FOUR AND TWO STROKE ENGINE

Engines based on the four-stroke cycle or Otto cycle have one power stroke for every four strokes (up-down-up-down) and are used in cars, larger boats and many light aircraft. They are generally quieter, more efficient and larger than their two-stroke counterparts. There are a number of variations of these cycles, most notably the Atkinson and Miller cycles. Most truck and automotive diesel engines use a four-stroke cycle, but with a compression heating ignition system. This variation is called the diesel cycle.

Engines based on the two-stroke cycle use two strokes (one up, one down) for every power stroke. Since there are no dedicated intake or exhaust strokes, alternative methods must be used to scavenge the cylinders. The most common method in spark-ignition two-strokes is to use the downward motion of the piston to pressurize fresh charge in the crankcase, which is then blown through the cylinder through ports in the cylinder walls.
Spark-ignition two-strokes are small and light (for their power output), and mechanically very simple; they are also generally less efficient and more polluting than their four-stroke counterparts. However in single cylinder small motor applications cc for cc, a two-stroke engine produces much more power than equivalent 4 strokes due to the enormous advantage of having 1 power stroke for every 360 degrees of crankshaft rotation (compared to 720 degrees in a 4 stroke motor).
Two-stroke engines have been less fuel-efficient than other types of engines because unspent fuel being sprayed into the combustion chamber can sometimes escape out of the exhaust duct with the previously spent fuel. Without special exhaust processing, this can produce high pollution levels. Whilst two-stroke motors remain popular in Europe and Asia, they are penalised in some American markets such as California for this reason.
Research continues into improving many aspects of two-stroke motors, including direct fuel injection amongst other things. Initial results have produced motors that are much cleaner burning than their traditional counterparts.
Two-stroke engines are widely used in snowmobiles, lawnmowers, weed-whackers, chain saws, jet skis, mopeds, outboard motors and many motorcycles.
The largest compression-ignition engines are two-strokes, and are used in some locomotives and large ships. These engines use forced induction to scavenge the cylinders. An example of this type of motor is the Wartsila-Sulzer turbocharged 2 stroke diesel as used in large container ships. It is the most efficient and powerful engine in the world, with over 50% thermal efficiency for comparison the most efficient small 4 stroke motors are around 43.0% thermal efficiency (SAE 900648), and size is an advantage for efficiency due to the increase in the ratio of volume to area.

HYDROGEN ENGINE

Some have theorized that in the future hydrogen might replace such fuels. Furthermore, with the introduction of hydrogen fuel cell technology, the use of internal combustion engines may be phased out. The advantage of hydrogen is that its combustion produces only water. This is unlike the combustion of fossil fuels, which produce carbon dioxide, a known green house gas GHG, carbon monoxide resulting from incomplete combustion, and other local and atmospheric pollutants such as sulfur dioxide and nitrogen oxides that lead to urban respiratory problems, acid rain, and ozone gas problems. However, free hydrogen for fuel does not occur naturally, burning it liberates less energy than it takes to produce hydrogen in the first place due to the second law of thermodynamics.
Although there are multiple ways of producing free hydrogen, those require converting combustible molecules into hydrogen, so hydrogen does not solve any energy crisis, moreover, it only addresses the issue of portability and some pollution issues. The disadvantage of hydrogen in many situations is its storage. Liquid hydrogen has extremely low density- 14 times lower than water and requires extensive insulation, whilst gaseous hydrogen requires heavy tankage. Although hydrogen has a higher specific energy, the volumetric energetic storage is still roughly five times lower than petrol, even when liquified. (The 'Hydrogen on Demand' process, designed by Steven Amendola, creates hydrogen as it is needed, but has other issues, such as the high price of the sodium borohydride, the raw material. Sodium borohydride is renewable and could become cheaper if more widely produced.)

