. For increase in durability at a surface of a skirt of pistons in modern firms put a layer of a special kolloidno-graphite covering. It much more improves прирабатываемость the piston to the cylinder and extends term of its work before major repairs. Similar processing of rubbing surfaces of pistons apply today and on petrol engines.

Except deterioration of surfaces of a skirt, flutes under компрессионные piston rings wear out also. The flute under маслосъемное a ring, though such deterioration usually Besides, wears out much less. At deterioration of flutes of a ring start to move more and more intensively downwards and upwards on flute height, and more and more notable there is a so-called pump action of rings. It is shown in more and more increasing expense картерного engine oils. Getting to the combustion chamber, it burns down there, forming the grey smoke leaving an exhaust pipe of the car. At considerable deterioration of flutes replacement of rings on new improves a situation a little. There comes objective necessity for replacement of all piston group with rather desirable расточкой cylinders for the repair size. All described kinds of deterioration is natural and, unfortunately, inevitable process.

With natural deterioration of the engine it is possible to struggle successfully, prolonging its working capacity. Americas here it is not necessary to open. It is just necessary to fulfil scrupulously requirements on car operation, to use qualitative oil and oil filters, competently to regulate fuel equipment. Good results are given by application of qualitative modifiers of oil and fuel, the preparations changing a microstructure of blankets of a friction of engines.

Along with it deterioration of the engine, as well as all car in whole, in many respects depends on the driver, from its qualification and technical literacy. After all knowingly cars of the same mark at one drivers serve long and smoothly, at others – are under repair almost every week. The skilled driver almost never supposes works of the engine with an overload, and furthermore – a detonation. He constantly listens, as its engine works, and immediately reacts to the overload accompanied booming, низкотональным by a sound on lowered frequency of rotation of a cranked shaft. The mode of dispersal of the car also is accompanied by the raised deterioration of the engine. The analogy to a horse and the equestrian Here arises: the careful owner without special need will not whip the canine friend, forcing it to run straight away, especially when the horse was not warmed up yet. Certainly, in critical situations the driver presumes to disperse the car dashing, extremely sharply. But if such abrupt style of driving becomes a habit, repair of the engine at such reckless driver is for certain provided twice earlier, than it is provided by specifications.

Other kind of deterioration not provided by any instructions is frequently observed also. It is emergency breakage of shatunno-piston group and, first of all, rings and crosspieces of ring flutes of the piston. In petrol engines it is connected first of all with a detonation. We will remind that the detonation is взрывоподобное gas mixture combustion in the cylinder, accompanied by spasmodic increase of pressure in the combustion chamber. It is to equivalently sharp blow by a weighty sledge hammer on the motionless piston and rings. Details, naturally, are not calculated on loading and can break, having damaged then sharp splinters a cylinder mirror. The reasons of a detonation a little. However the main things from them – this work of the engine on gasoline with lower, than it is provided by specifications, OCTANESым number, and also an overheat and work on a rich gas mixture. The skilled driver is obliged to hear detonation knocks at work of the engine and immediately to reduce fuel giving at dispersal, and then to eliminate the detonation reasons. The detonation sound is высокотональные the metal clicks coinciding on frequency with turns of a cranked shaft. They can be hardly audible against other sounds of the working engine, especially – at slightly early ignition, and to vanish at absolutely insignificant reduction of giving of fuel (gas). Such hardly appreciable detonation testifies to correctly adjusted coal of an advancing of ignition. But happens and so that detonation knocks appear at once by pressing a gas pedal that, certainly, is inadmissible, and to continue movement in such mode equivalently разбиванию a hammer of interiors of the engine.

Diesel engines are not so sensitive to change of structure of diesel fuel though and in them there are the troubles conducting to raised deterioration of details krivoshipno-shatunnoj groups. It, first of all, an overheat of the engine and the reduction of viscosity of oil connected with it, especially, if it of low quality. The raised deterioration can be a consequence and wrong adjustment of the pump high давлени, and deterioration of dispersion of fuel in chambers of combustion because of infringement of work of atomizers. And, certainly, much depends on the driver.

So, from all aforesaid it is possible to draw such generalised conclusions. The durability of your car, and no less than all vehicle as a whole, depends on two factors: from quality of manufacturing for which the firm-manufacturer, and from level of technical operation for which the driver, finally, is responsible is responsible. It is necessary to remember it both at car purchase, and by preparation and training of drivers.