Four-stroke engine

The duty cycle of a four-stroke engine occurs in four cycles, each of which constitutes one stroke of the piston between the dead points, and the engine passes the following phases:

• Inlet Lasts from 0 to 180 ° rotation of the crank. When the inlet piston moves down from the top dead center, the inlet valve is open. In the cylinder, a vacuum is formed, due to which a fresh charge is sucked into it. In the presence of a supercharger, the mixture is injected into the cylinder under pressure.
• Compression Tact . 180-360 ° rotation of the crank. The piston moves to TDC , while the charge is compressed by the piston to the pressure of the compression ratio. Due to compression, a higher specific power is achieved than an engine operating at atmospheric pressure could have (such as a Lenoir engine ), due to the fact that the entire charge of the working mixture is contained in a small volume. In addition, increasing the compression ratio increases the efficiency of the engine. In Otto engines of any design a combustible mixture is compressed, in diesels - clean air.

At the end of the compression stroke, charge is ignited in Otto engines or the start of fuel injection in Diesel engines.

• Work stroke 360-540 ° crank - piston movement in the direction of the lower dead point under the pressure of hot gases transmitted by the piston through the connecting rod to the crankshaft. In this case, the Otto engine undergoes the process of isochoric expansion; in the diesel engine, due to the continuing combustion of the working mixture, the heat supply continues as long as the injection of a portion of fuel lasts. Therefore, combustion in a diesel engine provides a process close to adiabatic , the expansion occurs at the same pressure.
• Release 540-720 ° rotation of the crank - cleaning the cylinder from the spent mixture. The exhaust valve is open, the piston moves towards the upper dead center, displacing exhaust gases.

In real engines, the timing phases are selected in such a way that the inertia of the gas flows and the geometry of the intake and exhaust paths are taken into account. As a rule, the beginning of the intake ahead of TDC from 15 to 25 °, the end of the intake lags by about the same distance from the BDC, since the inertia of the gas flow provides better filling of the cylinder. The exhaust valve is ahead of the NMT of the working stroke by 40 - 60 °, while the pressure of the burnt gases to the NMT drops and the back pressure on the piston during exhaust is lower, which increases efficiency. The closing of the exhaust valve is also attributed to the TDC intake for more complete removal of exhaust gases.

Since the combustion process and flame front propagation in Otto engines require a certain time, depending on the engine operation mode, and the maximum pressure, due to geometry considerations of the crank drive mechanism, is desirable to have from 40 to 45 ° from TDC of the beginning of the working stroke, ignition is carried out ahead of time - from 2 - 8 ° at idle up to 25 - 30 ° at full load.

The working process of a diesel engine differs from that described above in that the charge in the combustion chamber is clean air heated from compression to the ignition temperature. Some time before TDC, called the initiation time , liquid fuel starts to be injected into the combustion chamber, sprayed to droplets, each of which undergoes initiation , that is, it is heated, evaporating from the surface, during evaporation around each of the droplets a combustible mixture is formed and ignites in hot air . The initiation time for each diesel engine is stable, depends on the design features and changes only with its wear, therefore, unlike the ignition timing, the injection timing in the diesel engine is set once and for all when it is designed and manufactured. Since the mixture in the entire volume of the combustion chamber in a diesel engine is not formed, and the nozzle flame takes up a small volume of the chamber, the amount of air for each volume of injected fuel should be excessive, otherwise the combustion process does not end to end, and the exhaust gases contain a large amount of unburned carbon in the form of soot. The burning itself lasts as long as the injection of this particular batch of fuel lasts - from a few degrees after TDC at idle to 45-50 ° at full power. In high-power diesels, the cylinder can be equipped with several injectors.

• Gas exchange in the cylinder is almost completely provided by the movement of the working piston;
• A separate gas distribution mechanism is used to switch the cylinder cavity to the inlet and the exhaust;
• Each phase of gas exchange is performed during a separate half-turn of the crankshaft;
• The drive of gas distribution systems, ignition and fuel injection should rotate at a frequency twice as low as the engine speed. For this purpose, gear gearboxes and chain or belt drives can be used.

Otto cycle

The four-stroke engine was first patented by Alfon de Roche In 1861. Prior to that, around 1854-1857, two Italians (Eugenio Barsanti and Felice Matozzi) invented an engine that, according to available information, could be very similar to a four-stroke engine, but that patent was lost.