Once ignited and burnt, the combustion products, hot gases, have more available energy than the original compressed fuel/air mixture (which had higher chemical energy). The available energy is manifested as high temperature and pressure which can be translated into work by the engine. In a reciprocating engine, the high pressure product gases inside the cylinders drive the engine's pistons.
Once the available energy has been removed, the remaining hot gases are vented (often by opening a valve or exposing the exhaust outlet) and this allows the piston to return to its previous position (Top Dead Center - TDC). The piston can then proceed to the next phase of its cycle, which varies between engines. Any heat not translated into work is normally considered a waste product, and is removed from the engine either by an air or liquid cooling system.
Exhaust gas is flue gas which occurs as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel, fuel oil or coal. It is discharged into the atmosphere through an exhaust pipe or flue gas stack.
Although the largest part, by far, of most combustion gases is relatively harmless nitrogen (N2) and carbon dioxide (CO2), a relatively very small part of it is undesirable noxious or toxic substances, such as carbon monoxide (CO), hydrocarbons, nitrogen oxides (NOx), and particulate matter. When a hydrocarbon combusts, it also produces water, or H2O
Exhaust gas from an industrial plantEmission standards focus on reducing pollutants contained in the exhaust gases from vehicles as well as from industrial flue gas stacks and other air pollution exhaust sources in various large-scale industrial facilities such as petroleum refineries, natural gas processing plants, petrochemical plants and chemical production plants.
In steam engine terminology the exhaust is steam that no longer has the capacity to do useful work, i.e. literally exhausted.

CLASSIFICATION

The motion impulse of the engine is equal to the air mass multiplied by the speed at which the engine emits this mass:
I = m c where m is the air mass per second and c is the exhaust speed. In other words, the plane will fly faster if the engine emits the air mass with a higher speed or if it emits more air per second with the same speed. However, when the plane flies with certain velocity v, the air moves towards it, creating the opposing ram drag at the air intake:
m vMost types of jet engine have an air intake, which provides the bulk of the gas exiting the exhaust. Conventional rocket motors, however, do not have an air intake, the oxidizer and fuel both being carried within the airframe. Therefore, rocket motors do not have ram drag; the gross thrust of the nozzle is the net thrust of the engine. Consequently, the thrust characteristics of a rocket motor are completely different from that of an air breathing jet engine.
The air breathing engine is only useful if the velocity of the gas from the engine, c, is greater than the airplane velocity, v. The net engine thrust is the same as if the gas were emitted with the velocity c-v. So the pushing moment is actually equal to
S = m (c-v) The turboprop has a wide rotating fan that takes and accelerates the large mass of air but only till the limited speed of any propeller driven airplane. When the plane speed exceeds this limit, propellers no longer provide any thrust (c-v < 0).
The turbojets and other similar engines accelerate much smaller mass of the air and burned fuel, but they emit it at the much higher speeds possible with a de Laval nozzle. This is why they are suitable for supersonic and higher speeds.
From the other side, the propulsive efficiency (essentially energy efficiency) is highest when the engine emits an exhaust jet at a speed that is the same as the airplane velocity. The exact formula, given in the literature,[3] is
The low bypass turbofans have the mixed exhaust of the two air flows, running at different speeds (c1 and c2). The pushing moment of such engine is
S = m1 (c1 - v) + m2 (c2 - v) where m1 and m2 are the air masses, being blown from the both exhausts. Such engines are effective at lower speeds, than the pure jets, but at higher speeds than the turboshafts and propellers in general. For instance, at the 10 km attitude, turboshafts are most effective at about 0.4 mach, low bypass turbofans become more effective at about 0.75 mach and true jets become more effective as mixed exaust engines when the speed approaches 1 mach - the speed of sound.
Rocket engines are best suited for high speeds and altitudes. At any given throttle, the thrust and efficiency of a rocket motor improves slightly with increasing altitude (because the back-pressure falls thus increasing net thrust at the nozzle exit plane), whereas with a turbojet (or turbofan) the falling density of the air entering the intake (and the hot gases leaving the nozzle) causes the net thrust to decrease with increasing altitude. Rocket engines are more efficient than even scramjets above roughly Mach 15

