Engines

Internal combustion engines are most commonly used engines. In mobile scenarios internal combustion is advantageous, since it can provide high power to weight ratios together with excellent fuel energy-density. All internal combustion engines depend on the exothermic chemical process of combustion: the reaction of a fuel, typically with air, although other oxidizers such as nitrous oxide may be employed.

Classification of Internal Combustion Engine

Internal Combustion engines are classified as reciprocating or rotary, spark ignition or compression ignition, and two-stroke or four-stroke; the most familiar combination, used from automobiles to lawn mowers, is the reciprocating, spark-ignited, four-stroke gasoline engine.

The most common internal combustion engine type is gasoline powered. Others include those fueled by diesel, hydrogen, methane, propane, etc. Engines typically can only run on one type of fuel and require adaptations to adjust the air/fuel ratio or mix to use other fuels.

Classification according to Principles of operation

Reciprocating:

·         Two-stroke cycle

·         Four-stroke engine

·         Sleeve valve four-stroke

·         Bourke engine

·         Crude oil engine

Rotary:

·         Demonstrated:

·       Wankel engine

·         Proposed:

·        Orbital engine

·          Quasiturbine

Continuous combustion:

·         Gas turbine

·         Jet engine

·         Rocket engine

Classification according to Engine cycle

Two Stroke Engine

By suitable design it is possible to operate an Otto-cycle or diesel as a two-stroke or two-cycle engine with a power stroke every other stroke of the piston instead of once every four strokes. The power of a two-stroke engine is usually double that of a four-stroke engine of comparable size.

The general principle of the two-stroke engine is to shorten the periods in which fuel is introduced to the combustion chamber and in which the spent gases are exhausted to a small fraction of the duration of a stroke instead of allowing each of these operations to occupy a full stroke.

In the simplest type of two-stroke engine, the poppet valves are replaced by sleeve valves or ports (openings in the cylinder wall that are uncovered by the piston at the end of its outward travel). In the two-stroke cycle, the fuel mixture or air is introduced through the intake port when the piston is fully withdrawn from the cylinder. The compression stroke follows, and the charge is ignited when the piston reaches the end of this stroke. The piston then moves outward on the power stroke, uncovering the exhaust port and permitting the gases to escape from the combustion chamber.

Four Stoke

The ordinary Otto-cycle engine is a four-stroke engine; that is, in a complete power cycle, its pistons make four strokes, two toward the head (closed head) of the cylinder and two away from the head.

The four-stroke approach is also known as the Otto cycle, in honor of Nikolaus Otto, who invented it in 1867. They are:

·         Intake stroke

·         Compression stroke

·         Combustion stroke

·         Exhaust stroke

1.      During the first stroke of the cycle, the piston moves away from the cylinder head while simultaneously the intake valve is opened. The motion of the piston during this stroke sucks a quantity of a fuel and air mixture into the combustion chamber.

2.       During the next stroke, the piston moves toward the cylinder head and compresses the fuel mixture in the combustion chamber.

3.      At the moment when the piston reaches the end of this stroke and the volume of the combustion chamber is at a minimum, the fuel mixture is ignited by the spark plug and burns, expanding and exerting a pressure on the piston, which is then driven away from the cylinder head in the third stroke.

4.      During the final stroke, the exhaust valve is opened and the piston moves toward the cylinder head, driving the exhaust gases out of the combustion chamber and leaving the cylinder ready to repeat the cycle.

The efficiency of a modern Otto-cycle engine is limited by a number of factors, including losses by cooling and by friction. In general, the efficiency of such engines is determined by the compression ratio of the engine.

The compression ratio (the ratio between the maximum and minimum volumes of the combustion chamber) is usually about 8 to 1 or 10 to 1 in most modern Otto-cycle engines. Higher compression ratios, up to about 15 to 1, with a resulting increase of efficiency, are possible with the use of high-octane antiknock fuels. The efficiencies of good modern Otto-cycle engines range between 20 and 25 percent—in other words, only this percentage of the heat energy of the fuel is transformed into mechanical energy.

Classification according to Fuel and oxidizer types

Fuels used include gasoline, liquified petroleum gas, vapourized petroleum gas, compressed natural gas, hydrogen, diesel fuel, JP18 (jet fuel), landfill gas, biodiesel, peanut oil, ethanol, methanol (methyl or wood alcohol). Even fluidised metal powders and explosives have seen some use.

Engines that use gases for fuel are called gas engines and those that use liquid hydrocarbons are called oil engines. However, gasoline engines are also referred to as 'gas engines'.

Diesel engines are generally heavier, noisier and more powerful at lower speeds than gasoline engines. They are also more fuel-efficient in most circumstances and are used in heavy road-vehicles, some automobiles (increasingly more so for their increased fuel-efficiency over gasoline engines), ships and some locomotives and light aircraft.

Gasoline engines are used in most other road-vehicles including most cars, motorcycles and mopeds. Both gasoline and diesel engines produce significant emissions. There are also engines that run on hydrogen, methanol, ethanol, liquefied petroleum gas (LPG) and biodiesel. Paraffin and tractor vaporising oil (TVO) engines are no longer seen.