The first person to build the first practically used four-stroke engine was the German engineer Nikolaus Otto . Therefore, a four-stroke cycle is known as an Otto cycle , and a four-stroke engine using spark plugs is called an Otto engine .

Otto's ideal cycle consists of adiabatic compression, the message of heat at a constant volume, adiabatic expansion, and the return of heat at a constant volume. In the practical four-cycle Otto cycle, there are also isobaric compression (exhaust) and isobaric expansion (inlet), which are usually not considered, since in the idealized process they play no role either in conveying heat to the working gas or in performing gas work.

The attributive unit of the four-stroke engine, controls the gas exchange when changing cycles, providing alternate connection of the cylinder cavity to the intake and exhaust manifolds.

Gas distribution control can be carried out:

MECHANICAL:

- a distributing cam shaft or shaft with valves;

- cylindrical sleeve spools moving reciprocating or rotating in the cylinder head;

MICROPROCESSOR. In this case, the valve drive is carried out directly by powerful high-speed electromagnets (BMW) or using a hydraulic actuator (FIAT).

In the first case, the valves are controlled by a camshaft rotating half the slower than the crankshaft. The camshaft has several cams , each of which controls one intake or exhaust valve. From camshafts often include additional engine service devices - oil, fuel pumps, distributor of ignition, fuel pump, sometimes - mechanical superchargers, etc.

Different engines use one or more camshafts located near the crankshaft, above a row of cylinders or even above each row of valves. The drive of the camshafts is carried out from the crankshaft either by the distribution gears, or by a lamellar-roller chain, or by a toothed belt. In some old structures, rollers with bevel gears (B-2) were used. In any case, the shafts are synchronized with speeds of 1: 2.

In any case, the shaft located next to the crankshaft is called the lower one , in the head above or next to the valves - the upper one . The valves on the location relative to the combustion chamber can also be upper - located above the bottom of the piston, or lower - located next to the cylinders on the side. Bottom valves are driven from the lower shaft through short glass-like pushers. The upper valves are driven from the lower shaft, as a rule, by a sucker-like mechanism, from the upper either through rockers (rocker arms) or through glass-like pushers. Many engines use hydraulic tappets that automatically select the gaps in the valve pairs and make the timing distribution mechanism unattended.

The valve is a rod with a plate made of heat-resistant materials. The valve stem reciprocates in the guide sleeve, the plate with a conical sealing strap rests on the valve seat, also made of heat-resistant materials. Both the seat and the guide bushing are the contact surfaces through which the valve is cooled. This position is especially important for exhaust valves that constantly work in hot gas flows (and if the ignition or injection timing is incorrect, they are in a flame flow) and need an intensive heat sink. Therefore, to improve cooling, a cavity with heat-conducting material — with sodium and copper — can be located inside the valve stem. And the contacting surfaces themselves should be smooth and have the minimum possible clearances. Many valves have turning mechanisms that provide forced rotation around the longitudinal axis during operation.

Opening the valve provides the corresponding cam, closing - either the return valve spring / springs, or a special desmodromic mechanism (Daimler-Benz), which allows, due to the absence of springs, to achieve very high speeds of valve movement and, accordingly, significantly increase engine speed distribution mechanism. The fact is that the weaker the valve spring, the slower the return of the valve to the seat. Even when operating at relatively low speeds, weak springs allow the valves to “hang” and come into contact with the pistons (VAZ engines without an internal row of valve springs - at 5500-6000 rpm). The stronger the valve springs, the greater the stress that the timing parts experience, and the more high-quality oil should be used to lubricate it. The desmodromic mechanism allows the valves to be moved with a speed that is limited only by their moment of inertia, that is, substantially higher than the speeds that can be achieved for valves in real engines.

Electromagnetic or electro-hydraulic control with a microprocessor, above that, allows you to easily adjust the valve timing of the engine, achieving the most favorable distribution characteristics in each mode.

Some early engine models (Harley-Davidson, Peugeot) had inlet valves with weak springs that ensured “automatic” opening of the valve after the start of intake under vacuum under the piston.

For the timing correction in the timing with camshafts, various kinds of differentiating mechanisms are used, their design depends on the engine layout and the timing (which largely determines the layout of the entire engine).