Working

Compressing any gas raises its temperature, this is the method by which fuel is ignited in diesel engines. Air is drawn into the cylinders and is compressed by the pistons at compression ratios as high as 25:1, much higher than used for spark-ignite engines. Near the end of the compression stroke, diesel fuel is injected into the combustion chamber through an injector (or atomizer). The fuel ignites from contact with the air that, due to compression, has been heated to a temperature of about 700–900 °C (1300–1650 °F). The resulting combustion causes increased heat and expansion in the cylinder which increases pressure and moves the piston downward. A connecting rod transmits this motion to a crankshaft to convert linear motion to rotary motion for use as power in a variety of applications. Intake air to the engine is usually controlled by mechanical valves in the cylinder head. For increased power output, most modern diesel engines are equipped with a turbocharger, and in some derivatives, a supercharger to increase intake air volume. Use of an aftercooler/intercooler to cool intake air that has been compressed, and thus heated, by the turbocharger increases the density of the air and typically leads to power and efficiency improvements.
In cold weather, diesel engines can be difficult to start because the cold metal of the cylinder block and head draw out the heat created in the cylinder during the compression stroke, thus preventing ignition. Some diesel engines use small electric heaters called glow plugs inside the cylinder to help ignite fuel when starting. Some even use resistive grid heaters in the intake manifold to warm the inlet air until the engine reaches operating temperature. Engine block heaters (electric resistive heaters in the engine block) connected to the utility grid are often used when an engine is turned off for extended periods (more than an hour) in cold weather to reduce startup time and engine wear. Diesel fuel is also prone to 'waxing' in cold weather, a term for the solidification of diesel oil into a crystalline state. The crystals build up in the fuel (especially in fuel filters), eventually starving the engine of fuel. Low-output electric heaters in fuel tanks and around fuel lines are used to solve this problem. Also, most engines have a 'spill return' system, by which any excess fuel from the injector pump and injectors is returned to the fuel tank. Once the engine has warmed, returning warm fuel prevents waxing in the tank. Fuel technology has improved recently so that with special additives waxing no longer occurs in all but the coldest climates.
A vital component of all diesel engines is a mechanical or electronic governor, which limits the speed of the engine by controlling the rate of fuel delivery. Unlike Otto cycle engines, incoming air is not throttled and a diesel engine without a governor can easily overspeed. Mechanically governed fuel injection systems are driven by the engine's gear train. These systems use a combination of springs and weights to control fuel delivery relative to both load and speed. Modern, electronically controlled diesel engines control fuel delivery and limit the maximum RPM by use of an electronic control module (ECM) or electronic control unit (ECU). The ECM/ECU receives an engine speed signal from a sensor and controls the amount of fuel and start of injection timing through electric or hydraulic actuators.
Controlling the timing of the start of injection of fuel into the cylinder is a key to minimizing emissions, and maximizing fuel economy (efficiency), of the engine. The timing is usually measured in units of crank angle of the piston before Top Dead Center (TDC). For example, if the ECM/ECU initiates fuel injection when the piston is 10 degrees before TDC, the start of injection, or timing, is said to be 10 deg BTDC. Optimal timing will depend on the engine design as well as its speed and load.
Advancing the start of injection (injecting before the piston reaches TDC) results in higher in-cylinder pressure and temperature, and higher efficiency, but also results in higher emissions of oxides of nitrogen NOx through higher combustion temperatures. At the other extreme, delayed start of injection causes incomplete combustion and emits visible black smoke made of particulate matter (PM) and unburned hydrocarbon (HC).

Engine

An engine in the broadest sense, is something that produces an output effect from a given input. The origin of engineering however, came from the design, building and working of (military "engines") because before such devices came to be employed in battles there were very few mechanical devices used. Military engines included siege engines, large catapults, trebuchets, battering rams etc. So the first engineers were military engineers, then later as engineering developed, there came Civil engineers. These were engineers who dealt with designing, building and commissioning roads, bridges, docks and wharves, large public and private buildings etc. There also exists an overlap in English between two meanings of the word "engineer": "those who operate engines" and "those who design and construct new items."





An [engine] whose effect is to produce kinetic energy output from a fuel source is called a prime mover alternatively, a device that also produces kinetic energy, but from a preprocessed "fuel" such as electricity, a flow of hydraulic fluid or compressed air is called a motor. Unfortunately some everyday English language confusion about this delineation exists that sometimes leads to expensive errors. For example, a service specialist (an electrical engineer) might be called upon to travel some way to examine a faulty "motor" - to find on arrival at the site that the so-called "motor", is in fact, a rather large diesel engine that he has no knowledge of being outside his area of specialisation!





An ordinary car (up to the present time) has a starter motor, a windscreen wiper motor, windscreen washer motor, a fuel pump motor and motors to adjust the wing mirrors from within the car - but the power plant that propels the car is an engine. Again an aircraft will have many motors installed for operation of its many auxiliary operations and services, but aircraft are propelled by engines, in this case, jet engines.