Classification according to Cylinders

Internal combustion engines can contain any number of cylinders with numbers between one and twelve being common, though as many as 30 have been used. Having more cylinders in an engine yields two potential benefits:

·         First, the engine can have a larger displacement with smaller individual reciprocating masses (that is, the mass of each piston can be less) thus making a smoother running engine (since the engine tends to vibrate as a result of the pistons moving up and down).

·         Second, with a greater displacement and more pistons, more fuel can be combusted and there can be more combustion events (that is, more power strokes) in a given period of time, meaning that such an engine can generate more torque than a similar engine with fewer cylinders.

The down side to having more pistons is that, over all, the engine will tend to weigh more and tend to generate more internal friction as the greater number of pistons rub against the inside of their cylinders. This tends to decrease fuel efficiency and rob the engine of some of its power. For high performance gasoline engines using current materials and technology (such as the engines found in modern automobiles), there seems to be a break point around 10 or 12 cylinders, after which addition of cylinders becomes an overall detriment to performance and efficiency, although exceptions such as the W-16 engine from Volkswagen exist.

Most car engines have four to eight cylinders, with some high performance cars having ten, twelve, or even sixteen, and some very small cars and trucks having two or three. In previous years some quite large cars, such as the DKW and Saab 92, had two cylinder, two stroke engines.

Radial aircraft engines, now obsolete, had from five to 28 cylinders. A row contains an odd number of cylinders, so an even number indicates a two- or four-row engine.

Motor cycles commonly have from one to four cylinders, with a few high performance models having six.

Snowmobiles usually have two cylinders. Some larger (not necessarily high-performance, but also touring machines) have four.

Small portable appliances such as chainsaws, generators and domestic lawn mowers most commonly have one cylinder, although two-cylinder chainsaws exist.

Classification according to Ignition system

Internal combustion engines can be classified by their ignition system. Today most engines use an electrical or compression heating system for ignition. However outside flame and hot-tube systems have been used historically. Nikola Tesla gained one of the first patents on the mechanical ignition system with U.S. Patent 609250, "Electrical Igniter for Gas Engines", on 16 August 1898.

 

Classification according to Fuel systems

Often for simpler reciprocating engines a carburetor is used to supply fuel into the cylinder. However, exact control of the correct amount of fuel supplied to the engine is impossible.

Larger gasoline engines such as used in cars have mostly moved to fuel injection systems. LPG engines use a mix of fuel injection systems and closed loop carburetors. Diesel engines always use fuel injection.

Other internal combustion engines like jet engines use burners, and rocket engines use various different ideas including impinging jets, gas/liquid shear, preburners and many other ideas.

Classification according to Engine configuration

Internal combustion engines can be classified by their configuration which affects their physical size and smoothness (with smoother engines producing less vibration). Common configurations include the straight or inline configuration, the more compact V configuration and the wider but smoother flat or boxer configuration. Aircraft engines can also adopt a radial configuration which allows more effective cooling. More unusual configurations, such as "H", "U", "X", or "W" have also been used.

Multiple-crankshaft configurations do not necessarily need a cylinder head at all, but can instead have a piston at each end of the cylinder, called an opposed piston design. It was also used in single-bank locomotive engines, and continues to be used for marine engines, both for propulsion and for auxiliary generators. The Gnome Rotary engine, used in several early aircraft, had a stationary crankshaft and a bank of radially arranged cylinders rotating around it.

Classification according to Engine capacity

An engine's capacity is the displacement or swept volume by the pistons of the engine. It is generally measured in litres or cubic inches for larger engines and cubic centimetres (abbreviated to cc's) for smaller engines. Engines with greater capacities are usually more powerful and provide greater torque at lower rpms but also consume more fuel.

Apart from designing an engine with more cylinders, there are two ways to increase an engine's capacity. The first is to lengthen the stroke and the second is to increase the piston's diameter. In either case, it may be necessary to make further adjustments to the fuel intake of the engine to ensure optimal performance.

An engine's quoted capacity can be more a matter of marketing than of engineering. The Morris Minor 1000, the Morris 1100, and the Austin-Healey Sprite Mark II all had engines of the same stroke and bore according to their specifications, and were from the same maker. However the engine capacities were quoted as 1000cc, 1100cc and 1098cc respectively in the sales literature and on the vehicle badges.

Multiple cylinder, high speed, vertical engines are mostly used as automotive engines on farm tractors. Four stroke cycle engines are largely used both as stationary and automotive types. Two stroke engines have very limited use on farms.

To provide more frequent power impulses per revolution, tractors are also equipped with multiple cylinder engines. A few large size tractors are also equipped with six cylinder engines in which the power stroke occurs after every 120 degrees of the fly wheel travel. Six cylinders engines used on tractors have the firing order 1-5-3-6-2-4. This firing order is preferred as no two adjacent cylinders fired consecutively. It is also possible to provide nearly equal fuel-air mixtures distribution with this firing order. In all tractor engines, cylinders are numbered from the front side of the tractor.

Tractors are mostly equipped with high speed engines running at about 1500 rpm. Low and medium speed engines are usually employed for stationary jobs.