The work of the internal combustion engine is accompanied by the release of a significant amount of heat due to the high temperatures of the working gases and significant contact stresses in the friction parts. Therefore, to ensure the operation of the engine, the parts forming friction pairs must be cooled and lubricated, and the products of mechanical wear should be flushed out of the gaps between them. Lubricating oil, in addition to providing an oil wedge in the gaps, removes a significant amount of heat from the loaded rubbing surfaces. For cooling cylinder liners and engine head elements, an additional forced cooling system is used, which can be liquid and air.

The engine lubrication system consists of a container with oil; as such, the sump is often used - in a system with an oil sump or a separate oil tank - in a system with a dry sump . From the tank, the oil is sucked in by an oil pump , gear pump or, less commonly, a rotary pump , and through the channels comes under pressure to friction parts. In a system with an oil sump, cylinder liners and some minor parts are lubricated by spraying; dry sump systems require special lubricators to lubricate and cool these same parts. In engines of medium and high power, the lubrication system includes elements of oil cooling of the pistons in the form of coils poured into the bottoms or special nozzles pouring over the bottom of the piston from the crankcase. As a rule, the lubrication system contains one or more filters for cleaning oil from wear products of friction pairs and tarring the oil itself. Filters are used either with a cardboard curtain with a certain degree of porosity, or centrifugal. For oil cooling, air-oil radiators or water-oil heat exchangers are often used.

In the simplest case, the air cooling system is simply a massive finning of cylinders and heads. The incoming air flow from the outside and the oil from the inside cool the engine. If it is impossible to provide a heat sink with the oncoming flow, a fan with air ducts is included in the system. Along with such indisputable advantages as simplicity of the engine and relatively high survivability in adverse conditions, as well as relatively less weight, air cooling has serious disadvantages:

- a large amount of air blowing through the engine carries a large amount of dust, which accumulates on the fins, especially when oil leaks, which are unavoidable in operation, as a result, the cooling efficiency decreases sharply;

- low heat capacity of the air makes it blow through its engine significant volumes, which requires a significant power take-off for the cooling fan;

- the shape of the engine parts is poorly matched to the conditions of good airflow around it, and therefore it is very difficult to achieve uniform cooling of the engine components Due to the difference in operating temperatures in the individual structural elements, large thermal stresses are possible, which reduces the durability of the structure.

Therefore, air cooling is used infrequently in an internal combustion engine and, as a rule, either on low-cost structures or in cases where the engine runs under special conditions. So, on the front end conveyor of the ZAZ-967 , an air-cooled engine MeMZ-968 is used, the absence of a water jacket, sleeves and a radiator increases the survivability of the conveyor in a battlefield environment.

Liquid cooling has several advantages and is used on ICE in most cases. Benefits:

- the high heat capacity of the liquid contributes to the rapid and efficient removal of heat from the zones of heat generation;

- a much more uniform heat distribution in the structural elements of the engine, which significantly reduces thermal stresses;

- the use of liquid cooling allows you to quickly and efficiently regulate the flow of heat in the cooling system and, therefore, faster and much more evenly than in the case of air cooling, warm up the engine to operating temperature range;

- liquid cooling allows you to increase both the linear dimensions of the engine parts and its thermal stress due to the high efficiency of heat dissipation; поэтому все средние и крупные двигатели имеют жидкостное охлаждение, за исключением ПДП-двухтактных двигателей, у которых зона продувочных окон гильз охлаждается продувочным воздухом из соображений компоновки;

— специальная форма водо-воздушного или водо-водяного теплообменника позволяет максимально эффективно передавать тепло двигателя в окружающую среду.

Недостатки водяного охлаждения:

— повышение веса и сложность конструкции двигателя из-за наличия водяной рубашки;

— наличие теплообменника/радиатора;

— снижение надёжности агрегата из-за наличия стыков рукавов, шлангов и патрубков с возможными течами жидкости;

— обязательное прекращение работы двигателя при потере хотя бы части охлаждающей жидкости.

Современные системы жидкостного охлаждения используют в качестве теплоносителя специальные антифризы , замерзающие при низких температурах и содержащие пакеты присадок разного назначения — ингибиторы коррозии, моющие, смазывающие, антипенные, а иногда и герметизирующие места возможных течей. С целью повышения КПД двигателя системы герметизируют, при этом повышая рабочий диапазон температур к области кипения воды. Такие системы охлаждения работают при давлении выше атмосферного, их элементы рассчитаны на поддержание повышенного давления. Этиленгликолевые антифризы имеют высокий коэффициент объёмного расширения. Поэтому в таких системах часто применяются отдельные расширительные бачки или радиаторы с увеличенными верхними бачками.

С целью стабилизации рабочей температуры и для ускорения прогрева двигателя в системы охлаждения устанавливают термостаты . Для воздушного охлаждения термостат — сильфон , заполненный церезином или этиловым спиртом в сочетании с обоймой и системой рычагов, поворачивающих заслонки, обеспечивающие переключение и распределение воздушных потоков. В системах жидкостного охлаждения точно такой же термоэлемент осуществляет открытие клапана или переключение системы клапанов, направляющих жидкость либо в радиатор, либо в специальный канал, обеспечивающий циркуляцию нагреваемой жидкости и равномерное прогревание двигателя.

Радиатор или теплообменник охлаждения имеет вентилятор, продувающий через него поток атмосферного воздуха, с гидростатическим или электрическим приводом.

Двигатели Отто имеют термический КПД около 40 %, что с механическими потерями дает фактический КПД от 25 до 33%.

Современные двигатели могут иметь уменьшенный КПД для удовлетворения высоких экологических требований.

КПД ДВС можно повысить с помощью современных систем процессорного управления топливоподачей, зажиганием и фазами газораспределения. Степень сжатия современных двигателей, как правило, имеет значения, близкие к предельным (спорный момент, см. Цикл Миллера).

Мощность поршневого двигателя зависит от объёма цилиндров, объёмным КПД , потерь энергии — газодинамических, тепловых и механических, степени сжатия топливо-воздушной смеси, содержания кислорода в воздухе и частоты вращения. Мощность двигателя зависит также от пропускной способности тактов всасывания и выхлопа, а значит, от их проходных сечений, длины и конфигурации каналов, а также от диаметров клапанов, больше впускных. Это справедливо для любых поршневых двигателей. Максимальная мощность ДВС достигается при наивысшем наполнении цилиндров. Частота вращения коленвала в конечном счёте ограничена прочностью материалов и свойствами смазки. Клапана, поршни и коленчатые валы испытывают больши́е динамические нагрузки. На высоких оборотах двигателя могут происходить физические повреждения поршневых колец , механический контакт клапанов с поршнями, что приводит к разрушению двигателя. Поршневые кольца вертикально колеблются в канавках поршней. Эти колебания ухудшают уплотнение между поршнем и гильзой, что приводит к потере компрессии, падении мощности и КПД в целом. Если коленвал вращается слишком быстро, клапанные пружины не успевают достаточно быстро закрывать клапана. Это может привести к контакту поршней с клапанами и вызывать серьёзные повреждения, поэтому на скоростных спортивных двигателях используют привод клапанов без возвратных пружин. Так, «Даймлер-Бенц» серийно выпускает моторы с десмодромным управлением клапанами (с двойными кулачками, один открывает клапан, другой прижимает его к седлу), БМВ использует электромагнитное управление клапанами. На высоких скоростях ухудшаются условия работы смазки во всех парах трения.

Совокупно с потерями на преодоление инерции возвратно-поступательно движущихся элементов ЦПГ, это ограничивает среднюю скорость поршней большинства серийных двигателей 10 м/с.

Четырёхтактные двигатели могут быть как бензиновыми , так и дизельными . Они находят самое широкое применение в качестве первичных двигателей на стационарных и транспортных энергоустановках.

Как правило, четырёхтактные двигатели используются в тех случаях, когда имеется возможность более или менее широко варьировать соотношение оборотов вала со снимаемой мощностью и крутящим моментом либо тогда, когда это соотношение не играет роли при работе машины. Например, двигатель, нагруженный электрогенератором, в принципе может иметь любую рабочую характеристику и согласуется с нагрузкой только по рабочему диапазону оборотов, которые в принципе могут быть любыми, приемлемыми для генератора. Использование промежуточных передач вообще делает четырёхтактный двигатель более адаптированным к нагрузкам в самых широких пределах. Они же являются более предпочтительными в тех случаях, когда установка длительное время работает вне установившегося режима — благодаря более совершенной газодинамике их работа в переходных режимах и режимах со снятием частичной мощности оказывается более устойчивой.

When working on the shaft in a given rev range, especially low-speed (propeller shaft), it is preferable to use two-stroke engines, as having more favorable mass-power characteristics at low revs.

• Ricardo G.R. High-speed internal combustion engines. - M .: GNTI Engineering literature, 1960.